November/December 1997

More Discoveries from the Red Planet

by Damond Benningfield

Just four days after it entered orbit around Mars, the Mars Global Surveyor spacecraft beamed its first major discovery back to Earth: Mars is surrounded by a weak magnetic field. Although the present-day magnetic field is weak -- about one-tenth of one percent of the strength of Earth's -- it may have been stronger in the past. If so, it might have shielded the Martian surface from radiation, increasing the odds that life evolved on Mars.

Earth, Jupiter, and Saturn generate magnetic fields through the motions of their molten-metal cores. The cores are good conductors of electricity, and as the planets spin, the metallic cores generate electrical currents that produce magnetic fields. These magnetic fields deflect electrically charged particles from the Sun and cosmic rays from outside our solar system.

Sojourner analyzed the rock named "Chimp" in late September. Although it looks like a boulder, the rock is actually only a few inches tall. It's covered by a dark crust in some areas, a lighter surface in others. (NASA)

Global Surveyor's discovery suggests that Mars once generated a magnetic field in the same way. A stronger magnetic field could have protected nascent life on the planet. As Mars cooled, though, the field weakened. The magnetic field that Global Surveyor measured was a "fossil" of the earlier field; rocks at the surface became magnetized, and retain some of the planet's original field.

Several Russian probes had found hints of a magnetic field around Mars, but none of their findings could be confirmed. Global Surveyor carries more sensitive instruments, which discovered the magnetic field on September 15, just four days after the craft entered a highly elliptical orbit around Mars.

During its first month in orbit, Global Surveyor dipped into the upper fringes of the Martian atmosphere to lower its orbit. But engineers suspended these "aerobraking" maneuvers in mid-October when one of Global Surveyor's solar panels bent farther than expected.

The panel, which converts sunlight to electricity, failed to lock in position when it was deployed after the craft's launch. Engineers hoped the aerobraking maneuvers would force the panel into position, where latches would lock it in place. The panel moved into its fully deployed position, but it failed to lock into place, so it bent even farther. Project managers feared the panel and other parts of the spacecraft could be damaged by repeated maneuvers, so they suspended aerobraking to study the situation.

The delay could change Global Surveyor's final orbit. The craft will orbit Mars from pole to pole. Under the original mission plans, during each dayside pass, Global Surveyor would view the surface at 2 p.m. local time. That would provide consistent lighting for every picture, allowing scientists to accurately measure changes over time. Engineers are considering new orbits that will allow the craft to view Mars at different times of day.

If the problem can be corrected, Global Surveyor will begin mapping the Martian surface in March. Its camera -- a duplicate of the camera on the failed Mars Observer mission -- will provide the sharpest images of Mars ever recorded. Additional instruments will look for "hot spots" that could indicate the presence of liquid water below the surface.

Troubles on the Surface, Too

In the meantime, Global Surveyor's cousin, Mars Pathfinder, has troubles of its own. Scientists have not received new pictures or other data from the lander since September. Pathfinder's battery may have died, leaving it unable to track time and execute its instructions. Pathfinder's solar cells would continue to produce electricity during the day, but without a working battery, the craft would completely shut down at night. Ground stations on Earth received signals from Pathfinder on October 7, but engineers have been unable to fully diagnose the problem and work out a plan to allow Pathfinder to resume its work.

Pathfinder completed its 30-day primary mission and moved into an extended mission that could allow it to analyze more rocks and monitor the weather as the seasons change in the northern hemisphere.

Batteries on the Sojourner rover also died, but it is able to operate on solar power during the day. However, it communicates with Earth through the lander, so it is waiting for new instructions before continuing its explorations.

Sojourner has found two major types of rocks at Pathfinder's landing site in Ares Vallis, an ancient floodplain. One, which includes rocks such as Barnacle Bill and Shark, is rich in silica -- an indication that the rocks were remelted, like volcanic rock on Earth. These rocks, combined with Global Surveyor's magnetic field discovery, hint that Mars may have been more geologically active in the distant past than scientists had expected. The second type, like Yogi, Wedge, and Half Dome, are simpler rocks with a higher sulfur content.

If Sojourner is able to resume its journeys, future rock analyses may be less accurate than those conducted before its battery died because they will take place during the daytime. The instrument that "sniffs" the composition of Martian rocks is most efficient at colder nighttime temperatures.

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