Franck Lefèvre
email: franck.lefevre@latmos.ipsl.fr

Biosignatures for Astrobiology -Chapter 12 The enigma of methane on Mars

Between 2004 and 2012, four independent groups reported detections of low levels (10-60 ppbv) of methane on Mars. If true, these constitute the first observations of a potential biosignature on that planet and would be an important finding and addition to the inventory of minor species in its atmosphere. However, these claims for the presence of methane have been highly controversial. In 2014, the most robust search for methane on Mars was performed by the rover Curiosity. The latest measurements by Curiosity indicate a background CH4 level of 0.2-0.7 ppbv, except during a two-month period between November 2013 and January 2014, when high mixing ratios of around 7 ppbv were observed. These observations immediately raise the question of the origin of methane on Mars, but also pose fundamental challenges to our current understanding of Martian atmospheric physics and chemistry.

Observations

) detected varying amounts of methane between 0 and 30 ppbv over a few orbits of the Planetary Fourier Spectrometer (PFS) infrared sounder on board the Mars Express space-

craft. [START_REF] Geminale | Mapping methane in martian atmosphere with PFS-MEX data[END_REF] expanded on this using the same instrument and method over a six-year baseline. Their results revealed substantial seasonal variations of methane, with local enhancements of up to 70 ppbv located at high northern latitudes in summer. The mean value derived from PFS is about 15 ppbv. Using the CSHELL spectrometer at the IRTF (Hawaii), [START_REF] Mumma | Strong release of methane on Mars in northern summer 2003[END_REF] detected, during in January-March 2003, a strong local enhancement (~ 50 ppbv) in the form of a "plume" of methane at low latitudes over the Syrtis Major region (Fig. 12.1). No significant amounts of methane were found in their observations performed three years later in January-February 2006. [START_REF] Krasnopolsky | Search for methane and upper limits to ethane and SO2 on Mars[END_REF] reprocessed the CSHELL observations of February 2006 at Ls = 10° with refined analytical approaches and found about 10 ppbv of methane over the Valles Marineris region and ~3 ppbv outside this region. His observations for December 2009 at Ls = 20° and March 2010 at Ls = 70° showed no detection with an upper limit of 8 ppbv. [START_REF] Villanueva | A sensitive search for organics (CH4, CH3OH, H2CO, C2H6, C2H2, C2H4), hydroperoxyl (HO2), nitrogen compounds (N2O, NH3, HCN) and chlorine species (HCl, CH3Cl) on Mars using ground-based high-resolution infrared spectroscopy[END_REF] also used the CSHELL spectrometer in January 2006 but could only derive an upper limit of 7.8 ppbv, in contrast to the 10 ppbv detected by [START_REF] Krasnopolsky | Search for methane and upper limits to ethane and SO2 on Mars[END_REF] one month later. [START_REF] Villanueva | A sensitive search for organics (CH4, CH3OH, H2CO, C2H6, C2H2, C2H4), hydroperoxyl (HO2), nitrogen compounds (N2O, NH3, HCN) and chlorine species (HCl, CH3Cl) on Mars using ground-based high-resolution infrared spectroscopy[END_REF] did not detect methane in their later observations of November 2009 and April 2010. 'c', and 'd' were obtained in January-March 2003 between Ls = 121°-155°. From [START_REF] Mumma | Strong release of methane on Mars in northern summer 2003[END_REF].

were identified in Nili Fossae by Mars Express (30,31) and by the Mars Reconnaissance Orbiter (32) (Fig. 3). The observed morphology and mineralogy (33,34) of this region suggest that these bedrock outcrops, rich in hydrated minerals, might be connecting with reservoirs of buried material rich in volatile species. The characteristic arcuate ridges in the southeast quadrant of Syrtis Major were interpreted as consistent with catastrophic collapse of that quadrant, from interaction with a volatile-rich substrate (34).

The low mean abundance measured in early spring 2006 (profile a) provides an important constraint on the CH 4 lifetime. The plume seen in March 2003 (19,000 tons, Fig. 2C,profile d) implies a global mean mixing ratio of ~2 ppb, if later spread uniformly over the planet. The content of the central plume of profile c is similar. Combining data for the entire region mapped Heterogeneous (gas-grain) chemistry is a strong candidate. The presence of strong oxidants in the soil was suggested first by the labeled-release experiment on Viking landers, and laboratory simulations suggested that peroxides (such as H 2 O 2 ) were responsible; the apparent discovery of perchlorate (XClO 4 ) by the Phoenix lander (35,36) suggests the existence of another family of strong oxidants, although their presence at low latitudes has not been established. The lofting of oxidant-coated soil particles into the atmosphere could permit rapid oxidation of the CH 4 that collides with them. H 2 O 2 is also produced photochemically and is a known trace gas in the atmosphere ( 37) and (being polar) might bind to aerosol surfaces. Electrochemical processes in dust storms may produce additional peroxide efficiently (4,6,23,24) (SOM-6). Peroxide-coated grains might provide an effi-in Mars-analog cryoregimes such as permafrost (21). The annual release of CH 4 from an arctic tundra landscape on Earth (at 72°N) was measured to be 3.15 g m -2 , with midsummer CH 4 fluxes of typically 30 mg m -2 day -1 (40). If similar release rates applied to our midsummer plume, the tundra-equivalent area of (assumed uniform) release would be ~6000 km 2 compared with a plume footprint ~9.7 × 10 6 km 2 . If CH 4 release were uniform over the plume footprint, the mean release rate could be smaller than the arctic rate by a factor of ~1600.

Of special interest are the deep biocommunities that reduce sulfate to sulfide using H 2 or abiotic CH 4 as the electron donor, releasing H 2 S. These communities thrive at depths of 2 to 3 km in the Witwatersrand Basin of South Africa and have been isolated from the surface (and photosynthesis) for tens of millions of years (41, 42). It might be possible for analogous biota (methanogens or methanotrophs) to survive for eons below the cryosphere boundary on Mars, where water is again liquid, radiolysis can supply energy, and CO 2 can provide a carbon source. Gases accumulated in such zones might be released to the atmosphere if pores or fissures open seasonally, connecting these deep zones to the atmosphere at scarps, crater walls, or canyons. The location of CH 4 maxima over the Syrtis Major shield volcano and the nearby Nili Fossae district suggests a possible relation to serpentinization and/or to the phyllosilicates discovered there (43,44).

REPORTS

for each north-south spatial prooving southward by about 30° e of the peak, the mixing ratio factor of 2 in each case, and for thward gradient was similar to the These latitudinal gradients suggest the first centered near 30°N, 260°W and the second near 0°, 310°W. The vapor plume from each is consistent with ~60°in both latitude and longitude.

The amount of trace gas present in each plume can be estimated from these parameters (SOM-4). In the central plume of profile d (FWHM diam-For this case, the required source strength would be ~3.66 kg s -1 . The filling time and K h vary inversely, whereas K h and source strength vary proportionately. For a filling time of 0.5 MYs (~344 Earth days), K h ~1.1 × 10 8 cm 2 s -1 and the source strength is ~0.63 kg s -1 . A reasonable

B C

abundances, spatial profiles, and seasonal changes of CH 4 on tal CH 4 column density (in molecules per square meter) along (Sun to Mars surface and Mars surface to Earth) needed to easured lines (after correcting for terrestrial transmittance, (Fig. 1C) and R1 (Fig. 1B) lines were analyzed independently longitudes that spanned 46°but differed by 12°in mean for R0 versus 300°for R1, Table 1). The apparent differences density seen at low latitudes (30°N to 15°S) reflect (in part) ean topography sampled on the two dates. The confidence oth systematic uncertainty and random error; the systematic ts all extracted values in the same way whereas the random scatter among the individual points (SOM-2 and SOM-3). (B) ratio (in parts per billion) of CH 4 obtained from the column ch footprint, after correcting for two-way air mass on Mars and (SOM-3). The mixing ratios derived from R0 and R1 of CH 4 nfidence limits. The remaining systematic difference at low latitudes is consistent with stronger weighting of local sources (Fig. 3) on 20 March (R1), when the longitude range sampled was more nearly centered over them. The differences in mixing ratio (R0 versus R1) should then have decreased with increasing distance from the source or sources, as they did (compare values at 60°N, 40°N, and 25°S, 35°S, 47°S, and 62°S). (C) Geographic and temporal variability of Mars CH 4 . Latitudinal profiles of CH 4 mixing ratios for different longitudes and seasons are shown; the width of the color envelope represents the T1s confidence envelope. The areocentric seasons (L s ) are early northern spring (a: 17°), early northern summer (b and c: 122°), and late northern summer (d: 155°) (Table 1). These extracts are taken from spectra centered at the indicated longitude (CML), and the sub-Earth footprints span longitudelatitude ranges (Table 1) with these physical dimensions: a, 770 km × 535 km; b and c, 1274 km × 818 km; d, 948 km × 586 km. The mixing ratios shown in profile d are larger than those shown in Fig. 2B, owing to different longitudinal binning, and they reflect the longitudinal maximum of the plume (SOM-1, figs. S1 and S6, and Fig. 3).

REPORTS

on February 26, 2009 www.sciencemag.org nloaded from All of the above studies measured CH4 in its absorption band at 3.3 μm.

Using the band at 8 μm, [START_REF] Fonti | Mapping the methane on Mars[END_REF] performed a statistical analysis of the Thermal Emission Spectrometer (TES) spectra acquired from the Mars Global Surveyor spacecraft and extracted a weak signal attributed to methane absorption. They found seasonally variable methane at the ~ 10-30 ppbv level. It is important to recognise that the claimed detections at such low levels of methane were obtained at the limits of the instrumental capabilities. PFS and TES spacecraft instruments do not have the required sensitivity and spectral resolution for the unambiguous identification of CH4 [START_REF] Zahnle | Is there methane on Mars?[END_REF]. Instead, the detection of methane is made by summing together thousands of spectra, which does not suppress -and can even increaseinstrumental effects and systematic errors. Despite their much greater spectral resolution, ground-based observations are not simpler because Martian methane must then be viewed through the atmosphere of Earth, which contains ~10 4 more methane molecules above the observer than that which is retrieved on Mars. The measurements must therefore exploit the Doppler shift of the Martian lines when Mars is approaching or receding from Earth. However, even in these conditions, the claimed detections of methane are close to the noise level. In any case, the very different geographical distributions and seasonal variations of methane obtained from the Earth [START_REF] Mumma | Strong release of methane on Mars in northern summer 2003[END_REF]Krasnopolosky 2012) and from space [START_REF] Fonti | Mapping the methane on Mars[END_REF][START_REF] Geminale | Mapping methane in martian atmosphere with PFS-MEX data[END_REF] are puzzling and must be considered with great care. Because of the potential implications of the presence of methane, and in the light of the controversial observational dataset described above, the first in situ measurements promised by the Tunable Laser Spectrometer (TLS) aboard the Curiosity rover and its far superior detection capabilities were eagerly awaited. TLS uses an infrared laser at 3.27 microns to scan the same methane lines as PFS or from the Earth, but at ultra-high spectral resolution. TLS has two measurement modes: the direct ingest mode with error bars on the order of 2 ppbv and the enriched mode achieved by scrubbing out CO2 during a slow fill of the sample cell. This latter mode has error bars of only about 0.1 ppbv. The first atmospheric samples collected by TLS in Gale Crater (4°S, 137°E) spanned an eight-month period in spring-summer. By combining all of the individual measurements available at that time, it was concluded that methane was not detected, with an upper limit of only 1.3 ppbv [START_REF] Webster | Low upper limit to methane abundance on Mars[END_REF]. In a subsequent analysis, Webster et al (2015a) presented the entire TLS dataset reprocessed over a period of almost one Martian year (605 Martian days or sols). The results indicate detection of methane at two levels of abundance (Fig.

12.2). A "background" CH4 level of 0.7±0.2 ppbv, based on high-precision methane-enriched experiments, is observed during the first 8 months and last 4 months of the dataset. Between these two periods, two episodic enhancements of methane were observed. The first was detected in June 2013 with a single measurement indicating about 6 ppbv of CH4 followed by a drop to 2 ppbv one week later. The second was observed in November 2013-January 2014 (sols 466-526 in Figure 12.2). During that period -corresponding to the Martian spring (Ls = 56-82°) -four sequential measurements of TLS indicated a pulse of 7±2 ppbv of methane over two months. The duration of this event (60 sols) followed by a sudden drop to 0.5 ppbv 47 sols later was interpreted by Webster et al (2015a) to result from a local production of methane that, once terminated, dispersed quickly. One Martian year later (in Martian year 33, according to the calendar proposed by [START_REF] Clancy | An intercomparison of ground-based millimeter, MGS TES, and Viking atmospheric temperature measurements: Seasonal and interannual variability of temperatures and dust loading in the global Mars atmosphere[END_REF] which starts at Ls = 0° on 11 April 1955), no springtime pulse of methane was observed by TLS (Fig. 12.3). On the contrary, the instrument only detected a very small background level of 0.2 ppbv by means of a high-precision enriched run (Webster et al 2015b). This absence of reproducibility from one year to the next seems to rule out a seasonal effect. Thus, if the TLS findings are true, the in situ evidence for the presence of methane on Mars can be summarised as follows. 1) Methane is constantly present at very low levels (0.2-0.7 ppbv) in the Martian atmosphere. This equilibrium level requires a tiny source of less than 20 tonnes per year in order to balance the photochemical loss of methane over its at-mospheric lifetime of 300 years [START_REF] Lefèvre | Observed variations of methane on Mars unexplained by known atmospheric chemistry and physics[END_REF]. 2) TLS observed two pulses of methane ten times higher than the background methane level during its first Martian year on the surface. Such events were not observed in the second year. This suggests the presence of local and episodic releases of methane, which are not correlated to season. The occurrence of methane pulses at the surface has not yet been confirmed by TLS high-precision "enriched" runs.

Methane production

The observational claims for methane on Mars naturally raise the question of sources and sinks. On Mars, methane is not produced photochemically in the atmosphere and is destroyed by ultraviolet radiation and oxidants in approximately 300 years. Therefore, the presence of methane in the atmosphere, even in very small quantities, requires on-going (or very recent) emission from the surface. On Earth, more than 90% of methane has a biological origin, in the form of living beings, organic waste or fossilized matter. It is therefore tantalising to relate the existence of methane on Mars to a possible past or extant life on the planet. For instance, [START_REF] Krasnopolsky | Some problems related to the origin of methane on Mars[END_REF] calculated that methane originating only from the impacts of comets, meteorites, and interplanetary dust is insignificant, and further argued that the lack of volcanism, hot spots, and SO2 (which is more abundant than CH4 in terrestrial outgassing) favours the hypothesis of a biogenic origin for Martian methane. However, if a microbial source is a possibility to explain the existence of methane on Mars, it is by no means the only one. To be convinced of this, one merely has to consider the estimated mass of methane transferred to the Mars atmosphere each year -a few tens of tons are sufficient to explain the background level measured by TLSto the abiotic CH4 flux on Earth, which is estimated at several megatons per year [START_REF] Etiope | Abiotic methane on Earth[END_REF]. On Earth, methane can indeed be produced in several specific environments by chemical reactions that do not involve organic matter. These processes are found at high temperatures in volcanic or geothermal areas, and at low temperatures in aquifers via gas-water-rock interactions. In this latter case, [START_REF] Atreya | Methane and related trace species on Mars: Origin, loss, implications for life and habitability[END_REF] proposed that methane could be produced abiogenically by hydrothermal processes, such as serpentinization, i.e., the reaction between ultramafic (Mg, Fe-rich) silicates and water, producing serpentine and molecular hydrogen H2:

(Mg,Fe)2SiO4 + H2O → Mg3Si2O5(OH)4 + Mg(OH)2 + Fe3O4 + H2

The aqueous H2 produced above then reacts with CO2 via a Fischer-Tropsch-type reaction which in turn produces methane, e.g.:

CO2 + 4 H2 → CH4 + 2 H2O

At the Mid-Atlantic Ridge, a substantial production of methane by serpentinization has been detected at relatively mild temperatures (< 100 °C, Kelley et al 2005), which could also be found on Mars at depths of 2-3 km below the surface [START_REF] Oze | Have olivine, will gas: Serpentinization and the abiogenic production of methane on Mars[END_REF]. Although the presence of such aquifers is yet to be discovered on Mars, the discovery of methane in the atmosphere increases the possibility of active hydrothermal activity and a geologically "living" planet. Exogenous methane sources, such as comets, meteorites, or interplanetary dust particles, are also a possibility. [START_REF] Court | Investigating the contribution of methane produced by ablating micrometeorites to the atmosphere of Mars[END_REF] studied direct release of CH4 by ablation and pyrolysis of carbonaceous meteorites during their atmospheric entry. They found that this process accounted for less than 10 kg of methane annually, which is a negligible fraction of the mass required to maintain the abundance of methane observed by TLS. On the other hand, [START_REF] Keppler | Ultraviolet-radiation-induced methane emissions from meteorites and the Martian atmosphere[END_REF] argued that Murchison-type carbonaceous micrometeorites might be in sufficient quantity on the surface of Mars to become a significant abiogenic source of methane when exposed to ultraviolet radiation. The extrapolation of their laboratory experiment to the global scale suggests that 10-800 tons of methane could be produced annually at the surface of Mars, which could explain a substantial fraction -if not all -of the observed abundance of methane. However, it is important to note that none of the processes mentioned above can explain the temporal variability of Martian methane that seems to emerge from the observations. Since it has been shown that high methane episodes do not correlate with predicted meteor events (Roos-Serote et al 2016), we may discard the hypothesis of meteor showers as a source of methane variability on Mars.

Methane loss

According to conventional chemistry, which reproduces very well the observed methane distribution and variations on Earth, the photochemical loss of methane on Mars is accomplished either by photodissociation or by chemical reactions. The fields of action of both processes are clearly separated in altitude. In the upper atmosphere, methane is photolyzed in the Lyman-a line at 121.6 nm. The efficiency of this process peaks at about 80 km and represents 50-60 % of the total loss of methane integrated over the atmospheric column. In the lower atmosphere, as on Earth, the loss of Martian methane occurs through hydrogen abstraction reactions with OH and O( 1 D), which initiate the methane oxidation chain. On Mars, the importance of oxidation by OH over that by O( 1 D) depends strongly on the season but, overall, the sum of both reactions represents the remaining 40-50% of the total loss of methane. These processes lead to a global chemical lifetime of methane on Mars estimated by state-of-the-art models of 300-340 terrestrial years (Summers et al 2002;[START_REF] Krasnopolsky | Detection of methane in the martian atmosphere: evidence for life?[END_REF][START_REF] Lefèvre | Observed variations of methane on Mars unexplained by known atmospheric chemistry and physics[END_REF]. From this lifetime can be calculated the rate of globally supplied methane required to maintain a steady-state mixing ratio in the atmosphere: as mentioned above, less than 20 tons per year of methane transferred to the atmosphere are sufficient to maintain the steady-state value of about 0.5 ppbv measured by TLS. This mass may be compared with a terrestrial value on the order of 600x10 6 tons per year [START_REF] Ciais | Carbon and Other Biogeochemical Cycles[END_REF]. Can such a faint source create the variability of methane observed on Mars? [START_REF] Lefèvre | Observed variations of methane on Mars unexplained by known atmospheric chemistry and physics[END_REF] investigated this possibility by implementing highly localized and sporadic sources with the LMD global climate model (GCM). As expected from the 300-year lifetime of methane on Mars, their results showed that methane should be homogeneously mixed by atmospheric transport across most of the planet. A striking feature of GCM simulations of methane, however, is the large enrichment in methane that results from the condensation of CO2 gas at high latitudes in winter (Fig. 12.4). This is a well-established process, identical for all non-condensable species, as proven by observations of argon (e.g., [START_REF] Sprague | Mars' atmospheric argon: Tracer for understanding Martian atmospheric circulation and dynamics[END_REF] and carbon monoxide [START_REF] Encrenaz | Seasonal variations of the martian CO over Hellas as observed by OMEGA/Mars Express[END_REF]. Conversely, CH4 is depleted at high-latitudes when CO2 sublimates from the polar cap in summer and returns to the gas phase. At low latitudes, the slow seasonal modulation of methane driven by the condensationsublimation cycle of CO2 is greatly attenuated. This effect calculated by the LMD GCM at Gale Crater is shown in Figure 12.5. It induces a peak-topeak change in CH4 of about 25% and a maximum mixing ratio obtained shortly before equinox (Ls = 160°). This theoretical result is here compared only to the high-precision measurements of TLS that are representative of a low background level of CH4, which in principle should vary like those of any other non-condensable species. Figure 12.5 shows that the maximum value of background methane measured by TLS (0.9 ppbv) is also obtained near Ls = 160°. However, the change in background methane ob-served in the first half of the Martian year appears to be much stronger (by one order of magnitude) than what is expected from the GCM and from only the condensation-sublimation cycle of CO2. This puzzling fact will have to be confirmed by new observations covering a greater part of the Martian year. Using GCM simulations, [START_REF] Lefèvre | Observed variations of methane on Mars unexplained by known atmospheric chemistry and physics[END_REF] showed that reproducing the Earth-based observations of methane by [START_REF] Mumma | Strong release of methane on Mars in northern summer 2003[END_REF] required a methane lifetime shorter than ~200 days, and hence an unknown sink that is at least 600 times faster than the loss derived from the current kinetics data used by the atmospheric chemistry community. In what follows, we present GCM simulations of methane carried out in an attempt to reproduce the full set of TLS measurements, including the spike of 7 ppbv observed in the spring of Mars Year 32. For each model experiment, methane is continuously released from a single location chosen among nine sites located in the hemisphere centred on Gale Crater (Fig. 12.6, left). The production of methane is assumed to be continuous, since both serpentinisation (occurring a few kilometers below the surface) and the action of ul-traviolet radiation on organics (widespread on the surface) are unlikely to be episodic over short timescales. In a first step, the lifetime of methane is arbitrarily assumed to be 1 terrestrial year. This is similar to the suggestion by [START_REF] Lefèvre | Observed variations of methane on Mars unexplained by known atmospheric chemistry and physics[END_REF] to match the measurements of [START_REF] Mumma | Strong release of methane on Mars in northern summer 2003[END_REF]. In the case of the TLS measurements, a lifetime of 1 year implies an emission of 6000 tons yr -1 in order to maintain the background level of 0.7 ppbv suggested by the instrument. Such a source would be 40 times more significant than the largest abiotic gas seep known to date on Earth (Chimaera in Turkey, Etiope and Sherwood Lollar 2013). This hypothesis is unlikely but required in order to approach the TLS measurements with model simulations. Figure 12.6 (right) shows the methane mixing ratio obtained at Gale Crater when methane is continuously released from one of the nine sites of emission. With such a short lifetime for methane (1 year), atmospheric transport can produce pulses of methane at Gale crater that are superimposed onto the slow seasonal variation due to the CO2 condensation/sublimation cycle. The largest pulses simulated by the GCM (~1.6 ppbv) are obtained when the source is located at Apollinaris Patera, east of Gale Crater. However, they are 4 times smaller than those measured by TLS, and do not occur during the season observed by the instrument. A methane source located at Martz Crater (south of Gale Crater) provides a better result for seasonality, but requires a much larger release to match the pulse of 7 ppbv observed by TLS. As shown in Figure 12.7, a pulse peaking at 7 ppbv in the model simulation requires a methane lifetime of the order of 1 month in order to maintain the same background level of 0.7 ppbv. Such a short lifetime puts an enormous burden on the process that governs the destruction of methane on Mars, which must be faster than the conventional chemistry by three orders of magnitude. The fact that such a strong, unknown, chemical process would have been overlooked would be a surprise since conventional models do a rather good job at reproducing short-lived chemical species on Mars. For instance, species that are very sensitive to the oxidizing capacity of the atmosphere, such as hydrogen peroxide (H2O2) or ozone (O3), are measured in quantities that are quite consistent with model simulations including only the standard set of chemical reactions used in Earth photochemistry (e.g., [START_REF] Encrenaz | Seasonal variations of hydrogen peroxide and water vapor on Mars: Further indications of heterogeneous chemistry[END_REF][START_REF] Clancy | Daily global mapping of Mars ozone column abundances with MARCI UV band imaging[END_REF].

Several studies have explored the possibility of a fast-acting sink of methane in the specific atmospheric conditions of Mars. For instance, it has been proposed that methane could be destroyed by electrochemical reactions triggered by the strong electric fields generated by local 'dust devils' or during regional-scale dust storms [START_REF] Delory | Oxidant Enhancement in Martian dust devils and storms: electric fields and electron dissociative attachment[END_REF][START_REF] Farrell | Martian dust storms as a possible sink of atmospheric methane[END_REF].

During such events, energized electrons are expected to dissociate methane directly but also to dissociate H2O and produce vast amounts of H2O2. This latter effect is problematic, however, since the large-scale production of H2O2 by electric fields is contrary to the good agreement currently noted between observations and conventional models of H2O2 and O3. [START_REF] Lefèvre | Observed variations of methane on Mars unexplained by known atmospheric chemistry and physics[END_REF] used a GCM to demonstrate that electrochemistry should, in addition to H2O2, also produce large amounts of CO from the dissociation of CO2. Nonetheless, both of these facts are difficult to reconcile with current observations of H2O2 and CO, which show no significant enhancement during the dusty season. Fast destruction of methane at the global scale by electrochemical processes is therefore not supported for the moment by observations. [START_REF] Atreya | Oxidant Enhancement in Martian dust devils and storms: implications for life and habitability[END_REF][START_REF] Atreya | Methane and related trace species on Mars: Origin, loss, implications for life and habitability[END_REF] argued that the local excesses of H2O2 produced in dust storms could lead to its precipitation out of the atmosphere onto the Martian surface. Methane would then be scavenged by large amounts of H2O2 or other super-oxides embedded in the regolith. This process has the advantage of not altering the conventional atmospheric chemistry, but it is difficult to imagine that it could affect CH4 without processing O3 or CO in the same fashion. Furthermore, rapid loss of methane in the regolith is, at present, not supported by laboratory work: experiments on analogues of Martian soil or perchlorates show no fast oxidation of methane in the presence of H2O2 [START_REF] Gough | Can rapid loss and high variability of martian methane be explained by surface H2O2?[END_REF]. In addition, kinetic data on the reactions between CH4 with metal oxides and superoxide ions are extremely slow at Martian temperatures [START_REF] Krasnopolsky | Some problems related to the origin of methane on Mars[END_REF]. Another potential explanation for the apparent variability and short lifetime of methane on Mars was presented by [START_REF] Jensen | A sink for methane on Mars ? The answer is blowing in the wind[END_REF], who found that wind-driven agitation of quartz crystals could produce active sites that sequester methane temporarily. Crystalline quartz is very rare on Mars but is used in their experiments as an analogue for the silicate mineralso found in airborne dust or at the surface. The reverse reaction that releases CH4 back to the atmosphere has not been studied in the laboratory and, thus, the efficiency of these idealized experiments in the real Martian atmosphere is highly uncertain. Another possibility is that methane condenses in clathrates, however, according to laboratory work [START_REF] Trainer | Limits on the trapping of atmospheric CH4 in martian polar ice analogs[END_REF], the trapping of CH4 on polar ice analogues, including clathrates, appears to be negligible under modern Martian conditions and is not a viable explanation for the apparent variability of methane on Mars.

Open questions and future measurements

The detection of a very low background level (~0.5 ppbv) of methane in the atmosphere of Mars by the Curiosity rover raises the possibility of a planet that is biologically or geologically active now. However this detection does not constitute proof of such activity: we have seen that the production of methane required to explain the TLS observations is so minor that these results could equally be due to the slow UV-alteration of the organics delivered by inert meteoric or interplanetary dust material. In fact, for the atmospheric chemist, the variations of Martian methane pose a much greater challenge than just its presence. For robust theoretical reasons, based on knowledge well-established in the context of the atmosphere of the Earth, methane should have a 300-year lifetime on Mars. Due to this long lifetime with respect to mixing by transport, methane should therefore be uniformly distributed throughout the atmosphere. Similarly to other long-lived species observed on Mars (such as argon, or carbon monoxide), methane variations should only be controlled by the slow condensation/sublimation cycle of CO2, with peak-to-peak amplitudes of about 30% over one Martian year. Yet, the observations of Martian methane by TLS suggest variations of up to one order of magnitude in less than two months. This implies not only a local methane source situated by extraordinary coincidence very close to the Curiosity rover, but also that an unknown and extraordinarily fast mechanism of methane destruction is at work. Because this finding strongly challenges our current understanding of the atmospheric chemistry and physics of Mars, the discovery of variable amounts of methane needs to be confirmed by further detections. In particular, the question "is methane variable on Mars?" must now be addressed by mapping methane at the global scale, with a sensitivity better than 1 ppbv. Evidently the answer to this question is a prerequisite before tackling the following ones, such as "where are the emission sites of methane?", "how is methane destroyed?", and ultimately "how is methane produced?". Confirming the presence and variability of methane on Mars is one of the objectives of ESA-Roscomos Trace Gas Orbiter (TGO), which launched in March 2016 and arrived at Mars in October 2016. After a long aerobraking phase, the nominal science operations of TGO will have started in 2018. On board the platform, two different instruments, the Atmospheric Chemistry Suite (ACS) [START_REF] Korablev | The Atmospheric Chemistry Suite (ACS) of three spectrometers for the ExoMars 2016 Trace Gas Orbiter[END_REF] and NOMAD [START_REF] Vandaele | Science objectives and performances of NOMAD, a spectrometer suite for the ExoMars TGO mission[END_REF], will use the solar occultation technique to search for methane at all locations and seasons with a sensitivity hopefully better than 0.1 ppbv. It is therefore likely that a major advance is coming in our effort to unravel an enigma that may impact Mars science and astrobiology in a fundamental way.

Sources

Figure 12 . 1 .

 121 Figure 12.1. Methane mixing ratio (ppbv) observed from Earth by Mumma et al (2009). Left: regions where methane appeared enhanced during summer 2003 (Ls = 121°-155°). Right: latitudinal and temporal variability of methane. Profile 'a' was obtained in February 2006 at Ls = 17.2°. Profiles 'b', 'c', and 'd' were obtained in January-March 2003 between Ls = 121°-155°. From Mumma et al (2009).

Fig. 3 .

 3 Fig. 3. Regions where CH 4 appears notably localized in northern summer (A, B 1 , and B 2 ) and their relationship to mineralogical and geomorphological domains. (A) Observations of CH 4 near the Syrtis Major volcanic district. (B) Geologic map of Greeley and Guest (45) superimposed on the topographic shaded relief from the Mars Orbiter Laser Altimeter (46). The most ancient terrain units are dissected and etched Noachian plains (Npld and Nple) (~3.6 to 4.5 billion years old, when Mars was wet) and are overlain by volcanic deposits from Syrtis Major of Hesperian (Hs) age (~3.1 to 3.6 billion years old).

Figure 12 . 2 .

 122 Figure 12.2. Measurements of methane mixing ratio (ppbv) by the TLS instrument on the Curiosity rover, as presented by Webster et al (2015a). 1 sol = 1 Mars day = 24 hr 37 mn. Martian sol 1 was on August 6 th , 2012. Error bars represent ±1 standard error of the mean. All measurements were made at night, except for the two marked "D" ingested during the day. The values with smaller error bars labelled EN were retrieved from the "methane enrichment" runs. From Webster et al (2015a).

Figure 12 . 3 .

 123 Figure 12.3. Summary of TLS methane measurements vs. solar longitude for the Martian years 31 to 33 (October 2012-January 2016). Error bars represent ±1 standard error of the mean. From data published by Webster et al (2015a) and Roos-Serote et al (2016)

Figure 12 . 4 .

 124 Figure 12.4. Seasonal evolution of the zonally averaged methane mixing ratio calculated by a global climate-chemical model. The simulations include a local source at the surface near the Equator. The amount of methane released from the source balances the global photochemical loss integrated over the Martian year, assuming an equilibrium value of 10 ppbv. The lifetime of methane is 330 years as determined by conventional chemistry. From Lefèvre and Forget (2009).

Figure 12 . 5 .

 125 Figure 12.5. Seasonal evolution of the background methane levels measured at Gale Crater by the TLS instrument aboard the Curiosity rover (only high-precision "methane enriched" runs are included) and simulated by the LMD global climate model (GCM) assuming conventional methane chemistry. The slow variation of methane visible in the model simulation is only due to the condensation-sublimation cycle of CO2 at the Martian poles. TLS data are from Webster et al (2015a) and Roos-Serote et al (2016).

Figure 12 . 6 .

 126 Figure 12.6. Simulations of methane with the LMD global climatechemical model. Left: Location of each site of emission of methane tested for the nine model experiments. In each case the emission is continuous and the methane lifetime is arbitrarily assumed to be 1 terrestrial year. Right: Seasonal evolution of the mixing ratio of methane obtained at Gale Crater for each of the sites of emission.

Figure 12 . 7 .

 127 Figure 12.7. Seasonal evolution of the mixing ratio of methane (ppbv) at Gale Crater calculated by the LMD global climate model when methane is continuously released from Martz Crater. The results are shown for arbitrary lifetimes of methane of 1, 2, and 6 months and are compared to TLS measurements by Webster et al (2015a).

Figure 12 . 1 :

 121 Figure 12.1: after Mumma et al (2009) / permission Figure 12.2: after Webster et al (2015a) / permission Figure 12.3: adapted from Webster et al (2015a) and Roos-Serote et al (2016) Figure 12.4: after Lefèvre and Forget (2009) / permission Figure 12.5: original figure made for the chapter. TLS data from Webster et al (2015a) and Roos-Serote et al (2016) Figure 12.6: original figure made for the chapter.Figure 12.7: original figure made for the chapter. TLS data from Websteret al (2015a)

Figure 12 . 7 :

 127 Figure 12.1: after Mumma et al (2009) / permission Figure 12.2: after Webster et al (2015a) / permission Figure 12.3: adapted from Webster et al (2015a) and Roos-Serote et al (2016) Figure 12.4: after Lefèvre and Forget (2009) / permission Figure 12.5: original figure made for the chapter. TLS data from Webster et al (2015a) and Roos-Serote et al (2016) Figure 12.6: original figure made for the chapter.Figure 12.7: original figure made for the chapter. TLS data from Websteret al (2015a)


M R Combi
email: mcombi@umich.edu

T Mäkinen

J.-L Bertaux

E Quémerais

S Ferron

R Coronel

Comet 21P/Giacobini-Zinner: Water Production Activity over 20 Years with SOHO/SWAN Short Title: SOHO/SWAN Observations of comet 21P

Keywords: Comets, Comet 21P/Giacbini-Zinner, Ultraviolet observations, Comets, coma Comets, Cometary Atmospheres, Comet 21P/Giacobini-Zinner

), with new observations from 2012 and 2018. Unlike some comets that have faded over time, except for 2 outbursts seen in the 2012 results, the activity levels for Comet 21P have not changed consistently over time. A power-law fit to the pre-perihelion water production rate vs. heliocentric distance for all four apparitions gives a result of 3.75x10 28 x r -0.28±0.4 , and that for the postperihelion production rate, discounting 2012 data dominated by outbursts, gives 4.15

x 10 28 x r -10.2±0.9 . The production rates are in s -1 and heliocentric distances are in AU.

Introduction

Comet 21P/Giacobini-Zinner (hereafter G-Z) was discovered in 1900 by Michel Giacobini and recovered in later apparitions by Ernst Zinner. It has been widely observed on nearly every apparition ever since. The plasma tail of G-Z was the first comet visited by a spacecraft, the International Cometary Explorer [START_REF] Cowley | ICE encounters Giacobini-Zinner[END_REF], in 1985, which had a complement of particle and field instruments, including instrumentation to make ion composition measurements [START_REF] Ogilvie | Ion composition results during the International Cometary Explorer Encounter with Giacobini-Zinner[END_REF]). The photometric survey of comets by A' Hearn et al. (1995) classified G-Z as the archetype member of a group of comets that is depleted in C2 and C3 and slightly depleted in NH, all relative to CN. [START_REF] Fink | A taxonomic survey of comet composition 1985-2004 using CCD spectroscopy[END_REF] found it similarly depleted in C2 and C3 and also depleted in NH2. The agreement of the small depletion of NH and NH2 implies a depletion of the parent molecule NH3.

G-Z has also been observed in the UV and IR. During the 1985 apparition, G-Z was observed with the International Ultraviolet Explorer satellite, and water production rates were determined from observations of OH at 309 nm (McFadden et al. 1987) and H Lyman-alpha (Combi and Feldman 1992). H Lyman-alpha was also observed with the Pioneer Venus UVS instrument [START_REF] Combi | Pioneer Venus Lyman-a observations of comet P/Giacobini-Zinner and the life expectancy of cometary hydrogen[END_REF]). These results were summarized and compared with the 1998 and 2005 apparitions of H Lyman-alpha observed with the Solar Wind Anisotropies instrument (SWAN) on the SOlar and Heliospheric Observatory (SOHO) satellite by [START_REF] Combi | SOHO/SWAN observations of short-period spacecraft target comets[END_REF]. [START_REF] Weaver | An Infrared Investigation of Volatiles in Comet 21P/Giacobini-Zinner[END_REF] and [START_REF] Mumma | Detection of CO and Ethane in Comet 21P/Giacobini-Zinner: Evidence for Variable Chemistry in the Outer Solar Nebula[END_REF] showed that, like the carbon-chain radical depletion compared to OH in the visible, CH3OH and C2H6 were depleted compared to water. Later observations by [START_REF] Disanti | Highly Depleted Ethane and Mildly Depleted Methanol in Comet 21P/Giacobini-Zinner: Application of a New Empirical ν2-band Model for CH3OH near 50[END_REF] confirmed these depletions. [START_REF] Mumma | Detection of CO and Ethane in Comet 21P/Giacobini-Zinner: Evidence for Variable Chemistry in the Outer Solar Nebula[END_REF] found a rather substantial CO/H2O ratio in the range of 6%-15%, in contrast to a much lower upper limit by [START_REF] Weaver | An Infrared Investigation of Volatiles in Comet 21P/Giacobini-Zinner[END_REF]. All measurements summarized by Dello [START_REF] Russo | Emerging trends and a comet taxonomy based on the volatile chemistry measured in thirty comets with high-resolution infrared spectroscopy between 1997 and[END_REF] give a weighted average value for CO/H2O of only (2.2 ± 1.5)%. Shinnaka et al. ( 2020) found a very low CO2/H2O ratio in G-Z of only 1-2% and only upper limits or non-detections were available for other species, such as HCN, NH3, C2H2 and CH4. However, a recent and more sensitive set of IR observations of G-Z give improved measurements [START_REF] Roth | Probing the Evolutionary History of Comets: An Investigation of the Hypervolatiles CO, CH4, and C2H6 in the Jupiter-family Comet 21P/Giacobini-Zinner[END_REF] of CO (between 1% and 2%), CH4 (between 0.6% and 1.1%) and C2H6 (between 0.24% and 0.29%) compared to water. They also found some variation over the orbit of G-Z, indicating some possible compositional heterogeneity in the innermost ±1000 km of the coma toward either side of the nucleus, indicating possible seasonal effects comparing pre-, near-, and post-perihelion observations.

Infrared observations by

SOHO/SWAN Observations and Basic Analysis

The SWAN instrument on board the SOHO satellite makes nearly daily whole sky observations of the hydrogen Lyman-a 121.5-nm emission. As such, its major investigation is to measure the 3-dimensional structure of the solar wind and solar radiation fields via their interaction and resulting depletion of the interstellar neutral hydrogen streaming through the solar system [START_REF] Bertaux | SWAN: a study of solar wind anisotropies on SOHO with Lyman alpha sky mapping[END_REF]. The sensitivity of SWAN, as well as its location on the SOHO spacecraft in a halo orbit around the Sun-Earth L1 Lagrange point, make it an ideal observatory to view the large atomic hydrogen comae surrounding comets [START_REF] Bertaux | Lyman-alpha observations of comet Hyakutake with SWAN on SOHO[END_REF][START_REF] Combi | A survey of water production in 61 comets from SOHO/SWAN observations of hydrogen Lyman-alpha: Twenty-one years[END_REF]. SWAN enables daily 1 o by 1 o resolution imaging of bright enough comets (typically brighter than mV~10) independent of north/south hemisphere location or (mostly) the relative positions of the comet, Sun and Earth, a definite advantage over observations made from the ground or from low Earth orbit. Other impediments to obtaining water production rates from comet observations or affecting the resulting uncertainties are caused by one or more of the following: bright stars near the comet (especially when the comet in is or near the galactic equator), SWAN's solar avoidance area, or when portions of the spacecraft block the line-of-sight to the comet.

Because photodissociation of water and its main daughter radical, OH, produces two H atoms per water molecule, modeling an observation of the H coma and its variation along a comet's orbit has proven to yield a consistent set of water production rates for many comets. The methodology, our time-resolved model (or TRM), as well as background theoretical and observationally-verified parameterizations have been explained in detail and documented in several previous papers [START_REF] Combi | Gas Dynamics and kinetics in the coemtary coma : Theory and Observations[END_REF][START_REF] Mäkinen | Temporal deconvolution of the hydrogen coma. I. A Hybrid model[END_REF][START_REF] Combi | Temporal deconvolution of the hydrogen coma. II. Pre-and post-perihelion activity of Comet Hyakutake (1996 B2)[END_REF][START_REF] Combi | A survey of water production in 61 comets from SOHO/SWAN observations of hydrogen Lyman-alpha: Twenty-one years[END_REF]) and need not be discussed in detail yet again here.

Total uncertainties in SWAN water production rates result from the fitting process of the comet model and the background interplanetary hydrogen to the observation, noise/uncertainty in the measurement as well as brightness calibration and errors associated with the model and model parameters. The smaller listed and plotted 1-s uncertainties result only from the noise in the data and the fitting process. Total uncertainties resulting from the SWAN calibration, the solar measurements of integrated Lyman-alpha and model associated errors are expected to be on the order of ~30%. In this paper, owing to the fact that observations are of a moderately weak Jupiter Family comet (JFC), G-Z, we give single-image water production rates obtained from each image. The filling time of the field of view used (a circular radius of 8 degrees) by hydrogen atoms streaming outward at speeds of 8-20 km s -1 , combined with the photodissociation times of H2O and OH, ~80,000 and ~150,000 s at a heliocentric distance of 1 AU and spacecraft-comet distances of ~0.4-1 AU result in the H coma representing the water production over the previous 2-3 days.

The solar Lyman-alpha g-factors are calculated from the daily disk-integrated solar Lyman-alpha fluxes from the LASP web site at the University of Colorado (http://lasp.colorado.edu/lisird/lya/), in combination with the solar Lyman-alpha velocity dependent line profile (Lemaire et al. 1998). Daily-fluxes are measured from the Earth-facing disk of the Sun, so the value at the comet, which is in a different direction, are taken from the nearest time the same face of the Sun was seen by the comet, accounting for the 27-day rotation of the Sun. Typically, variations of solar Lyman-alpha caused by active regions on the Sun repeat on subsequent rotations.

Water Production Rates in Comet 21P/Giacobini-Zinner

G-Z was observed by SWAN during the last four apparitions, 1998, 2005, 2012, and 2018. A summary of the data set is given in Table 1. The resulting water production rates, observational circumstances and ancillary data for the 1998 and 2005 apparitions were published in a paper by [START_REF] Combi | SOHO/SWAN observations of short-period spacecraft target comets[END_REF] and also appear in the SOHO/SWAN database of over 70 comets in the NASA Planetary Data System Small Bodies Node [START_REF] Combi | Gas Dynamics and kinetics in the coemtary coma : Theory and Observations[END_REF](Combi , 2020)). The daily-average water production rates, formal model-fitting uncertainties, g-factors and observational circumstances for the 2012 and 2018 apparitions are given in Table 2.

Figure 1 shows a plot of the water production rate as a function of time in days from perihelion for all four apparitions. There is clearly some irregularity in activity within each apparition, as well as from apparition to apparition. There is, however, no evidence of consistent fading over 20 years as there has been in comets 103P/Hartley 2, 41P/Tuttle-Giacobini-Kresak and 46P/Wirtanen (Combi et al. 2020).

The irregularity in activity over time is also apparent in visual magnitude variations (Yoshida 2020). In particular, the water production rate variations are noteworthy during the 2012 apparition, which has some of the larger departures from the mean activity, both during the pre-and post-perihelion legs of the orbit. In particular, there were large increases in production rate in 2012 around 34 days before perihelion and 30 days after perihelion, indicating likely outbursts at the level of a factor of two.

q = perihelion distance (AU)
The water production rate levels and characteristic variation peaking at about 40 days before perihelion and then decreasing through perihelion and up to 40 days after is very similar to and agrees with that reported by [START_REF] Schleicher | Photometric observations of comet P/Giacobini-Zinner[END_REF] from photometric data of OH obtained during the 1985 apparition. Their data cover a larger range of times, spanning from approximately 80 days before to 80 days after perihelion. Before 40 days before perihelion their results show that the production rate does continue to decrease, as one might expect, but from 40 to 80 days after perihelion the OH production rate levels off at ~10 28 s -1 .

Figure 2 shows the production rates from all four apparitions plotted as a function of heliocentric distance. The previous paper that included the results from the 1998 and 2005 apparitions [START_REF] Combi | SOHO/SWAN observations of short-period spacecraft target comets[END_REF] found pre-and post-perihelion power-law fits to the production rate of 5.9 x 10 28 r -1.7±0.4 and 6.8 x 10 28 r -11.9±3.5 , respectively, with r being the heliocentric distance in AU, and the production rate at 1 AU in s -1 . The combination of all four apparitions, leaving out the 2012 postperihelion data dominated by the outburst, gives a pre-perihelion result of 3.75 x 10 28 x r -0.28±0.4 , and for the post-perihelion production rate give 4.2 x 10 28 x r -10.2±0.9 .

It appears that the large outburst values from 2012 significantly influence the power laws. While the pre-to-post perihelion asymmetry remains similar, with a much flatter slope before perihelion and a very steep slope after. The post-perihelion fit for just the 2018 is 4.0 x 10 28 r -9.2±1.4 , which is to within uncertainties the same as that for 1998, 2005 and 2018.

It is notable that there is a decided change in slope (exponent) separating the pre-perihelion data before -40 days. After -40 days the production rate is essentially flat, while before -40 days the production rates decrease as one might expect, and as is seen in the ground-based photometry of [START_REF] Schleicher | Photometric observations of comet P/Giacobini-Zinner[END_REF].

If we fit a power law only to the data before -40 days we obtain a slope of -3.9, or an expression of 7.07 x 10 28 r -3.9±1.0 . This is a more typical drop in production rate with heliocentric distance compared with other Jupiter Family Comets [START_REF] Combi | A survey of water production in 61 comets from SOHO/SWAN observations of hydrogen Lyman-alpha: Twenty-one years[END_REF]) and highlights the strong seasonal variation of water production rate.

Summary

We present here the analysis of hydrogen Lyman-alpha observations of comet 21P/Giacobini-Zinner obtained with the all-sky SWAN camera on the SOHO spacecraft obtained during the four apparitions since the SOHO launch, 1998SOHO launch, , 2005SOHO launch, , 2012SOHO launch, , and 2018. With the exception of what appear to be two outbursts during the 2012 apparition raising the water production rate by a factor of two for a short period, the water production activity appears to be quite stable over this period of time. Comparison with OH results even from the 1985 apparition look very similar.

  Figure 1. Single-image water production rates in comet 21P/Giacobini-Zinner are plotted as a function of time from perihelion in days. The error bars correspond to the respective 1s formal uncertainties. The red x's show the values from the 1998 apparition, the blue triangles from 2005, the green diamonds from 2012, and the black squares from 2018. The vertical lines show the formal model fitting uncertainties.

Figure 2 .

 2 Figure 2. Single-image water production rates in comet 21P/Giacobini-Zinner are plotted as a function of heliocentric distance in AU. The pre-perihelion data are in the left half and the post-perihelion data in the right. Note that the perihelion distance is 1.03 AU and is responsible for the gap. In addition, there are small changes in perihelion distance over the 20 years. The straight lines represent the power-law fits to all four apparitions, leaving out the post-perihelion 2012 data dominated by the outburst. The best-fit coefficients for the power-law fits are given in the text. The x's (red) show the values from the 1998 apparition, the triangles (blue) from 2005, the diamonds (green) from 2012, and the squares (black) from 2018.

Table 1 .

 1 Summary of SOHO/SWAN Observations, Comet 21P/Giacobini-Zinner

	Perihelion Date	q(AU)	# of Images	rH range (AU)
	21.320 Nov. 1998	1.024	18	1.034 -1.108
	2.761 Jul. 2005	1.038	39	1.038 -1.284
	11.735 Feb. 2012	1.030	19	1.058 -1.213
	10.278 Sep. 2018	1.013	71	1.013 -1.284
	rH = heliocentric distance (AU)

Acknowledgements: SOHO is an international mission between ESA and NASA. M.


M R Combi
email: mcombi@umich.edu

T Mäkinen

J.-L Bertaux

E Quémerais

S Ferron

Water Production Rate of C/2020 F3 (NEOWISE) from SOHO/SWAN over Its Active Apparition

Keywords: Comets, Comae, Neutral coma gases, Long period comets, comet origins, Oort cloud

C/2020 F3 (NEOWISE) was discovered in images from the Near Earth Object program of the Wide-Field Infrared Survey Explorer (NEOWISE) taken on 27 March 2020 and has become the Great Comet of 2020. The Solar Wind ANisotropies (SWAN) camera on the Solar and Heliospheric Observatory (SOHO) spacecraft, located in a halo orbit around the Earth-Sun L1 Lagrange point, makes daily full-sky images of hydrogen Lyman-alpha. Water production rates were determined from the SWAN hydrogen Lyman-alpha brightness and spatial distribution of the comet measured over a 4-month period of time on either side of the comet's perihelion on 3

July 2020. The water production rate in s -1 was moderately asymmetric around perihelion and varied with the heliocentric distance, r, in au as (6.9±0.5) x 10 28 r -2.5±0.2 and (10.1±0.5) x 10 28 r -3.5±0.1 before and after perihelion, respectively. This is consistent with the comet having been through the planetary region of the solar system on one or more previous apparitions. A water production rates as large as 5.27 x 10 30 s -1 were determined shortly after perihelion, once the comet was outside the solar avoidance area of SWAN, when the comet was 0.324 au from the Sun.

Introduction

Comet C/2020 F3 (NEOWISE), hereafter comet NEOWISE, is an Old Long-Period comet, using the A' Hearn et al. (1995) classification, discovered in images taken on 27 March 2020 by the Near Earth Object program of the Wide-Field Infrared Survey Explorer (NEOWISE). Its orbit had an inbound semi-major axis of 270 au and is projected to have an outbound semi-major axis of 255 au. It reached perihelion on 3 July at a distance of 0.295 au from the Sun. It became a spectacular visual naked-eye object for a few weeks near perihelion. Its semimajor axis indicates that it is definitely not a dynamically new comet directly from the Oort cloud and so has been through the planet region of the solar system on previous apparitions.

The Solar Wind ANisotropies (SWAN) camera on the Solar and Heliospheric Observatory (SOHO) spacecraft observed the Lyman-alpha emission from atomic hydrogen throughout its apparition. SWAN makes all-sky observations of the Lyman-alpha emission of interstellar atomic hydrogen that streams through the solar system and is removed by solar radiation and solar charged particle impact leaving a characteristic distribution that is bright in the direction of incoming hydrogen atoms and fainter in the downstream direction of outgoing hydrogen atoms [START_REF] Bertaux | [END_REF].

The typical interplanetary hydrogen Lyman-alpha brightness ranges from about 0.5 kilorayleighs up to several kilorayleighs in directions near the Sun.

SWAN's 5 by 5 one-degree detectors scan across the sky daily, making a full sky image, excluding an avoidance region around the Sun and another region blocked by parts of the spacecraft itself. SOHO has been located in a halo orbit around the L1 Earth-Sun Lagrange point since a few months after its launch in December of 1995. Most instruments are still operating nominally, including SWAN. A comet is detectable when its hydrogen Lyman-alpha brightness is larger than about 100 Rayleighs so it is distinguishable by its spatial distribution above that of the interplanetary signal. This typically happens for comets that are brighter than between magnitude 12 to 10 in the visible. Cometary signals as bright as 20-30 kilorayleighs, which do not saturate the detectors, can be reliably detected. Observations at the faint end are typically limited by the interplanetary brightness, especially near the Sun, and by the occurrence of nearby bright stars, especially when a comet is near the galactic equator.

Model Analysis

Observations of a comet's hydrogen coma are analyzed in order to calculate the water production rate of the comet, relying on the fact that water is typically the most abundant volatile gas in the coma when comets are within about 3 AU from the Sun (Mäkinen and Combi 2005;Combi et al. 2005). Water is photodissociated into a well-known distribution of possible fragment H and O atoms, H2 molecules, OH radicals and their ions [START_REF] Combi | Gas dynamics and kinetics in the cometary coma: theory and observations[END_REF]). Since SOHO's launch in 1995 water production rates have been determined from SWAN observations of over 70 comets (Bertaux et al. 1998;Combi et al. 2019Combi et al. , 2020a)).

Comet NEOWISE was detectable and first distinguishable from background stars in SWAN images beginning on 12 May 2020. It was detected from then until 19 June when it entered the solar avoidance area. Enough of the coma to be useful for analysis was clearly detectable beginning on 6 July and remained detectable until 2 September.

Figure 1 shows an image of the H Lya coma observed by SWAN on 9 July 2020 when it was near its maximum. Since the solar fluorescence rate of hydrogen Lyman-a is rather large, radiation pressure on the hydrogen atoms produces what is essentially a broad hydrogen "tail" pointed away and lagging behind the direction of the comet in its orbit around the Sun. The extent of the asymmetry of and the general shape of the hydrogen coma is determined by velocity distributions of H atoms leaving the inner partially collisional coma (Combi & Smyth 1988;Combi et al. 2000). While H atoms are produced at a dominant velocity of ~18 km s -1 upon photodissociation of water and ~8 km s -1 upon photodissociation of OH, when coma production rates are large, and especially for a smaller heliocentric distance when photochemical lifetimes are smaller, collisions of the fast nascent hydrogen atoms with the slow outflowing heavy molecules partially thermalizes and slows part of the distribution of hydrogen atoms. For a large production rate comet like C/2020 F3 (NEOWISE) at small heliocentric distances (0.3-0.5 au) a significant fraction of the hydrogen atoms are slowed to velocities from 8-18 km s -1 to 1-4 km s -1 , elongating and narrowing the antisunward distribution of the hydrogen tail. Figure 2 shows the profile cut through the hydrogen coma indicated by the straight line in Figure 1. In the model that we use, the nascent velocity distribution is determined solely by the known photodissociation branches of water and OH, and the velocity distribution of the atoms exiting the inner partially collisional coma is determined by the level of the water production rate. The velocity distribution then is not a fitting parameter, but a consequence of the level of the production rate and the heliocentric distance. Therefore, the fact that the model reproduces the spatial distribution of the coma is a demonstration that the fixed parameterization of the coma physics is correct given the linear relationship between the observed brightness and abundance as well as the nonlinear relationship between the spatial distribution of the brightness and the photochemical and collisional physics in the model.

Water production rates were calculated from all hydrogen coma images using the method described in detail by Mäkinen and Combi (2005). Water production rates were calculated from each SWAN image of comet F3 (NEOWISE) for 74 days beginning on 13 May 2020 and running through 2 September 2020. There was a large data gap from 18 June until 7 July when the comet was too close to the Sun in the sky for SWAN to obtain images of the region of the sky where the comet was located. The first post-perihelion production rate obtained when the comet was on 0.324 au from the Sun on 7 July was 5.15 x 10 30 s -1 , one of the largest water production rates obtained by SWAN since comet C/1995 O1 (Hale-Bopp) (Combi et al. 2000). For Hale-Bopp Combi et al. (2000) explained that for a brightness less than ~30 kilorayleighs optical depth effects in the SWAN aperture could be neglected. For F3 (NEOWISE) the brightest pixel centered on the nucleus has a comparable brightness, but this is only because the smallest heliocentric distance is ~0.33 au when the comet was nearly this bright and the illuminating flux from the Sun ~8 times larger than it was for Hale-Bopp at perihelion and column densities similarly many times lower. Therefore, optical depth effects for F3 (NEOWISE) are much less of an issue.

Table 1 gives the observational circumstances, g-factors, the water production rates, and their formal 1-sigma uncertainties. Figure 3 shows the variation of the water production rate as a function of time in days measured from perihelion on 3 July 2020. The vertical lines through each point give the formal 1-sigma error bars that result from the fitting procedure to the hydrogen coma model and the interplanetary hydrogen Lyman-alpha sky background. Actual systematic uncertainties resulting from the model, model parameters and the absolute calibration of SWAN, the LASP solar Lyman-alpha irradiances and the solar Lyman-alpha line profile are estimated to be on the order of 30%. It appears that the water production rate at perihelion when SWAN could not observe it was on the order of nearly 10 31 s -1 .

Discussion

It is often instructive to look at a power-law fit of the water production rate as a function of heliocentric distance both before and after perihelion. This means that the nucleus is only slightly larger than that of the Rosetta target comet 67P/Churyumov-Gerasimenko, which had a mean diameter of ~4 km. The water production rate of 67P was in the range of 1-3 x 10 28 s -1 (Bertaux et al. 2014, Biver et al. 2019;Combi et al. 2020b) when it reached its perihelion of 1.24 AU. When C/2020 F3 (NEOWISE) was at a heliocentric distance of 1.2 -1.3 AU its water production rate was in the range of 4 -6 x 10 28 s -1 (Table 1).

Whereas the production rate of NEOWISE was a factor 2-4 times larger than that of 67P at the same heliocentric distance, it's surface area is about 56% larger.

So, while 67P had an active surface fraction in the range of 4 -8%, that of NEOWISE was on the order of 8 -20%.

Summary

Comet C/2020 F3 (NEOWISE) was a spectacular naked object in late June and early July 2020. The SOHO/SWAN all-sky camera observed its Lyman-alpha hydrogen coma on 74 days during the period from 13 May through 6 September 2020. When it was recovered after exiting the SOHO solar avoidance area a few days after perihelion, the hydrogen coma displayed a broad and extended tail produced by solar Lyman-alpha radiation pressure that was over 15 o long. This was well reproduced by our analysis model that accounts for the nascent velocity distribution and the partial collisional thermalization (and slowing) of atoms as they exit the inner coma. This model was used to calculate water production rates for each image. A maximum water production rate of 5.27 x 10 30 s -1 was recorded on 8 July when the comet was at a heliocentric distance of 0.337 au.

While this was one of the largest water production rates ever recorded by SOHO/SWAN (Combi et al. 2019), once the comet was at a heliocentric distance of 1.2-1.3 au, comparable to the perihelion measurements of the Rosetta target comet 67P/Churyumov-Gerasimenko, its production rate was only a factor of ~3 larger. This, combined with its determined radius of ~5 km, compared with that of 67P of ~4 km, implied that its active fraction was only about a factor of 2 larger than 67P, a rather typical Jupiter family comet.

The orbit of F3 NEOWISE, having an inbound semi-major axis of 270 au, implies that it is an Old Long-Period comet, indicates that it has been through the planetary region of the solar system at least once, and possibly many times.

The heliocentric distance dependence, described by power-law fits of production rates as a function of heliocentric distance of (6.9±0.5) x 10 28 r -2.5±0.2 and (10.1±0.5) x 10 28 r -3.5±0.1 , respectively, before and after perihelion, are consistent with this dynamical age. There were no noticeable outbursts, quiescent periods, or evidence of any fragmentation throughout the time covered by the SWAN observations. ACKNOWLEDGEMENTS: SOHO is an international mission between ESA and NASA. M.

Combi acknowledges support from NASA grant 80NSSC18K1005 from the Solar System Observations Program. T.T. Mäkinen was supported by the Finnish Meteorological the production rate as a function of heliocentric distance for the pre-and post-perihelion legs of the orbit giving values of (6.9±0.5) x 10 28 r -2.5±0.2 and (10.1±0.5) x 10 28 r -3.5±0.1 , respectively. g: Solar Lyman-a g-factor (photons s -1 ) at 1 AU Q: Water production rates for each image (10 28 s -1 ) dQ: internal 1-sigma uncertainties (10 28 s -1 )

Figure 4

 4 Figure4shows the water production rate plotted as a function of heliocentric distance with pre-and post-perihelion data plotted in separate panels. The straight lines are the best-fit power laws. In the survey of 61 comets observed by SOHO/SWAN,Combi et al. (2019) have shown there is a consistent pattern of variation of the average and range of pre-and post-perihelion power-law exponents as a function of the dynamical age of long period comets, with the flattest slopes found in Dynamically New comets (1/ao < 50 x 10 -6 ) and gradually more steeper slopes in Young Long-Period (50 x 10 -6 1/ao < 2000 x 10 -6 ) and Old Long-Period comets (1/ao > 2000 x 10 -6 ), using the A'Hearn et al. (1995) dynamical age classification. The power-law fits to the water production rates are (6.9±0.5) x 10 28 r -2.5±0.2 and (10.1±0.5) x 10 28 r -3.5±0.1 before and after perihelion, respectively, with the production rate in s -1 and the heliocentric distance, r, in au. The pre-perihelion power-law exponent determined for C/2020 F3 (NEOWISE) is near the middle of the range of comets, in the same dynamical Old Long-Period classification from the survey. The post-perihelion exponent is also consistent, being in the same range with comets that are similarly Old Long-Period in the survey.The deviations of the actual production rate determination from the power-law fits are surprisingly small, showing that there were no major outbursts or quiescent episodes.[START_REF] Bauer | American Astronomical Society, DPS Meeting #52[END_REF] have estimated a nucleus diameter of ~5 km for this comet from WISE spacecraft observations by subtracting a fitted dust coma model.

Figure 1 .

 1 Figure 1. SOHO/SWAN image of the hydrogen coma of comet C/2020 F3 (NEOWISE) observed on 9 July 2020. The field of view is 30 o across. The straight line from left to right and slightly upward shows a profile cut along the maximum extent of the hydrogen "tail" whose profile is shown in Figure 2. The dark region toward the right and lower right corresponds to the solar avoidance area of SWAN with the direction to the to the right Sun.

Figure 2 .

 2 Figure 2. Model-Data comparison profile along the long extent of the hydrogen "tail" of comet C/2020 F3 (NEOWISE) observed by SOHO/SWAN on 9 July 2020. The white curved line corresponds to the observed brightness along the cut shown in Figure 1 along the maximum extent of the hydrogen tail. The flat straight green line near the bottom corresponds to the fitted interstellar hydrogen background. The curved green line is the fit of the coma model to the observed image. The red line toward the lower right is where the observed profile cuts through a field star that has been marked to be ignored. Note that the shape of the model profile follows the asymmetric coma distribution that is produced by radiation pressure from the resonance scattering of the

Figure 4 .

 4 Figure 4. Water production rate of comet C/2020 F3 (NEOWISE) plotted as a function of heliocentric distance. The left half is pre-perihelion and right half is post-perihelion. The straight lines correspond to the power-law fits to

Institute (FMI). J.-L. Bertaux (http://lasp.colorado.edu/lisird/lya/). We acknowledge the personnel that have been keeping SOHO and SWAN operational for over 20 years, in particular Dr.

Walter Schmidt at FMI. same solar Lyman-alpha photons that enable us to observe the hydrogen coma.

The asymmetric shape itself is a property of the model parameterization itself and not a free fitting parameter. The fact that it reproduces the data indicates that the model works well even under these extreme conditions of high production rate and small heliocentric distance.