Thermal Inertia Values Research Articles

Eclipsing binary Trojan asteroid Patroclus: Thermal inertia from Spitzer observations

We present mid-infrared ( 8 – 33 μ m ) observations of the binary L5-Trojan system (617) Patroclus–Menoetius before, during, and after two shadowing events, using the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope. For the first time, we effectively observe changes in asteroid surface temperature in real time, allowing the thermal inertia to be determined very directly. A new detailed binary thermophysical model is presented which accounts for the system’s known mutual orbit, arbitrary component shapes, and thermal conduction in the presence of eclipses. We obtain two local thermal-inertia values, representative of the respective shadowed areas: 21 ± 14 J s - 1 / 2 K - 1 m - 2 and 6.4 ± 1.6 J s - 1 / 2 K - 1 m - 2 . The average thermal inertia is estimated to be 20 ± 15 J s - 1 / 2 K - 1 m - 2 , potentially with significant surface heterogeneity. This first thermal-inertia measurement for a Trojan asteroid indicates a surface covered in fine regolith. Independently, we establish the presence of fine-grained (<a few μ m ) silicates on the surface, based on emissivity features near 10 and 20 μ m similar to those previously found on other Trojans. We also report V-band observations and report a lightcurve with complete rotational coverage. The lightcurve has a low amplitude of 0.070 ± 0.005 mag peak-to-peak, implying a roughly spherical shape for both components, and is single-periodic with a period ( 103.02 ± 0.40 h ) equal to the period of the mutual orbit, indicating that the system is fully synchronized. The diameters of Patroclus and Menoetius are 106 ± 11 and 98 ± 10 km , respectively, in agreement with previous findings. Taken together with the system’s known total mass, this implies a bulk mass density of 1.08 ± 0.33 g cm - 3 , significantly below the mass density of L4-Trojan asteroid (624) Hektor and suggesting a bulk composition dominated by water ice. All known physical properties of Patroclus, arguably the best studied Trojan asteroid, are consistent with those expected in icy objects with devolatilized surface (extinct comets), consistent with what might be implied by recent dynamical modeling in the framework of the Nice Model.

Thermal inertia and bolometric Bond albedo values for Mimas, Enceladus, Tethys, Dione, Rhea and Iapetus as derived from Cassini/CIRS measurements

Spectra taken by Cassini’s Composite Infrared Spectrometer (CIRS) between 10 and 600 cm −1 (17–1000 μm) of surface thermal emission of Mimas, Enceladus, Tethys, Dione, Rhea and Iapetus have been used to derive the thermal inertia and bolometric Bond albedo values. Only an upper limit for the bolometric Bond albedo of Iapetus’ dark leading side could be determined due to the insensitivity of the thermal model to albedo when albedos are very low. The thermal inertia in this region however is better constrained. The CIRS coverage of Enceladus is extensive enough that the latitudinal variation in these values from 60°S to 70°N has been determined in 10° wide bins. The bolometric Bond albedos determined here are consistent with literature values which show the surface of the saturnian icy moons to be covered in ice contaminated to varying degrees. The thermal inertia of the moons is shown to be in the range 9– 20 J m - 2 K - 1 s - 1 2 , approximately 2–6 times lower than that of the Galilean satellites, implying a less well consolidated and more porous surface. The thermal inertias of Iapetus and Phoebe are somewhat higher, suggesting that the very low thermal inertias of satellites from Rhea inwards may be related to their probable coating of E-ring material. Latitudinal variations on the surface of Enceladus show that the bolometric Bond albedo and thermal inertia increase towards the active plume source at the south pole.

HiRISE views enigmatic deposits in the Sirenum Fossae region of Mars

HiRISE images together with other recent orbital data from Mars define new characteristics of enigmatic Hesperian-aged deposits in Sirenum Fossae that are mostly 100–200 m thick, drape kilometers of relief, and often display generally low relief surfaces. New characteristics of the deposits, previously mapped as the “Electris deposits,” include local detection of meter-scale beds that show truncating relationships, a generally light-toned nature, and a variably blocky, weakly indurated appearance. Boulders shed by erosion of the deposits are readily broken down and contribute little to talus. Thermal inertia values for the deposits are ∼200 J m −2 K −1 s −1/2 and they may incorporate hydrated minerals derived from weathering of basalt. The deposits do not contain anomalous amounts of water or water ice. Deflation may dominate degradation of the deposits over time and points to an inventory of fine-grained sediment. Together with constraints imposed by the regional setting on formation processes, these newly resolved characteristics are most consistent with an eolian origin as a loess-like deposit comprised of redistributed and somewhat altered volcanic ash. Constituent sediments may be derived from airfall ash deposits in the Tharsis region. An origin directly related to airfall ash or similar volcanic materials is less probable and emplacement by alluvial/fluvial, impact, lacustrine, or relict polar processes is even less likely.

Estimation and mapping of wintertime increase in water ice content of the Martian surface soil based on seasonal Thermal Emission Spectrometer thermal inertia variations

Results of the Mars Global Surveyor Thermal Emission Spectrometer (TES) thermal inertia seasonal variations analysis show significant increase of the thermal inertia during autumn‐winter periods in the middle latitudes. This observed increase occurred with regular repetition during each of the three Mars years of the TES observations. Two climatic factors (atmospheric dust loading and water ice (frost) formation in the soil) might lead to the real and/or apparent seasonal increase in thermal inertia. We compare maps of summertime and wintertime thermal inertia at similarly low atmospheric dust opacity within the latitude belt ±50°, outside of the seasonal CO2 ice cover. On the basis of the results of such comparison we developed a new method for estimating and mapping the wintertime increase of the water ice within the surface soil layer corresponding to the daily thermal skin depth of 2–10 cm. We use both the nomograms of relation between the thermal inertia for dry and icy soil compiled for different water ice content and an analytical approach. Comparison of the mapped wintertime TES thermal inertia values with computed values of the parameter for icy soil at different ice amounts shows that the wintertime thermal inertia values in the latitude ranges 40°–50°N and 40°–50°S are consistent with the presence of the water ice amount in the soil from 4 up to 17 vol % (locally), whereas at lower latitudes the ice amount is mainly less than 1 vol %. Mapping results show that the zone with a soil water ice amount of &gt;3 vol % is much more in the northern hemisphere than in the southern one.

Albedo, Size, and Surface Characteristics of Hayabusa-2 Sample-Return Target 162173 1999 JU3 from AKARI and Subaru Observations

Abstract Mid-infrared observations of the Apollo-type asteroid 162173 1999 JU3 were performed in 2007 May during its close Earth approach with the AKARI space telescope and in 2007 August with the Subaru telescope. A thermophysical model analysis of the obtained data resulted in a radiometric diameter of 0.92$\pm$0.12km and a visual geometric albedo of 0.063$^{+0.020}_{-0.015}$. The geometric albedo is typical for C-type asteroids. The solutions for the thermal inertia of 1999 JU3's surface material were not as clear as for the asteroid 25143 Itokawa due to less-favorable observing conditions. But the most likely solutions are connected to high thermal inertia values, indicating that the surface is predominantly covered by boulders and bare rocks, while areas with thick dust regolith are less common.

Characterization and modeling of the dynamic behavior of a liquid–vapor phase change actuator

The dynamic behavior of a liquid–vapor phase change actuator has been investigated. A modular liquid–vapor phase change actuator was designed and fabricated. The actuator consists of a cavity filled with a two-phase fluid bounded on the bottom by a thin membrane into which heat is added and on the top by a cover slip which is displaced by the expansion of the vapor. A parametric study of geometric and operation parameters was conducted. A lumped parameter mathematical model of the actuator was developed to predict the dynamic behavior of the actuator. The model was validated against measured data. Based on the parameter study, actuators with smaller bubbles experienced higher heat loss. Thicker actuators were associated with higher thermal inertia. Thermal inertia, as well as heat losses, had a major effect on the dynamic behavior of the actuator. The model showed that faster response times and higher sensitivity can be achieved if the thermal inertia and heat loss coefficient values are reduced. Time constants less than 2.5 ms and displacement sensitivity higher than 30 μm/W are feasible.

A survey of Karin cluster asteroids with the Spitzer Space Telescope

The Karin cluster is one of the youngest known families of main-belt asteroids, dating back to a collisional event only 5.8 ± 0.2 Myr ago. Using the Spitzer Space Telescope we have photometrically sampled the thermal continua (3.5–22 μm) of 17 Karin cluster asteroids of different sizes, down to the smallest members discovered so far, in order to make the first direct measurements of their sizes and albedos and study the physical properties of their surfaces. Our targets are also amongst the smallest main-belt asteroids observed to date in the mid-infrared. The derived diameters range from 17.3 km for 832 Karin to 1.5 km for 75176, with typical uncertainties of 10%. The mean albedo is p v = 0.215 ± 0.015 , compared to 0.20 ± 0.07 for 832 Karin itself (for H = 11.2 ± 0.3 ), consistent with the view that the Karin asteroids are closely related physically as well as dynamically. The albedo distribution ( 0.12 ⩽ p v ⩽ 0.32 ) is consistent with the range associated with S-type asteroids but the variation from one object to another appears to be significant. Contrary to the case for near-Earth asteroids, our data show no evidence of an albedo dependence on size. However, the mean albedo is lower than expected for young, fresh “S-type” surfaces, suggesting that space weathering can darken main-belt asteroid surfaces on very short timescales. Our data are also suggestive of a connection between surface roughness and albedo, which may reflect rejuvenation of weathered surfaces by impact gardening. While the available data allow only estimates of lower limits for thermal inertia, we find no evidence for the relatively high values of thermal inertia reported for some similarly sized near-Earth asteroids. Our results constitute the first observational confirmation of the legitimacy of assumptions made in recent modeling of the formation of the Karin cluster via a single catastrophic collision 5.8 ± 0.2 Myr ago.

Regional differences in gully occurrence on Mars: A comparison between the Hale and Bond craters

The observation of gullies on Mars raised questions about the presence of liquid water in the recent past. In some regions like Hale and Bond crater, gullies occur in one crater (Hale) but do not in another crater nearby (Bond). These regional differences have been interpreted as an argument for a formation of the gullies related to groundwater. The formation of gullies on Earth depends on rainfall and/or melting of snow as well as on several parameters such as the presence of steep slopes and sufficient amounts of fines and debris. We investigated the Hale/Bond region for differences in crater wall morphology and texture, slopes, and thermal properties to determine whether the gully formation is dependent on factors such as steep slope angles and availability of fine-grained material. Morphologically there exist two kinds of gullies in the Hale crater: Gullies on the south- and east-facing crater slopes have a pristine appearance with deep channels eroded into the talus material and well-preserved aprons. Gully-like features on the north- and west-facing slopes are degraded and superposed by craters, indicating that they are old in comparison to the pristine ones. However, their formation process is unclear and might be due to debris flows, surface runoff or dry mass wasting processes or a combination of these processes. The crater walls of Bond do not show gullies. Their morphology is most likely consistent with a degraded mantle deposit. Slope measurements reveal that the gullies in Hale crater occur on slopes between ∼20° and ∼30° in contrast to the slopes without gullies in Bond that are between ∼10° and ∼20° steep. Mean thermal inertia values on slopes with younger gullies are ∼175 J m −2 K −1 s −1/2 corresponding to higher amounts of fine-grained material. At slopes with older gully-like features mean thermal inertia values are ∼315 J m −2 K −1 s −1/2 corresponding to higher amounts of bedrock or possibly indurated grain sizes. Mean thermal inertia values of the Bond crater walls are ∼230 J m −2 K −1 s −1/2 indicating more consolidated terrain possibly due to the cementation of the dissected mantle material. From our investigation we conclude that the occurrence of gullies in the Hale/Bond region most likely depends on the distribution of unconsolidated material and steep slopes. The regional and local gully distribution on Mars likely varies due to differences in topography and surface material properties. Their proposed clustered distribution on Mars is not an argument for a groundwater formation mechanism of the gullies.

Composition and thermal inertia of the Mawrth Vallis region of Mars from TES and THEMIS data

Clay mineral-bearing deposits previously discovered on Mars with near infrared ( λ = 0.3 – 5 μm ) remote sensing data are of major significance for understanding the aqueous history, geological evolution, and past habitability of Mars. In this study, we analyzed the thermal infrared ( λ = 6 – 35 μm ) surface properties of the most extensive phyllosilicate deposit on Mars: the Mawrth Vallis area. Clay mineral-bearing units, which in visible images appear to be relatively light-toned, layered bedrock, have thermal inertia values ranging from 150 to 460 J m −2 K −1 s −1/2. This suggests the deposits are composed of a mixture of rock with sand and dust at 100-meter scales. Dark-toned materials that mantle the clay-bearing surfaces have thermal inertia values ranging from 150 to 800, indicating variable degrees of rockiness or induration of this younger sedimentary or pyroclastic unit. Thermal Emission Spectrometer (TES) spectra of the light-toned rocks were analyzed with a number of techniques, but none of the results shows a large phyllosilicate component as has been detected in the same surfaces with near-infrared data. Instead, TES spectra of light-toned surfaces are best modeled by a combination of plagioclase feldspar, high-silica materials (similar to impure opaline silica or felsic glass), and zeolites. We propose three hypotheses for why the clay minerals are not apparent in thermal infrared data, including effects due to surface roughness, sub-pixel mixing of multiple surface temperatures, and low absolute mineral abundances combined with differences in spatial sampling between instruments. Zeolites modeled in TES spectra could be a previously unrecognized component of the alteration assemblage in the phyllosilicate-bearing rocks of the Mawrth Vallis area. TES spectral index mapping suggests that (Fe/Mg)-clays detected with near infrared data correspond to trioctahedral (Fe 2+) clay minerals rather than nontronite-like clays. The average mineralogy and geologic context of these complex, interbedded deposits suggests they are either aqueous sedimentary rocks, altered pyroclastic deposits, or a combination of both.

Thermal inertia of main belt asteroids smaller than 100 km from IRAS data

Recent works have shown that the thermal inertia of km-sized near-Earth asteroids (NEAs) is more than 2 orders of magnitude higher than that of main belt asteroids (MBAs) with sizes (diameters) between 200 and 1000 km. This confirms the idea that large MBAs, over hundreds millions of years, have developed a fine and thick thermally insulating regolith layer, responsible for the low values of their thermal inertia, whereas km-sized asteroids, having collisional lifetimes of only some millions years, have less regolith, and consequently a larger surface thermal inertia. Because it is believed that regolith on asteroids forms as a result of impact processes, a better knowledge of asteroid thermal inertia and its correlation with size, taxonomic type, and density can be used as an important constraint for modeling of impact processes on asteroids. However, our knowledge of asteroids’ thermal inertia values is still based on few data points with NEAs covering the size range 0.1–20 km and MBAs that > 100 km . Here, we use IRAS infrared measurements to estimate the thermal inertia values of MBAs with diameters < 100 km and known shapes and spin vector, filling an important size gap between the largest MBAs and the km-sized NEAs. An update to the inverse correlation between thermal inertia and diameter is presented. For some asteroids thermophysical modeling allowed us to discriminate between the two still possible spin vector solutions derived from optical lightcurve inversion. This is important for (720) Bohlinia: our preferred solution was predicted to be the correct one by Vokrouhlický et al. [2003. The vector alignments of asteroid spins by thermal torques. Nature 425, 147–151] just on theoretical grounds.

Spitzer Space Telescope observations of the nucleus of comet 67P/Churyumov-Gerasimenko

Context. Comet 67P/Churyumov-Gerasimenko is the target of the Rosetta mission, and an early characterization of its nucleus is required to assist in preparing the orbital strategy of the spacecraft, the delivery of the Philae surface module and the science operations. We detected the nucleus using the Hubble Space Telescope in March 2003, but had to assume an albedo to derive its size from its observed magnitudes. Aims. It is important to derive an additional constraint for independently determining both the comet size and albedo. Methods. We implemented the well-known “radiometric method”, which combines visible and infrared photometry. Sixteen thermal images of 67P/C-G were obtained by the Multiband Imaging Photometer (MIPS) 24 μ m channel of the Spitzer Space Telescope (SST) on 25 February 2004: the observations spanned a time interval of ~12.5 h, which sampled the rotational light curve of its nucleus. The comet was then outbound at a heliocentric distance of 4.48 AU, at a distance of 4.04 AU from SST, and at a solar phase angle of 12.1°. The nucleus conspicuously appeared as a bright point source superimposed on a dust trail; it was necessary to apply the point-spread function fitting technique using an adequate model of the trail to correctly determine the thermal flux from the nucleus. The data were analyzed using a standard thermal model that incorporated the thermal inertia. Results. Our preferred solution with a low thermal inertia has overall dimensions measured along the principal axis of 4.40–5.20 km, 4.16–4.30 km, and 3.40–3.50 km, corresponding to an effective radius of a sphere with the same volume in the range of 1.93–2.03 km. Larger values of thermal inertia produce larger sizes but the effective radius cannot exceed ~2.3 km. The albedo is in the range 0.039–0.043, remarkably consistent with the canonical value of 0.04 for cometary nuclei. The success of the landing of the Philae surface module remains critically dependent upon the bulk density of the nucleus: it would be safe if close to 0.35 g cm-3 , but a larger value, for instance 0.5 g cm-3 , would present some risks.

MGS‐TES thermal inertia study of the Arsia Mons Caldera

Temperatures of the Arsia Mons caldera floor and two nearby control areas were obtained by the Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES). These observations revealed that the Arsia Mons caldera floor exhibits thermal behavior different from the surrounding Tharsis region when compared with thermal models. Our technique compares modeled and observed data to determine best fit values of thermal inertia, layer depth, and albedo. Best fit modeled values are accurate in the two control regions, but those in the Arsia Mons' caldera are consistently either up to 15 K warmer than afternoon observations, or have albedo values that are more than two standard deviations higher than the observed mean. Models of both homogeneous and layered (such as dust over bedrock) cases were compared, with layered‐cases indicating a surface layer at least thick enough to insulate itself from diurnal effects of an underlying substrate material. Because best fit models of the caldera floor poorly match observations, it is likely that the caldera floor experiences some physical process not incorporated into our thermal model. Even on Mars, Arsia Mons is an extreme environment where CO2 condenses upon the caldera floor every night, diurnal temperatures range each day by a factor of nearly 2, and annual average atmospheric pressure is only around one millibar. Here, we explore several possibilities that may explain the poor modeled fits to caldera floor and conclude that temperature dependent thermal conductivity may cause thermal inertia to vary diurnally, and this effect may be exaggerated by presence of water‐ice clouds, which occur frequently above Arsia Mons.

Ignition of New Zealand Wood Products in the LIFT, RIFT and ISO 5657 Apparatus using the ASTM E 1321-97 Protocol

The results from ignition tests conducted using the Lateral Ignition and Flame spread Test (LIFT) apparatus, Reduced scale Ignition and Flame spread Test (RIFT) apparatus, and ISO 5657 ignitability apparatus from nine different types of New Zealand wood-based products are used to obtain the critical ignition heat flux and thermal inertia using the procedure specified in ASTM E 1321-97a. It is found that vertically oriented samples in the LIFT and RIFT has a higher minimum ignition heat flux compared with the horizontal samples in the ISO 5657 apparatus. There is only a limited agreement between the three test methods for thermal inertia values.

Coordinated analyses of orbital and Spirit Rover data to characterize surface materials on the cratered plains of Gusev Crater, Mars

Comparison of the Mars Exploration Rover Spirit's Pancam (0.4 to 1.0 μm) and Mars Express Observatoire pour la Mineralogie l'Eau, les Glaces et l'Activité (OMEGA) (0.4 to 2.5 μm) spectral reflectance data over Spirit's traverses shows that Gusev cratered plains are dominated by nanophase ferric‐oxide‐rich dust covering weakly altered basaltic sands. This interpretation is also consistent with both observations from OMEGA data covering plains beyond the traverse region and interpretations of data from the other payload instruments on the Spirit Rover. OMEGA observations of relatively low albedo regions where dust has presumably been stripped by dust devils show negative spectral reflectance slopes from 1.5 to 2.5 μm and moderately masked spectral features which are indicative of olivine or pyroxene. High‐albedo regions north and south of the Spirit landing site have flat spectral reflectance slopes and few spectral features, although all spectra have a nanophase ferric‐oxide absorption edge between 0.4 and 0.75 μm. Comparison of THEMIS‐derived thermal inertia values with OMEGA‐derived spectral parameters shows that although the dust cover can be optically thick (0.4 to 2.5 μm wavelength region) in some areas, it is not thick enough (∼1 cm) to mask the thermal inertia of the underlying substrate for areas included in this study.

Geologic characteristics of relatively high thermal inertia intracrater deposits in southwestern Margaritifer Terra, Mars

Twenty‐one craters in southwestern Margaritifer Terra exhibit unusually warm interior deposits in nighttime Thermal Emission Imaging System (THEMIS) infrared images. These deposits exhibit nighttime temperatures as high as 223 K and are 5–18° warmer than the surrounding plain. Thermal inertia values, derived from Thermal Emission Spectrometer (TES) data, are greater for the deposits than the plains, with maximum values between ∼455 and 675 J m−2 K−1 s−1/2. Analysis of THEMIS thermal inertia data having nearly an order of magnitude better spatial resolution shows that the deposits can have a thermal inertia as high as ∼1060 J m−2 K−1 s−1/2. Albedo and dust cover index values suggest that both the deposits and the surrounding region are generally dust‐free. Compositional analysis with TES and THEMIS data show that though a small number of the deposits may have isolated compositional differences, the majority of the deposits have a composition similar to that of the surrounding plains. The geomorphology of the craters as viewed from Mars Orbiter Camera (MOC) images shows that the deposits are coherent units, rather than sand deposits, an observation that is consistent with the relatively high thermal inertia values. If these deposits are coherent rock units, as the results of our study suggest, possible methods of formation include emplacement of primary igneous material or lithification of sediments from surrounding terrain.

Submillimeter lightcurves of Vesta

Thermal lightcurves of Asteroid Vesta with significant amplitude have been observed at 870 μm (345 GHz) using the MPIfR 19-channel bolometer of the Heinrich–Hertz Submillimeter Telescope. Shape and albedo are not sufficient to explain the magnitude of this variation, which we relate to global variations in thermal inertia and/or other thermophysical parameters. Vesta's lightcurve has been observed over several epochs with the same general shape. However, there are some changes in morphology that may in part be related to viewing geometry and/or asteroid season. Inconsistent night-to-night variations exhibit the inherent difficulties in photometry at this wavelength. We are able to match the observed brightness temperatures with a relatively simple thermal model that integrates beneath the surface and assumes reasonable values of thermal inertia, loss tangent and refractive index, and without having to assume low values of emissivity in the submillimeter. High flux portions of the submillimeter lightcurve are found to correspond to regions with weak mafic bands observed in Hubble Space Telescope images.

Thermophysical properties of the Isidis basin, Mars

We investigated the thermal properties of the Isidis basin in order to understand the high values (&gt;450 J m−2 K−1 s−1/2) located in the southern region of the basin. Thermal inertia data were compared to a variety of complementary data sets, including radar, visible, and results from mesoscale atmospheric simulations. We considered four mechanisms for creating the high thermal inertia, including (1) the thinning of a dust mantle, (2) the presence of unconsolidated, coarse‐grained material, (3) high rock abundance, and (4) a high degree of induration. Induration is the scenario most consistent with the data, although we cannot rule out unconsolidated materials and it is likely that rocks contribute to values of thermal inertia to a lesser degree. We also investigated three mechanisms for controlling the geographical distribution of the high thermal inertia values, including (1) the influence of topography, (2) the role of surface morphology, and (3) present aeolian processes. Topography plays a significant role along the southern boundary of the basin but not within the basin itself. THEMIS data show a complex relationship between the thermal inertia and morphology. The wind patterns modeled by the Mars Regional Atmospheric Modeling System (MRAMS) are not fully consistent with the wind directions implied by streaks in the thermal data but are consistent with a second group of streaks observed in the visible data. This suggests that small‐scale (tens of kilometers) streaks observed in the thermal data did not form under present‐day, nominal winds.

Thermal behavior of horizontally mixed surfaces on Mars

Current methods for deriving thermal inertia from spacecraft observations of planetary brightness temperature generally assume that surface properties are uniform for any given observation or co-located set of observations. As a result of this assumption and the nonlinear relationship between temperature and thermal inertia, sub-pixel horizontal heterogeneity may yield different apparent thermal inertia at different times of day or seasons. We examine the effects of horizontal heterogeneity on Mars by modeling the thermal behavior of various idealized mixed surfaces containing differing proportions of either dust, sand, duricrust, and rock or slope facets at different angles and azimuths. Latitudinal effects on mixed-surface thermal behavior are also investigated. We find large (several 100 J m −2 K −1 s −1/2) diurnal and seasonal variations in apparent thermal inertia even for small (∼10%) admixtures of materials with moderately contrasting thermal properties or slope angles. Together with similar results for layered surfaces [Mellon, M.T., Putzig, N.E., 2007. Lunar Planet. Sci. XXXVIII. Abstract 2184], this work shows that the effects of heterogeneity on the thermal behavior of the martian surface are substantial and may be expected to result in large variations in apparent thermal inertia as derived from spacecraft instruments. While our results caution against the over-interpretation of thermal inertia taken from median or average maps or derived from single temperature measurements, they also suggest the possibility of using a suite of apparent thermal inertia values derived from single observations over a range of times of day and seasons to constrain the heterogeneity of the martian surface.

High‐resolution thermal inertia derived from the Thermal Emission Imaging System (THEMIS): Thermal model and applications

Thermal inertia values at 100 m per pixel are determined using nighttime temperature data from the Thermal Emission Imaging System (THEMIS) on the Mars Odyssey spacecraft, producing the highest‐resolution thermal inertia data set to date. THEMIS thermal inertia values have an overall accuracy of ∼20%, a precision of 10–15%, and are consistent with both Thermal Emission Spectrometer orbital and Miniature Thermal Emission Spectrometer surface thermal inertia values. This data set enables the improved quantification of fine‐scale surface details observed in high‐resolution visible images. In the Tharsis region, surface textures and crater rims observed in visible images have no corresponding variation in the THEMIS thermal inertia images, indicating that the dust mantle is pervasive at THEMIS scales and is a minimum of a few centimeters and up to 1–2 m thick. The thermal inertia of bed form material indicates particle diameters expected for aeolian sediments, and these materials are likely currently saltating. Variations in the thermal inertia within interior layered deposits in Hebes Chasma can be distinguished, and the thermal inertia is too low to be consistent with bedrock or a lava flow. Thus a secondary emplacement of volcanic material or a volcanic ash deposit is a more likely method of formation. Higher‐resolution THEMIS thermal inertia enables the identification of exposed bedrock on the Martian surface. In Nili Patera and Ares Vallis, bedrock material corresponds to distinct compositional and morphologic surfaces, indicating that a specific unit is exposed and is likely currently being kept free of unconsolidated material by aeolian processes.

Possibility of mapping physiographic zones by thermal survey from space

Physiographic zones of the Earth are “major units of the geographic (landscape) sphere, which give way to each other in a regular and definite order depending mainly on the ratio of heat and moisture” [1]. The rise in the Earth’s annual mean temperature recorded for the instrumental observation period [2] may provoke variations in the global distribution of the heat/moisture ratio and, as a consequence, in displacement of the position and configuration of physiographic zones. The variations may be accompanied by negative socioeconomic impacts. To minimize the negative consequences, it is expedient to carry out a timely forecasting based on data of the space monitoring of physiographic zones. Such monitoring requires analysis of a giant body of data. Therefore, it is necessary to elaborate automatic methods for the mapping of these zones by using satellite-borne survey. Currently, several quantitative criteria (indices) have been suggested to identify physiographic zones, e.g., Budyko’s radiational aridity index [3], which characterizes the ratio between the annual radiation balance and annual precipitation. The space version of this method is limited by the need for obtaining continuous yearlong series of observations over elements of the radiation balance of the day surface and atmospheric precipitation. Cloudiness prevents the recording of such continuous series. Therefore, we have studied the possibility of applying another method for monitoring the position of physiographic zones, namely mapping of the thermal inertia of the Earth’s surface based on a satellite-borne thermal survey data [4‐11]. 1 1 Thermal inertia p is the resistance of a surface to heating (or cooling) under the influence of a periodic heat source. p = ( λ · c · ρ ) 1/2 , J/(m 2 · s 1/2 · K) = 1 TIU, where λ is the thermal conductivity factor, W/(m · K); c is specific heat, J/(kg · K), and ρ is density, kg/m 3 . The method we applied for the remote mapping of thermal inertia (TI); daily mean evaporation rate (ER) from the Earth’s surface, mm/day; and heat flux (HF), W/m 2 involved the mathematical model of the daily variation of the Earth’s surface temperature (ESTs) [10], which took into account basic factors affecting the EST formation. The model has the following assumptions: the metrological conditions and concentration of optically active gases in the atmosphere within the whole area under study are identical; emittance, albedo of the Earth, and TI do not vary during the whole period of the satellite-borne thermal survey. To determine TI and ER, we carried out the thermal multispectral space survey for 3‐10 days under stable meteorological conditions in the absence of rainfall in order to characterize the daily EST dynamics more completely. Moreover, mapping of TI and ER was based on the following parameters of routine expedited meteorological observations over parameters involved in the mathematical EST model: total solar radiation; air temperature, moisture, and wind velocity at a height of 2 m above the surface; atmospheric pressure; and cloudiness. To solve the inverse problem, we performed mathematical EST modeling for all possible combinations of TI, ER, HF, and albedo, i.e., the “library” of ideal ESTs. Then, the measured EST values were compared to ideal values. The TI and ER values sought were deduced from the assigned fitting criterion of measured and ideal EST values. Algorithm errors of solving the inverse problem are given in the table.