It's official: The next Mars rover will explore a 5-kilometre-high mountain of sediment inside a crater called Gale, NASA announced today.
The Curiosity rover is scheduled to launch between 25 November and 18 December. It will examine Martian rocks and soil to learn about the history of the planet's climate and to look for chemical traces of life.
There has been vigorous debateMovie Camera about where to send the rover. At a meeting in May, competing camps made their final arguments for each of the four sites then on the shortlist.
"These are all like different flavours of ice cream – all fantastic but slightly different," said John Grant of the National Air and Space Museum in Washington DC, one of the organisers of the May meeting, at a NASA press conference today.
In recent weeks, NASA said it had narrowed that list to two craters: Gale and Eberswalde.
Eberswalde contains a beautifully preserved fan of sediment from an ancient river delta. On Earth, organic material gets concentrated in such sediment, so this would be a good place to look for signs of past life on Mars.
But on Friday, NASA announced it will send Curiosity, also known as the Mars Science Laboratory (MSL), to Gale crater instead.
There, a giant mound of layered sediment rises 5 kilometres from the crater floor. It is not known exactly how it got there, but the sediment contains clays, a sure sign that it was exposed to liquid water at some point.
"If you start at the bottom of the pile of layers and you go to the top, it's like reading a novel," said John Grotzinger, the rover's project scientist at the California Institute of Technology in Pasadena. "And we think Gale crater is going to be a great novel about the early environmental evolution of Mars."
It is too dangerous to land on the mountain itself, so Curiosity will touch down on a flatter part of the crater floor, then drive up the mountain.
But first, it will take a look at sediment deposited at the landing site by a river that once flowed into the crater. That sediment and the clays in the mountain are two of the places in Gale that may have been habitable and left organics behind.
But organics could also be preserved in several other places where life may once have existed. One is an ancient river canyon that cuts into the mountain. There are also layers in the mountain containing sulphate minerals, which require water to form. And there are also cracks in the mountain that appear to have been waterlogged in the past.
"There was a real preference for Gale in that it's not a one-trick pony," said Michael Meyer of NASA Headquarters in Washington DC. "There's several different environmental settings that can be explored [there], any one of which might have some possibility of preserving organics."
Grotzinger agrees: "It has exceptionally high diversity for different kinds of habitable environments and it is possible that some of those might preserve organic carbon."
"Organic carbon" means any complex carbon-based molecules, whether they come from living things or some other source. For example, Mars has probably received organic carbon from meteorite impacts.
Grotzinger noted that finding organic carbon will be "a very, very difficult challenge" based on experience with Earth geology. Even though life is abundant on our home planet, ancient rocks very rarely preserve organic material, he says.
"We hope to be able to look for organic carbon," he said. "What we can promise to deliver with MSL is an understanding of the environmental history of Mars."