HR: 0830h
AN: P21B-0046    [PDF]
TI: Change of Martian surface height associated with polar cold spots
AU: * Ford, P G
EM: pgf@space.mit.edu
AF: MIT Center for Space Research, 77 Massachusetts Avenue, Cambridge, MA 02139-4307 United States
AU: Pettengill, G H
EM: ghp@space.mit.edu
AF: MIT Center for Space Research, 77 Massachusetts Avenue, Cambridge, MA 02139-4307 United States
AU: Pettengill, G H
EM: ghp@space.mit.edu
AF: MIT Dept of Earth, Atmospheric, and Planetary Sciences, 77 Massachusetts Avenue, Cambridge, MA 02139-4307 United States
AB: For the past 30 years, orbiting microwave radiometers have observed anomalously low emission temperatures during Martian polar winters. While the physical surface temperature cannot drop significantly below 148K---the point at which CO$_2$ starts to condense---radiometric temperatures of 110K or lower at 25$\mu$ wavelength are commonly found in isolated ``cold spots'' throughout both northern and southern polar winters. These form roughly circular patches, tens to hundreds of km in diameter, and persist for no more than a few days. Three models have been proposed to account for them: (a) an atmospheric effect that accompanies CO$_2$ snowfall; (b) fresh surface deposits of CO$_2$ snow; or (c) a change in the properties of CO$_2$ slab ice. Following the success of Smith \emph{et al.}$^1$ in using the MOLA laser altimeter aboard Mars Global Surveyor to directly measure the growth of the winter polar caps, we have applied the same technique to ask whether cold spots are accompanied by a sudden change in surface height. To identify the cold spots, we first examined all polar observations made by the TES radiometer that was co-boresited with MOLA, and made gridded images of $\partial T / \partial \lambda$, the derivative of the brightness temperature \emph{wrt} wavelength, $20\mu \le \lambda \le 25\mu$, over $5^\circ$ ranges in $L_s$. A total of 169 cold spots were readily located in the 3 winters (one northern, two southern) during which MOLA operated, and their times of first appearance were noted. We then examined the individual MOLA tracks that crossed these regions from 30 days before, to 30 days after, the cold spot appearances. Three sets of crossing points were assembled: (a) both pairs of tracks were made before the cold spot appearance, (b) both after the appearance, and (c) one before and the other after. For each crossing point, the surface height was interpolated from the 3 nearest altimeter footprints in each of the two tracks. The difference between the resulting pair of heights was averaged over all crossing points, and the RMS variance of the height differences was used as a measure of the statistical error in the measurement. Preliminary results show small height differences \emph{before} the appearance of a cold spot, as expected, but no abrupt jump in surface height immediately \emph{after} one appears. This suggests that the cold spots are not formed by deep ($>50$cm) CO$_2$ snow deposits, but it cannot help us decide between the alternatives of CO$_2$ snowfall or a change in slab ice properties. $^1$ Smith, Zuber, and Neumann, \emph{Science}, \underline{294}, 2141-2146, 2001.
DE: 5416 Glaciation
DE: 5462 Polar regions
DE: 5470 Surface materials and properties
DE: 6225 Mars
SC: Planetary Sciences [P]
MN: 2003 Fall Meeting