Mars Global Surveyor

Thermal Emission Spectrometer

An Overview of TES Polar Observations to Date

H.H. Kieffer, T. Titus, K. Mullins, P.R. Christensen

Second International Conference on Mars Polar Science and Exploration
August 21-25, 2000

Poster Paper # 4104

The Thermal Emission Spectrometer (TES) has made observations of the Martian polar regions over the last three years. These observations are a combination of low resolutions scans (aerobraking observations of the south pole) and high resolution ``noodles'' (aerobraking observations of the north pole and all mapping phase observations). This review summarizes important results to date [1,2], which include:

Both polar caps are mostly dark ice (not frost) prior to exposure to solar insolation. Figure 1 shows the brightening of the seasonal cap after sunrise.
The asymmetric recession of the south polar cap is dominated by albedo variations, especially the Cryptic region, which remains a dark slab of CO₂ throughout its sublimation. [Figures 2a - 2d]
Seasonal cap appearance is largely determined by frost grain size. The geographic patterns repeat each year. [Figures 2a - 3a]
Cold spots observed during the northern winter are a spectral-emissivity effect mainly due to surface accumulation of fine-grained frost or snow; their kinetic temperatures are not exceptional, as indicated by the 18 um brightness temperatures. The surface temperatures vary as expected from the variations of topography as measured with MOLA. [Figures 4a - 4e]
Cold spots are concentrated near topographic features, eg. craters, chasma, and slopes of the perennial cap. [Figure 5]
Mapping data has constrained the characteristic time scales of cold spot formation and dissipation during the polar night; both are a few days. [Figures 6a - 6c]

References:

[1] Kieffer et al., Jour. Geophys. Res., 105, 9653-9699, 2000.

[2] Titus et al., submitted to Jour. Geophys. Res., 2000.

[3] Smith et al., Science, 279, 1686--1692, 1998.

[4] Barlow, N.G., Icarus, 75, 285--305, 1988.

Figure 1

Lambert Albedo vs. Latitude. The color indicates the season, where purple is Ls=180° and black is Ls=190°. Shortly after sunrise, a rapid brightening of the seasonal cap is seen over a period of 20 sols.

Figure 2a:

South Polar Crocus Date based on 1999-2000 mapping data. The cap recedes quickly through across the cryptic region (outlined in yellow in the lower right quadrant). The latitude lines are spaces 10° apart and 0° longitude is up.

Figure 2b

IRTM Albedo from 1977. The Ls is 222°. The dark feature inside the seasonal cap is the cryptic region.

Figure 2c:

TES Albedo data acquired in 1997. The Ls is 221°.

Figure 2d

TES Albedo data acquired in 1999. The Ls is 221°.

Figure 3aAlbedo vs T₁₈-T₂₅ for Ls=218° -222°. This plot is a 2-dimensional histogram showing the correlation between T₁₈ -T₂₅ and albedo. If one assumes that the dust abundance in the CO₂ is relatively constant, then this correlation is due to variations in the CO₂ grain sizes, where dark CO₂ is coarse-grained (or ice) and bright CO₂ is finer-grained.

Figure 4a

Location and spectra of four observed cold spots. The cold spots are labeled A (Ls 253°, rev 89), B (Ls=261°, rev 102), C (Ls=306°, rev 222) and D (Ls =233°, rev 233). (a) The width of the TES track is exaggerated here. T₁₈-T₂₅ along tracks of four revs plotted on MOLA elevation data [3]. The latitude lines are 80° N and 70° N. The zero degree longitude line is down, increasing clockwise. (b) The cold spot , at Ls= 261°. The latitude line is 70° N and the longitude line is 210° W. North is the lower left corner. (c) Mean spectra from each of the four cold spots: A (black), B (red), C (green), and D (blue).}

Figure 4b

Cold spots at Ls=253° (Rev 89). The blue data are T₁₈, the brightness temperature near 18 um, which is a good indicator of the surface kinetic temperature. The black data are T₂₅, the brightness temperature in the middle of the 25 um CO₂ transparency band, where the emissivity is largely affected by CO₂ grain size. The strongest cold spot in this figure is Cold Spot A in Figure 4a.

Figure 4c

Cold spots at Ls=261° (Rev 102). The color scheme is the same as Figure 4b. The strongest cold spot in this figure is Cold Spot B in Figure 4a

Figure 4d

Cold spots at Ls=306° (Rev 222). The color scheme is the same as Figure 4b. The strongest cold spot in this figure is Cold Spot C in Figure 4a. This cold spot is mostly the result of recent mid-altitude atmospheric condensation. This data track also shows that cold spots are very abundant during late winter.

Figure 4e

Cold spots at Ls=309° (Rev 233). The color scheme is the same as Figure 4b. The strongest cold spot in this figure is Cold Spot D in Figure 4a

Figure 5

North Polar Cold Spot Spatial Distribution. Only cold spots where [T₁₈-T₂₅] > 15° are are shown. (See color bar) The data was acquired over the Ls ranges 183-218° , 278-317°, and 11-13°. The latitude lines are 10° apart. 0° longitude is down. The craters are from the Barlow [4] crater inventory.

Figure 6a

The Current South Polar Ring Distribution of Cold Spots: The latitude range is 87° S to 87.2° S. The seasonal range of the data is Ls=21°~ through Ls=35°. Regions in the polar ring with warm colors have a higher frequency of cold spots than the cool colors. The Cold Spot Index is T₁₈-T₂₅, where warm colors (yellow, orange, and red) are cold spots.

Figure 6b

South Polar Ring Distribution of Cold Spots: The latitude range is 87° S to 87.2° S and the seasonal range is Ls=21°~ through Ls=35°. It is clear that certain longitudes have a higher frequency of cold spots (warm colors) than other longitudes (cool colors).

Figure 6c

Southern Polar Cold Spot Lifetime. This cold spot is located at latitude -87°, longitude 18.2° W. The peak occurs at Ls = 23.77°. This cold spot appears in a few sols and decays at a slower rate, but has a half-life of approximately 4 or 5 sols.

Questions should be addressed to Tim Titus
This page last updated Mar 21, 2001.

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