Mars Odyssey Spacecraft Research Articles

Bounding the unknowns of martian crustal heat flow from a synthesis of regional geochemistry and InSight mission data

The thermal evolution of terrestrial planets is primarily modulated by the distribution of heat-producing elements (HPE) within the crust and mantle. Chemical data from Martian meteorites suggest that Mars differentiated early, which led to an early partitioning of incompatible heat-producing elements in the crust. Previous estimates of Martian crustal heat flow that used the bulk regolith abundances of HPEs from the Gamma-Ray Spectrometer (GRS) suite on Mars Odyssey spacecraft have further corroborated this view of Mars, albeit with poorly known crustal column representativeness. Here we couple the GRS-derived chemical maps of Mars with estimates of crustal thickness and density from the InSight lander to revise the estimated Martian crustal heat flow. The mean crustal heat flow values range from 3.0 to 13.9 mW m−2 for the endmember gravity-derived crustal thickness models anchored by constraints from InSight. We also estimate the crustal heat flow from other factors such as the distribution of HPEs with depth and the Uranium content of the crust. Our results suggest that the mean crustal heat flow varies substantially across models, with the highest mean values being associated with higher densities and an increased enrichment of HPEs with depth. Further work is needed to constrain the crustal thickness of Mars, as the largest uncertainties in the estimate of crustal heat flow stem from uncertainty in the crustal thickness estimates, not geochemical variability. The results from this work corroborate previous estimates of a strong fractionation of heat-producing elements into the Martian crust.

Numerical modeling of mapping of Martian epithermal neutron emission: Applications to FREND investigation onboard ESA’s Trace Gas Orbiter

Neutron telescope FREND is operating onboard ESA TGO mission from May 2018 till present. The instrument measures epithermal neutron flux from the Martian surface. Measured variations of the neutron flux may be used to determine hydrogen abundance in the soil usually interpreted as a water/water ice. The unique design of the instrument provides measurements of the neutron flux with a spatial resolution that is much higher than the resolution of previous neutron spectrometers operating on orbit around Mars on board the NASA Mars Odyssey spacecraft. It is known that the spatial resolution of a neutron spectrometer in an orbit around Mars is determined not only by the parameters of the detectors, presence or absence of additional modules to improve the resolution, the orbital altitude of the spacecraft, but also by the fact that Mars has an atmosphere that scatters some of the neutrons that emerge from the Martian surface. One of the ways to increase the spatial resolution of orbital neutron detectors is the passive collimation method using neutron-absorbing substances. In this paper we consider the results of numerical modeling of parameters of the collimated instrument in order to determine its spatial resolution, taking into account the presence of the Martian atmosphere. The contribution of neutrons passing through the collimator walls to the total counting rate and how they affect the estimation of the hydrogen/water concentration in the soil are also considered. Therefore, a method was developed to take into account the imperfect absorbing properties of the collimator and to clear the measured signal from the contribution of neutrons that passed through the collimator wall. Comparison of the spatial resolution of non-collimated and collimated instruments is discussed. Moreover, the results obtained with the conventional method of data processing are compared with the results of developed method, which takes into account the contribution of non-collimated neutrons. The advantage of the results obtained by the developed method is shown.

Observations of neutron radiation environment during Odyssey cruise to Mars

In April 2001, Mars Odyssey spacecraft with the High Energy Neutron Detector (HEND) onboard was launched to Mars. HEND/Odyssey was switched on measurement mode for most of transit to Mars to monitor variations of spacecraft background and solar activity. Although HEND/Odyssey was originally designed to measure Martian neutron albedo and to search for Martian subsurface water/water ice, its measurements during cruise phase to Mars are applicable to evaluate spacecraft ambient radiation background. The biological impact of the neutron component of this radiation background should be understood, as it must be taken into account in planning future human missions to Mars. We have modeled the spacecraft neutron spectral density and compared it with HEND measurements to estimate neutron dose equivalent rates during Odyssey cruise phase, which occurred during the maximum period of solar cycle 23. We find that the Odyssey ambient neutron environment during May – September 2001 yields 10.6 ± 2.0 μSv per day in the energy range from 0 to 15 MeV, and about 29 μSv per day when extrapolated to the 0–1000 MeV energy range during solar quiet time (intervals without Solar Particle Events, SPEs). We have also extrapolated HEND/Odyssey measurements to different periods of solar cycle and find that during solar minimum (maximum of GCR flux), the neutron dose equivalent rate during cruise to Mars could be as high as 52 μSv per day with the same shielding. These values are in good agreement with results reported for a similar measurement made with an instrument aboard the Mars Science Laboratory during its cruise to Mars in 2011–2012.

New estimation of non-thermal electron energetics in the giant solar flare on 28 October 2003 based on Mars Odyssey observations

A new estimation of the total number and energy of the non-thermal electrons produced in the giant (>X17) solar flare on 2003 October 28 is presented based on the analysis of the observations of the hard X-ray (HXR) emission by the High Energy Neutron Detector (HEND) onboard the Mars Odyssey spacecraft orbiting Mars. Previous estimations of the non-thermal electron energy based on the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) data were incomplete since RHESSI missed the peak of the flare impulsive phase. In contrast, HEND observed the whole flare. We used two models to estimate the energy of the non-thermal electrons: the cold thick target model and the warm thick target model. More specifically, in the second case we employed an approximation which relates the pitch-angle averaged injection spectrum with the electron spectrum integrated over the emitting source. We found that, depending on the model used and the low-energy cutoff (Ec) of the non-thermal electrons, the estimate of their total energy in the entire flare can vary from 2.3×1032 to 6.2×1033 ergs. The lowest estimate, 2.3×1032 ergs, obtained within the cold thick target model and fixed Ec=43 keV, is consistent with the previous estimate. In this case, non-thermal electrons accelerated in the peak of the flare impulsive phase missed by RHESSI contained approximately 40% of the total energy of non-thermal electrons of the entire flare. The highest value, 6.2×1033 ergs, obtained with the cold thick target model and fixed Ec=10 keV, looks abnormally high, since it exceeds the total non-potential magnetic energy of the parent active region and the total bolometric energy radiated in the flare. Our estimates also show that the total number and energetics of the HXR-producing electrons in the flare region is a few orders of magnitude higher than of the population of energetic electrons injected into interplanetary space.

Inventory of CO2 available for terraforming Mars

We revisit the idea of ‘terraforming’ Mars — changing its environment to be more Earth-like in a way that would allow terrestrial life (possibly including humans) to survive without the need for life-support systems — in the context of what we know about Mars today. We want to answer the question of whether it is possible to mobilize gases present on Mars today in non-atmospheric reservoirs by emplacing them into the atmosphere, and increase the pressure and temperature so that plants or humans could survive at the surface. We ask whether this can be achieved considering realistic estimates of available volatiles, without the use of new technology that is well beyond today’s capability. Recent observations have been made of the loss of Mars’s atmosphere to space by the Mars Atmosphere and Volatile Evolution Mission probe and the Mars Express spacecraft, along with analyses of the abundance of carbon-bearing minerals and the occurrence of CO2 in polar ice from the Mars Reconnaissance Orbiter and the Mars Odyssey spacecraft. These results suggest that there is not enough CO2 remaining on Mars to provide significant greenhouse warming were the gas to be emplaced into the atmosphere; in addition, most of the CO2 gas in these reservoirs is not accessible and thus cannot be readily mobilized. As a result, we conclude that terraforming Mars is not possible using present-day technology.

Catalog of hard X-ray solar flares detected with Mars Odyssey/HEND from the Mars orbit in 2001–2016

The study of nonstationary processes in the Sun is of great interest, and lately, multiwavelength observations and registration of magnetic fields are carried out by means of both ground-based telescopes and several specialized spacecraft (SC) on near-Earth orbits. However the acquisition of the new reliable information on their hard X-ray radiation remains demanded, in particular if the corresponding SC provide additional information, e.g. in regard to the flare observations from the directions other than the Sun-Earth direction. In this article we present a catalog of powerful solar flares registered by the High Energy Neutron Detector (HEND) device designed in the Space Research Institute (IKI) of Russian Academy of Sciences. HEND is mounted onboard the 2001 Mars Odyssey spacecraft. It worked successfully during the flight to Mars and currently operates in the near-Mars orbit. Besides neutrons, the HEND instrument is sensitive to the hard X-ray and gamma radiation. This radiation is registered by two scintillators: the outer one is sensitive to the photons above 40 keV and the inner one to the photons above 200 keV. The catalog was created with the new procedure of the data calibration. For most powerful 60 solar flares on the visible and on the far sides of the Sun (in respect to a terrestrial observer), we provide time profiles of flare radiation, summed over all the channels of X-ray and in some cases of gamma-ray bands as well as the spectra and characteristics of their power law approximation. We briefly discuss the results of the previous articles on the study of the Sun with HEND instrument and the potential of the further use of these data.

Interannual similarity and variation in seasonal circulation of Mars' atmospheric Ar as seen by the Gamma Ray Spectrometer on Mars Odyssey

More than 3 Mars' years (MY) of atmospheric argon (Ar) measurements are used to study annual and seasonal variations in atmospheric transport and mixing. Data are obtained over the period 20 May 2002 to 4 May 2008 by the Gamma Subsystem (GS) of the Gamma Ray Spectrometer (GRS) on the Mars Odyssey spacecraft in orbit around Mars. Here we augment previous studies of Mars' Ar in which strong seasonal variations were observed and horizontal meridional mixing coefficients for the southern hemisphere were computed. Comparison of year‐to‐year seasonal abundance shows strong similarity but also some short‐period (∼15°–30° Ls) and interannual variations. Evidence for short periods of strong eddy transport is exhibited during autumn and winter. The seasonal change in Ar concentration for southern latitudes is relatively gradual and well defined, but seasonal changes at high northern latitudes are chaotic and indicate that atmospheric disturbance is ubiquitous. Major topographic landforms (Elysium, Tharsis, Noachis Terra, Hellas) apparently have little control over seasonal Ar concentration at the spatial resolution of the GRS data set. Some indication of local enhanced Ar concentration is present from 30°N to 60°N for the Hellas and Tharsis sectors in late winter and early spring. The data show some significant (3σ) differences between MY 26 and MY 27 in geographical sectors that are likely produced by local weather. The GS data do not show seasonal variation of Ar at equatorial and low‐latitude zones, in contrast to those from the Alpha Particle X‐ray Spectrometer (APXS) measurements from the Mars Exploration Rovers.

Feasibility of infrared Earth tracking for deep-space optical communications

Infrared (IR) Earth thermal tracking is a viable option for optical communications to distant planet and outer-planetary missions. However, blurring due to finite receiver aperture size distorts IR Earth images in the presence of Earth's nonuniform thermal emission and limits its applicability. We demonstrate a deconvolution algorithm that can overcome this limitation and reduce the error from blurring to a negligible level. The algorithm is applied successfully to Earth thermal images taken by the Mars Odyssey spacecraft. With the solution to this critical issue, IR Earth tracking is established as a viable means for distant planet and outer-planetary optical communications.

Erratum to: “Culmination of the flare activity of group 10786 in July 2005: X-ray observations from near-Mars and near-Earth orbits”

A detailed study of two major solar flares that occurred in Group 10786 at the time of its disappearance behind the western limb is presented. The flares of July 14, 2005 were previously studied fairly poorly, as no RHESSI hard X-ray observations were available for themaxima of the twomost powerful of these flares. Observations carried out using the HEND equipment (on the Mars Odyssey spacecraft) developed at the Institute for Space Research in Moscow are used here to fill this gap. In the first flare, an intense, impulsive burst occurred at 07:23 UT, about 1.5 h after the onset of a weak, prolonged event. While processes in the neighborhood of the northern spot dominated in the flares of July 5–9, a powerful impulsive energy release on July 14 emerged when the flare process that originated in the North reached the southern spot. Our analysis of the flare activity of this medium-sized group reveals a gradual enhancement of the flare activity and a strong interaction between the acceleration above the magnetic-field neutral line and in the immediate vicinity of the spots. At the time of the culmination of the flare activity in the group on July 13 and 14, the pattern of nonstationary processes changes: fast coronal mass ejections form after a series of impulsive energy-release events. Spacecraft observations of the burst of July 14 after 11 UT at points separated in longitude (on RHESSI and Mars Odyssey) revealed clear anisotropy of the flare emission at energies exceeding 80 keV.

Martian surface heat production and crustal heat flow from Mars Odyssey Gamma-Ray spectrometry

[1] Martian thermal state and evolution depend principally on the radiogenic heat-producing element (HPE) distributions in the planet's crust and mantle. The Gamma-Ray Spectrometer (GRS) on the 2001 Mars Odyssey spacecraft has mapped the surface abundances of HPEs across Mars. From these data, we produce the first models of global and regional surface heat production and crustal heat flow. As previous studies have suggested that the crust is a repository for approximately 50% of the radiogenic elements on Mars, these models provide important, directly measurable constraints on Martian heat generation. Our calculations show considerable geographic and temporal variations in crustal heat flow, and demonstrate the existence of anomalous heat flow provinces. We calculate a present day average surface heat production of 4.9 ± 0.3 × 10−11 W · kg−1. We also calculate the average crustal component of heat flow of 6.4 ± 0.4 mW · m−2. The crustal component of radiogenically produced heat flow ranges from <1 mW · m−2 in the Hellas Basin and Utopia Planitia regions to ∼13 mW · m−2 in the Sirenum Fossae region. These heat production and crustal heat flow values from geochemical measurements support previous heat flow estimates produced by different methodologies.

Thermal history of Mars inferred from orbital geochemistry of volcanic provinces

Reconstruction of the geological history of Mars has been the focus of considerable attention over the past four decades, with important discoveries being made about variations in surface conditions. However, despite a significant increase in the amount of data related to the morphology, mineralogy and chemistry of the martian surface, there is no clear global picture of how magmatism has evolved over time and how these changes relate to the internal workings and thermal evolution of the planet. Here we present geochemical data derived from the Gamma Ray Spectrometer on board NASA's Mars Odyssey spacecraft, focusing on twelve major volcanic provinces of variable age. Our analysis reveals clear trends in composition that are found to be consistent with varying degrees of melting of the martian mantle. There is evidence for thickening of the lithosphere (17-25 km Gyr(-1)) associated with a decrease in mantle potential temperature over time (30-40 K Gyr(-1)). Our inferred thermal history of Mars, unlike that of the Earth, is consistent with simple models of mantle convection.

THEMIS-VIS observations of clouds in the martian mesosphere: Altitudes, wind speeds, and decameter-scale morphology

We present measurements of the altitude and eastward velocity component of mesospheric clouds in 35 imaging sequences acquired by the Mars Odyssey (ODY) spacecraft’s Thermal Emission Imaging System visible imaging subsystem (THEMIS-VIS). We measure altitude by using the parallax drift of high-altitude features, and the velocity by exploiting the time delay in the THEMIS-VIS imaging sequence. We observe two distinct classes of mesospheric clouds: equatorial mesospheric clouds observed between 0° and 180° L s ; and northern mid-latitude clouds observed only in twilight in the 200–300° L s period. The equatorial mesospheric clouds are quite rare in the THEMIS-VIS data set. We have detected them in only five imaging sequences, out of a total of 2048 multi-band equatorial imaging sequences. All five fall between 20° south and 0° latitude, and between 260° and 295° east longitude. The mid-latitude mesospheric clouds are apparently much more common; for these we find 30 examples out of 210 northern winter mid-latitude twilight imaging sequences. The observed mid-latitude clouds are found, with only one exception, in the Acidalia region, but this is quite likely an artifact of the pattern of THEMIS-VIS image targeting. Comparing our THEMIS-VIS images with daily global maps generated from Mars Orbiter Camera Wide Angle (MOC-WA) images, we find some evidence that some mid-latitude mesospheric cloud features correspond to cloud features commonly observed by MOC-WA. Comparing the velocity of our mesospheric clouds with a GCM, we find good agreement for the northern mid-latitude class, but also find that the GCM fails to match the strong easterly winds measured for the equatorial clouds. Applying a simple radiative transfer model to some of the equatorial mesospheric clouds, we find good model fits in two different imaging sequences. By using the observed radiance contrast between cloud and cloud-free regions at multiple visible-band wavelengths, these fits simultaneously constrain the optical depths and particles sizes of the clouds. The particle sizes are constrained primarily by the relative contrasts at the available wavelengths, and are found to be quite different in the two imaging sequences: r eff = 0.1 μm and r eff = 1.5 μm. The optical depths (constrained by the absolute contrasts) are substantial: 0.22 and 0.5, respectively. These optical depths imply a mass density that greatly exceeds the saturated mass density of water vapor at mesospheric temperatures, and so the aerosol particles are probably composed mainly of CO 2 ice. Our simple radiative transfer model is not applicable to twilight, when the mid-latitude mesospheric clouds were observed, and so we leave the properties of these clouds as a question for further work.

GRS evidence and the possibility of paleooceans on Mars

The Gamma Ray Spectrometer (Mars Odyssey spacecraft) has revealed elemental distributions of potassium (K), thorium (Th), and iron (Fe) on Mars that require fractionation of K (and possibly Th and Fe) consistent with aqueous activity. This includes weathering, evolution of soils, and transport, sorting, and deposition, as well as with the location of first-order geomorphological demarcations identified as possible paleoocean boundaries. The element abundances occur in patterns consistent with weathering in situ and possible presence of relict or exhumed paleosols, deposition of weathered materials (salts and clastic minerals), and weathering/transport under neutral to acidic brines. The abundances are explained by hydrogeology consistent with the possibly overlapping alternatives of paleooceans and/or heterogeneous rock compositions from diverse provenances (e.g., differing igneous compositions).

MARTIAN SOIL GETS BAKED

AFTER THREE WEEKS of flexing its mechanical joints on the surface of Mars, the Phoenix lander is doing some science. The soil sample scooped by the probe’s robotic arm has not yet yielded any surprises—it contains carbon dioxide, but no water, for example—but the analyses are just beginning, mission scientists say. The testing finally began after a few tense days during the week of June 8, when engineers worked to coax a clumpy scoop of martian dirt into one of eight ovens on the probe. There, the soil is undergoing cycles of increased heating, and evolved gas will then be analyzed by a mass spectrometer. The goal of the mission is to study the possible ancient role of water near Mars’s north pole, a region where NASA’s Mars Odyssey spacecraft detected fields of subsurface ice several years ago. Images from Phoenix show a stark white substance about 5 cm down in the trenches ...

Martian South Polar Layered Deposit stratigraphy and implications for accumulation history

We examine the stratigraphy of the Martian South Polar Layered Deposits (SPLD) exposed on scarp walls throughout the deposit using THEMIS visible (17 m/pxl) images from the Mars Odyssey spacecraft. By correlating marker layers and packets of layers among multiple locations, we are able to identify three sequences of layers within the SPLD, each with a different areal extent. While certain layers are correlatable over hundreds of km, implying widespread deposition of polar material, the absence of several identified layer sequences in some regions, notably the Promethei Lingula region, as well as the presence of several unconformities, indicates that deposition and sublimation or erosion of these deposits has not been uniform around the SPLD. This is further supported by the fact that individual layers display variations in thickness greater than a factor of 4 over hundreds of kilometers. Using these observations we propose an internal structure for the SPLD, with individual layers roughly following a dome‐like shape toward the center of the deposit, and becoming more planar toward the margins. We infer that the SPLD underwent at least three major periods of layer accumulation; each period is distinguished by a change in areal extent of net accumulation. The changes in areal extent of the layer sequences may be due to changes in Martian climate causing episodes of significant erosion of the SPLD between the deposition of each sequence or causing wide variations in accumulation zones of the SPLD over the history of these deposits.

Root‐mean‐square surface slopes of Phoenix landing sites with 75‐cm bistatic radar received by Mars Odyssey

Between August and December 2005, we conducted 76 oblique‐incidence scattering experiments using the SRI International 46‐m antenna in the Stanford foothills to illuminate Mars for 20‐min periods with an unmodulated, 75‐cm λ, circularly polarized wave. The direct signal and a Martian surface echo, separated by differential Doppler frequency shifts, were received simultaneously by the one‐bit receiver on board the Mars Odyssey spacecraft. Four of the proposed Phoenix landing regions, denoted A–D, were probed by at least one experiment. The surface echoes are characterized by both fluctuating amplitude and varying spectral width, which are responses to surface reflectivity and roughness variations. Our analysis of the echo data is based on quasi‐specular scattering theory and makes use of high‐resolution Mars Orbiter Laser Altimeter (MOLA) topographic maps to model the scattering surface in three dimensions along the specular track of the echo. We find MOLA topography sufficient to model scattering at 75‐cm λ, indicating that the effective horizontal scales being sensed by the radar are at or larger than the MOLA grid spacing of 150–500 m. We find that the RMS surface slopes for the proposed Phoenix landing regions, as determined using the Hagfors scattering law and applicable to those scales, rarely exceed 1°, except in the presence of large craters and other surface irregularities.

Long-term observations of the evolution of the southern seasonal cap of Mars: Neutron measurements by the HEND instrument onboard the 2001 Mars Odyssey spacecraft

We present the results of five-year observations of the southern seasonal cap of Mars based on neutron spectroscopy of the surface fulfilled by the Russian HEND instrument onboard the NASA 2001 Mars Odyssey spacecraft. The numerical modeling of the observational data allowed us to reconstruct the curves of the variations of the total mass of the southern seasonal cap of Mars for different years (three Martian years) and to find the year-to-year variations of the seasonal cycle.

Water ice permafrost on Mars: Layering structure and subsurface distribution according to HEND/Odyssey and MOLA/MGS data

To elucidate the nature of permafrost in the shallow subsurface of Mars, we analyze jointly neutron albedo from the High Energy Neutron Detector (HEND) on the Mars Odyssey spacecraft and near‐IR (1064‐nm) surface reflectance from the Mars Orbiter Laser Altimeter (MOLA) on Mars Global Surveyor. The first dataset measures the content of hydrogen (in the form of water or hydroxyl) in the soil, and the second yields the flux of absorbed solar energy by the surface. We identify a statistically–significant negative cross‐correlation between these data at latitudes poleward of 40° latitude in the northern hemisphere and in the latitude band 40°–60° in the southern hemisphere, which we interpret as evidence for the presence of stable water ice under a dry equilibrium top layer (ETL). We deduce an empirical relation between near‐IR reflectance and thickness of this ETL, which allows the burial depth of the water ice table to be estimated with km‐scale spatial resolution. We observe no correlation between neutron and near‐IR albedo within the southern hemisphere poleward of 60° latitude. While it is known from previously analyzed neutron and gamma‐ray data that subsurface water ice is present within this region and is covered by a layer of dry regolith, the absence of a correlation indicates that the thickness of this layer is not controlled by an equilibrium condition between the ice table and atmosphere.

Observations of the north polar water ice annulus on Mars using THEMIS and TES

The Martian seasonal CO 2 ice caps advance and retreat each year. In the spring, as the CO 2 cap gradually retreats, it leaves behind an extensive defrosting zone from the solid CO 2 cap to the location where all CO 2 frost has sublimated. We have been studying this phenomenon in the north polar region using data from the THermal EMission Imaging System (THEMIS), a visible and infra-red (IR) camera on the Mars Odyssey spacecraft, and the Thermal Emission Spectrometer (TES) on Mars Global Surveyor. Recently, we discovered that some THEMIS images of the CO 2 defrosting zone contain evidence for a distinct defrosting phenomenon: some areas just south of the CO 2 cap edge are too bright in visible wavelengths to be defrosted terrain, but too warm in the IR to be CO 2 ice. We hypothesize that we are seeing evidence for a seasonal annulus of water ice (frost) that recedes with the seasonal CO 2 cap, as predicted by previous workers. In this paper, we describe our observations with THEMIS and compare them to simultaneous observations by TES and OMEGA. All three instruments find that this phenomenon is distinct from the CO 2 cap and most likely composed of water ice. We also find strong evidence that the annulus widens as it recedes. Finally, we show that this annulus can be detected in the raw THEMIS data as it is collected, enabling future long-term onboard monitoring.

High-resolution subsurface water-ice distributions on Mars

Theoretical models indicate that water ice is stable in the shallow subsurface (depths of <1-2 m) of Mars at high latitudes. These models have been mainly supported by the observed presence of large concentrations of hydrogen detected by the Gamma Ray Spectrometer suite of instruments on the Mars Odyssey spacecraft. The models and measurements are consistent with a water-ice table that steadily increases in depth with decreasing latitude. More detailed modelling has predicted that the depth at which water ice is stable can be highly variable, owing to local surface heterogeneities such as rocks and slopes, and the thermal inertia of the ground cover. Measurements have, however, been limited to the footprint (several hundred kilometres) of the Gamma Ray Spectrometer suite, preventing the observations from documenting more detailed water-ice distributions. Here I show that by observing the seasonal temperature response of the martian surface with the Thermal Emission Imaging System on the Mars Odyssey spacecraft, it is possible to observe such heterogeneities at subkilometre scale. These observations show significant regional and local water-ice depth variability, and, in some cases, support distributions in the subsurface predicted by atmospheric exchange and vapour diffusion models. The presence of water ice where it follows the depth of stability under current climatic conditions implies an active martian water cycle that responds to orbit-driven climate cycles. Several regions also have apparent deviations from the theoretical stability level, indicating that additional factors influence the ice-table depth. The high-resolution measurements show that the depth to the water-ice table is highly variable within the potential Phoenix spacecraft landing ellipses, and is likely to be variable at scales that may be sampled by the spacecraft.