Thermal inertias in the upper millimeters of the Martian surface derived using Phobos' shadow

Abstract

Abstract. The first thermal images of Phobos' shadow on the surface of Mars, in addition to simultaneous visible images, were obtained by the Phobos '88 Termoskan instrument. The best observed shadow occurrence was on the flanks of Arsia Mons. For this occurrence, we combined the observed decrease in visible illumination of the surface with the observed decrease in brightness temperature to calculate thermal inertias of the Martian surface. The most realistic of our three models of eclipse cooling improves upon our preliminary model by including nonisothermal initial conditions and downward atmospheric flux. Most of our derived inertias fall within the range 38 to 59 J m ': s 4n K 4 (0.9 to 1.4 10 -3 cal cm ': s 4a K4), corresponding to dust-sized particles (for a homogeneous surface), consistent with previous theories of Tharsis as a current area of dust deposition. Viking infrared thermal mapper (IRTM) inertias are diurnally derived and are sensitive to centimeter depths, whereas the shadow-derived inertias sample the upper tenths of a millimeter of the surface. The shadowderived inertias are lower than those derived from Viking IRTM measurements (84 to 147), however, uncertainties in both sets of derived inertias make conclusions about layering tenuous. Thus, near-surface millimeter versus centimeter layering may exist in this region, but if it does, it is likely not very significant. Both eclipse and diurnal inertias appear to increase near the eastern end of the shadow occurrence. We also analyzed a shadow occurrence near the crater Herschel that showed no observed cooling. This analysis was limited by cool morning temperatures and instrument sensitivity, but yielded a lower bound of 80 on eclipse inertias in that region. Based upon our results, we strongly recommend future spacecraft thermal observations of Phobos' shadow, and suggest that they will be most useful if they improve upon Termoskan's geographic and temporal coverage and its accuracy.

The Nature of M-Class Asteroids from 3-μm Observations

“…We used six different thermophysical models for each asteroid, five of which differed only in thermal inertia (P), with values of P equal to 0.0006, 0.002, 0.01, 0.05, and 0.4 cal s −1/2 cm −2 K −1 , corresponding to the values for 1 Ceres, the Moon (and Mercury, Phobos, and Mars), loose pumice, basalt, and iron, respectively (Rees 1990, Betts et al 1995, Chase et al 1976, Ksanfomality et al 1991, Morrison 1976). The final thermophysical model had the same thermal inertia as the iron model, but it had an emissivity ( ) equal to 0.6, rather than 0.9 as with the other model.…”

Spectroscopic Identification of Carbonate Minerals in the Martian Dust

“…Most moderate-to high-albedo regions on Mars have a thin (less than a few millimeters) layer of particles with diameters Յ10 m (16,21,22), probably due to airfall dust. The 0-to 10-m particle size fraction of the laboratory magnesite-labradorite mixture closely matches this estimate.…”

High-Resolution Thermal Inertia Mapping from the Mars Global Surveyor Thermal Emission Spectrometer

“…While the geographic coverage is limited, their results indicate a great deal of variability of the surface at this smaller scale. Betts et al (1995) reported the thermal inertia of the south flank of Arsia Mons derived from the TER-MOSCAN temperature measurements in shadow of the moon Phobos. Most recently, Jakosky et al (2000) reported comparisons of a Viking-based thermal inertia map with thermal inertia maps derived from the MGS Thermal Emission Spectrometer aerobraking and science-phasing mission data at 2…”