Methane clathrate hydrates as a potential source for martian atmospheric methane

Small amounts of methane have been detected in the atmosphere of Mars, though the actual sources of the gas remain unknown. Thermodynamic conditions on Mars suggest that gas clathrate hydrate deposits might exist at the polar caps and in some areas of the planetary subsurface. We review the literature available on the detection of methane in the martian atmosphere and the presence of gas clathrate hydrates on Mars. The possibility of martian methane clathrate deposits is established, and initial sources for the sequestered methane are discussed. Based on correlated data and information from disparate sources, we conclude that subsurface methane clathrate deposits are a possible immediate source for the observed atmospheric methane on Mars.     

Martian water vapor: Mars Express PFS/LW observations

We present the seasonal and geographical variations of the martian water vapor monitored from the Planetary Fourier Spectrometer Long Wavelength Channel aboard the Mars Express spacecraft. Our dataset covers one martian year (end of Mars Year 26, Mars Year 27), but the seasonal coverage is far from complete. The seasonal and latitudinal behavior of the water vapor is globally consistent with previous datasets, Viking Orbiter Mars Atmospheric Water Detectors (MAWD) and Mars Global Surveyor Thermal Emission Spectrometer (MGS/TES), and with simultaneous results obtained from other Mars Express instruments, OMEGA and SPICAM. However, our absolute water columns are lower and higher by a factor of 1.5 than the values obtained by TES and SPICAM, respectively. In particular, we retrieve a Northern midsummer maximum of 60 pr-μm, lower than the 100-pr-μm observed by TES. The geographical distribution of water exhibits two local maxima at low latitudes, located over Tharsis and Arabia. Global Climate Model (GCM) simulations suggest that these local enhancements are controlled by atmospheric dynamics. During Northern spring, we observe a bulge of water vapor over the seasonal polar cap edge, consistent with the northward transport of water from the retreating seasonal cap to the permanent polar cap. In terms of vertical distribution, we find that the water volume mixing ratio over the large volcanos remains constant with the surface altitude within a factor of two. However, on the whole dataset we find that the water column, normalized to a fixed pressure, is anti-correlated with the surface pressure, indicating a vertical distribution intermediate between control by atmospheric saturation and confinement to a surface layer. This anti-correlation is not reproduced by GCM simulations of the water cycle, which do not include exchange between atmospheric and subsurface water. This situation suggests a possible role for regolith-atmosphere exchange in the martian water cycle.     

Investigation of the water sorption properties of Mars-relevant micro- and mesoporous minerals

Encouraged by recent results of the Mars Odyssey spacecraft mission and the OMEGA team (Mars Express) concerning water in equatorial latitudes between ±45° on Mars and the possible existence of hydrated minerals, we have investigated the water sorption properties of natural zeolites and clay minerals close to martian atmospheric surface conditions as well as the properties of Mg-sulfates and gypsum. To quantify the stability of hydrous minerals on the martian surface and their interaction with the martian atmosphere, the water adsorption and desorption properties of nontronite, montmorillonite, chabazite and clinoptilolite have been investigated using adsorption isotherms at low equilibrium water vapor pressures and temperatures, modeling of the adsorption equilibrium data, thermogravimetry (TG), differential scanning calorimetry (DSC), and proton magic angle spinning nuclear magnetic resonance measurements (¹H MAS NMR). Mg-sulfate hydrates were also analyzed using TG/DSC methods to compare with clay mineral and zeolites. Our data show that these microporous minerals can remain hydrated under present martian atmospheric conditions and hold up to 2.5-25 wt% of water in their void volumes at a partial water vapor pressure of 0.001 mbar in a temperature range of 333-193 K. Results of the ¹H MAS NMR measurements suggest that parts of the adsorbed water are liquid-like water and that the mobility of the adsorbed water might be of importance for adsorption-water-triggered chemistry and hypothetical exobiological activity on Mars.     

Roles of methane and carbon dioxide in geological processes on Mars

We discuss in this paper possible roles of methane and carbon dioxide in geological processes on Mars. These volatiles in the martian crust may migrate upward from their sources either directly or via various traps (structural, sedimentary, ground ice, gas hydrates). They are then likely emitted to the atmosphere by seepage or through diverse vent structures. Though gas hydrates have never been directly detected on Mars, theoretical studies favor their presence in the crust and polar caps; they could have played an important role as significant gas reservoirs in the subsurface. The martian gas hydrates would possibly be a binary system of methane and carbon dioxide occupying clathrate cavities. Landforms such as mud volcanoes with well-known linkage to gas venting are extensively distributed on Earth, and methane is the primary gas involved. Thus, identification of these landforms on Mars could suggest that methane and possibly carbon dioxide have contributed to geological processes of the planet. For example, we present a newly identified field in Chryse Planitia where features closely resembling terrestrial mud volcanoes occur widely, though with no observable activity. We also present results of a preliminary search for possible recent or present-day, methane-emission zones in the regions over which enrichments of atmospheric methane have been reported.     

The impact of martian mesoscale winds on surface temperature and on the determination of thermal inertia

Radiative control of surface temperature is a key characteristic of the martian environment and its low-density atmosphere. Here we show through meteorological modeling that surface temperature can be far from radiative equilibrium over numerous sloping terrains on Mars, where nighttime mesoscale katabatic winds impact the surface energy budget. Katabatic circulations induce both adiabatic atmospheric heating and enhancement of downward sensible heat flux, which then becomes comparable to radiative flux and acts to warm the ground. Through this mechanism, surface temperature can increase up to 20 K. One consequence is that warm signatures of surface temperature over slopes, observed through infrared spectrometry, cannot be systematically associated with contrasts of intrinsic soil thermal inertia. Apparent thermal inertia maps retrieved thus far possibly contain wind-induced structures. Another consequence is that surface temperature observations close to sloping terrains could allow the validation of model predictions for martian katabatic winds, provided contrasts in intrinsic thermal inertia can be ruled out. The thermal impact of winds is mostly discussed in this paper in the particular cases of Olympus Mons/Lycus Sulci and Terra Meridiani but is generally significant over any sloped terrains in low thermal inertia areas. It is even general enough to apply under daytime conditions, thereby providing a possible explanation for observed afternoon surface cooling, and to ice-covered terrains, thereby providing new insights on how winds could have shaped the present surface of Mars.     

Stability of hydrous minerals on the martian surface

The presence of water-bearing minerals on Mars has long been discussed, but little or no data exist showing that minerals such as smectites and zeolites may be present on the surface in a hydrated state (i.e., that they could contain H₂O molecules in their interlayer or extra-framework sites, respectively). We have analyzed experimental thermodynamic and X-ray powder diffraction data for smectite and the most common terrestrial zeolite, clinoptilolite, to evaluate the state of hydration of these minerals under martian surface conditions. Thermodynamic data for clinoptilolite show that water molecules in its extra-framework sites are held very strongly, with enthalpies of dehydration for Ca-clinoptilolite up to three times greater than that for liquid water. Using these data, we calculated the Gibbs free energy of hydration of clinoptilolite and smectite as a function of temperature and pressure. The calculations demonstrate that these minerals would indeed be hydrated under the very low-P (H₂O) conditions existing on Mars, a reflection of their high affinities for H₂O. These calculations assuming the partial pressure of H₂O and the temperature range expected on Mars suggest that, if present on the surface, zeolites and Ca-smectites could also play a role in affecting the diurnal variations in martian atmospheric H₂O because their calculated water contents vary considerably over daily martian temperature ranges. The open crystal structure of clinoptilolite and existing hydration and kinetic data suggest that hydration/dehydration are not kinetically limited. Based on these calculations, it is possible that hydrated zeolites and clay minerals may explain some of the recent observations of significant amounts of hydrogen not attributable to water ice at martian mid-latitudes.     

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 cold and wet Mars

Water on Mars has been explained by invoking controversial and mutually exclusive solutions based on warming the atmosphere with greenhouse gases (the “warm and wet” Mars) or on local thermal energy sources acting in a global freezing climate (the “cold and dry” Mars). Both have critical limitations and none has been definitively accepted as a compelling explanation for the presence of liquid water on Mars. Here is considered the hypothesis that cold, saline and acidic liquid solutions have been stable on the sub-zero surface of Mars for relatively extended periods of time, completing a hydrogeological cycle in a water-enriched but cold planet. Computer simulations have been developed to analyze the evaporation processes of a hypothetical martian fluid with a composition resulting from the acid weathering of basalt. This model is based on orbiter- and lander-observed surface mineralogy of Mars, and is consistent with the sequence and time of deposition of the different mineralogical units. The hydrological cycle would have been active only in periods of dense atmosphere, as having a minimum atmospheric pressure is essential for water to flow, and relatively high temperatures (over ∼245 K) are required to trigger evaporation and snowfall; minor episodes of limited liquid water on the surface could have occurred at lower temperatures (over ∼225 K). During times with a thin atmosphere and even lesser temperatures (under ∼225 K), only transient liquid water can potentially exist on most of the martian surface. Assuming that surface temperatures have always been maintained below 273 K, Mars can be considered a “cold and wet” planet for a substantial part of its geological history.     

A 1 Gyr climate model for Mars: new orbital statistics and the importance of seasonally resolved polar processes

New results from a 1 Gyr integration of the martian orbit are presented along with a seasonally resolved energy balance climate model employed to illuminate the gross characteristics of the long-term atmospheric pressure evolution. We present a new analysis of the statistical variation of the martian obliquity and precession prior to and subsequent to the formation of the Tharsis uplift, and explore the long term effects on the martian climate. We find that seasonal polar cycles have a critical influence on the ability for the regolith to release CO₂ at high obliquities, and find that the atmospheric CO₂ actually decreases at high obliquities due to the cooling effect of polar deposits at latitudes where seasonal caps form. At low obliquity, the formation of massive, permanent polar caps depends critically on the values of the frost albedo, A_frost, and frost emissivity, ?_frost. Using our model with values of A_frost=0.67 and ?_frost=0.55, matched to the NASA Ames General Circulation Model (GCM) results (Haberle et al., 1993, J. Geophys. Res. 98, 3093-3123, and Haberle et al., 2003, Icarus 161, 66-89), we find that permanent caps only form at low obliquities (<13°), suggesting that any permanent deposits on the surface of Mars today may be residuals left over from a period of very low obliquity, or are the result of mechanisms not represented by this model. Thus, contrary to expectations, the martian atmospheric pressure is remarkable static over time, and decreases both at high and low obliquity. Also, from our one billion year orbital model, we present new results on the fraction of time Mars is expected to experience periods of low obliquity and high obliquity.     

Loss of water from Mars:: Implications for the oxidation of the soil

The evolution of the martian atmosphere with regard to its H₂O inventory is influenced by thermal loss processes of H, H₂, nonthermal atmospheric loss processes of H⁺, H₂⁺, O, O⁺, CO₂, and O₂⁺ into space, as well as by chemical weathering of the surface soil. The evolution of thermal and nonthermal escape processes depend on the history of the intensity of the solar XUV radiation and the solar wind density. Thus, we use actual data from the observation of solar proxies with different ages from the Sun in Time program for reconstructing the Sun's radiation and particle environment from the present to 3.5 Gyr ago. The correlation between mass loss and X-ray surface flux of solar proxies follows a power law relationship, which indicates a solar wind density up to 1000 times higher at the beginning of the Sun's main sequence lifetime. For the study of various atmospheric escape processes we used a gas dynamic test particle model for the estimation of the pick up ion loss rates and considered pick up ion sputtering, as well as dissociative recombination. The loss of H₂O from Mars over the last 3.5 Gyr was estimated to be equivalent to a global martian H₂O ocean with a depth of about 12 m, which is smaller than the values reported by previous studies. If ion momentum transport, a process studied in detail by Mars Express is significant on Mars, the water loss may be enhanced by a factor of about 2. In our investigation we found that the sum of thermal and nonthermal atmospheric loss rates of H and all nonthermal escape processes of O to space are not compatible with a ratio of 2:1, and is currently close to about 20:1. Escape to space cannot therefore be the only sink for oxygen on Mars. Our results suggest that the missing oxygen (needed for the validation of the 2:1 ratio between H and O) can be explained by the incorporation into the martian surface by chemical weathering processes since the onset of intense oxidation about 2 Gyr ago. Based on the evolution of the atmosphere-surface-interaction on Mars, an overall global surface sink of about 2×10⁴² oxygen particles in the regolith can be expected. Because of the intense oxidation of inorganic matter, this process may have led to the formation of considerable amounts of sulfates and ferric oxides on Mars. To model this effect we consider several factors: (1) the amount of incorporated oxygen, (2) the inorganic composition of the martian soil and (3) meteoritic gardening. We show that the oxygen incorporation has also implications for the oxidant extinction depth, which is an important parameter to determine required sampling depths on Mars aimed at finding putative organic material. We found that the oxidant extinction depth is expected to lie in a range between 2 and 5 m for global mean values.     

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).     

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.     

Monte-Carlo simulations of atmospheric muon production: Implication of the past martian environment

In order to evaluate the obliquity-driven atmospheric-density path length effect on nuclide production rate on Mars, we performed a Monte-Carlo simulation to produce the number of secondary particles such as muons, neutrons and protons in the martian atmosphere and to simulate that production of ¹⁰Be and ³⁶Cl in the martian regolith by muons and neutrons depends on how much atmosphere had been present for the past 10 million years. The vertical profile of the present martian atmosphere to generate secondary particles has been determined based on the data provided by the Viking missions. For other thickness profiles, we scaled Linsley's atmospheric model. Atmospheric shower has been generated with the SIBYLL 2.1 for high-energy hadronic interactions and EHSA for low energy photonuclear interactions. With increasing atmospheric thickness, more primary interactions occur in the atmosphere. Consequently the proton flux is reduced and the secondary cosmic ray flux, such as muons or energetic neutrons increases at surface. The result indicates that the muon production is more sensitive to obliquity-driven atmospheric variations than proton reduction. A thicker atmosphere would result in enhanced nuclide production at a place deeper than 5 m below the surface and the nuclides present in detectable concentrations. Application to the polar deposit is described.     

Radio occultation measurements and MGCM simulations of Kelvin waves on Mars

We have derived new results concerning thermal tides on Mars from a combination of radio occultation measurements and numerical simulations by a Mars General Circulation Model (MGCM). This investigation exploits a set of concurrent observations by Mars Express (MEX) and Mars Global Surveyor (MGS) in mid-2004, when the season on Mars was midspring in the northern hemisphere. The MEX occultations sampled the atmosphere near the evening terminator at latitudes ranging from 54° N to 15° S. The MGS occultations provided complementary coverage near the morning terminator at latitudes of 35° N and 71° S. The geopotential field derived from these measurements contains distinctive modulation caused by solar-asynchronous thermal tides. Through careful analysis of the combined observations, we characterized two prominent wave modes, obtaining direct solutions for some properties, such as the amplitude and phase, as well as constraints on others, such as the period, zonal wave number, and meridional structure. We supplemented these observations with MGCM simulations. After evaluating the performance of the MGCM against the measurements, we used the validated simulation to deduce the identity of the two tidal modes and to explore their behavior. One mode is a semidiurnal Kelvin wave with a zonal wave number of 2 (SK2), while the other is a diurnal Kelvin wave with a zonal wave number of 1 (DK1). Both modes are known to be close to resonance in the martian atmosphere. Our observations of the SK2 are more complete and less ambiguous than any previous measurement. The well-known DK1 is the dominant solar-asynchronous tide in the martian atmosphere, and our results confirm and extend previous observations by diverse instruments.     

Influences of atmospheric variations on Mars's record of small craters

Planetary atmospheres influence cratering rates at small diameters (∼2-250 m) by filtering impactor populations via ablation, aerobraking and breakup of entering objects. The atmosphere of Mars undergoes rapid and drastic obliquity-driven variations in density, corresponding to pressure variations between zero and several tens of millibars. Here a simulation is used to assess the fate of a large population of impactors interacting with the present and predicted past and future martian atmospheres. We find that even Mars's present atmosphere significantly reduces crater production rates at small diameters (<30 m) and past denser atmospheres would have affected cratering even more strongly, and to considerably larger diameters. These effects are increased if the inner Solar System's small impactor population contains significant numbers of icy, cometary bodies. Evidence of recent atmospheric density variations may be detectable in the martian small cratering record with future planned imaging capabilities. Because of martian atmospheric effects and variations, surface ages derived from counts of craters of less than about 250 m on Mars may be underestimated.     

Martian atmospheric particulate spectral end-members recovery from PFS and IRIS data

We present an application of a multivariate analyses technique on data returned by the Planetary Fourier Spectrometer (PFS) instrument on board the ESA’s Mars Express (MEX) spacecraft in order to separate the atmospheric contribution from the observed radiation. We observe that Thermal/Far Infrared spectra returned from Mars, covering almost a whole martian year, can be represented by a linear model using a limited set of end-member spectra. We identify the end-members as the suspended mineral dust and water ice clouds, but no surface signature was found. We improve previous studies performed with data from the Thermal Emission Spectrometer (TES) thanks to the higher spectral resolution of PFS. This allows for distinguishing narrow gaseous bands present in the martian atmosphere. Furthermore, the comparison of results from PFS and TES with data collected in 1971 by the Mariner 9 Infrared Interferometer Spectrometer (IRIS) shows an atmospheric dust component with similar spectral behavior. This might indicate homogeneity of the dust source regions over a time period of more than 30 years.     

Propagation of tropospheric gravity waves into the upper atmosphere of Mars

We study the propagation of gravity waves in the martian atmosphere using a linearized one-dimensional full-wave model. Calculations are carried out for atmospheric parameters characteristic of Mars Orbiter Laser Altimeter (on Mars Global Surveyor MGS) observations of apparent gravity waves in high latitude clouds and MGS radio occultation measurements of temperature variations with height suggestive of gravity wave activity. Waves that reach the thermosphere produce fluctuations in density comparable in amplitude with the density variations detected in Mars Odyssey aerobraking data. Gravity waves of modest amplitude are found to deposit momentum and generate significant heating and cooling in the martian atmosphere. The largest heating and cooling effects occur in the thermosphere, at altitudes between about 130 and 150 km, with heating occurring at the lower altitudes and cooling taking place above.     

Volatile transport on inhomogeneous surfaces: I – Analytic expressions,with application to Pluto’s day

The two orders of magnitude drop between the measured atmospheric abundances of non-radiogenic argon, krypton and xenon in Earth versus Mars is striking. Here, in order to account for this difference, we explore the hypothesis that clathrate deposits incorporated into the current martian cryosphere have sequestered significant amounts of these noble gases assuming they were initially present in the paleoatmosphere in quantities similar to those measured on Earth (in mass of noble gas per unit mass of the planet). To do so, we use a statistical-thermodynamic model that predicts the clathrate composition formed from a carbon dioxide-dominated paleoatmosphere whose surface pressure ranges up to 3 bars. The influence of the presence of atmospheric sulfur dioxide on clathrate composition is investigated and we find that it does not alter the trapping efficiencies of other minor species. Assuming nominal structural parameters for the clathrate cages, we find that a carbon dioxide equivalent pressure of 0.03 and 0.9 bar is sufficient to trap masses of xenon and krypton, respectively, equivalent to those found on Earth in the clathrate deposits of the cryosphere. In this case, the amount of trapped argon is not sufficient to explain the measured Earth/Mars argon abundance ratio in the considered pressure range. In contrast, with a 2% contraction of the clathrate cages, masses of xenon, krypton and argon at least equivalent to those found on Earth can be incorporated into clathrates if one assumes the trapping of carbon dioxide at equivalent atmospheric pressures of ～2.3 bar. The proposed clathrate trapping mechanism could have then played an important role in the shaping of the current martian atmosphere.     

Origin of the martian dichotomy and Tharsis from a giant impact causing massive magmatism

The origin of the ancient martian crustal dichotomy and the massive magmatic province of Tharsis remains an open problem. Here, we explore numerically a hypothesis for the origin of these two features involving both exogenic and endogenic processes. We propose a giant impact event during the late stage of planetary formation as the source of the southern highland crust. In a second stage, the extraction of excess heat by vigorous mantle convection on the impacted hemisphere leads to massive magmatism, forming a distinct Tharsis-like volcanic region. By coupling short-term and long-term numerical simulations, we are able to investigate both the early formation as well as the 4.5 Gyr evolution of the martian crust. We demonstrate numerically that this exogenic-endogenic hypothesis is in agreement with observational data from Mars.     

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.