Follow the Water

In late 1997, Mars Global Surveyor (MGS) arrived, and for a while--until Pathfinder’s batteries died--two US missions were operating together on and over Mars. MGS continues to this day: though orbiting high above the planet, it has returned amazing images and information about the history of water on Mars.

One of the instruments on MGS is an Orbiting Laser Altimeter, "MOLA" for short. By sending down beams of light, and timing how long they take to bounce off the surface and back to the spacecraft, MOLA reveals fine detail of the topography, or surface elevation, of the planet. In false color views it’s clear the Northern hemisphere is low and flat: still more detailed views show channels running out of Vallis Marineris into the northern lowlands. Some researchers believe the incredible flatness of this region--the flattest plains in all the solar system--means that perhaps we’re looking at the bottom of a vast and ancient ocean, with the smooth surface resulting from long periods of sedimentation.

Also on board MGS is an instrument rather like Mars Odyssey’s THEMIS, called TES for "Thermal Emission Spectrometer." TES is mapping the chemical composition of the Martian surface. In some places it has seen an abundance of hematite, a mineral which down here on Earth is associated with water. (In fact the twin 2003 Rover missions, MER, is considering a landing site known as "Hematite" to land and explore this area in detail.)

Odyssey’s THEMIS will have much greater resolution than TES and the hope is to see still more detail of such regions. Odyssey also carries a Gamma Ray Spectrometer, or "GRS": GRS can detect the elements and chemistry of the surface rocks, and even see down one meter below the surface, looking for evidence of hydrogen, one of the prime constituents of water.