C Lantz

R Brunetto

M A Barucci

S Fornasier

D Baklouti

J Bourc ¸ois

M Godard

Ion irradiation of carbonaceous chondrites: a new view of space weathering on primitive asteroids

Keywords: Asteroids, Surfaces, Spectroscopy, Meteorites, Experimental techniques

We present an experimental study on ion irradiation of carbonaceous chondrites, simulating solar wind irradiation on primitive asteroids, to better constrain the space weathering processes of low albedo objects. The irradiations were performed on pressed pellets of the CV Allende, CO Frontier Mountain 95002 and Lancé, CM Mighei, CI Alais, and ungrouped Tagish Lake meteorites, as well as on some silicate samples (olivine and diopside). We used 40 keV He + with fluences up to 6•10 16 ions/cm 2 corresponding to timescales of 10 3 -10 4 years for an object in the Main Belt. Reflectance spectra were acquired ex situ before and after irradiations in the visible to mid-infrared range (0.4 -16 µm). Several spectral modifications are observed. In the MIR range, we observe a shift of the phyllosilicates (near 3 and 10 µm) and silicates (near 10 µm) bands toward longer wavelength. In the visible-NIR range, spectral darkening and reddening are observed for some samples, while others show spectral brightening and blueing. Results are also compared with previous irradiation on ordinary and carbonaceous chondrites. We find that the spectral modifications in the visible range are correlated with the initial albedo/composition. We propose a model for space weathering effects on low albedo objects, showing that those with initial albedo between 5 and 9 % shall not suffer SpWe effects in the visible range. These experiments provide new clues on spectroscopic features modifications within the visible-infrared ranges that could be detected in situ by future sample return missions (Hayabusa-2/JAXA and OSIRIS-REx/NASA).

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Introduction

Surface alteration processes -also known as space weathering (SpWe), i.e. ion irradiation and micrometeorite impacts -on atmosphere-less small bodies induce variations of the spectral properties of surface materials, thus preventing the identification of sub-surface primitive materials preserved since Solar System formation. Understanding how these modifications act on asteroids and change their spectra will cast light on a better compositional interpretation of remote observations, and can provide valuable support for the forthcoming space missions planned to collect samples of primitive objects and bring them back to Earth (Hayabusa-2/JAXA [Takagi et al., 2011] and OSIRIS-REx/NASA [START_REF] Lauretta | Handbook of Cosmic Hazards and Planetary Defense, chap[END_REF]). Primitive asteroids (C-complex; [START_REF] Bus | [END_REF]DeMeo et al., 2009]) are especially studied because they are thought to contain organic compounds and some water, key elements for life on Earth.

SpWe effects on silicates are pretty well understood with the role of nanophase reduced iron particles (npFe 0 ), producing darkening and reddening (increasing slope toward longer wavelength) of their surfaces [Clark et al., 2002a;[START_REF] Brunetto | Asteroids IV[END_REF] -and observed in situ on S-type asteroids Eros [Clark et al., 2001] and Itokawa [Ishiguro et al., 2007]. However, there is no general conclusion on what is the spectral trend of C-complex asteroids when exposed to SpWe. Dedicated studies based on spectroscopic observations showed both reddening [START_REF] Lazzarin | [END_REF]Fornasier et al., 2016;[START_REF] Kaluna | Lunar and Planetary Science Conference[END_REF] and blueing [Nesvorný et al., 2005;Lantz et al., 2013] effects on spectral slopes. SpWe effects can be simulated in the laboratory using ion (solar wind simulation) or laser (micrometeorite impacts simulation) irradiations on meteorite samples,

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A N U S C R I P T and most of these experiments on primitive materials were studied on few samples at the same time (see Table 1 for a resume). Hiroi et al. [2013] [Lantz et al., 2015]. Here we perform a systematic experimental study on ion irradiation of CCs to better constrain the SpWe of dark objects. Thereby we use the same sample preparation, the same irradiation process and the same spectroscopic measurement method for all the studied samples. This allows us to focus on SpWe effects without being strongly affected by variations of other parameters (powder/pellet, grain size, phase angle, or temperature effects) blurring comparisons between different meteorites compositional classes. This new set of irradiations is dedicated to study the SpWe effects of a wide variety of CCs: CV Allende, CO Frontier Mountain 95002 and Lancé, CM Mighei, CI Alais, and ungrouped Tagish Lake meteorites. We also analyze natural silicates (olivine and diopside) to compare with S-type asteroids and previous

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experiments on ordinary chondrites (OCs) [Brunetto et al., 2006;Fu et al., 2012].

The diopside was not studied as an analog of pyroxene found within OC (the Ca content being very different) but was an easily available sample to test our experimental set up. We expanded on the work of previous studies [Brunetto et al., 2014;Lantz et al., 2015] by adding an additional irradiation step at a higher dose and also included a new sample of Allende. This sample will allow comparison with our previous results on Allende and validate our new experimental setup.

Experimental techniques

We crushed the silicate and CC samples into powder using agate mortar and pestle without constraining the grains size (range 1-100 µm) and we prepared pellets by pressing ∼100 mg of meteorite on a KBr substrate (samples are held vertically; diameter 13 mm). KBr only supports the meteorite powder (∼200 µm thickness): no ion (implantation depth is a few hundred nanometers [Brunetto et al., 2014]) nor IR light reach the KBr substrate interface. Ion irradiation of the pellets was performed on the IRMA platform (CSNSM, Orsay) with He + at 40 keV using the following fluences: 5•10 15 , 1•10 16 , 3•10 16 and 6•10 16 ions/cm 2 .

The whole pellet surface is irradiated at each step (additive doses: to reach the value of 1•10 16 we used twice the dose of 5•10 15 ions/cm 2 , and so on) except a corona of 500 µm thick on the pellet edge that was hidden by the sample holder.

Irradiation was performed in a vacuum chamber (P ∼10 -7 mbar), and the samples were placed under vacuum (P ∼ 10 -2 mbar) between irradiation sessions and ex situ MIR reflectance measurements.

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This sample preparation and irradiation protocol was chosen with respect to our previous studies. Here we only focused on the Helium ions because of experimental time limitations. Helium is the second most abundant ion in solar wind.

See Brunetto et al. [2014] and Lantz et al. [2015] for details.

Bidirectional reflectance spectroscopy in the visible (VIS, 0.4 -1.05 µm) and near-infrared (NIR, 0.9 -2.5 µm) ranges was performed using the INGMAR setup (IrradiatioN de Glaces et Météorites Analysées par Réflectance VIS-IR, IAS-CSNSM/Orsay). Spectra were collected through VIS-NIR fibers coupled to a grating spectrometer in the VIS range (Maya2000Pro, Ocean Optics) and a Fourier Transform spectrometer in the NIR range (Tensor37, Bruker) at ambient temperature. We used a fixed collection angle e = 15˚and illumination angles i = 15˚(phase angle was 20˚because of 3D-geometry) in the VIS, or i = 20˚(phase angle was 15˚) in the NIR. VIS and NIR spectra are acquired on the same pellet location with spectral resolutions of 0.5 nm and 16 cm -1 respectively. The collecting spot size is 3-4 mm, larger than the pellet heterogeneity, and thus representative of all constituents of the meteorite (matrix and chondrules). The VIS and NIR spectral ranges were combined by overlapping the spectra, merging the two wavelength regions at the common wavelengths (0.92-0.98 µm). Reference spectra were acquired using a Spectralon standard (with 99 % Lambertian reflectance; Labsphere) to calibrate the absolute reflectance of our measurements.

The samples were installed on the sample holder and never moved from there between irradiations and measurements. The sample holder is mounted first on the INGMAR chamber to collect spectra before irradiation, then on the IRMA ion beamline, and finally back again on INGMAR to collect spectra after the first

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irradiation. We repeat this scheme until the last irradiation step. The INGMAR configuration allows to have reproducible measurements: spectra are collected at the same position on the pellet, with an error of less than 100 µm. Thus, the error estimated on the reflectance is about 1 % in relative value for each sample. This is calculated by changing the collecting spot position on the sample and collecting a spectrum each time, to finally evaluate the reflectance level difference. Within the above mentioned uncertainties, there was no change in the slope.

A micro-analysis was performed from Near to Mid Infrared (NIR-MIR, 2 -16 µm) at the SMIS beamline (Spectroscopy and Microscopy in the Infrared using Synchrotron; Dumas et al. [2006]) of the Synchrotron SOLEIL (France) at ambient temperature using a microscope (Nicolet iN10, Thermo Scientific) with the internal Globar light source (focused beam of 200 µm, confocal reflection).

Reference spectra were collected on a gold sample and the spectral resolution was 4 cm -1 . These measurements were performed after the overall protocol presented above to avoid too many manipulations of the samples from the lab to SOLEIL, back and forth, and we thus only have measurements with the highest fluence of irradiation for this spectral range. Spectra are an average of ten or more measurements acquired on different regions of the sample (on each zone, virgin then irradiated), thus we have to deal with possible heterogeneity of the samples at that scale.

Measurements were performed every 100 µm and the spot size was 200•200 µm 2 covering a grid. These spectra were analyzed individually to estimate the error in the determination of all band peak positions associated to sample heterogeneity estimated to about 0.02 µm ( the spectral resolution is about 0.001 µm at 2.7 µm and about 0.01 µm at 10 µm). Allende is an oxidized CV3 chondrite displaying non equilibrated mineral clusters [Rubin & Wasson, 1987]. It contains large chondrules (mm size) and large refractory inclusions within an abundant matrix (40 vol%). The carbon content is low (0.3 wt%) [START_REF] A C C E P T E D M A N U S C R I P T Kerridge | [END_REF]. Frontier Mountain 95002 (FM 95002) is a CO3 having few or no hydrated material [Rubin & Wasson, 1987;[START_REF] Keller | [END_REF]Zolensky et al., 1993]. The matrix is 30-40 vol%, and the carbon content is about 0.6 wt% [Pearson et al., 2006]. Lancé (CO3.5 type) has the same overall features.

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Mighei, a typical CM2, has small chondrules (∼200 µm) and refractory inclusions, an abundant matrix (∼70 vol%) and lots of hydrated minerals (∼75 vol%) [Howard et al., 2009]. It contains about 2 wt% of carbon [START_REF] A C C E P T E D M A N U S C R I P T Kerridge | [END_REF], mainly in the form of insoluble organic matter, but the soluble part shows a large diversity of organic compounds [Schmitt-Kopplin et al., 2010]. Alais, as a CI1, has the most abundant matrix (99 vol%, no chondrule) and largely underwent aqueous alteration with 50-60 vol% of phyllosilicates [Cloutis et al., 2011a]. The carbon

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content is about 5.4 wt% [Pearson et al., 2006]. Tagish Lake is an ungrouped meteorite having petrological similarity to the CIs and CMs, eventhough Blinova et al. [2014a] showed it is not a simple mixture of CI and CM material. It also underwent aqueous alteration (it is often described as C2 type) but with different degrees, leading to a great lithological heterogeneity for the Tagish Lake meteorite [START_REF] A C C E P T E D M A N U S C R I P T Izawa | [END_REF]Blinova et al., 2014b]. However, this peculiar CC has a lower density (1.7 g/cm 3 [Gounelle et al., 2001]) and is the most carbon-rich chondrite (5.5 wt%) [Grady et al., 2002] and the darkest one.

Results

VIS-NIR

The CC sample spectra are presented in Fig. 1 and 2 while the case of terrestrial silicates is discussed in Appendix A.

The Allende, Lancé, and FM 95002 meteorite spectra before irradiation look similar in shape: strong UV drop-off, a maximum of reflectance around 0.6 µm, and a decrease toward longer wavelengths, accompanied with a 1 µm band due to silicates [Cloutis et al., 2011b]. The spectra are bluish because of the sample preparation: pellets have bluer spectra than unpacked powder (see Append. B).

The absolute reflectance at 0.55 µm (at 20˚phase angle; referred as R 0.55 in the following text) is 12, 15 and 17 % respectively (see Table 2 for the whole values), in agreement with literature data on CV and CO chondrites [Cloutis et al., 2012b,a] (though pellet spectra tend to be brighter than the powder ones; see Append. B). Dark meteorites are also known to show a strong opposition effect: we should expect an increase in the reflectance at lower phase angles [Beck et al.,

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After irradiation, R 0.55 decreases by about 2 % in absolute reflectance for the three pellets and the slope increases (e.g. a linear fit between 0.55 and 2.45 µm on Allende gives a slope of -0.20 µm -1 before irradiation and goes to -0.12 µm -1 after). We notice darkening and reddening effects on these samples.

The three other CCs (Mighei, Alais, and Tagish Lake) are darker before irradiation ( R 0.55 of 5, 4 and 2 % respectively) and rich in phyllosilicates (see MIR spectra further), even though we do not see any feature associated to the aqueous alteration process in the visible range (see e.g. Cloutis et al. [2011b] for Mighei spectra having the 0.7 µm band). Our Alais spectrum is consistent with those published in Cloutis et al. [2011a]; the same for our Tagish Lake sample compared to Hiroi et al. [2004] data. After irradiation, R 0.55 increases by about 1 % in absolute reflectance for these samples. The slope gets slightly bluer on Mighei and Alais, but the effect is strong on Tagish Lake (slope on the 0.55-2.45 µm range is -0.01 µm -1 before and -0.13 µm -1 after irradiation).

The dichotomy between the brightest and darkest CCs are summarized in Fig. 3 where we compare the two endmembers (in brightness level) of our set of samples, FM 95002 and Tagish Lake. With increasing irradiation dose, the reflectance and the slope are affected differently for these objetcs. The effects of darkening and brightening are visible with naked eye on the pellets, as shown in Fig. 4.

The red/blue slope variation is a very well known effect of grain size (see the review of CCs spectroscopic study by Cloutis et al. [2011a]). Because we did not select a size for the study, our samples represent on average an asteroid surface with a wide distribution of grains (even if all meteorites do not grind up the same way). Irradiation cannot change grain size (from microscopic

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A N U S C R I P T point of view) but it may affect roughness (at nanometric scale). So the variations noticed here before and after irradiation are effects of simulated SpWe.

NIR-MIR

Spectra in the 8 -16 µm range are dominated by silicate bands, and in the 2.4 -3.3 µm range by the hydrated minerals features. The organics near 3.4 µm were hardly detected on our samples. We also recall that, in the MIR, the silicate bands are intense and features appear upside down (reststrahlen peaks) thanks to a rapid increase of refractive index leading to strong surface scattering regime [START_REF] Salisbury | [END_REF]].

On the three hydrated chondrites (Mighei, Alais, and Tagish Lake), a large absorption band is seen around 3 µm (see Fig. 5): a sharp feature at 2.71 ±0.01 µm associated to Mg-rich phyllosilicates (OH stretching) plus a contribution of adsorbed/interlayer molecular water. This is in agreement with spectra published by Beck et al. [2010] on Mighei and Alais, and Hiroi et al. [2001] and Izawa et al. [2008] on Tagish Lake. After irradiation, the band minimum shifts to 2.80, 2.78, and 2.77 ± 0.01 µm, respectively, and the sharp feature disappears. Spectra before irradiation of Allende, Lancé, and FM 95002 (see Fig. 6) all have the same five peaks, with the olivine bands corresponding to a forsterite number (abundance of magnesium silicate, defined as Mg/(Mg+Fe), from 0 to 100) of Fo ≈ 50-55 and shifting toward Fo ≈ 30-35 after irradiation (olivine bands number 2 and 4 following the calibrations by Hamilton [2010]). Similar values were found with previous measurements of an irradiated Allende sample performed by our team [Brunetto et al., 2014]. The bands seen on Allende are in agreement with those measured by Beck et al. [2014]. On Mighei, Alais and Tagish Lake (see 7), the phyllosilicates dominate the MIR range. Alais has an intense peak at 9.83 ± 0.01 µm (as observed by Beck et al. [2010]) shifting to 10.12 ± 0.01 µm and broadening after irradiation. Mighei presents a broad structure with a maximum at 10.20 ± 0.01 µm shifting to 10.81 ± 0.01 µm after irradiation. The structure is made by the superposition of the phyllosilicates contribution at 9.9 µm, of the olivine peaks at 10.4 and 11.2 µm, and possibly of the enstatite at 11.5 µm,

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similarly to what we measured on the other CM Murchison [Lantz et al., 2015].

Tagish Lake has two distinct peaks: the first one, similar to that observed on Alais, at 9.82 ± 0.01 µm (due to phyllosilicates) shifting to 10.21 ± 0.01 µm after irradiation, and a second one at 11.53 ± 0.01 µm (probably enstatite too) shifting to 11.56 ± 0.01 µm after irradiation. Alais and Tagish Lake also have a peak around 8.65 ± 0.03 µm, and Tagish Lake has two peaks above 13 µm, all shifting the same way.

The case of terrestrial silicates is also discussed in Appendix A.

Raman spectroscopy (see Brunetto et al. [2014] for details on the Raman technique and data reduction) shows the enlargement and displacement of carbon D and G bands for all the samples, similarly to what has been observed in the case of Allende [Brunetto et al., 2014] and Murchison [Lantz et al., 2015]: there is an amorphization of the poly-aromatic structure. In Fig. 8

Discussion

The silicate features

After irradiation of each sample, we observe a systematic of the silicate/phyllosilicate bands around 10 µm toward a spectral region associated with iron-richer minerals (using calibrations on olivine by Hamilton [2010]). The same effect is observed on the aqueous alteration band at 2.7 µm (hydrated minerals only seen on Mighei, Alais, and Tagish Lake). These results confirm what we saw on Allende [Brunetto et al., 2014] and Murchison [Lantz et al., 2015], and the new measurement on

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Tagish Lake is in agreement with the study by Vernazza et al. [2013] who also noticed the shift and a certain broadening of the MIR bands for this meteorite.

The spectral shift of the IR bands could be due to a loss of magnesium, as a result of preferential sputtering (a differential sputtering of atoms near the surface; the volatile Mg being more easily sputtered backward than the heavier Fe) [Hapke et al., 1975] possibly leading to amorphization (seen thanks to the broadening of the bands, and confirmed by Raman) [Brucato et al., 2004;Demyk et al., 2004],

or preferential amorphization of Mg-rich olivine. See Brunetto et al. [2014] for the full discussion. Because of preferential sputtering, there is more Mg loss compared to Fe, but Fe is also a bit sputtered and, as it is heavier, it can be more easily The bottom panels of Fig. 6, 7, and 16 show the shift for each band as a function of the initial position. It is interesting to note that the brightest CCs, i.e. Allende, Lancé, and FM 95002, have the same shift for similar bands. We note that the shift on CCs (Fig. 6) is smaller than on the olivine sample (Fig. 16): this is because olivine before irradiation has a higher forsterite number, so that more Mg-minerals can be amorphized/sputtered than the CCs. However, one has to remember that in meteorites the olivine is not pure. With the Fig. 9, we confirm that a relation exists between the measured band shift and the reflectance of each band for a sample: the stronger the absorption, the larger the band shift. We already noticed this trend during our previous study dedicated to Allende [Brunetto et al., 2014]. We proposed that it is due to the balance between ion penetration depth and IR probe depth. A material having a strong imaginary index, like olivine in its strongest IR bands, almost acts like a mirror and a surface scattering regime takes place. So photons in the strongest bands probe a thin layer that is affected by irradiation. The smaller bands have photons emerging from a deeper layer that is not fully modified by irradiation and effects are less noticeable. As an interesting fact, we notice that the most intense peak of olivine (on CCs or terrestrial olivine sample; corresponding to band number 3) exceeds the position of a crystalline olivine with a forsterite number of 0 (no Mg content = 100 % fayalite) [Hamilton, 2010]: this is because Mg loss is also accompanied by amorphization [Demyk et al., 2001;Carrez et al., 2002].

One has to note that these band shifts do not disagree with the possible formation of nanophase reduced iron particles (npFe 0 ) [Hapke, 2001]: the shift toward Fe-rich spectral region could also be due to these npFe 0 (enrichment of Fe). As a matter of fact, Dukes et al. [2015] saw an increase of the Fe/Si ratio on their Tagish Lake sample after ion irradiation. [START_REF] Keller | Space Weathering of Airless Bodies: An Integration of Remote Sensing Data[END_REF] and Matsuoka et al. [2015] did not see any shift of the NIR and MIR bands, but they both used STEM (Scanning Transmission Electron Microscopy) and observed amorphization of phyllosilicates (serpentine group). In the first case

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A N U S C R I P T thus it will be impossible to use this spectroscopic information of SpWe from remote sensing observations. The thermal region near 10 µm is even less studied from the ground and data are prone to errors due to highly variable telluric ozone structure (9.2 -10.2 µm) [Lim et al., 2005] and requires the use of space telescopes like Spitzer did [Emery et al., 2005]. These SpWe effects on MIR spectra will be hardly seen with telescopic observations, but they could be used during the characterization of asteroid surfaces by space missions and help choosing sampling sites. For example, we detect band shifts of at least 50 nm at 2.7 µm: resolution of VIR onboard Dawn/NASA is 10 nm at this wavelength [de Sanctis et al., 2011] and is about 20 nm for OVIRS on OSIRIS-REx/NASA [Reuter & Simon-Miller, 2012] and NIRS3 on Hayabusa-2/JAXA. Around 10 µm, resolution of OTES on OSIRIS-REx spacecraft is 10 cm -1 [Hamilton & Christensen, 2014], that is about 10 nm while we dectect band shift of at least 20 nm for the most intense bands.

In the future, the James Webb Space Telescope could provide more numerous and accurate data for both NIR and MIR ranges.

The darkest surfaces case

VIS-NIR trends for irradiated CCs as a function of the initial composition

With the new experiment, we obtain and we confirm the VIS-NIR darkening/reddening trend for the CO and CV types of carbonaceous chondrites [START_REF] Lazzarin | [END_REF]Brunetto et al., 2014]. Our new irradiations of olivine and diopside also confirm this very well-known effect of ion/laser irradiation on silicates and OCs [Yamada et al., 1999;Sasaki et al., 2001Sasaki et al., , 2002;;Brunetto & Strazzulla, 2005;Marchi et al., 2005;Strazzulla et al., 2005;Brunetto et al., 2006;Loeffler et al., 2009;Fu et al., 2012] (see Append. A). The importance of npFe 0 in these

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modifications has been underlined by many authors [Hapke, 2001;Noble et al., 2007] and their presence confirmed on S-type NEA Itokawa (Hayabusa/JAXA mission) [Hiroi et al., 2006;Yoshikawa et al., 2007] as a result of solar wind implantation accompanied with amorphization of target minerals and redeposition of sputtered materials [Noguchi et al., 2011[Noguchi et al., , 2014]]. Finally, here we show that Mighei, Alais and Tagish Lake become brighter and bluer after SpWe simulation.

The response to SpWe of the darkest CCs is obviously different.

In our previous attempt dedicated to understand SpWe effects on primitive asteroids, and considering other studies, we proposed a composition dependent trend [Lantz et al., 2015], to explain the non convergent conclusion on reddening/blueing and darkening/brightening debate. If we strictly compare experiments done in the same way, the new set of irradiations made here allows us to separate the effect of the initial composition from other effects such as sample preparations, experimental setups, and viewing geometries. Then, we focus on the weathering effect by performing the ratio of altered spectrum divided by the unaltered spectrum. These ratios allow to quickly visualize how the SpWe change the spectra with increasing irradiation: the reflectance ratio is lower than 1 when surface darkens and ratio slope has a positive value if the spectrum reddens. The resulting reflectance and slope of the ratioed spectra are reported in Fig. 10. They indicate that the spectral reponse for SpWe of CCs, and in extent of C-complex asteroids, are related to the initial composition and the carbon content. With higher irradiation doses, some CCs will become redder and darker like OCs (chondrules richer), while the most primitive (matrix and carbon richer) will become bluer and brighter.

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Both the blueing and the brightening behaviors of the darkest surfaces have already been explored by Moroz et al. [2004a] who performed ion irradiation of complex hydrocarbons. The authors explain that carbonization process -hydrogen loss and carbon structural changes (growth of large aromatic clusters) -is responsible of the blueing and brightening effects. The carbonization leads to an optical gap decrease and the material gets metal-like optical properties (the importance of optical gap modifications in hydrocarbon materials is discussed by Jones [2012]). Thus, ion irradiation causes a gradual growth of refractive index imaginary part and surface scattering dominates at small wavelengths: this could explain the reflectance increase in the VIS range. Because the absorption coefficients of bitumens are much lower in the NIR region, the spectral behavior is controlled mostly by volume scattering, and carbonization should lead to darkening in the NIR range. Brunetto & Roush [2008] performed ion irradiation of methane and derived the optical constants. The imaginary index on the irradiated sample (around 1•10 -2 in the VIS-NIR range) is bigger than on pure methane (around 5•10 -4 ). The imaginary index value has been calculated for the Tagish Lake meteorite by [START_REF] Roush | [END_REF] and is around 5•10 -3 in this region. Thereby, ion irradiation of CCs could lead to a carbonization process of the organic component inducing growth of the imaginary index, and then explain the blueing and brightening effects described by Moroz et al. [2004a]. On Tagish Lake, Dukes et al.

[2015] showed that there is an important loss of C: this could also explain the brightening of the surface.

More recent studies tend also to emphasize the role of initial composition.

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Gillis-Davis et al. [2015] performed laser irradiation (simulation of micrometeorite impact heating) of Allende and Murchison, as well as graphite to investigate the extent to which spectral changes in carbon, as a function of laser irradiation, might be observed in the meteorites. Spectra of laser irradiated Allende responded like OCs and S-type asteroids: decrease of reflectance and reddening of the slope accompanied with mitigation of the bands in the 0.4 -2.4 µm range, while a blueing effect was observed on the short wavelengths (< 0.55 µm). Murchison spectra became darker too, with a progressive loss of the 0.7 µm band for increasing dose of irradiation, but without significant spectral slope changes. The graphite sample became brighter after some irradiations and then slightly darkened, but it did not show systematic change in the slope. A TEM x-ray mapping showed a carbon phase being deposited within grain boundaries and vesicles. Although [START_REF] Kaluna | Lunar and Planetary Science Conference[END_REF] presented laser irradiation while we deal here with ion irradiation (the effects would probably differ, and indeed we do not have the same result for surface brightness on the CM sample), they showed that Murchison and Allende exhibit distinct spectral responses to laser SpWe that may be due to the different graphitic carbons content in matrix. [START_REF] Kaluna | Lunar and Planetary Science Conference[END_REF] also used laser irradiation but focused on phyllosilicate minerals, widely present in some CC classes, and that could also drive the spectral modifications of such primitive materials. They compared two serpentine end-members, cronstedtite (Fe 2+ 2 Fe 3+ (Si,Fe 3+ O 5 )(OH) 4 ) and lizardite (Mg 3 Si 2 O 5 (OH) 4 ), with olivine (anhydrous silicate; (Mg 2+ , Fe +2 ) 2 SiO 4 ). Unsurprisingly, the olivine sample became redder and darker in the whole 0.5 -2.5 µm range. Lizardite became slightly darker in the VIS part (and so reddened). Cronstedtite became also a bit darker in the VIS, but reflectance decreased a lot in the NIR so a blueing

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of the slope was measured. Because µm-size (larger than 50 nm) iron particles production can lead to darkening without reddening (as opposed to nm-size) [Noble et al., 2007;Lucey & Riner, 2011], the authors proposed that such particles are created first in the cronstedtite sample to explain the blueing trend. They also postulate the possibility that the continuous darkening in the NIR is due to the creation of an opaque phase, spectrally neutral. CCs of types 1 and 2 (that underwent aqueous alteration process) are mainly made of phyllosilicates with varying amount of Fe and Mg elements [Tomeoka et al., 1989]. In particular, CMs show the widest amount of cronstedtite (40-50 % of detected hydrated minerals) than Mg-rich serpentine (25-30 %) [Howard et al., 2009]. The blueing effect we measure on Mighei, Alais, and Tagish Lake is similar to this cronstedtite blueing under irradiation. One has to note, again, that here we compare laser and ion irradiations.

Anyway, [START_REF] Kaluna | Lunar and Planetary Science Conference[END_REF] also proved that two Fe-Mg end-member compositions exhibit a divergent set of spectral trends: this would imply that, whatever the technique used for irradiation, initial sample composition influences the modification pathways. Our results support these conclusions.

Carbon vs. npFe 0

However, the role of npFe 0 when formed within a dark matrix is not yet understood. The accumulation of these opaque reduced iron particles on semitransparent minerals (like S-type asteroids and OCs) leads to the darkening (these are absorbing in the VIS) and reddening (reflectance remains high in the NIR) effects [Hapke, 2001]. But CCs already have finely-dispersed opaque particles within the matrix (imparting an overall dark appearance) [Cloutis et al., 2011b] and it is possible that opaque materials mask the optical effects of SpWe prod-

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ucts on low-albedo objects [Pieters et al., 2000]. Furthermore, some authors proposed that because npFe 0 have higher albedos than C-type asteroids (and CCs), one could expect an increase of reflectance plus a blueing toward NIR [Clark et al., 2002a;[START_REF] Rivkin | Asteroids III[END_REF].

It is proved that the carbonaceous phase is modified after irradiation, both from the point of view of the backbone structure with amorphization (probed with Raman spectroscopy; see Fig. 8, Brunetto et al. [2014], and Lantz et al. [2015]) and from the chemistry because CH bonds are affected by ion irradiation processes [Godard et al., 2011]. We have also shown with MIR spectroscopy that silicates suffer strong modifications like amophization and Mg loss by back-scattered sputtering. Raman and MIR spectroscopies are sensitive to different compounds so that these experimental techniques allow us to distinguish between several effects.

In the VIS-NIR range, the aromatic-rich amorphous carbon phase and the npFe 0 can have the same effects on the spectra: both have strong absorption properties (higher imaginary index compared to the matrix) leading to absorbing pattern, especially in the VIS, even if they are present in small amount, finely dispersed (small particles with size lower than the wavelength) within a silicate matrix.

Hapke [2001] has developed a spectral model to explain the spectral modifications induced by SpWe. This model is based on the Maxwell-Garnett effective medium theory, to calculate the absorption coefficient of a semi-transparent silicate host medium containing inclusions of small metallic iron spheres (the so called npFe 0 ). In the model, this homogeneous medium is then used as a coating layer on top of unaltered materials. It has been widely used to describe the effects of SpWe on reflectance spectra of silicates [Brunetto et al., 2007] and S-type aster-

A C C E P T E D M

A N U S C R I P T oids with the example of Itokawa [Hiroi et al., 2006;Bonal et al., 2015]. We also tested this model using npFe 0 (optical constants from Johnson & Christy [1974]) or inclusions of dehydrogenated carbon (Raman spectrocopy proved it becomes amorphous under ion irradiation; with optical constants from Jager et al. [1998]) as contaminating inclusions within the silicate matrix. The test (see Fig. 11) has shown that the spectral changes are very similar and we cannot conclude on which compound is dominating the SpWe effects within the VIS range. However, we notice that it needs more carbon than iron particles to affect the spectra because the absorbing coefficient is higher for iron. The Maxwell-Garnett theory was also used in a model for TNOs by Grundy [2009]: when little carbonaceous particles smaller than the wavelength are mixed with ice (transparent matrix), that material becomes highly absorbing (the R albedo decreases of more than 50% with 1% of these particules). The fact that the carbon content has an important role in the SpWe modifications we are simulating is confirmed by the correlation we observe between reflectance variation after irradiation and the carbon content of the meteorites (as shown in Fig. 12). We do need additionnal measurements with other techniques, from the microscopic point of view, to resolve this issue for the role of carbon vs. npFe 0 .

Implication for SpWe of asteroids

We propose a simple model of SpWe expected effects on low albedo objects.

To do so, we fit the values of reflectance and slopes from the ratio plots (that allow to easily represent the way the spectra change under irradiation) with a linear function depending on the initial value of reflectance at 20˚phase angle

A C C E P T E D M A N U S C R I P T before irradiation (R 0.55 ): Val = a + b • ln(R 0.55 ) (1)
where Val is the ratio value of reflectance at 0.55 µm or spectral slopes on the range 0.55 -1.05 µm, as shown in Fig. 13. We present the result of the study for both lower and higher doses of irradiation (ions with 40 keV energy) in Table 3.

The most important parameter is the factor b giving an information on the function inclination: it will increase (in absolute value) with increasing irradiation dose (as shown by the comparison of the two doses used). The Fig. 14 shows the evolution of this factor b as a function of the dose for both level of reflectance and slope variation. These fits allow to describe the behavior of CCs under ion irradiation:

the brightest members with R 0.55 ≥ 4.5-9 % (derived from the model; explanations in Fig. 13) will have a darker surface and a redder slope following the given functions, while the darkest members will have the opposite spectral response.

This model is valid under specific conditions. We underline the different behaviors that CCs have, depending on their initial composition, for a given irradiation fluence and phase angle. The phase angle is an important parameter on the spectral measurements: when it goes to 0˚there is an opposition effect and the reflectance strongly increases [Beck et al., 2012], while when the phase angle grows a spectral reddening has been noticed on asteroids [Clark et al., 2002b], meteorites [Gradie et al., 1980], and comet 67P/ChuryumovGerasimenko [Fornasier et al., 2015]. Furthermore, grain size effect is not negligible: the finer grain, the redder spectrum slope [Johnson & Fanale, 1973]. Measurements at different temperatures can also lead to differences in the spectra (band shift for example) [Moroz et al., 2000]. Brunetto et al. [2014] noticed that the optical effects of irradiation on Allende are correlated with the nuclear energy loss. This kind of information can be useful to rescale the laboratory dose to the astrophysical distribution of ions. We showed that the VIS slope reaches an asymptotic value starting from 3•10 16 He + /cm 2 (same energy of 40 keV used here). With the new step of irradiation at 6•10 16 He + /cm 2 we can confirm that an asymptote is reached for both VIS slope and R 0.55 (see Fig. 3: parameter a, giving the asymptote, is very close to the data point of the last fluence) on the brightest CCs. The highest dose we use corresponds to timescales of about 10 3 -10 4 years for an object at 3 AU, and the implantation depth is a few hundred nanometers [Brunetto et al., 2014]. This is consistent with the SpWe rims formation timescale obtained by direct measurements of the exposure to energetic ions of the Itokawa particles brought to Earth by the Hayabusa mission and consisting of thin layer of vapor/sputtered deposited on a larger amorphous layer with npFe 0 and npFeS [START_REF] Nagao | [END_REF]Noguchi et al., 2014]. Berger & Keller

A C C E P T E D M A N U S C R I P T
[2015] derive surface exposure ≤10 5 years of three Itokawa grains from solar flare particle track densities.

It is less obvious to derive timescales from the three other CCs, the darkest ones, especially for the slope: the asymptote is not clearly reached. One can wonder if this is because these samples are carbon-richer, and that modifications of the carbonaceous phase have higher timescales with respect to npFe 0 formation. Sample return missions targeting low albedo asteroids will provide important clues to solve this issue.

Nevertheless, SpWe features were found on several sides of Itokawa grains [Matsumoto et al., 2015], showing that grain motion due to impacts [Shestopalov et al.,

A C C E P T E D M A N U S C R I P T
2013], seismic shaking [Richardson et al., 2005], or YORP effect [Walsh et al., 2008] for example, can be quite efficient to rejuvenate a surface. Furthermore, some regolith can be formed by thermal fatigue [Delbo et al., 2014]. All of these processes are counterbalancing the fast SpWe rate on surface particles.

One of the astrophysical implications of our experimental results is that there must be a large fraction of asteroids whose albedo lies in the 4.5-9 % range, that should not show VIS-NIR variations upon irradiation. We checked for possible confirmation of this prediction. Clark et al. [1999] published photometric results on Mathilde, the darkest asteroid observed by a spacecraft (mission NEAR/NASA), with a normal reflectance values ranging from 0.041 to 0.053: the authors assessed that the very little color variation seen in Mathilde data (typical normal reflectance constrasts are on the order of 6 % from the mean) indicates that SpWe processes have not altered the optical properties of the surface materials. Considering our new results on spectroscopy of irradiated CCs, this assumption appears plausible because this reflectance value lies within the range of no or weak effects due to SpWe. Let us recall that the Murchison sample we studied previously [Lantz et al., 2015] did not show clear modifications under irradiation: so that Murchison could be the low endmember of the population for which initial composition/albedo does not allow SpWe to be efficient, like what was observed on Mathilde.

Some recent studies tried to highlight SpWe effects from observational data comparing young and old primitive asteroid families [START_REF] Kaluna | Lunar and Planetary Science Conference[END_REF]Fornasier et al., 2016] but it appears hard to distinguish between SpWe and other hypothesis like spectral heterogeneity within a family. We shall build stronger statistics observations. This implies that resurfacing processes must be also very efficient, as suggested by recent studies on regolith motions. However, SpWe studies must be pursued to decipher the physical, chemical and mineralogical changes it causes at nanometric scale.

A C C E P T E D M A N U S C R I P T

The olivine shows a weak band at 0.65 µm, and the broad and deep one around 1 µm. Both bands are seen on the diopside sample, plus the typical pyroxene band centered at 2.3 µm. With increasing irradiation dose, we clearly see the darkening (14 and 9 % decrease in absolute reflectance; almost 20 % in relative, same order of magnitude than the CV/CO spectra) and reddening (slope on the 0.55-2.45 µm range is 0.10 µm -1 before irradiation and goes to 0.24 µm -1 after irradiation for the olivine, values of 0.03 and 0.15 µm -1 for the diopside; calculated with a linear fit) effects on the spectra. However the band depth is slightly affected by irradiation: the 1 µm band goes from 53.5 to 49 % depth on the olivine, and from 23 to 20.5 % on the diopside.

If we compare our results to other studies, olivine samples have been irradiated

with He + by Fu et al. [2012] and Ar + by Brunetto et al. [2006]. The first authors used ion energy and fluence close to ours and showed a strong decrease of the reflectance in the NIR part of the spectrum, while here we only notice the decrease within the VIS range. The 1 µm band also suffered a stronger decrease in depth (from 30 to 15 %). The second authors used higher energy (200 keV) but lower fluence (factor 10) and the reflectance decrease mainly happens in the VIS. They also performed ion irradiation of an orthopyroxene and our results are consistent (one has to notice that here we study a clinopyroxene: bands are not located at the same wavelengths). Furthermore, Kohout et al. [2014] published a VIS-NIR spectroscopic study of several olivine samples containing different amount of npFe 0 (artificially produced) and showed that these particles induce a slope reddening, a reflectance decrease and the mitigation of the band. We plan to probe our samples using TEM to highlight their possible presence after irradiation.

A C C E P T E D M A N U S C R I P T

The silicate MIR spectra (see Fig. A2) before irradiation are in agreement with literature data [START_REF] Salisbury | Infrared (2.1-25 um) spectra of minerals, Johns Hopkins studies in earth and space sciences[END_REF]. On the olivine, four bands are detected (attributed to SiO 4 symmetric and antisymmetric stretchings [Jaeger et al., 1998])

and locations correspond to a Forsterite number of Fo ≈ 70-80 [Hamilton, 2010].

After irradiation, we notice a shift toward longer wavelengths and Fo ≈ 50-60

(iron-rich region). The pyroxene signatures on the diopside spectra move toward the same region. The band displacement is not the only effect observed here: the most intense peak on olivine suffers a distorsion while some peaks on diopside vanish after irradiation. The mineral structure is strongly affected. As cited in the main text, and as proposed by Brunetto et al. [2014], this shift toward longer wavelengths could be due to a preferential Mg loss within minerals. Demyk et al.

[2001] and Carrez et al. [2002] noticed amorphization of olivine (Fo = 90) and decrease of Mg/Si ratio after He + irradiation (at the same energy than used in this study), and Demyk et al. [2004] saw the same amorphization effect on pyroxenes.

One has to note that Carrez et al. [2002] proposed that a reduction process leading to formation of npFe 0 could also explain the surface enrichment in Fe.

A C C E P T E D M

A N U S C R I P T He + /cm 2 ). R 0.55 is the absolute reflectance value at 0.55 µm. The slopes (µm -1 ) are evaluated with a linear fit on the whole range 0.55 -2.45 µm (VISNIR), and on the range 0.55 -1.05 µm (VIS). The associated error on R 0.55 is about 1 % of the given absolute value, and less than 0.01 µm -1 for the slopes. Table 3: Values of the parameters for the linear fit Val = a+b•ln(R 0.55 ) with the minimal and maximal irradiation doses. We give the equation for all the tests we made: reflectance, and VIS, VISNIR and NIR slopes. Among the slopes, we chose to visually present only the VIS one in Fig. 13 because this is the most affected spectral parameter.

  provide the only available example of multiple irradiation showing a differential trend depending on meteorite classes. Carbonaceous chondrites (CCs) are composed of chondrules (mostly silicate [Lauretta et al., 2006]), Ca-Al-rich inclusions, Fe-, Ni-metal, and fine-grained matrix. The matrix contains the organic matter [Pizzarello et al., 2006] and shows evidence of metamorphism [Huss et al., 2006] or aqueous alteration [Brearley, 2006]. Proportions of these materials will change with the meteorite type. Thus CCs contain various amounts of different compounds coexisting and it is still not clear how these component characteristics determine the response of different CC classes to SpWe. In a previous study, we have already noticed that VIS-NIR spectral modifications are function of albedo

  according to their mineralogy and chemical compositon: matrix-or chondrules-rich, the amount of metal, carbon (carbonates, graphite, organic material), oxygen, etc. Then the petrographic type is used to describe the thermal history, from a largely aqueously altered object (with phyllosilicates; type 1-2) to one that went through some metamorphism (type up to 6; type 3 remaining pristine) [see a review in Weisberg et al., 2006]. Here are some details for the samples used in this study.

  , we show a spectrum before irradiation and a spectrum of the most disordered component after the highest irradiation dose. One can also notice the amorphization of the silicates as seen on the Lancé sample: the olivine peaks at about 850 and 820 cm -1 almost disappear after irradiation on the Raman spectrum. This doublet is the dominant feature of the olivine Raman spectrum that arises from coupled A C C E P T E D M A N U S C R I P T symmetric and asymmetric stretching vibrational modes of the constituent SiO 4 tetrahedra [Kuebler et al., 2006]. These modes will be efficient under Raman with an olivine crystalline bone, while an amorphous structure will be hardly detected explaining why the peaks disappear after irradiation. The ion irradiation is known to amorphize silicates, as shown by IR spectroscopy analysis [Demyk et al., 2004]. Raman spectroscopy is a technique that is more sensitive to the aromatic carbon content (and to crystalline phase), explaining that one can see better the carbon bands compared to the olivine ones -as opposed to what is seen with IR spectroscopy. The D and G bands are due to the sp 2 sites of carbon atoms, and those have a gap falling in the range of the Raman excitation [see e.g. Ferrari & Robertson, 2001, and references therein]. The Raman cross section being much larger for aromatic carbon than for minerals, the carbon D and G bands dominate the spectra once few aromatic carbons are present [see e.g. Fig.4 in Brunetto et al., 2011].

  photo-electron spectroscopy), while experiments on olivine showed Mg/Si ratio decrease [Demyk et al., 2001; Carrez et al., 2002].

  On the darkest and hydrated CCs, phyllosilicate structures are efficiently modified by irradiation seen as clear broadening of the bands. One can see that the shift on Mighei is bigger than on Alais or Tagish Lake: this might be due to a different initial compo-A C C E P T E D M A N U S C R I P T sition of the phyllosilicates. Indeed, the Mighei spectrum clearly differs from the two others even before irradiation.

  compositions influencing the reflectance effects measured here. On one hand, we propose that because the CO/CV chondrites are mainly composed of chondrules (typically made of Si compounds), their spectral answer is close to that of OCs and S-type asteroids with predominating npFe 0 action. On the other hand, the darkest chondrites are matrix dominated: the carbonaceous phase could dominate the optical effects, and we do observe modifications as a clear linear relationship exists between the albedo variation upon irradiation and the carbon content of the meteorite. Finally, the role of hydrated minerals can not be excluded, and the influence of npFe 0 on the darkest minerals reflectance response have not yet been explored.Our global study on several CCs suggests an empirical model of SpWe effects on the darkest meteorites indicating clearly that the initial composition rules the trends on reflectance and slope in the VIS-NIR. We predict that asteroids initially darker than 5 % become brighter by about 0.5-1.5 % and bluer, while asteroids brighter than 9 % become darker by about 1.5-3 % and redder like OCs and S-types -our model can explain the absence of albedo variations osberved on asteroid Mathilde -at short timescales (10 3 -10 4 years) consistent with Itokawa

Figure 1 :

 1 Figure 1: Visible to near-infrared bidirectional diffuse reflectance spectra of CV Allende (top), COs Lancé (middle), and Frontier Mountain 95002 (bottom) before (black line; a) and after irradiation with 5•10 15 (b), 1•10 16 (c), 3•10 16 (d), and 6•10 16 He + /cm 2 (lightest grey; e).

Figure 2 :Figure 3 :Figure 4 :

 234 Figure 2: Same as Fig.1for CM Mighei (top), CI Alais (middle) and ungrouped Tagish Lake (bottom). One can note some artifacts on the spectra of Mighei and Alais (between 0.6 and 0.9 µm) probably due to the specular component of the reflected light.

Figure 5 :

 5 Figure 5: NIR confocal reflectance spectra around OH-stretching bands for the non-irradiated area (black line) and the highest fluence of irradiation (grey line) for Mighei (left), Alais (central), and Tagish Lake (right). The dotted line indicates the 2.71 µm position on each plot corresponding to the band minimum.

Figure 6 :

 6 Figure 6: Mid-Infrared confocal reflectance spectra of Allende (top), Lancé (middle up), and Frontier Mountain 95002 (middle down) on a non-irradiated area of the pellets (black line), and irradiated at the highest fluence (grey line). Spectra are scaled to unity at strongest peak to facilitate the comparison. The bottom panel shows the band shift as a function of the initial position (same x-axis than the other panels). Error on the initial position is within the symbol size.

Figure 7 :

 7 Figure 7: Same as Fig. 6 for Mighei, Alais, and Tagish Lake.

Table 1 :

 1 Resume of SpWe laboratory experiments on slopes and reflectance. These are made on pellets, exceptKeller et al. [2015a] who worked on a thin section. Red and Blue refer to reddening and blueing of the spectral slopes, while dark refers to the surface brightness decrease after irradiation. VIS for visible part of the spectrum, from 0.4 to 1.0 µm, NIR for the infrared to 2.5 µm. Vernazza et al.[2013] saw both reddening and blueing of the slope using two different experiments.Matsuoka et al. [2015] noticed that the reflectance increases below 0.4 µm. InLantz et al. [2015], the changes are smaller than either effects due to sample preparation.

	Sample	Experiment	Effect	Reference
	Allende	ion ion	red in the VIS dark + red in the VIS	Lazzarin et al. [2006] Brunetto et al. [2014]
	FM 95002	ion	red in the VIS	Lazzarin et al. [2006]
	Mighei	laser	dark + red in the VIS	Moroz et al. [2004b]
	Tagish Lake	ion 1 ion 2	red in the VISNIR blue in the VISNIR	Vernazza et al. [2013]
		laser	dark + blue in the VISNIR	Hiroi et al. [2013]
	ALH 83108 (CO3) ALH 85002 (CK4) MAC 88100 (CM2) Orgueil + Ivuna (CI1 mix.)	laser	dark + red in the VISNIR dark + blue in the VISNIR	Hiroi et al. [2013]
		ion	dark + red in the NIR	Keller et al. [2015a]
	Murchison (CM2)	ion	small changes	Lantz et al. [2015]
		laser	dark + blue in the VISNIR Matsuoka et al. [2015]

Table 2 :

 2 Evolution of VIS-NIR spectral parameters before and after irradiation with the highest dose (6•10 16

Preprint submitted to IcarusDecember 16, 2016

Acknowledgments

We warmly thank the Natural History Museum of Vienna (Dept. of Mineralogy and Petrography; Lancé sample NHMW ID 3593 [3] (von A958) and Al-

A C C E P T E D M A N U S C R I P T

the authors observed also a change in the Fe 2+ /Fe 3+ ratio, with Fe 2+ becoming more abundant in the irradiated material, while in the second case, the authors observed S-and Fe-rich small amorphous silicate particles (with diameters 20-1000 nm) formed and deposited on the surface after irradiation.

On one hand, Matsuoka et al. [2015] performed laser irradiation so the modification process (microstructural and chemical effects) is different (for example melting) and it is hard to compare with our experiment. On the other hand, Keller et al. [2015a] used ion irradiation but at lower energy (1•10 18 He + /cm 2 at 4 keV): one can assume that the spectral changes we see, and in particular the 2.7 µm band shift, take place in the volume probed by the reflectance and thus require higher ion penetration depth to be seen (a SRIM calculation gives a penetration depth for 40 keV He ions 8.5 times bigger than for 4 keV). The energy of 40 keV used in our study is about ten times higher than the slow solar wind energy of helium ions, but it is close to the average slow solar wind energy for heavier ion such as argon. Moreover, higher energy helium ions are emitted from the Sun through active regions and solar flare. Thus, because there is a wide distribution of energies and ions in the solar wind, we should detect this shift also on asteroids. For example, Matsumoto et al. [2015] proposed that 14keV He + might have played a role in the formation of a thick spaceweathered layer they detected on an Itokawa particle.

The amount of available data on asteroids in

Conclusions

In this paper we presented reflectance spectra of several meteorites probed by visible and infrared spectroscopy. We studied ion irradiation effects as a simulation of the solar wind bombardment of asteroid surfaces. For the first time, the same experiment is done using the same method on six CCs spanning a wide range of initial albedo and chondrules, carbon and hydrated mineral contents. In the VIS-NIR range, the CV and CO classes have a similar spectral answer to SpWe than OCs and S-type asteroids (darkening/reddening) while the darkest and most aqueously altered chondrites (CM/CI/ungrouped Tagish Lake) show a clear surface brightening and a certain blueing. It confirms that a relationship exists between the spectral variations observed and the initial composition [Lantz et al., 2015]. As already noticed in Brunetto et al. [2014] and Lantz et al. [2015], there is a systematic shift of phyllosilicate and olivine bands near 2.7 and 10 µm toward Fe-rich spectral region (longer wavelengths) that may be attributed to the amorphization and/or loss of Mg due to preferential sputtering, and/or presence of npFe 0 .

The formation of these npFe 0 are the common thought mechanism for space weathering on S-type asteroids in the VISNIR range, but there is a significant

Appendix A. The silicate samples

Before irradiation, the VIS-NIR spectra (see Fig. A1) of olivine and diopside are fully consistent with what is expected for these silicates in terms of reflectance level (68 and 56 % respectively at 0.55 µm) and band shape [Clark et al., 2007].

Appendix B. Pellet vs. powder spectra

The Fig. B1 shows bidirectional reflectance spectra (i = 45˚and e = 0˚) of the Allende meteorite (provided by L. Folco, Museo Nazionale dell´Antartide, Siena, Italy; same sample as the one presented in Brunetto et al. [2014]). We took spectra on the powder (grain size 1-100 µm) and on a pellet made from that powder. Several differences can be noticed. First, the pellet spectrum is brighter (reflectance of 13 % at 0.55 µm) than the powder one (7.5 %). This is because the pressed pellet has a smoother surface and acts almost like a mirror. Second, the slope is redder

on the powder (the cause of a blue slope in solid samples is also due to surface roughness). The spectral variations observed with respect to sample preparation effects are addressed in Cloutis et al. [2012b] and references therein. Another example of variation between pellet and powder can be found by comparing our Tagish Lake VISNIR spectrum before irradiation and those measured on powder by Izawa et al. [2015]. These authors find redder slopes than ours but reflectance values are comparable. The ratio is also made with a derived irradiated spectrum taking into account the Hapke model: with iron (dashed grey line) or carbon (dashed/dotted black line) inclusions. The computation gives a value φ•d, where φ is the volume fraction of inclusions in the host matrix and is a function of the concentration of inclusions and relative densities of inclusion and host materials, and d is the mean optical path length. Here d is estimated to be the same for both kinds of inclusions, so that more carbon is necessary to fit the SpWe effects. (5•10 15 He + /cm 2 ; grey data) and last (6•10 16 He + /cm 2 ; dark data) steps. See Table 3 for the equation parameters. See Fig. 14 for the behavior of parameter b (related to fit slope) as a function of the irradiation dose. These fits allow us to derive the values of initial R 0.55 for which we could expect no effect of space weathering on primitive bodies: by resolving the equation y = 1 for the reflectance or y = 0 for the slope, we find 8.2 and 5.3 % respectively (fit with the highest dose).


Diemo Schwarz

Guillaume Lemaitre

Mitsuko Aramaki

Richard Kronland-Martinet

Effects of Test Duration in Subjective Listening Tests

In perceptual listening tests, subjects have to listen to short sound examples and rate their sound quality. As these tests can be quite long, a serious and practically relevant question is if participants change their rating behaviour over time, because the prolonged concentration while listening and rating leads to fatigue. This paper presents first results of and hypotheses about changes in the rating behaviour of subjects taking a long-lasting subjective listening test evaluating different algorithms for environmental sound texture synthesis. We found that ratings present small but statistically significant upwards tendency towards the end of the test. We put forward the hypotheses that this effect is due to the accustomation of the subjects to the artefacts present in the test stimuli. We also present the analysis of a second test evaluating wind noises in interor car recordings and find similar effects.

INTRODUCTION

In perceptual listening tests, subjects have to listen to short sound examples and rate their sound quality. The sound examples would typically be several variants of a speech or sound synthesis algorithm under test, in order to find the best methods or parameters. As these tests can be quite long (usually more than 15 minutes, up to two hours), a serious and practically relevant question is if participants change their rating behaviour over time, possibly because the prolonged concentration while listening and rating leads to fatigue or other long term effects. This is a real and original research question relevant to countless researcher's daily work, but it is rarely treated specifically in the literature. We will present analyses of two data sets: A first data set (section 3) with sound quality ratings of five different environmental sound texture synthesis algorithms [START_REF] The | Evaluation of examplebased sound texture synthesis algorithms of the physis project[END_REF][START_REF] Schwarz | Concatenative Sound Texture Synthesis Methods and Evaluation[END_REF], and a second data set (section 4) from a listening test of unpleasantness of wind noise in car interiors [START_REF] Lemaitre | A psychoacoustical study of wind buffeting noise[END_REF].

From an analysis of data set 1, we found that ratings present small but statistically significant upwards tendency in sound quality rating towards the end of the test. We put forward the hypothesis that this effect is due to the accustomation of the subjects to the artefacts present in the test stimuli.

Copyright: c 2016 Diemo Schwarz et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License 3.0 Unported, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Data set 2 presents a downwards tendency in the pleasantness rating for certain types of stimuli. Here the hypothesis is that listening fatigue could be the main factor.

Of course a good test design would randomise the order of presentation of sounds in order to cancel out these effects for calculation of the mean score for the different stimuli, but they do augment the standard deviation of the results.

PREVIOUS AND RELATED WORK

Despite the practical relevance of this question, existing literature on this subject is rather rare. Neither Bech and Zacharov [START_REF] Bech | Perceptual audio evaluation-Theory, method and application[END_REF], nor Pulkki and Karjalainen [START_REF] Pulkki | Communication Acoustics: An Introduction to Speech, Audio and Psychoacoustics[END_REF] treat this question specifically. This observation was corroborated by the reaction of three researchers experienced in designing and carrying out listening test asked by the authors, who all showed surprise at the first hints of an effect. In experimental psychology, Ackerman and Kanfer [START_REF] Ackerman | Test length and cognitive fatigue: An empirical examination of effects on performance and test-taker reactions[END_REF] studied cognitive fatigue in SAT-type tests of 3 to 5 hours, which is much too far from our use case. We have to look in fields such as usability testing to find relevant research: Schatz et al. [START_REF] Schatz | The impact of test duration on user fatigue and reliability of subjective quality ratings[END_REF] study the duration effect of a 90 min test (including a 10 min break) of video transmission quality and web site usability on user ratings. They find little difference of the mean scores of control questions repeated at the beginning and end of the test, although physiological measurements of fatigue (eye blink rate, heart rate mean and variation) and subjective task load index (TLX) questionnaires show clear signs of strain. However, they admit that "pure audio or speech tests might even cause stronger boredom and fatigue (due to higher monotony) than mixed task profiles". Here we can argue further that the mental strain in our experiment 1 is higher, since the decision rate, i.e. the number of ratings to decide on is very high-after every stimulus of 7 s, two ratings were required-and more concentrated listening was asked for, whereas in the above studies, rather few judgments from the subjects were required. We also have to note that above study took place in a lab, and subjects were payed to participate. Our experiment 1 is on-line and unpayed, and the subjects' motivation is thus much lower.

EXPERIMENT 1

Data set 1 was collected in a subjective listening test [START_REF] The | Evaluation of examplebased sound texture synthesis algorithms of the physis project[END_REF][START_REF] Schwarz | Concatenative Sound Texture Synthesis Methods and Evaluation[END_REF], comparing 5 different algorithms for extending an environmental sound texture recording for an arbitrary amount of time, using synthesis based on granular and spectral sound representations, with and without the use of audio descriptors. These algorithms were developed in the course of the PHYSIS collaborative research project 1 . Their details are not subject of this article and can be found in [START_REF] Schwarz | Concatenative Sound Texture Synthesis Methods and Evaluation[END_REF][START_REF] O'leary | A montage approach to sound texture synthesis[END_REF][START_REF]A two level montage approach to sound texture synthesis with treatment of unique events[END_REF][START_REF] Liao | On the modeling of sound textures based on the STFT representation[END_REF][START_REF] Liao | Modelling and transformation of sound textures and environmental sounds[END_REF][START_REF] Schwarz | Smooth granular sound texture synthesis by control of timbral similarity[END_REF]. See also the state-of-the-art overview on sound texture synthesis [START_REF] Schwarz | State of the art in sound texture synthesis[END_REF] for further discussion and a general introduction of sound textures.

The 5 algorithms under test are evaluated in an ongoing listening test accessible online. 2 The experiment setup is briefly described in the following, full details can be found in [START_REF] The | Evaluation of examplebased sound texture synthesis algorithms of the physis project[END_REF][START_REF] Schwarz | Concatenative Sound Texture Synthesis Methods and Evaluation[END_REF].

Sound Base

The sounds to be tested stem from 27 original environmental sound texture examples that cover scenes relevant for games and audio-visual applications, such as nature sounds, human crowds, traffic, city background noises, etc. Each original sound of 7 s length is resynthesised by 5 different sound textures algorithms.

Experimental Procedure

The subjects take the experiment via a web-based form where first the instructions, and then the 27 sounds are presented in random order. For each sound example, the original, and the 6 test stimuli of 7 s length are presented. The stimuli contain in randomised order 5 syntheses, and the original as hidden reference. For each stimulus, the subject is asked to rate the aspects of sound quality and naturalness on a scale of 0-100.

Experiment 1 Results and Evaluation

Project members and members of the wider research teams were invited by email to take the listening test. There were 17 responders, 16 listening on headphones or earplugs, 1 on studio loudspeakers. None reporting hearing impairments, 5 reported not being familiar with listening tests. We removed one responder from the statistics (reporting not being familiar with listening tests) who left 80% of the quality and all similarity ratings at the default setting of the web form of 50, and rated the quality of the rest of the stimuli as less than 50.

Figure 1 shows the mean quality and similarity ratings, over all responses and sounds, for the different algorithms. Table 1 shows that the inter-rater reliability, measured by Cronbach's α, is very high (i.e. subjects agree to a high degree in their ratings), with SDIS being slightly lower.

Effects of Order on Ratings in Experiment 1

As the perceptual listening test was quite long (the minimal listening time for 27 sounds, each with 6 stimuli and one original, is already 27 • 7 • 7 s = 22 min, the actual test time would be closer to 35 min), the question is if participants change their rating behaviour over time, because the prolonged concentration while listening and rating leads to fatigue.

Figure 2 shows the linear regression fit for all ratings for all synthesised stimuli. The quality ratings show a slight correlation significant at the 1% level with p = 0.0008. The slope models a 0.24 and 0.22 increase in quality and similarity rating, respectively, per presentation order.

Figure 3 shows, for each stimulus type, a linear regression fit of the ratings versus the order of presentation of the sound example. We do observe a general trend for the ratings to rise towards the end of the test. For Descr and Random, the model is significant at the 5% level for quality ratings, for similarity ratings just above 5%, and for Montage quality rating at the 10% level. However, only a small fraction of the data is explained by the order, which is good, since we can conclude that the subjects in the test really made an effort to rate the stimuli with concentration and dedication throughout the long perceptual test.

The effect of presentation order is associated to a 0.24 slope that corresponds to a model difference of 6.5 rating points between the first and the last example. For the Descr and Random quality ratings, we found a 0.27 and 0.28 slope, respectively, that corresponds to a difference of 7.5 points.

Figure 3 also shows the standard deviation of ratings for each stimulus type over order of presentation, and a linear regression fit. These fits show in general a falling trend (the subjects converging towards common values), except for algorithm SDIS, which stands out also because it is always rated much lower.

Hypotheses for Experiment 1

The fact that the rise in ratings is only statistically significant for some of the algorithms, and only for their respective quality ratings, hints at a possible accustomation of the listeners to the artefacts of some of the algorithms.

EXPERIMENT 2

Data set 2 is from a psychoacoustic listening test [START_REF] Lemaitre | A psychoacoustical study of wind buffeting noise[END_REF] examining the unpleasantness of wind buffeting noises in the interior of 19 car models. The cars were recorded in a wind tunnel under three different conditions of a buffeting generating device. The test duration was 36 min on average (from 10 to 97 min), and each subject gave 121 ratings in 11 sets of 11 sounds. The experiment design foresaw a lower and an upper anchor reference recording that was present in each set of sounds to rate. In the following we will examine the mean of these ratings only as this eliminates the possibly confounding factors of the 19 different car models and 3 experimental conditions. Note that the original rating of "unpleasantness" on a range from 0 to 1 has been inverted and rescaled here to a "pleasantness" rating from 0 to 100 to align with the moreis-better valence of experiment 1. slope p-value R 2 adj. R 2 Global 0.01 0.9127 0.00% -0.02% lower anchor -0.86 0.0001 84.04% 82.27% upper anchor -0.12 0.7146 1.56% -9.38% While the global results in table 3 show that the randomisation evens out the ratings, the regressions for the anchor sounds, visible in figure 4, show no duration effect for the upper anchor, but a highly significant downwards trend of the pleasantness rating for the lower anchor, that makes for a theoretical difference of 9.5 points between the first and last example.

Hypotheses for Experiment 2

The sound stimuli for this experiment were all real recordings of car interiors, therefore the hypothesis for experiment 1 of accustomation to artefacts of synthesis algorithms can not apply. We hypothesise instead that the downward trend of the pleasantness rating for the lower anchor is due to accumulation of annoyance with the par- ticularly bad sound of this car, while the upper anchor's much more pleasant sound didn't provoke annoyance in the long term.

Order of Presentation

CONCLUSIONS AND FUTURE WORK

From the analysis of the two data sets we can conclude that there can be effects of changes in the rating behaviour of the subjects in perceptive listening tests over the duration of the tests. These effects vary depending on the type of stimuli and setup of the test. Although randomisation of the order of presentation cancels out these effects for calculation of the mean score for the stimuli, if we were to understand them better, we might reach more contrasted results of the tests, or could devise ways of designing the tests in order to minimise these effects. More research and the analysis of more data sets is necessary to see if the findings presented here generalise to other experiments and setups.

For further work, we could record more precisely the subject behaviour (listening activity and timing), and finally, a closer observation of the physiological and mental state of subjects while taking the test, e.g. via EEG, EMG, or heart rate sensors, could reveal relations between attention signals derived from the sensor data and the hypothesised effects of test duration.

Figure 1 .

 1 Figure 1. Box plots of the quality and similarity ratings per type of stimulus, showing the median (middle line), quartile range (box), min/max (whiskers), and outliers (crosses).

Figure 2 .

 2 Figure2. Scatter plots and linear regression fit of all 1215 ratings of experiment 1 for synthesised sounds, explained by order of the sound example. The parameters of the regression models can be found in table 2.

Figure 3 .Figure 4 .

 34 Figure 3. Per-stimulus scatter plots and linear regression fit of ratings of experiment 1 explained by order, overlaid with bar plots of standard deviation and linear regression fit.

Table 1 .

 1 Inter-rater reliability of experiment 1 (standardized Cronbach's α) for all ratings, and per stimulus type.

		Quality Similarity
	Overall	0.9874	0.9915
	Orig	0.9672	0.9789
	Descr	0.9431	0.9686
	Montage	0.9560	0.9695
	AudioTexture 0.9410	0.9628
	Random	0.9337	0.9615
	SDIS	0.8944	0.8979

1 https://sites.google.com/site/physisproject 2 http://ismm.ircam.fr/sound-texture-synthesis-evaluation

  The parameters of the regression models can be found in table 2.

		Quality			Similarity
		slope p-value	R 2 adj. R 2 slope p-value	R 2 adj. R 2
	Global	0.24 0.0008 0.51%	0.46%	0.22 0.0030 0.40%	0.36%
	Orig	0.11 0.3385 0.21% -0.02%	0.12 0.2680 0.28%	0.05%
	Descr	0.27 0.0366 1.00%	0.77%	0.24 0.0604 0.81%	0.58%
	Montage	0.24 0.0673 0.77%	0.54%	0.16 0.2402 0.32%	0.09%
	AudioTexture	0.21 0.1057 0.60%	0.37%	0.17 0.2163 0.35%	0.12%
	Random	0.28 0.0388 0.98%	0.75%	0.27 0.0516 0.87%	0.64%
	SDIS	0.21 0.1537 0.47%	0.24%	0.25 0.1383 0.50%	0.28%

Table 2 .

 2 Linear regression fit results for experiment 1: slope of the regression line m, p-value of the regression model, and percentage of the variation explained by the model R 2 and adjusted R 2 .

Table 3 .

 3 Linear regression fit results for experiment 2: slope of the regression line m, p-value of the regression model, and percentage of the variation explained by the model R 2 and adjusted R 2 .

		α
	Global	0.7681
	lower anchor 0.9373
	upper anchor 0.8900

Table 4 .

 4 Inter-rater reliability of experiment 2 (standardized Cronbach's α) for all ratings, and per condition.


Franck Poirier
email: franck.poirier@univ-ubs.fr

Keywords: Mobile Text entry, Onscreen Keyboard, Touchscreen, Connected Object, Smartwatch, Internet of Things. ACM Classification H.5.2 [Information interfaces and presentation]: User Interfaces -Input Devices and Strategies, Interaction Styles Design, Performance, Experimentation, Human factors

Text entry is face to a new challenge with the Internet of Things wave. Connected objects have very few resources and especially a tiny screen. Obviously, full keyboard or keyboard using a lot of resources are not well suited. In this position paper, we propose a minimum 3-key keypad to text entry on connected objects. We argue that using 3 keys is a right approach to improve the transfer of thoughts into writing according to all the theoretical models and laws in HCI.

Introduction

It is not possible to summarize in this paper our research in text entry. Thus we will only explain the logic of our approach for mobile text entry and present an adaptation for smart objects, particularly for connected watches. For more details, refer to the original articles.

We are working for more than 10 years on mobile text entry. In 2007, we proposed UniGlyph, a text entry method for handheld devices like PDAs [START_REF] Poirier | Glyph: A New Stroke-Alphabet for Stylus-Based or Key-Based Text Entry[END_REF], [START_REF] Poirier | UniGlyph: only one keystroke per character on a 4-button minimal keypad for key-based text entry[END_REF], [START_REF] Poirier | Text Entry Methods For Handheld Devices Or For AAC Writing System[END_REF].

On shown figure 1, UniGlyph is based on the analysis of Latin capital letters that are composed from three basic shapes: (1) diagonal stroke, (2) curve and (3) horizontal or vertical line. On this basis, the Uniglyph keypad consists of three keys called respectively diagonalshape key, loop-shape key and straight-shape key.

Each letter of the English alphabet is represented by only one primitive shape. Each letter is input by a single keystroke on the coded key.

In order to recall the coded key, the user needs to follow a very simple rule: -if the capital letter contains a diagonal stroke, then click on the diagonal-shape key (1); -otherwise, if it contains a loop or a curving stroke, then click on the loop-shape key (2); -otherwise, click on the straight-shape key (3).

Smartwatches

Smartwatches are wearable computers on the wrist that in addition to providing the time give access to some functionalities of the smartphone. Since two or three years, more and more IT companies have put smartwatches on the market (Samsung, Apple, Sony, LG, Motorola…). However, it is clear that these kind of smart devices have not yet achieved much success.

The lack of usable text entry keyboard is probably a key reason for the current failure of wearable devices.

From our point of view, text entry should be present on all mobile or wearable devices [START_REF] Mackenzie | Text Entry Systems: Mobility, Accessibility, Universality[END_REF]. That is why text entry on smartwatch is a major research challenge.

If it is not so difficult to display notifications on the tiny screen of a smart object, it is much more difficult for the user to produce information. It is impossible to finger tap on a full keyboard with a tiny 1 to 2-inchscreen in order to enter text.

A smartwatch screen can just contain a small number of keys, buttons or icons. In fact, it is acceptable to enter text on a so tiny touchscreen only if the soft keyboard contains just a few buttons.

Related works

For the last three years, different text input methods for smartwatches have been proposed.

Few text entry methods for smartwatches are on the market, e.g. Fleksy [START_REF]Fleksy keyboard[END_REF], Minuum [START_REF]Minuum keyboard[END_REF], Swype [START_REF]Swype keyboard[END_REF]. These three methods are based on a full QWERTY keyboard. Due to the fat finger problem, it seems that static QWERTY keyboard is not the right solution.

ZoomBoard [START_REF] Oney | ZoomBoard: A Diminutive QWERTY Soft Keyboard Using Iterative Zooming for Ultra-Small Devices[END_REF] is one of the first methods based on a zooming user-interface paradigm. It provides a full QWERTY keyboard. The letter keys around the finger press are iteratively enlarged, the user refines the finger position in order to point to the desired key.

Dunlop and al. [START_REF] Dunlop | Towards High Quality Text Entry on Smartwatches[END_REF] propose to divide the watch screen into seven zones, six big ambiguous keys, three at the top of the screen and three at the bottom and a center zone for the input entry field.

DragKeys [START_REF] Cho | A Text Technique for Wrist-worn Watches with Tiny Touchscreens[END_REF] is a circular keyboard composed of 8 ambigouous keys arranged around the text cursor. At most five letters are assigned to each key. To enter a letter, two dragging gestures must be made.

The analog keyboard project [START_REF]The Analog Keyboard Project[END_REF] explores direct handwriting on the small touch screen.

Other approaches use IR proximity sensors to capture gestures performed above the device, for example, Gesture Watch [START_REF] Kim | The Gesture Watch: a wireless contact-free gesture based wrist interface[END_REF] and HoverFlow [START_REF] Kratz | Hoverflow: exploring around-device interaction with IR distance sensors[END_REF]. These approaches have the advantage to reduce screen occlusion but are not very discrete.

Smaller is harder

Smartwatches have much smaller touchscreens than PDAs or smartphones. For example, by comparing an Apple Watch (model 1.65'') to a iPhone 6, the screen is 8 times smaller; comparing an Apple Watch (model 1.5'') to a iPhone 6 Plus, the screen is 15 times smaller.

Clearly, the fat finger problem on touch screens becomes a big fat finger problem on smartwatches!

We understand that QWERTY-like keyboard and even reduced but not-too-reduced keyboards are not well suited to text entry on smartwatches.

According to Hick-Hyman's law, the more the keyboard will be reduced, the more keystroke will be fast.

According to Fitts' law, the more the keyboard will be reduced, the more keys will be big, the more key selection will be fast.

A strongly reduced keyboard is a good solution to the fat finger problem. By greatly reducing the fine motor control problem, we can expect to improve the transfer of thoughts into writing.

So we believe that an adaptation of UniGlyph is a pragmatic response to the problem of text entry on smart objects.

UniWatch

UniWatch is the adaptation of UniGlyph for tiny connected devices. We have explored different design approaches to adapt UniGlyph [START_REF] Poirier | UniWatch -Some approaches derived from UniGlyph to allow text input on tiny devices such as connected watches[END_REF]. In this paper we only consider the more direct adaptation based on keypresses on a 3-key keypad.

Other adaptations based on flick gesture or direct finger drawing have been developed. The evaluation [START_REF] Poirier | UniWatch -Some approaches derived from UniGlyph to allow text input on tiny devices such as connected watches[END_REF] have shown that the keypress approach is preferable because it implies a better feedback (the coded letters are recalled on the key), it is easier to use (compared to the flick gesture approach), and it is quicker and more reliable (compared to the finger drawing approach).

UniWatch uses only 3 keys, the original command-key of UniGlyph is replaced by touch-based gestures for controlling the whole entry process. The three keys are placed on the lower side of the screen. The user interaction is limited to single taps on these keys. Due to the size of the keys the risk of error is very low.

As with the method proposed by Dunlop [START_REF] Dunlop | Towards High Quality Text Entry on Smartwatches[END_REF] each key is ambiguous, a disambiguation engine gives word completion and word prediction. Most often the desired word is at the beginning of the prediction line.

The UniWatch prototype has been implemented on a Sony Smartwatch.

Figure 2 shows the UniWatch keypad. Note that the three keys occupied the quarter of the screen, the complete entry window with the text line and the prediction line nearly occupies the three quarter of the screen.

Input sentence (hello world)

3-key keypad

Word prediction list

Questions and research perspectives

The QWERTY keyboard has survived all of the new devices like computers, PDAs, smartphones, tabs… despite the many virtual keyboards that have been developed over the years. It's pretty sad when you think that this keyboard dates back to 1878! May be the rise of smart objects with tiny screen will sound the death of the full keyboards for text entry on the go.

In my opinion, original text input methods, different from QWERTY-like keyboards, which use very few screen resources and exploit the intrinsic characteristics of handheld devices are required.

If we fail to find a well-suited solution for smart objects, the user will have to use vocal assistants (S Voice on Android, Siri on IOS…). Is it the solution knowing that voice communication is not always possible or appropriate to the context of user interaction [START_REF]The Analog Keyboard Project[END_REF], [START_REF] Sawhney | Nomadicradio: Speechand Audio Interaction for Contextual Messaging in Nomadic Environment[END_REF]… and also the fact that most users don't want to speak to their device?

Figure 1 .

 1 Figure 1. The UniGlyph character set and the associated input keys: (1) diagonal-shape key, (2) loop-shape key, (3) straight-shape key.

Figure 2 .

 2 Figure 2. The UniWatch keypad: text entry approach by button tapping.