NASA's Mars Global Surveyor operated in orbit around Mars for nine years and 52 days, longer than any other spacecraft to Mars
and long enough to complete three extensions of its original two-year mission.
A few of the mission's many important discoveries about Mars include:
- Before-and-after images revealing bright new deposits in two gullies on Mars suggest liquid water has carried sediment down them in the past 7 years. These pictures from the spacecraft's camera are the strongest evidence to date that water still flows occasionally on the surface of Mars, in brief episodes.
- The mineral-mapping infrared spectrometer found concentrations of a mineral that often forms under wet conditions, fine-grained hematite. This discovery led to selection of a hematite-rich region as the landing site for NASA's Mars Exploration Rover Opportunity.
- Laser altimeter measurements have produced an unprecedented global topographic map of Mars. The instrument revealed a multitude of highly eroded or buried craters too subtle for previous observation, and mapped canyons within the polar ice caps.
- The magnetometer found localized remnant magnetic fields, indicating that Mars once had a global magnetic field like Earth's, shielding the surface from deadly cosmic rays.
- Before-and-after images identified 20 new impact craters in 2006 that had not been present seven years earlier. These observations allowed calibrating calculations widely used for estimating the ages of planetary surfaces from crater densities.
- The camera found a fan-shaped area of interweaving, curved ridges interpreted as evidence of an ancient river delta resulting from persistent flow of water over an extended period in the planet's ancient past.
- A long life also allowed Global Surveyor to track changes through repeated annual cycles. For three Martian summers in a row, deposits of carbon-dioxide ice near Mars' South Pole shrunk from the previous year's size, suggesting a climate change in progress.