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Thermophysical Analysis of Gale Crater using TES andTHEMIS observations.

Edward M. Barratt (1,2) and Nathaniel E. Putzig (1)(1) Southwest Research Institute, Boulder, Colorado, 80302, USA, (2) University of Colorado, Boulder, Colorado,80309, USA. (eddy@boulder.swri.edu)

AbstractWe present an analysis of thermophysical propertieswithin and surrounding Gale Crater using data fromthe Mars Global Surveyor (MGS) Thermal EmissionSpectrometer (TES) and the Mars Odyssey ThermalEmission Imaging System (THEMIS). THEMIS im-ages provide higher spatial resolution (∼100 m perpixel) but larger absolute uncertainty in temperaturerelative to TES (∼3 km per pixel). We derive appar-ent thermal inertia from TES data and THEMIS im-ages, using the TES results to analyse seasonal thermalinertia variations and the THEMIS results to analysesmaller scale spatial variations. We compare the varia-tions to those found for horizontal mixtures and layersof different materials to better constrain the physicalheterogeneity of surfaces in and around Gale Crater.

1. IntroductionThermal inertia (I) is a bulk property that controlshow a volume of material stores and conducts heat.Theoretically, the surface temperature of an objectwith negligible thermal inertia would respond instan-taneously to radiative forcing, depending solely on in-cident radiation and the object’s albedo (A). In reality,geologic material has non-zero conductivity (k) thatspreads heat into its interior, while its density (ρ) andheat capacity (c) allow it to store heat. Together, thesethree properties comprise the thermal inertia:

I ≡√kρc (1)

We use the SI derived unit of thermal inertia, tiu:

tiu ≡ Jm−2K−1s−1/2 (2)

The temperature of a surface is determined by a bal-ance of the upward radiated heat from the surface withdownwelling heat flux due to solar insolation, atmo-spheric radiation, subsurface heat conduction, and anyother heat sources. In the case of Mars, another heat

source that must be considered is latent heat due toseasonal CO2 condensation and sublimation. For geo-logic materials under Martian surface conditions, ther-mal inertia generally increases with grain size, provid-ing a means to assess the physical properties of thenear-surface using observations of temperature.

Together with surface brightness temperature, theTES instrument provided albedo (A) and atmosphericdust opacity (τD), which were used in our derivationsof thermal inertia. Albedo within Gale Crater wasseen to change from year to year, with changes occur-ring after major dust storms. THEMIS measurementstaken after the failure of the TES spectrometer rely onseasonal forecasts of τD, and assume that A has re-mained unchanged since the last global dust storm ofthe MGS operational period. A complete descriptionof the thermal-inertia derivation technique is describedby [1] and references therein.

2 Surface HeterogeneityIn the technique used here to derive thermal iner-tia, the surface properties are assumed to be homo-geneous within the area sampled by the measurement(i.e. within each 3-km pixel for TES and each 100-mpixel for THEMIS), both laterally and vertically to afew seasonal thermal skin depths. Because of its non-linear relationship to temperature, the derived thermalinertia over heterogeneous regions will not be a simpleaverage of thermal inertia in that region. In fact, whensampling the same pixel of a heterogeneous region,different values of derived thermal inertia may resultdepending on the season and the time of day. Suchchanges in derived thermal inertia between observa-tions can be used to constrain the sub-pixel and sub-surface distribution of thermal inertia. Fig. 1 presentsmaps of thermal inertia derived from TES nighttimemeasurements for two different seasons and demon-strates that Gale Crater is considerably heterogeneous,at least for the 20 pixel-per-degree TES sampling.

Model curves for seasonal apparent thermal inertiavariations, under a few simple layering schemes, areproduced and compared to derived apparent thermalinertia within Gale Crater. Preliminary analysis sug-gests that Gale Crater’s bulk thermophysical proper-ties may be too complex to be described by a simpletwo-component model of horizontally mixed or layermaterials.

3 Themis InterpretationIn Fig. 2, we present a typical THEMIS derivedthermal-inertia image in the vicinity of the landing sitefor Mars Science Laboratory (MSL) Curiosity, demon-strating the improved lateral resolution compared tothe TES-derived results. Using our THEMIS resultsand supplementary information from the Mars Recon-naissance Orbiter Context Camera (CTX) and HighResolution Imaging Science Experiment (HiRISE) wewill compare observed features with earlier THEMISresults by [2], with the geological mapping carried outby [3] and with later work using observations fromMSL [4].

4. SummaryAnalysing the seasonal variations in apparent thermalinertia allows us to place constraints on sub pixel het-erogeneity of the surface, while the presence of MSLallows us to evaluate findings at one location withground truths.

This research was carried out thanks to funding pro-vided by NASA MFRP Grant NNX11AM44G.

References[1] Putzig, N.E., and Mellon, M.T.: Apparent thermal in-

ertia and the surface heterogeneity of Mars, Icarus, Vol.191, pp. 68-94, 2007.

[2] Ferguson, R.L., et al.: Surface Properties of the MarsScience Laboratory Candidate Landing Site..., Space Sci-ence Reviews, Vol. 170, pp. 739-773, 2012.

[3] Anderson, R.B., and Bell III, J.F.: Geologic mappingand characterisation of Gale Crater and implications forits potential as a Mars Science Laboratory landing site,Mars, Vol. 5, pp. 76-128, 2010.

[4] Hamilton, V.E., et al.: Preliminary Results from theMars Science Laboratory REMS ground temperaturesensor at Rocknest, LPSC, 18–22 March 2013, TheWoodlands, Texas, 2013.

Figure 1: Derived thermal inertia from nighttime TESmeasurements for Top: 240◦ < Ls < 280◦, Bottom:000◦ < Ls < 040◦. Plotted over a mosaic of MROcontext camera images. The red star shows the landinglocation of MSL.

Figure 2: A detail of the thermal inertia derived fromTHEMIS image I32325004, plotted over the nighttime200◦ < Ls < 240◦ TES derived thermal inertia, andthe MRO context camera mosaic. Blue star shows thelanding location of MSL. Ellipse shows approximateoutline of landing ellipse. Black lines show mappedboundaries of Peace Vallis, differentiating between theLow and High thermal inertia fans described by [3].