Since July 1997, the orbital spacecraft known as Mars Global Surveyor has been transmitting an array of revealing images that are helping scientists to unravel the mysteries of Mars' early history, its interior processes, and its atmosphere. The spacecraft has captured the evolution of a martian dust storm (for the first time ever) as well as images of deeply layered terrain and highly magnetized crustal features.

The first set of formal results comes from data obtained in October and November 1997, when the spacecraft was just beginning to use the drag of Mars' upper atmosphere to lower and circularize its highly elliptical orbit in a process called aerobraking. At the time, a dust storm was brewing on Mars and had grown to about the size of the South Atlantic Ocean.

High-resolution images of dunes, sandsheets, and drifts are helping to reveal earlier chapters of martian history. Landforms shaped by erosion appear to be common, and dust spilling down the slopes of these ridges has invited comparisons to the snow-covered Rocky Mountains of Colorado.

The martian crust also exhibits more layering at greater depths than was expected. The steep walls of canyons, valleys, and craters show the martian crust to be stratified at scales of a few tens of yards. "At this point we simply do not know whether these layers represent piles of volcanic flows or sedimentary rocks that might have formed in a standing body of water," said mission scientist Arden Albee of the California Institute of Technology.

In March, Surveyor ceased aerobraking and began its summer-long session of observations from an interim elliptical orbit. During these passes, Surveyor will attempt to take images of several features of public interest, including the Mars Pathfinder and Viking mission landing sites. In early April, Surveyor snapped new images of the Cydonia region, site of the controversial feature known as the "Face on Mars."

"Most scientists believe that everything we've seen on Mars is of natural origin," said Carl Pilcher, acting science director for Solar System Exploration in NASA's Office of Space Science. "However, we also believe it is appropriate to seek to resolve speculation about features in the Cydonia region by obtaining images when it is possible to do so."

Capturing images from the two landing sites may prove more difficult, since the locations of the sites are not precisely known and because the landers are so small. The spacecraft is scheduled to resume aerobraking in September and continue braking until March 1999, when it will settle into a final, circular orbit for its prime mapping mission.

(Right) Extensive wind-swept plains of the
Medusae Fossae formation on Mars.
This southern subframe image, frame
33104, is of a 3.0 × 4.7-kilometer
area centered near 2.0 degrees north,
163.8 degrees west.

(Left) The first image shows the full processed swath of the Cydonia region viewed by Mars Global Surveyor at 14:02:17 UT (10:02:17: AM EDT) on April 14, 1998, on orbit 239. The second and third images show the top and bottom of the swath.
(Below) Layered features in Valles Marineris — Most remarkable about this Mars Orbiter Camera image is the discovery of light and dark layers in the rock outcrops of the canyon walls. In the notable, triangular mountain face (at center), some 80 layers, typically alternating in brightness and varying in thickness from 5 to 50 meters (16 to 160 feet), are clearly visible. This sheer mountain cliff, over 1000 meters (3200 feet) tall, is only one of several outcrops that, together, indicate layering almost the entire depth of the canyon.

This type of bedrock layering has never been seen before in Valles Marineris. It calls into question common views about the upper crust of Mars (for example, that there is a deep layer of rubble underlying most of the martian surface), and argues for a much more complex early history for the planet.

Go to top of page     Back to Contents