Electron reflectometry in the martian atmosphere

The technique of electron reflectometry, a method for remote estimation of planetary magnetic fields, is expanded from its original use of mapping crustal magnetic fields at the Moon to achieving the same purpose at Mars, where the presence of a substantial atmosphere complicates matters considerably. The motion of solar wind electrons, incident on the martian atmosphere, is considered in detail, taking account of the following effects: the electrons' helical paths around the magnetic field lines to which they are bound, the magnetic mirror force they experience due to converging field lines in the vicinity of crustal magnetic anomalies, their acceleration/deceleration by electrostatic potentials, their interactions with thermal plasma, their drifts due to magnetic field line curvature and perpendicular electric fields and their scattering off, and loss of energy through a number of different processes to, atmospheric neutrals. A theoretical framework is thus developed for modeling electron pitch angle distributions expected when a spacecraft is on a magnetic field line which is connected to both the martian crust and the interplanetary magnetic field. This framework, along with measured pitch angle distributions from the Mars Global Surveyor (MGS) Magnetometer/Electron Reflectometer (MAG/ER) experiment, can be used to remotely measure crustal magnetic field magnitudes and atmospheric neutral densities at ∼180 km above the martian datum, as well as estimate average parallel electric fields between 200 and 400 km altitude. Detailed analysis and full results, concerning the crustal magnetic field and upper thermospheric density of Mars, are left to two companion papers.     

Oxygen and carbon isotope ratios in the martian atmosphere

Oxygen and carbon isotope ratios in the martian CO₂ are key values to study evolution of volatiles on Mars. The major problems in spectroscopic determinations of these ratios on Mars are uncertainties associated with: (1) equivalent widths of the observed absorption lines, (2) line strengths in spectroscopic databases, and (3) thermal structure of the martian atmosphere during the observation. We have made special efforts to reduce all these uncertainties. We observed Mars using the Fourier Transform Spectrometer at the Canada–France–Hawaii Telescope. While the oxygen and carbon isotope ratios on Mars were byproducts in the previous observations, our observation was specifically aimed at these isotope ratios. We covered a range of 6022 to 6308 cm⁻¹ with the highest resolving power of ν/δν=3.5×10⁵ and a signal-to-noise ratio of 180 in the middle of the spectrum. The chosen spectral range involves 475 lines of the main isotope, 184 lines of ¹³CO₂, 181 lines of CO¹⁸O, and 119 lines of CO¹⁷O. (Lines with strengths exceeding 10⁻²⁷ cm at 218 K are considered here.) Due to the high spectral resolution, most of the lines are not blended. Uncertainties of retrieved isotope abundances are in inverse proportion to resolving power, signal-to-noise ratio, and square root of the number of lines. Laboratory studies of the CO₂ isotope spectra in the range of our observation achieved an accuracy of 1% in the line strengths. Detailed observations of temperature profiles using MGS/TES and data on temperature variations with local time from two GCMs are used to simulate each absorption line at various heights in each part of the instrument field of view and then sum up the results. Thermal radiation of Mars' surface and atmosphere is negligible in the chosen spectral range, and this reduces errors associated with uncertainties in the thermal structure on Mars. Using a combination of all these factors, the highest accuracy has been achieved in measuring the CO₂ isotope ratios: ¹³C/¹²C = 0.978 ± 0.020 and ¹⁸O/¹⁶O = 1.018 ± 0.018 times the terrestrial standards. Heavy isotopes in the atmosphere are enriched by nonthermal escape and sputtering, and depleted by fractionation with solid-phase reservoirs. The retrieved ratios show that isotope fractionation between CO₂ and oxygen and carbon reservoirs in the solid phase is almost balanced by nonthermal escape and sputtering of O and C from Mars.     

Isotopic fractionation of methane in the martian atmosphere

The existence of methane in the martian atmosphere may be an indicator of subsurface life. Biological processes are known to fractionate the common isotopologues of methane, and hence measuring these isotopic ratios may yield constraints on the nature of the methane source. Any measurement of the isotopic ratios of atmospheric methane must consider the additional fractionation due to photochemistry in order to quantify the isotopic ratios of the source. Using a one-dimensional photochemical model, we find that photochemistry has a small (4.5‰) contribution to δ¹³C(CH₄) but has a large (114‰) contribution to δD(CH₄). Confirmation of these fractionation values will require additional laboratory data on key model inputs, particularly the ultraviolet absorption cross sections of ¹³CH₄ and kinetic rate coefficients for the reactions of ¹³CH₄ and CH₃D with OH and O(¹D) at pressures and temperatures relevant to the martian atmosphere.     

MAWD observations revisited: seasonal behavior of water vapor in the martian atmosphere

Viking/MAWD experiment and more recent MGC/TES observations have provided to date the most detailed information about the annual atmospheric water cycle on Mars. Their data agree in major details but still reveal some disagreements. These disagreements turn out to be most significant in the perihelion season and especially during the major dust storms. We consider the potential influence of aerosol scattering on 1.38 μm water retrieval under various types of observation geometry. In order to obtain new retrievals of water vapor abundance from MAWD data, we apply radiative transfer calculations. The resulting seasonal and spatial distribution of water turns out to be more consistent with TES results, implying a remarkable stability of the martian seasonal water cycle. Mapping data corresponding to particular seasons reveals a distinct wave structure in the global distribution of the water column. We interpret it as a manifestation of a strong control over the water cycle on Mars from the atmospheric circulation.     

A sensitive search for SO2 in the martian atmosphere: Implications for seepage and origin of methane

Mars was observed near the peak of the strongest SO₂ band at 1364-1373 cm⁻¹ with resolving power of 77,000 using the Texas Echelon Cross Echelle Spectrograph on the NASA Infrared Telescope Facility. The observation covered the Tharsis volcano region which may be preferable to search for SO₂. The spectrum shows absorption lines of three CO₂ isotopomers and three H₂O isotopomers. The water vapor abundance derived from the HDO lines assuming D/H = 5.5 times the terrestrial value is 12±1.0 pr. μm, in agreement with the simultaneous MGS/TES observations of 14 pr. μm at the latitudes (50° S to 10° N) of our observation. Summing of spectral intervals at the expected positions of sixteen SO₂ lines puts a 2σ upper limit on SO₂ of 1 ppb. SO₂ may be emitted into the martian atmosphere by seepage and is removed by three-body reactions with OH and O. The SO₂ lifetime, 2 years, is longer than the global mixing time 0.5 year, so SO₂ should be rather uniformly distributed across Mars. Seepage of SO₂ is less than 15,000 tons per year on Mars which is smaller than the volcanic production of SO₂ on the Earth by a factor of 700. Because CH₄/SO₂ is typically 10⁻⁴-10⁻³ in volcanic gases on the Earth, our results show seepage is unlikely to be the source of the recently discovered methane on Mars and therefore strengthen its biogenic origin.     

The effects of topographically-controlled thermal tides in the martian upper atmosphere as seen by the MGS accelerometer

Mars Global Surveyor accelerometer observations of the martian upper atmosphere revealed large variations in density with longitude during northern hemisphere spring at altitudes of 130-160 km, all latitudes, and mid-afternoon local solar times (LSTs). This zonal structure is due to tides from the surface. The zonal structure is stable on timescales of weeks, decays with increasing altitude above 130 km, and is dominated by wave-3 (average amplitude 22% of mean density) and wave-2 (18%) harmonics. The phases of these harmonics are constant with both altitude and latitude, though their amplitudes change significantly with latitude. Near the South Pole, the phase of the wave-2 harmonic changes by 90° with a change of half a martian solar day while the wave-3 phase stays constant, suggesting diurnal and semidiurnal behaviour, respectively. We use a simple application of classical tidal theory to identify the dominant tidal modes and obtain results consistent with those of General Circulation Models. Our method is less rigorous, but simpler, than the General Circulation Models and hence complements them. Topography has a strong influence on the zonal structure.     

Derivation of martian surface slope characteristics from directional thermal infrared radiometry

Directional thermal infrared measurements of the martian surface is one of a variety of methods that may be used to characterize surface roughness and slopes at scales smaller than can be obtained by orbital imagery. Thermal Emission Spectrometer (TES) emission phase function (EPF) observations show distinct apparent temperature variations with azimuth and emission angle that are consistent with the presence of warm, sunlit and cool, shaded slopes at typically ∼0.1 m scales. A surface model of a Gaussian distribution of azimuth independent slopes (described by θ-bar) is combined with a thermal model to predict surface temperature from each viewing angle and azimuth of the TES EPF observation. The models can be used to predict surface slopes using the difference in measured apparent temperature from 2 separate 60-70° emission angle observations taken ∼180° in azimuth relative to each other. Most martian surfaces are consistent with low to moderate slope distributions. The slope distributions display distinct correlations with latitude, longitude, and albedo. Exceptionally smooth surfaces are located at lower latitudes in both the southern highlands as well as in high albedo dusty terrains. High slopes are associated with southern high-latitude patterned ground and north polar sand dunes. There is little apparent correlation between high resolution imagery and the derived θ-bar, with exceptions such as duneforms. This method can be used to characterize potential landing sites by assuming fractal scaling behavior to meter scales. More precisely targeted thermal infrared observations from other spacecraft instruments are capable of significantly reducing uncertainty as well as reducing measurement spot size from 10s of kilometers to sub-kilometer scales.     

Investigations of the variability of dust particle sizes in the martian atmosphere using the NASA Ames General Circulation Model

We present a Mars General Circulation Model (GCM) numerical investigation of the physical processes (i.e., wind stress and dust devil dust lifting and atmospheric transport) responsible for temporal and spatial variability of suspended dust particle sizes. Measurements of spatial and temporal variations in airborne dust particles sizes in the martian atmosphere have been derived from Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) spectral and emission phase function data [Wolff, M.J., Clancy, R.T., 2003. J. Geophys. Res. (Planets) 108 (E9), doi:10.1029/2003JE002057. 1-1; Clancy, R.T., Wolff, M.J., Christensen, P.R., 2003. J. Geophys. Res. (Planets) 108 (E9), doi:10.1029/2003JE002058. 2-1]. The range of dust particle sizes simulated by the NASA Ames GCM is qualitatively consistent with TES-derived observations of effective dust particle size variability. Model results suggest that the wind stress dust lifting scheme (which produces regionally confined dust lifting) is the process responsible for the majority of the dust particle size variability in the martian atmosphere. Additionally, model results suggest that atmospheric transport processes play an important role in the evolution of atmospheric dust particles sizes during substantial dust storms on Mars. Finally, we show that including the radiative effects of a spatially variable particle size distribution significantly influences thermal and dynamical fields during the dissipation phase of the simulated global dust storm.     

Forced waves in the martian atmosphere from MGS TES nadir data

We have analyzed the temperature retrievals from Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) nadir spectra to yield latitude-height resolved maps of various atmospheric forced wave modes as a function of season for a full Mars year. Among the isolated wave modes is the zonal mean, time mean temperature, which we used to derive zonal mean zonal winds and stationary wave quasi-geostrophic indices of refraction, diagnostic of their propagation. The diurnal Kelvin wave was isolated in the data, with results roughly consistent with models (Wilson and Hamilton, 1996, J. Atmos. Sci. 33, 1290-1326). The s = 1 and s = 2 stationary waves were found to have significant amplitude in ducts extending up the winter polar jets, while the s = 3 stationary wave was found to be confined to near the surface. The s = 1 stationary wave was found to have little phase tilt with height during northern winter, but significant westward phase tilt with height in the southern winter. This indicates that the wave carries heat poleward, slightly more than that found in Barnes et al. (1996; J. Geophys. Res. 101, 12,753-12,776). The s = 1 stationary wave is likely the dominant mechanism for eddy meridional heat transport for the southern winter. We noted that the phase of the s = 2 stationary wave is nearly constant with time, but that the s = 1 stationary wave moved 90° of longitude from fall to winter and back in spring in the North. While interannual variability is not yet addressed, overall, these results provide the first comprehensive benchmark for forced waves in Mars’s atmosphere against which future atmospheric models of Mars can be compared.     

Interannual variability of water ice clouds over major martian volcanoes observed by MOC

Using Mars Global Surveyor Mars Orbiter Camera daily global maps, cloud areas have been measured daily for water ice clouds associated with the topography of the major volcanoes Olympus Mons, Ascraeus Mons, Pavonis Mons, Arsia Mons, Elysium Mons, and Alba Patera. This study expands on that of Benson et al. [Benson, J.L., Bonev, B.P., James, P.B., Shan, K.J., Cantor, B.A., Caplinger, M.A., 2003. Icarus 165, 34-52] by continuing their cloud area measurements of the Tharsis volcanoes, Olympus Mons and Alba Patera for an additional martian year (August 2001-May 2003) and by also including Elysium Mons measurements from March 1999 through May 2003. The seasonal trends in cloud activity established by Benson et al. [Benson, J.L., Bonev, B.P., James, P.B., Shan, K.J., Cantor, B.A., Caplinger, M.A., 2003. Icarus 165, 34-52] for the five volcanoes studied earlier are corroborated here with an additional year of coverage. For volcanoes other than Arsia Mons, interannual variations that could be associated with the large 2001 planet encircling dust storm are minimal. At Arsia Mons, where cloud activity was continuous in the first two years, clouds disappeared totally for ∼85° of LS (LS=188°-275°) due to the dust storm. Elysium Mons cloud activity is similar to that of Olympus Mons, however the peak in cloud area is near LS=130° rather than near LS=100°.     

Traveling waves in the martian atmosphere from MGS TES Nadir data

We have characterized the annual behavior of martian atmospheric traveling waves in the MGS TES data set from the first two martian years of mapping. There is a high degree of repeatability between the two years. They are dominated by strong low zonal wavenumber waves with high amplitudes near the polar jets, strongest in late northern fall and early northern winter. The m=1 waves have amplitudes up to about 20 K, are vertically extended, and occasionally extend even into the tropics. Periods for m=1 range from 2.5 to 30 sols. Much weaker waves were identified in the south, with amplitudes less than about 3.5 K. Traveling waves with m=2 and m=3 are also seen, but their amplitudes are typically limited to less than 4 K, and are generally more confined near the surface. In the north, they are more evident in fall and spring rather than winter solstice, which is clearly dominated by m=1 waves. Some evidence of storm tracks has been identified in the data, with accentuated weather-related temperature perturbations near longitudes 200° to 320° E for both the southern and northern hemispheres near latitude ±65° at the surface. Some evidence was also found for a sharpening of longitudinal gradients into what may be frontal systems. EP flux divergences show the waves extracting energy from the zonal mean winds. When the m=1 waves were strongest, decelerations of the zonal jet of order 30 m/(s sol) were measured. Above 1 scale height, the waves extract energy from the jet predominately through barotropic processes, but their character is overall mixed barotropic/baroclinic. Inertial instabilities may exist at altitude on the equatorward flanks of the polar jets, and marginal stability extends through to the tropics. This may explain the coordination of the tropical behavior of the waves with that centered along the polar jet, consistent with the ideas expressed in Wilson et al. (2002, Geophys. Res. Lett. 29, #1684) and similar to those in Barnes et al. (1993, J. Geophys. Res. 98, 3125-3148). Throughout the year, there exist large regions with the meridional gradient of PV less than zero, but they are strongest near winter solstice. Poleward of the winter jet, the regions of instability reach the surface, equatorward they do not. These regions, satisfying a necessary criterion for instability, likely explain the genesis of the waves, and perhaps also their bimodal character between surface (faster waves) and altitude (slow m=1 waves).     

Simultaneous mapping of H2O and H2O2 on Mars from infrared high-resolution imaging spectroscopy

New maps of martian water vapor and hydrogen peroxide have been obtained in November-December 2005, using the Texas Echelon Cross Echelle Spectrograph (TEXES) at the NASA Infra Red Telescope facility (IRTF) at Mauna Kea Observatory. The solar longitude Ls was 332° (end of southern summer). Data have been obtained at 1235-1243 cm⁻¹, with a spectral resolution of 0.016 cm⁻¹ (R=8×¹⁰⁴). The mean water vapor mixing ratio in the region [0°-55° S; 345°-45° W], at the evening limb, is 150±50 ppm (corresponding to a column density of 8.3±2.8 pr-μm). The mean water vapor abundance derived from our measurements is in global overall agreement with the TES and Mars Express results, as well as the GCM models, however its spatial distribution looks different from the GCM predictions, with evidence for an enhancement at low latitudes toward the evening side. The inferred mean H₂O₂ abundance is 15±10 ppb, which is significantly lower than the June 2003 result [Encrenaz, T., Bézard, B., Greathouse, T.K., Richter, M.J., Lacy, J.H., Atreya, S.K., Wong, A.S., Lebonnois, S., Lefèvre, F., Forget, F., 2004. Icarus 170, 424-429] and lower than expected from the photochemical models, taking in account the change in season. Its spatial distribution shows some similarities with the map predicted by the GCM but the discrepancy in the H₂O₂ abundance remains to be understood and modeled.     

Continuous monitoring of nightside upper thermospheric mass densities in the martian southern hemisphere over 4 martian years using electron reflectometry

Details are presented of an improved technique to use atmospheric absorption of magnetically reflecting solar wind electrons to constrain neutral mass densities in the nightside martian upper thermosphere. The helical motion of electrons on converging magnetic field lines, through an extended neutral atmosphere, is modeled to enable prediction of loss cone pitch angle distributions measured by the Magnetometer/Electron Reflectometer (MAG/ER) experiment on Mars Global Surveyor at 400 km altitude. Over the small fraction of Mars' southern hemisphere (∼2.5%) where the permanent crustal magnetic fields are both open to the solar wind and sufficiently strong as to dominate the variable induced martian magnetotail field, spherical harmonic expansions of the crustal fields are used to prescribe the magnetic field along the electron's path, allowing least-squares fitting of measured loss cones, in order to solve for parameters describing the vertical neutral atmospheric mass density profile from 160 to 230 km. Results are presented of mass densities in the southern hemisphere at 2 a.m. LST at the mean altitude of greatest sensitivity, 180 km, continuously over four martian years. Seasonal variability in densities is largely explained by orbital and latitudinal changes in dayside insolation that impacts the nightside through the resulting thermospheric circulation. However, the physical processes behind repeatable rapid, late autumnal cooling at mid-latitudes and near-aphelion warming at equatorial latitudes is not fully clear. Southern winter polar warming is generally weak or nonexistent over several Mars years, in basic agreement with MGS and MRO accelerometer observations. The puzzling response of mid-latitude densities from 160° to 200° E to the 2001 global dust storm suggests unanticipated localized nightside upper thermospheric lateral and vertical circulation patterns may accompany such storms. The downturn of the 11-year cycle of solar EUV flux is likely responsible for lower aphelion densities in 2004 and 2006 (Mars years 27 and 28).     

Metastable methane clathrate particles as a source of methane to the martian atmosphere

The observations of methane made by the PFS instrument onboard Mars Express exhibit a definite correlation between methane mixing ratio, water vapor mixing ratio, and cloud optical depth. The recent data obtained from ground-based telescopes seem to confirm the correlation between methane and water vapor. In order to explain this correlation, we suggest that the source of gaseous methane is atmospheric, rather than at the solid surface of the planet, and that this source may consist of metastable submicronic particles of methane clathrate hydrate continuously released to the atmosphere from one or several clathrate layers at depth, according to the phenomenon of “anomalous preservation” evidenced in the laboratory. These particles, lifted up to middle atmospheric levels due to their small size, and therefore filling the whole atmosphere, serve as condensation nuclei for water vapor. The observed correlation between methane and water vapor mixing ratios could be the signature of the decomposition of the clathrate crystals by condensation-sublimation processes related to cloud activity. Under the effect of water condensation on crystal walls, metastability could be broken and particles be eroded, resulting in a subsequent irreversible release of methane to the gas phase. Using PFS data, and according to our hypothesis, the lifetime of gaseous methane is estimated to be smaller than an upper limit of 6 ± 3 months, much smaller than the lifetime of 300 yr calculated from atmospheric chemical models. The reason why methane has a short lifetime might be the occurrence of heterogeneous chemical decomposition of methane in the subsurface, where it is known since Viking biology experiments that oxidants efficiently decompose organic matter. If true, it is shown by using existing models of H₂O₂ penetration in the regolith that methane could prevent H₂O₂ from penetrating in the subsurface, and further oxidizing the soil, at depths larger than a few millimeters. The present source of methane clathrate, acting over the last few hundred thousand or million years, could have given rise to the thin CO₂-ice layer covering the permanent water ice south polar cap. The hypothesis proposed in this paper requires, to be validated, a number of laboratory experiments studying the stability of methane clathrates in martian atmospheric conditions, and the kinetics and amplitude of clathrate particle erosion in presence of condensing water vapor. Detailed future observations of methane, and associated modeling, will allow to more accurately quantify the production rate of methane clathrate, its temporal variability at seasonal scale, and possibly to locate the source(s) of clathrates at the surface.     

Photochemistry of the martian atmosphere: Seasonal, latitudinal, and diurnal variations

There is a significant progress in the observational data relevant to Mars photochemistry in the current decade. These data are not covered by and sometimes disagree with the published models. Therefore we consider three types of models for Mars photochemistry. A steady-state model for global-mean conditions is currently the only way to calculate the abundances of long living species (H₂, O₂, and CO). However, our model does not fit the observed CO abundance using gas-phase chemistry and reasonable values of heterogeneous loss of odd hydrogen on the water ice aerosol. The second type of the calculated models is steady-state models for local conditions. The MGS/TES data on temperature profiles, H₂O, and dust are input parameters for these models. The calculations have been made for nine seasonal points spread over the martian year and for twelve latitudes with a step of 10° for each season. The only adopted heterogeneous reaction is a weak loss of H₂O₂ on water ice with probability of 5×¹⁰⁻⁴. The results are in good agreement with the recent observations of the O₂ dayglow at 1.27 μm and the O₃ and H₂O₂ abundances. Global maps of the seasonal and latitudinal behavior of these species have been made. The third type of models is a time-dependent model for local conditions. These models show that odd hydrogen quickly converts to H₂O₂ at the nighttime and the chemistry is switched off while the association of O, the heterogeneous loss of H₂O₂, and eddy diffusion continue. This requires significant changes in the global-mean and local steady-state models discussed above, and these changes have been properly done. The calculated diurnal variations of Mars photochemistry are discussed. The martian photochemistry at low and middle latitudes is significantly different in the aphelion period at LS=10°-130° from that in the remaining part of the year.     

Methane adsorption on a martian soil analog: An abiogenic explanation for methane variability in the martian atmosphere

Recent observations suggest methane in the martian atmosphere is variable on short spatial and temporal scales. However, to explain the variability by loss reactions requires production rates much larger than expected. Here, we report results of laboratory studies of methane adsorption onto JSC-Mars-1, a martian soil simulant, and suggest that this process could explain the observations. Uptake coefficient (γ) values were measured as a function of temperature using a high-vacuum Knudsen cell able to simulate martian temperature and pressure conditions. Values of γ were measured from 115 to 135 K, and the data were extrapolated to higher temperatures with more relevance to Mars. Adsorptive uptake was found to increase at lower temperatures and larger methane partial pressures. Although only sub-monolayer methane surface coverage is likely to exist under martian conditions, a very large mineral surface area is available for adsorption as atmospheric methane can diffuse meters into the regolith. As a result, significant methane may be temporarily lost to the regolith on a seasonal time scale. As this weak adsorption is fully reversible, methane will be re-released into the atmosphere when surface and subsurface temperatures rise and so no net loss of methane occurs. Heterogeneous interaction of methane with martian soil grains is the only process proposed thus far which contains both rapid methane loss and rapid methane production mechanisms and is thus fully consistent with the reported variability of methane on Mars.     

The effects of the martian regolith on GCM water cycle simulations

This paper describes General Circulation Model (GCM) simulations of the martian water cycle focusing on the effects of an adsorbing regolith. We describe the 10-layer regolith model used in this study which has been adapted from the 1-D model developed by Zent, A.P., Haberle, R.M., Houben, H.C., Jakosky, B.M. [1993. A coupled subsurface-boundary layer model of water on Mars. J. Geophys. Res. 98 (E2), 3319-3337, February]. Even with a 30-min timestep and taking into account the effect of surface water ice, our fully implicit scheme compares well with the results obtained by Zent, A.P., Haberle, R.M., Houben, H.C., Jakosky, B.M. [1993. A coupled subsurface-boundary layer model of water on Mars. J. Geophys. Res. 98 (E2), 3319-3337, February]. This means, however, that the regolith is not able to reproduce the diurnal variations in column water vapour abundance of up to a factor of 2-3 as seen in some observations, with only about 10% of the atmospheric water vapour column exchanging with the subsurface on a daily basis. In 3-D simulations we find that the regolith adsorbs water preferentially in high latitudes. This is especially true in the northern hemisphere, where perennial subsurface water ice builds up poleward of 60° N at depths which are comparable to the Odyssey observations. Much less ice forms in the southern high latitudes, which suggests that the water ice currently present in the martian subsurface is not stable under present conditions and is slowly subliming and being deposited in the northern hemisphere. When initialising the model with an Odyssey-like subsurface water ice distribution the model is capable of forcing the simulated water cycle from an arbitrary state close to the Mars Global Surveyor Thermal Emission Spectrometer observations. Without the actions of the adsorbing regolith the equilibrated water cycle is found to be a factor of 2-4 too wet. The process by which this occurs is by adsorption of water during northern hemisphere summer in northern mid and high latitudes where it remains locked in until northern spring when the seasonal CO₂ ice cap retreats. At this time the water diffuses out of the regolith in response to increased temperature and is returned to the residual water ice cap by eddie transport.     

First ENA observations at Mars: ENA emissions from the martian upper atmosphere

The neutral particle detector (NPD) on board Mars Express has observed energetic neutral atoms (ENAs) from a broad region on the dayside of the martian upper atmosphere. We show one such example for which the observation was conducted at an altitude of 570 km, just above the induced magnetosphere boundary (IMB). The time of flight spectra of these ENAs show that they had energies of 0.2-2 keV/amu, with an average energy of ∼1.1 keV/amu. Both the spatial distribution and the energy of these ENAs are consistent with the backscattered ENAs, produced by an ENA albedo process. This is the first observation of backscattered ENAs from the martian upper atmosphere. The origin of these ENAs is considered to be the solar wind ENAs that are scattered back by collision processes in the martian upper atmosphere. The particle flux and energy flux of the backscattered ENAs are and , respectively.     

Convective instability in the martian middle atmosphere

Dry convective instabilities in Mars’s middle atmosphere are detected and mapped using temperature retrievals from Mars Climate Sounder observations spanning 1.5 martian years. The instabilities are moderately frequent in the winter extratropics. The frequency and strength of middle atmospheric convective instability in the northern extratropics is significantly higher in MY 28 than in MY 29. This may have coupled with changes to the northern hemisphere mid-latitude and tropical middle atmospheric temperatures and contributed to the development of the 2007 global dust storm. We interpret these instabilities to be the result of gravity waves saturating within regions of low stability created by the thermal tides. Gravity wave saturation in the winter extratropics has been proposed to provide the momentum lacking in general circulation models to produce the strong dynamically-maintained temperature maximum at 1-2 Pa over the winter pole, so these observations could be a partial control on modeling experiments.