The Mars rover Spirit has been stuck in sand for some 18 months, but that hasn't stopped JPL scientists from making new discoveries.
Analysis of soil layers that the rover exposed while trying to break free of a Martian sand trap has turned up new evidence that liquid water once existed on the Red Planet.
Just below the planet's dusty surface, water-soluble minerals were found layered below less-dissolvable minerals, leading the rover team to hypothesize that the water-soluble minerals were dissolved and then deposited by water percolating down from melting ice or snow.
"We see this kind of gradient of change in these trace elements as you go down into the soil over the few tens of centimeters we had to look at," said Bruce Banerdt, the JPL-based project scientist for the Mars Exploration Rover Project.
The evidence of liquid water that scientists have previously detected on Mars rocks could date back to as far as several billion years ago. These current findings, however, would place water on the planet much more recently — on the geologic timescale, at least.
"It couldn't have been too long ago, because it's all in looser dirt, like the kind you would kick around in your backyard. And you wouldn't expect loose dirt to be lying there undisturbed for millions of years," Banerdt said.
There is no evidence that liquid water would exist on Mars today because temperatures there seldom remain above freezing for long and the planet's thin atmosphere would cause liquid water to instantly vaporize.
Tilts of the planet's axis that occur at intervals of tens and 100s of thousands of years do, however, warm up parts of the planet's surface occasionally.
"We have a rough relative chronology consistent with wet, warm conditions early in Mars history, then the planet becoming very dry and cold, but then we also know of variations on the current climate where there are possible conditions [to support water in liquid form] as the axis of the planet wobbles around," said Banerdt. "Our hypothesis is that tens to hundreds of thousands of years ago, the climate on Mars could have been a little warmer and support precipitation or condensation on the surface that was stable enough to percolate into these soil layers."
Perhaps even more dazzling than this potential rewrite of Mars' history is the unexpected lifespan of the rovers Spirit and Opportunity.
The twin rovers were designed for a mission of only three months, which they completed in April 2004, but have kept going for far longer than NASA engineers predicted.
"By the time a year had passed, the expectation was the rovers' solar panels would be covered with dust" and therefore unable to draw power, said Banerdt. "With spring and summer wind and dust devils, dirt accumulates but then we wake up one day and it's like someone came and cleaned them off."
Operational all 687 days of the Martian year, Opportunity is slowly making its way to an ancient crater about five miles from its current location along the planet's equator.
Spirit, however, has remained stationary on the other side of the planet and at 15 degrees below the equator since April 2009, when its left wheels became stuck in soft sand. The rover was unable to reposition its solar panels for the low sunlight of Mars' winter, and on March 22 powered down into a hibernation mode.
"All rover systems are turned off, including the radio and survival heaters," JPL's John Callas, project manager for the rovers, said in a recent statement by NASA. "All available solar array energy goes into charging the batteries and keeping the mission clock running."
Since the rover shut down, scientists have been analyzing data collected about the soil exposed during attempts to free Spirit, which by that time had use of only four of its six wheels.
Over the Martian winter, Spirit has likely been exposed to potentially damaging temperatures as cold as 150 (Fahrenheit) degrees below zero, but Banerdt remains hopeful that Spirit will be able to "wake herself up" and communicate again come next month's start of Martian spring.
Although Spirit was able to move about a foot after being trapped in sand, Banerdt said there is little chance the rover will be able to break free. If the rover remains stationary, it could be use to track the wobbly rotation of Mars with enough precision to calculate the mass and density of the planet's core.