More Discoveries from the Red Planet
(From the November/December 1997 issue of StarDate magazine)
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 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 liquid 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.
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. Scientists aren't sure whether Mars continues to generate a magnetic field through this process, or whether the spacecraft found a "fossil" magnetic field in the Martian crust.
Several Russian probes had detected 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.
Durings its first four months in orbit, Global Surveyor will slowly dip into the upper fringes of the Martian atmosphere to change its orbit. By mid-January, it should settle into a circular orbit that allows it to scan the entire planet.
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.
In the meantime, Global Surveyor's cousin, Mars Pathfinder, has completed its 30-day primary mission and moved into an extended mission that will allow it to analyze more rocks and monitor the weather as the seasons change in the northern hemisphere.
Batteries on the Sojourner rover have died, so it can operate only during the day, when its solar cells can convert sunlight into electricity.
Sojourner has found two major types of rocks at Pathfinder's landing site in Ares Vallis, an ancient floodplain. One type, 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.
Future rock analyses may be less accurate than those conducted before Sojourner's battery died because they will take place during the daytime, when temperatures are much warmer. The instrument that "sniffs" the composition of Martian rocks is most efficient at colder nighttime temperatures. -- — Damond Benningfield
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