The Mars Orbiter Laser Altimeter (MOLA) is the mapping instrument on the Mars Global Surveyor spacecraft in orbit around Mars. The need to note the date when discussing topography of Mars provides an indication of the rate at which this topic is changing. MOLA’s two year mission officially began March 1999. A high resolution map representing 27 million elevation measurements has been assembled into a global map. Already MOLA’s map is making history. “We now have a definitive picture of the shape of the whole planet.”¹ These new maps are enabling planetary geologists to settle issues of Martian geology that have been contested for 25 years. While Mars is 100 million km away from Earth, we now know Mars at the planet level better than we know parts of Earth. MOLA returns Martian topography to an accuracy of 13 meters, but some parts of Earth are know only to 100 meters.

We now know the landscape relief of Mars to be a range in elevation of 30 km, the largest of the terrestrial planets. On Earth relief is only 20 km.

The Tharsis volcanic region is now seen as two zones. Olympus Mons appears to have grown independently to the west of the larger Tharsis dome. “This argues

for a broader mantle heat source for Tharsis than was previously thought.”² Magnetic stripes recently detected on Mars’s surface and the implications of possible plate tectonics, suggests that an internal, heat-driven process shaped Mars’s spectacular topography.

The southern hemisphere is dominated by the Hellas impact basin. An asteroid strike excavated the basin 2,300 km across and about 9 km deep. The basin is surrounded by a ring of material rising 2 km above regional average height and reaching out 4,000 km from center. MOLA’s map shows that material removed by the Hellas impact accounts for a significant amount of the high topography in the southern hemisphere. In the image of Hellas below, the black line corresponds to the zero elevation contour. Dark orange represents elevations 3.5 km above zero. The dark purple corresponds to -8 km below zero.

The process that produced the low northern hemisphere occurred early in Martian history. The north-south elevation difference must have dominated the transport of water on Mars throughout its history. Both Martian polar ice caps are predominantly water. The south cap also has a carbon dioxide ice component. MOLA data indicates that the northern ice cap is larger than the southern. Evidence suggests a major portion of the south polar ice cap may be buried beneath a mantle of dust deposits. The MOLA team has estimated a maximum ice volume of the two poles at 4.7 million km^3, (1.5 times the volume of the Greenland ice cap). This volume is one-third less than that estimated to have been necessary to cause water-influenced topographical features. The implication is that much of Mars’s water has either escaped to space or is currently stored beneath the surface regolith.

The new data will serve as a reference of geologic history and how water has flowed across the planet’s surface during the past four billion years. MOLA data reveals that for much of Mars’ history, three major closed basins stored water or ice. The low northern hemisphere is most likely the site of the early Martian ocean. The northern ocean watershed serves as an area of drainage for three-fourths of the planet’s surface. Hellas, the second basin, is far smaller in area than the northern plains basin, but it’s great depth, gives it a volume that is almost equal. Crater Argyre has a small volume, compared with Hellas, but has a watershed that is as large.

There is global south to north drainage indicated by the direction of outflow channels and valley networks. Many valley networks formed in response to subsurface effects rather than drainage. Some of the Valles Marineris canyons have been considered source areas of outflow channels. MOLA data has enabled accurate calculation of floor slopes and their relation to the adjacent Chryse outflow channels. Data shows that water in Valles would flow eastward from the height of land resulting from the eastern flank of Tharsis until it reaches the Coprates Chasma (purple color in image below). Corprates, at about 300^oE, is roughly 11 km deep. Continuing east the Valles floor rises gently for the next 1500 km. Water flow to the northeast outflow channels would be blocked by Corprates Chasma unless depth was sufficient to overcome this relief of about 1 km.
Tectonics such as rift deepening or tilting as well as sediment in-filling, could explain the east to west rise in floor gradients. These events could have post-dated the outflow channels’ formation.

MOLA will continue collecting 864,000 elevation measurements a day. Researchers are now signing up to use the data for topics ranging from locating ancient water reservoirs to the selection of spacecraft landing sites. “I just can’t wait until people have the opportunity to use this map.”²



QUOTATIONS
1 Smith, David, MOLA co-investigator, Goddard Space Flight Center
2 Zuber, Maria, MOLA co-investigator, Massachusetts Institute of
Technology

REFERENCES
“The Global Topography of Mars and Implications for Surface Evolution,” David Smith, Maria Zuber, Science, May 28, 1999, pp. 1495-1501.

“A New Look at the Martian Landscape,” Bernice Wuethrich, Science, May 28, 1999, pp. 1441-1442.

BACKGROUND RESOURCES
MOLA Science Investigation (http://ltpwww.gsfc.nasa.gov/tharsis/mola.html)

CREDITS
Scientists - Stephanie Stockman - GSFC
Teachers - Dave Hixson, Bruce Taliaferro