Thermal and visible studies of Mars using the Termoskan data set

Mars, Phobos '88, Phobos 2, Termoskan, thermal infrared, ejecta, Phobos shadow, channels, valleys

Betts, Bruce Harold ; Murray, Bruce C.

University:California Institute of Technology

Department:Geological and Planetary Sciences

关键词: Mars, Phobos '88, Phobos 2, Termoskan, thermal infrared, ejecta, Phobos shadow, channels, valleys;   

Others  :  https://thesis.library.caltech.edu/7472/1/Betts%20%20%201994.pdf

美国|英语

来源: Caltech THESIS

【 摘 要 】

In February and March, 1989, the Termoskan instrument on board the Phobos '88spacecraft of the USSR acquired the highest spatial resolution thermal data ever obtainedfor Mars, ranging in resolution from 300 m to 3 km per pixel. It simultaneously obtainedbroad band visible channel data. The panoramas cover a large portion of the equatorialregion from 30°S to 6°N. New and unique analyses facilitated by Termoskan arepresented here. In addition, this thesis describes the instrument, data, and validation.Termoskan thermal data shows good temperature agreement with Viking IRTM.However, conversion of Termoskan visible data to bolometric albedo is problematic.

Utilizing the Termoskan data, I recognized a new feature on Mars: ejecta blanketdistinct in the thermal infrared (EDITH). Virtually all of the more than 100 such featuresdiscovered in the Termoskan data are located on the plains near Valles Marineris. Icompiled a data base of 110 EDITH and non-EDITH craters ranging in diameter from 4.2km to 90.6 km. EDITHs have a startlingly clear dependence upon terrains of Hesperianage, and show almost no other correlations within the data base. The Hesperian terraindependence cannot be explained by either atmospheric or impactor variations. Windpatterns or locally available aeolian material cannot provide a single overall explanation forthe observed variations. I postulate that most of the observed EDITHs are due toexcavation of thermally distinctive Noachian age material from beneath a relatively thinlayer of younger, more consolidated Hesperian volcanic material. The plausibility of thistheory is supported by much geological evidence for relatively thin near-surface Hesperiandeposits overlying massive Noachian megabreccias on the EDITH-rich plains units. Isuggest that absence of thermally distinct ejecta blankets on Noachian and Amazonianterrains is due to absences of distinctive near-surface layering. Thermally distinct ejecta blankets are excellent locations for future landers and remote sensing because of relatively dust free surface exposures of material excavated from depth.

Also included in the thermal images are observations of several major channel andvalley systems including significant portions of Shalbatana, Ravi, Al-Qahira, and Ma'adimValles, the channel south of Hydraotes Chaos, channel material in Eos Chasma, and smallportions Simud, Tiu, and Ares Valles and channel material in Gangis Chasma.Simultaneous broad band visible data exists for all but Ma'adim Vallis. I find that most ofthe channels and valleys have higher inertias than their surroundings, consistent withprevious thermal studies of martian channels. I show for the first time that thermal inertiaboundaries closely match all flat channel floor boundaries. Using Viking albedos,Termoskan temperatures, and thermal modelling, I derive lower bounds on typical channelthermal inertias ranging from 8.4 to 12.5 (10^(-3) cal cm^(-2) s^(-1/2) K^(-1). Lower bounds oninertia differences with the surrounding heavily cratered plains range from 1.1 to 3.5.Atmospheric and geometric effects are not sufficient to cause the inertia enhancements. Iagree with previous researchers that localized, dark, high inertia areas within channels arelikely aeolian in nature. However, thermal homogeneity and strong correlation of thermalboundaries with the channel floor boundaries lead me to favor non-aeolian overallexplanations. Small scale aeolian deposition or aeolian deflation may, however, play somerole in the inertia enhancement Channel floor inertia enhancements are stronglyassociated with channels showing fretted morphologies such as wide, flat floors and steepscalloped walls. Therefore, I favor fretting processes over catastrophic flooding forexplaining the inertia enhancements. Fretting may have emplaced more blocks on channelfloors or caused increased bonding of fines due to increased availability of water.Alternatively, post-channel formation water that may have been preferentially present dueto the low, flat fretted floors may have enhanced bonding of original fines or dust falloutThe coupling of both EDITHs and channel inertias to morphology is unlike most sharpMartian inertia variations which are decoupled from observed surface morphology.

Termoskan observed morning limb brightening in the thermal channel, but not inthe visible channel. The thermal morning limb brightening is likely due to a water ice ordust haze that is warmer than the surface at the time of the observations. A water ice hazewith a scale height of 5 km could match the observations. Visible scattering is observed tobe significant on morning and evening limbs out to 60 or 70 km. Localized high altitudestratospheric clouds are observed in the visible channel.

The Termoskan data show that the highland-lowland boundary in the AeolisQuadrangle appears strongly correlated with a high-low thermal inertia boundary. Thesharpness of that boundary varies from less than 4 km to more than 50 km. In all cases,inertias continue to decrease gradually for many tens of km into the lowlands. Severalother large scale thermal boundaries are also observed in the data.

Termoskan observed fine thermal structure on the flanks of Arsia Mons andelsewhere, which represent examples of interesting and significant thermal variations seenat the limit of Termoskan's spatial resolution. Sharp variations and boundaries imply therecannot be global scale dust blanketing deeper than about one centimeter, if that.

Termoskan obtained the first ever thermal images of Phobos' shadow on thesurface of Mars, along with simultaneous visible images. The best observed shadowoccurrence was on the flanks of Arsia Mons. For this occurrence, I combined theobserved decrease in visible illumination of the surface with the observed decrease inbrightness temperature to calculate thermal inertias of the Martian surface. Most of thederived inertias fall within the range 0.9 to 1.4, corresponding to 5 to 10 micron dustparticles for a homogeneous surface. Dust at the surface is consistent with previoustheories of Tharsis as a current area of dust deposition. Shadow derived inertias aresensitive to mm depths, whereas diurnally derived inertias are sensitive to cm depths. Theshadow derived inertias are very similar to Haberle and Jakosky [1991] atmosphericallycorrected Palluconi and Kieffer [1981] Viking IRTM diurnally derived inertias. Thus, ifnear surface layering exists at all in this region, it is not very significant.