Although there is plenty of evidence of water on Mars in the ancient past, now there is none.

Data from a Mars Global Surveyor science duo -- a magnetometer and electron reflectometer (MAG/ER) -- revealed why this might be. The two instruments confirmed that the Red Planet has no internally generated magnetic field. Without one, MAG/ER principal investigator Mario Acuña said, the solar wind would have blown every bit of water off the planet quite quickly.

"The solar wind, at a million miles per hour, has direct access to Mars," Acuña said. "It erodes the surface, atom by atom, ion by ion. It can pick up atoms and particles and blows them away."

Even more insightful perhaps, is that MAG/ER found magnetically charged rocks and crust in the ridges of the Southern Hemisphere. These rocks are four billion years old, proving that Mars did once have an internally generated magnetic field.

This finding tells Acuña and other scientists that a major geologic event, only 300 million years after the formation of Mars, destroyed its magnetic field and provoked the loss of its water seas.

Mars Global Surveyor's primary mission ended in January, but scientists hope its data and discoveries will be improved upon by Odyssey's suite of higher resolution instruments.

And the earlier probe will not be abandoned any time soon as engineers plan to use its radio transmitter to beam data Earthward from two twin rovers set to arrive on Mars in 2004.

The profusion of data delivered by Mars Global Surveyor makes it impossible to pinpoint its most important discovery, Albee and others agree.

‘We have to say the greatest thing has been the incredible success of the mission," Albee said, "and that was to have a whole set of global data to have for many, many years."