The Mars Orbiter Laser Altimeter payload on the Global Surveyor spacecraft has generated the first comprehensive three-dimensional map of the martian north pole. The altimeter's 50-cm-diameter telescope gathered more than 2.5 million laser pulses reflected off the surface of Mars over 206 orbits. In the Dec. 11, 1998, issue of Science, mission scientists reported that the surface profile reconstructed from the altimeter data shows an icecap 3 km thick, containing about 1.5 million cubic kilometers of ice, half the volume of the Greenland icecap.This three-dimensional reconstruction of the martian north polar icecap shows fissures and chasms that have no analog in Earth's icecaps. The altimetry data were gathered with the Mars Orbiter Laser Altimeter (right) on the Mars Global Surveyor. Photos courtesy of NASA Goddard Space Flight Center.The altimeter makes surface- height measurements by measuring the time of flight for a pulse from its 50-mJ Q-switched 1.06-µm Nd:YAG laser. The 3.5-cm output beam gives a surface spot size between 70 and 330 m, depending on altitude and pointing angle. The instrument's telescope routes the reflected light to a silicon avalanche photodiode, which is electronically gated to select the range window. The time-of-flight measurements are correlated with orbital position to produce a topographical map with 0.6-km spatial resolution and 5- to 30-m vertical accuracy. "When this instrument was conceived in 1988 it was new technology," said David E. Smith, a project scientist for the altimeter. "It's no longer new, but it is providing the first precision altimetry of any planet." At 10-m global vertical accuracy, the altimeter has begun to answer some interesting questions. For example, the long-term cycling -- the history -- of water on Mars is unknown. One proposal suggested the hydrostatic pressure of a large polar icecap would be sufficient to drive groundwater from beneath the poles to the equator, which could have a significant effect on martian climate. The altimeter data show that the current icecap is not large enough to drive that process. The altimeter has examined other martian features as well, such as impact craters and clouds, but range restrictions have limited the study to the Northern Hemisphere. When the Global Surveyor's areobraking is complete this month, the circular orbit will allow the altimeter to generate an accurate map of the full martian topography using more than 700 million laser pulses.