NASA's Curiosity rover, which has been on Mars for almost a year and a half now, just delivered its largest and most important downlink so far of findings, discoveries, and conclusions. (Also see video: "Mission to Mars.")
While the six papers released by the journal Science report on different aspects of Mars and do it from different scientific perspectives, together they present the beginnings of a new understanding of the red planet, especially in its early epochs, three to four billion years ago.
Early Mars, it has become increasingly apparent, was in many important ways similar to Earth. (See also "Did Life on Earth Come From Mars?")
"If you put together all that we're learning about Gale Crater and Mars, you really begin to chip away the rock and the sculpture inside emerges," said Pan Conrad, an astrobiologist with NASA's Goddard Space Flight Center and a co-author on several of the papers.
As described in the six Science papers, that hidden but slowly materializing landscape looks something like this:
1. Early Mars was habitable, perhaps for a long time.
After concluding that rivers and streams once flowed into Gale Crater, the Curiosity team has now reported that a lake existed there as well. That surface water, as well as groundwater that likely went down hundreds of meters, possessed all that was needed to support microbial life.
The period when Gale Crater was warmer, wetter, and habitable was broadly between 3.5 and 4 billion or more years ago. That period is when life on Earth is understood to have arisen.
Was Mars once home to primitive extraterrestrial life? Curiosity can't and won't make that determination, but its discoveries have made the possibility of Martian life significantly more plausible.
2. Water once flowed on many parts of Mars.
And that water was there at times and in forms that scientists didn't believe to be possible not long ago.
One of the major achievements of Curiosity so far has been to "ground-truth" observations made in recent years by the satellites orbiting the planet. Those instruments found strong hints that Mars had a watery past, and at Gale Crater they were found to be on target.
As a result, the mission has given greater credibility to the view that thousands of additional fossil formations of what appear to be ancient streams, channels, deltas, and lakes likely are just what they appeared to be.
3. "Follow the carbon" has been vindicated.
The search for Martian carbon-based organic compounds—one of the major goals of the Curiosity mission—has been and will continue to be complicated and trying.
While as many as six different organic compounds have been identified so far by the miniature chemistry lab called Sample Analysis at Mars (SAM), their origin remain unclear.
"There's no doubt that SAM has identified organics, but we can't say with confidence yet that they are Martian in origin," said Douglas Ming of NASA's Johnson Space Center, and author of one of the six Curiosity papers in Science.
There are at least two sources of the confounding results: The presence on Mars of a chlorine-based compound that, when heated in the SAM oven with any organic material, largely destroys the compounds. And the leak into the SAM ovens of an organic solvent brought to Mars as part of a wet chemistry experiment.
The search for Martian organics is nonetheless making progress. With each collection of Martian sand or rock, the number of organics found and their concentration has increased—suggesting that different samples of Martian material are leading to different results. If the organics were all contaminants from Earth, those concentrations would likely be more steady.
"SAM is probably the most complex and important instrument ever brought to another planet," Ming said. "Inevitably, it has taken time to figure out how best to work with it."
4. Mars gets pounded by radiation.
Galactic cosmic rays and solar eruptions bombard Mars, and their high-energy particles break the bonds that allow organisms to survive. The Radiation Assessment Detector on Curiosity has made the first measurements ever of radiation on the surface of Mars, and the results are sobering.
In another Science paper, RAD principal investigator Donald Hassler reports that the radiation would almost certainly be fatal within a few million years to any microbial life on the surface or less than several meters deep.
The RAD team used as their model the terrestrial bacteria Deinococcus radiodurans, which is capable of withstanding enormous doses of radiation, to make that assessment.
Notwithstanding the high levels of radiation, Hassler reported that life could theoretically still survive on Mars today under special circumstances. If a bacteria similar to D. radiodurans appeared when early Mars was wetter, warmer, and had a more protective atmosphere, he wrote, it could have survived over the epochs through long periods of dormancy.
5. Mars radiation also damages normal chemistry.
Many on the Curiosity team point to radiation damage of all carbon chemistry on Mars as a major reason why it has been so difficult to identify organics on the surface.
That effort, however, may have gotten a significant boost from one of the most unexpected developments to come out of the mission so far―a method to date how long surfaces have been exposed to the sky on Mars.
Using measurements of radioactive decay also employed on Earth, Kenneth Farley of the California Institute of Technology reports that the surface of Yellowknife Bay has been exposed for some 80 million years.
The discovery points to a method for finding places for the rover to drill where there has been less chemistry-damaging radiation exposure.
Essentially, Farley said, the team has to look for cliffs or overhangs being undercut by the surface wind―as happened in Yellowknife Bay―and where radiation would be blocked by the rocks above. "If we find that kind of formation," he said, "drill there."
6. Detours sometimes turn out to be interesting.
The Curiosity rover was scheduled to head for the scientifically alluring Mount Sharp in the center of Gale Crater soon after landing. More than 480 days into the mission, the rover is still months away from its prime destination.
The detour to Yellowknife Bay is the main reason why, and it has turned out to be a gold mine of data. But now the rover is on what is called the "rapid transit route" to three-mile-high Mount Sharp, driving most of the time and passing many potentially interesting sites.
Having already found and analyzed the first potentially habitable environment ever discovered on Mars, the Curiosity team will be looking for more. And with their increased knowledge about which potential drill sites are likely to have been protected from radiation, the search for Martian organics will go into high gear as the rover approaches the target-rich Mount Sharp.