One of the first three-dimensional views of the north pole of Mars shows a polar ice cap cut by deep troughs. Mapped by the Mars Orbiter Laser Altimeter (MOLA), this topographic view was assembled from approximately 2.6 million laser pulse measurements taken during periods since September 1997 when the Mars Global Surveyor was within 800 km of the surface, the limit of the instrument's range. The spacecraft has been dipping into the atmosphere to reduce its elliptical orbit and achieved a circular orbit of 400 km in February.
The MOLA consists of a diode-pumped, Q-switched Nd:YAG laser emitting 45-mJ pulses at 1.064 µm, with a pulse-repetition rate of 10 Hz. The receiver is a 50-cm-diameter parabolic mirror and silicon avalanche photodiode detector. The laser spot on the planet's surface is about 100 m in diameter, depending on the orbiter's altitude, and the individual spots are spaced about 300 m apart. With the spacecraft in a circular orbit, the vertical accuracy of the measurements should be within 5 to 10 m. The vertical resolution of the instrument is 30 cm. "We're hoping to be able to actually see the change in the ice cap," said NASA's David E. Smith, principal investigator for MOLA. The depth of the cap thickens by a few tens of centimeters in the winter—just on the edge of instrument noise—when carbon dioxide frost condenses from the atmosphere.
Studying the orbiter's data, NASA scientists have already determined that the cap is mostly water ice mixed with rock and dust. It has an estimated volume of 1.2 million km3, less than half that of the ice cap in Greenland and about 4% of the Antarctic ice sheet. That's too small, by about a factor of 10, for scientists who were hoping the ice sheet might explain where the water in a presumed prehistoric Martian ocean went.
Now in the desired orbit, the MOLA will spend two years mapping the whole planet. One of its first targets will be the southern polar region, allowing NASA scientists to pick an appropriate site to set down the Mars lander launched in January and scheduled to descend in December 1999. Running constantly for two years will test the limits of the MOLA. "Will this diode-pumped laser last for 7, 8 million shots nonstop?" wonders Smith. "We'll be interested to see. It's the first time anything like this has been done."