Far left: This Mars Global Surveyor image of an anonymous crater wall near 38.7 degrees south latitude, 263.3 degrees west longitude in the Centauri Montes region was taken on August 30, 1999.
Left: Another image of the same spot taken on September 10, 2005 shows a fresh, bright deposit whose downslope end branches out like fingers of water would around obstacles. If the flow was, in fact, water, it would amount to some five to 10 swimming pools’ worth, says Edgett.

Mars Global Surveyor—Lost, But Not Forgotten

On November 21, NASA announced that Mars Global Surveyor’s operating career was likely over. The news came almost three weeks after the last signal was received from the 10-year-old spacecraft, better known as MGS. One possibility is that the spacecraft lost the power to communicate because it could no longer pivot its solar panel to collect enough sunlight to recharge its batteries. Efforts are still under way to regain contact by taking photos of the spacecraft from the Mars Reconnaissance Orbiter. Knowledge of the detailed orientation of MGS may permit JPL to regain radio contact and reestablish control of the spacecraft. Arden Albee, MGS project scientist, former chief scientist at JPL, and Caltech professor of geology and planetary science, emeritus, says the odds are against recovering it, but that it will endure for many years in its orbit at 400 kilometers above the surface of Mars.

Our story really begins with the loss of the Mars Observer in 1993, a year after launch, as it entered Mars orbit. MGS in a sense rose from the ashes of the Observer, as it was assembled quickly from spare Observer parts. The Surveyor was half the size and mass of its forebear, but carried much of the same equipment—narrow- and wide-angle cameras, a thermal-emission spectrometer, magnetometers with an electron reflectometer, a laser altimeter, and a radio system with an ultrastable oscillator.

MGS was launched two days after the 1996 presidential election. Ten months later it pulled into an elliptical orbit around Mars. While pioneering the technique called aerobraking, in which the spacecraft would dip into and out of the Mars atmosphere repeatedly in order to slow down and reach a circular orbit, a solar-panel hinge was damaged. This incident meant the spacecraft needed to brake more slowly to reduce pressure on the panel and avoid further damage, and may have contributed to MGS’s ultimate loss. MGS remained in an elliptical orbit, decelerating slowly, for one Mars year (two Earth years).

This delay yielded unexpected bonuses. It had long been thought that Mars had at best a very weak magnetic field, suggesting that, unlike Earth, Mars did not have an actively convecting nickel-iron core. The eventual circular orbit would measure such a field, if it existed. But the elliptical orbit dipped under Mars’s ionosphere and revealed remnant magnetism in the oldest parts of the crust, suggesting that early in its history Mars had an internal dynamo resembling that of our own planet.

MGS also gathered detailed information about the Martian atmosphere during its delayed descent. The circular orbit that MGS was meant to enter was aligned so that the local surface time below the spacecraft was always 2 p.m. (2 a.m. on the night side), a compromise between the optimal lighting times for camera photography and spectral imagery. Rather than the 2 a.m./2 p.m. measurements MGS would have been restricted to in a circular orbit, the local time changed continuously in the elliptical orbit. During the prolonged aerobraking process, the Martian atmosphere was determined to vary greatly with altitude, and this information guided MGS—as well as later spacecraft—during entry into its ultimate orbiting altitude of 378 kilometers above Mars’s surface.

The MGS photos, which number over 240,000, have provided exciting insights into the Martian surface, suggesting a past in which water flowed through gullies and ancient river deltas. The discovery of the water-associated mineral hematite near the Martian equator guided the selection of the landing site for the Mars Exploration Rover Opportunity. Atmospheric measurements allowed MGS to report the “weather” to other incoming spacecraft. Repeated observations and measurements over five Martian years have revealed the changing surface of a planet nearly 60 million kilometers away—for a while we even had better global topographic coverage of Mars than we had of Earth. “Surveyor changed the planet into a known object,” Albee says. “Second-grade kids read about Mars as if it were Earth because of the information that came from Surveyor.”

We now know that weather systems blow from west to east on Mars just as they do on Earth, and that Mars has a winter during which it snows dry ice at the poles, followed by a summer during which the ice retreats. In its final days, MGS cemented its fame as a comparison of new and old photos showed a fresh, gully-like feature in the side of a formerly smooth crater. The lack of topographic relief of the gully suggests it arose from recently flowing water rather than a landslide. According to Michael Malin (PhD ’76), president of Malin Space Science Systems, the sediments deposited along the gully were diverted around obstacles and ended in finger-like branches, just as would happen to water-laid sediments on Earth. Ken Edgett, a Malin staff scientist, was quoted in the Los Angeles Times as saying, “You have all heard of a smoking gun; this is a squirting gun.” The possibility of liquid water on Mars has boosted the hopes of many who believe life does exist on other planets.

MGS reached many milestones in its lengthy career. The first came after 28 days of data reception provided the first systematic global portrait of Mars. It satisfied all its mission objectives after one Mars year in orbit. Measured by these standards, MGS has far outlived the dreams of the space scientists who designed it and sent it on its way. And now, 10 years later, during which the spacecraft and all its instruments operated nonstop for 24 hours a day, seven days a week, MGS has finally entered a deep sleep, perhaps someday to be reawakened by a radio “kiss” from Earth. —EN

Meanwhile, on the Martian surface, Spirit is on the move again as the days of spring lengthen. The rover had spent the winter strategically parked on a low ridge in order to get maximum solar power for its instruments, which performed a thorough study of its surroundings. Many mysteries remain, however, including the nature of the light material lying just beneath the surface that was exposed by the rover’s wheels (below) en route to its winter quarters.
And on the opposite side of the planet, JPL controllers are looking for a route to get Opportunity, Spirit’s twin, to the bottom of Victoria Crater.