The gift that keeps on giving ... everything from Opportunity over the past eight years has been a bonus.
WHILE many obsessed over speculation that NASA's newest Mars rover, Curiosity, had dug up signs of life – it had not – it is the agency's older, smaller jalopy, Opportunity, that has been exploring a more intriguing plot of Martian real estate.
"This is our first glimpse ever at conditions on ancient Mars that clearly show us a chemistry that would have been suitable for life," said Steven Squyres, the principal investigator for Opportunity, said at a news conference last week at a meeting of the American Geophysical Union in San Francisco.
Opportunity could be sitting on rocks chock-full of organic molecules – but the rover and the scientists back on Earth would never know. Unlike Curiosity, Opportunity is not carrying instruments that can detect those kinds of molecules. But the scientists are not complaining. Everything from Opportunity over the past eight years has been a bonus for a mission that was to have ended long ago.
Opportunity landed on Mars in January 2004, for what was supposed to be a three-month mission. Yet the rover continues operating in good condition. (Its twin, Spirit, died in 2010, stuck in a sand-trap and unable to point its solar arrays in the correct direction to survive winter, outliving its planned lifetime by almost six years.)
Last year, Opportunity arrived at a 23-kilometre wide crater named Endeavour, where NASA's Mars Reconnaissance Orbiter has spotted clays from above. Clays generally form in the presence of water.
The clay signal pointed to a hill, which the scientists named after Jacob Matijevic, an engineer on the rover team who died this year.
As Opportunity approached, "We started seeing things that looked really, really different," Squyres said. The most common rock there was light-coloured, fine-grained, very soft, and nothing like any that Opportunity had come across before.
"It is right in the sweet spot of where the clay signature is present," Squyres said. "It has got to be the clay-bearing stuff."
But when the rover looked at the elements in the rock, it was the same mix of elements in a typical Martian rock. "What's unusual is that it's not unusual," Squyres said. "This puzzled us at first. I was expecting something dramatic and instead what you see here kind of looks like average Mars."
Another instrument could have identified minerals in the rock, but the radioactive cobalt it relies on has long decayed away.
If Curiosity were at the Endeavour crater, its instruments could directly look for the carbon-based molecules known as organics that are the building blocks of life. But Curiosity, which landed in August, is more than 8000 kilometres away exploring a different crater where clays have also been spotted from orbit – and it is still months away from reaching there.
In its nearly nine years on Mars, Opportunity has driven more than 30 kilometres, crossing a Martian plain and stopping by several smaller craters. On its travels it came across minerals that pointed to flowing water in Mars' past, but these minerals formed in highly acidic conditions.
"Battery acid kind of numbers," Squyres said. "And that's a challenging place for life."
Clays only form in more benign conditions.
"The thing that's different here is that these clay minerals point towards a neutral chemistry – water you can drink," Squyres said. "And that's a different story, a different world."
These rocks appear to date to the early warm and wet era of Mars, perhaps when the planet was more hospitable to life.
Then Opportunity took a look at another outcrop on Matijevic Hill made of darker, harder minerals. Close up, it saw tiny spheres embedded in the rocks, very similar in appearance to iron-rich spheres nicknamed "blueberries" that the rover had observed earlier.
Except these did not contain much iron.
"These are something totally different," Squyres said. "I've been calling them newberries, because there's something new."
Even though Opportunity cannot find organics, Squyres said there were many questions that it would be able to answer that would tell scientists what this part of early Mars was like. What are the newberries made of? How did they form? Did wind or water shape the rocks of Matijevic Hill?
Opportunity will spend several months at Matijevic Hill to try to unravel as many of these puzzles as it can, using its remaining instruments.
"What we have stumbled upon here at Matijevic Hill, drawn here by that clay signature, is what's turning out to be one of the most delightful geologic puzzles that we have ever found with this rover on Mars," Squyres said. "It's fascinating. It's a work in progress."
The New York Times