IN DEPTH: Mars Rover Closes in on Crater

Sept. 22, 2006 — It's been a long, slow haul, with many stops to explore and a few unplanned breaks due to equipment glitches, but a robotic Mars scout is about to reach paydirt.

The place is called Victoria Crater, and like most of Mars doesn't look like much at first blush: a big hole in the ground, about a half-mile wide and 230 feet or so deep.

But scientists catching a glimpse of Victoria from the Mars Global Surveyor, an orbiting reconnaissance spacecraft, have seen enough to devote the rover, named Opportunity, to what will likely be its final resting place.

"This has been the destination ever since Opportunity left Endurance Crater more than 18 months ago," said NASA's Guy Webster, a spokesman at the Jet Propulsion Laboratory in Pasadena, Calif., which manages the program.

Victoria's walls are layered with exposed rock, a stack of geologic history measuring more than 100 feet thick.

Opportunity Lands

By deciphering rock and soil content and structure at Opportunity's initial landing spot, Eagle Crater, scientists came up with the first direct evidence of a shallow, ancient ocean on Mars.

The search for water has been NASA's holy grail in its ongoing quest to learn if life ever arose on Mars.

One of the biggest questions left unanswered by the findings at Eagle was if the water had been around long enough for life to form. Scientists had hoped for more answers at the rover's next stop, Endurance Crater, where it spent nine months exploring a stack of exposed rocks 23 feet thick.

From there, Opportunity set out for Victoria, stopping along the way to study rocks and soils which together told a story of shallow lakes, shifting dunes and variable levels of groundwater — all very acidic.

It is at Victoria that scientists have their best shot, at least during this ground foray of Mars, to fill in some of the missing history.

"Eagle Crater is about the size of your garage. It's just a tiny little hole in the ground, and yet at the same time we were very excited to see layers of rock that were about a foot thick," said rover science team member John Grotzinger, with the California Institute of Technology in Pasadena.

The next stop, Endurance Crater, was about the size of baseball park, Grotzinger explained, with rock layers several feet thick.

All Eyes on Victoria

With Victoria, "we're talking about something that's really big," he said. "When we get to the side of the crater, it will be difficult to see the other side, it's so big, but what we're going to see are impressive cliffs of rock that are possibly 100 feet thick — much larger than anything we've seen previously," he said.

"This is going to give us a much better chance to understand the early wet history of Mars," Grotzinger said.

In particular, researchers hope to learn if Victoria's rocks were deposited differently than those studied by Opportunity so far. They also want to know if there was ever a time when the water was less acidic, and perhaps more suitable for habitation.

"We are very interested in whether the rocks continue to show evidence for having been formed in shallow lakes," said Ray Arvidson of Washington University in St. Louis, the deputy principal investigator for Opportunity and its twin, Spirit.

The two roving geology stations landed on Mars more than 2 1/2 years ago for what were expected to be 90-day surveys. Instead, the rovers, which are located on opposite sides of Mars' equator, have been operating for more than 10 times longer than that.

"We can't say how long the rovers will last, but we will push to get the best possible science out of these national treasures as long as they keep operating," said rover project manager John Callas, at NASA's Jet Propulsion Laboratory in Pasadena, Calif.

Two Robots Better Than One

Opportunity's arrival at Victoria coincides with the beginning of science operations by a new, sharp-eyed, orbiting mapper called Mars Reconnaissance Orbiter (MRO).

The probe last week reached its final orbital perch after six months of delicate maneuvering to dip into the thin Martian atmosphere and shed speed. Now the low point of the spacecraft's orbit will stay about 155 miles above the planet's south pole, with an apogee, or orbital high point, at about 196 miles above the north pole.

From there, MRO will use its sophisticated suite of imagers and ground-penetrating radar to look for past and present signs of water, as well as landing sites for future Mars rovers and science laboratories.

Science operations are scheduled to begin in November, following a thorough instrument checkout. Victoria Crater will be high on the priority list of targets.

"By combining the data from Opportunity and Mars Reconnaissance Orbiter, we'll be able to do some fantastic coordinated analysis," Arvidson said.

Added Callas, "Victoria could very well be the most productive and exciting science of the entire mission."