North Pole Cold Spots
In the 1970s, spacecraft observations of the polar regions of Mars revealed polar brightness temperatures that were significantly below the expected kinetic temperatures for CO2 sublimation. For the past few decades, we have speculated as to the nature of these Martian polar cold spots. Are the cold spots surface or atmospheric effects? Do the cold spots behave as blackbodies, or do they have emissivities less than unity? Two developments have allowed us to answer these questions: (1) the measurement of the optical constants of CO2 by Gary Hansen (1997) and (2) direct thermal spectroscopy of the north polar cap by MGS TES (Kieffer et al., 1998).
With a few possible exceptions, cold spots are surface effects. The CO2 frost in cold regions of the polar cap show a strong absorption feature at 25 microns that is indicative of fine-grained CO2, thus explaining the low brightness temperatures observed by the Viking IRTM. Brightness temperatures at 18 microns are usually consistent with expected kinetic surface temperatures. In many cases, the brightness temperatures at 15 microns reveals an atmosphere that is too warm for CO2 condensation to occur.
Cold spot formation is strongly dependent on topography, forming preferentially near craters and on slopes of the perennial cap. While cold spots are surface effects, the formation of the fine-grained CO2 deposits is not entirely restricted to surface formation. TES data, combined with MOLA cloud data (Ivanov and Muhleman, 1999), suggest that at least a few of these cold spots were formed from atmospheric condensates.
Another major component to the north polar cap composition is slab CO2 ice. Slab ice has near unity spectral emissivity (Kieffer et al.,1999;Hansen, 1998) and appears to have a low albedo. Two explanations for the low albedo are that the slab ice is intrinsically dark or the slab ice is transparent and we are seeing through to the underlying substrate. Regions of the cap where T18-T25 < 5 degrees have slab ice. Slab ice is the dominant endmember of the polar cap at latitudes south of the polar night.
TES Observations of the South Pole
The Mars Polar Lander arrived at Mars on December 3, 1999. TES analysis of recent data from the mapping phase demonstrates that the spacecraft landing site was bare ground, free of -128° C (-200° F) dry ice that completely covered this region during the winter. The image to the left shows the 2pm and 2am temperatures of data within the landing site on December, 2, 1999. The plus sign marks the landing site. The thick white line shows the location of the polar layered deposits. Temperatures are given in Celsius. The temperature of CO2 frost (dry ice) on Mars is 145K (-128° C), approximately -200° F. Temperatures above 200K show the absence of CO2 frost. These temperatures were calculated from thermal radiation at a wavelength of 30µm.
The recession of the south polar cap has been observed telescopically and from spacecraft in both the visible and thermal regions. Although a simple cap-edge versus time plot has commonly been used, without regard as to the longitude of measurement, Mariner 9, Viking, and HST observations clearly show that the retreating edge is irregular and asymmetric.
The data used in this analysis is from the Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES). Repeated TES coverage over the period Ls 185 through Ls 270 acquired much of the cap recession.
During this period of time, TES was taking data in the array normal spin (ANS) mode, scanning acrossed the planet as the satellite made one complete roll every 100 minutes. Therefore, the data was irregularly sampled in both space and time. Also, because of the changes in the spacecraft orbit, the spatial resolution of the data is variable, ranging from 25 to 125 kilometers.
We have constructed a map of the south polar region that contains the date when the last CO2 sublimates, hereafter called the crocus date. The crocus date is based on sliding a representative temperature - versus - time curve along the observations for each location in the polar region and selecting the season of maximum temperature change.
Recessions in the classic area “ Mountains of Mitchell ” are delayed significantly, disappearing approximately at Ls 260. High resolution (26 Km) brightness temperature data at Ls 244 confirms that solid CO2 is the dominant cold component.
One region (approx. 72-80 S, 180-250 W) within the annual polar cap became dark long before the temperatures begin to rise; in comparison with most areas that showed a gradual increase in brightness until a rapid darkening as the temperature rose well above CO2 frost value. This dark region, here after called the Cryptic region, appears to be a major contributor to the asymmetric polar recession. The cause of the Cyrptic region's unexpected behavior is currently under study.
South Pole Cryptic Region
The early part of the Mars Global Surveyor mission provided good TES coverage of the Mars south polar region. These data allow mapping of the polar cap recession, surface and atmospheric temperatures, and albedo features found within the seasonal cap itself (Kieffer et al, 1998, Titus et al, 1998).
During the period observed, the seasonal south polar cap retreated continuously and asymmetrically around the geographic pole, much the way Viking observed in 1976-1977 (Kieffer et al., 1977). One of the most dominant albedo features on the seasonal cap is a region that appears almost as dark as bare ground, but yet remains cold. (See Figure 1.) We refer to this region, generally located between latitudes 85° S and 75° S and longitudes 150° W and 310° W, as the Cryptic region.
A re-examination of the IRTM data revealed that the Cryptic region was not unique to the TES era, but also was quite appearant during the Viking era. (See Figure 2.) Interesting enough, Antoniadi (Blunck, 1977) observed dark regions forming on the season cap that loosely correlates to the Cryptic region: Depressio Magna (1909) and Depressio Parva (1929). These depressios were located at 270° W, 78° S and 166° W, 76° S, respectively.
Analysis of both the TES and IRTM data indicate that the Cryptic region is unique in its thermophysical properties relative to the rest of the cap. The region is a repeatable event that occupies the same general area from year to year. It is darker and slightly warmer than the rest of the south polar cap. Even though the Cryptic region is slightly warmer, it must still be CO2 buffered since it remains “ cold ” for several days.
Spectral analysis of the TES longward of the 15 micron atmospheric band shows that the Cryptic region shows less spectral than the rest of the polar cap. This suggests that the region may be composed of “ ice ”, as opposed to snow or frost (Hansen, 1998). Further spectral analysis is on going.