SAN FRANCISCO — NASA’s Curiosity rover is zeroing in on its quarry. The latest results from the probe tell scientists that the site it’s exploring at Gale Crater was once an ancient and habitable lakebed on Mars. Moreover, the findings could point to the best places for scientists to find telltale organic molecules, if they exist.
Curiosity arrived more than a year ago on Mars and roved right onto an ancient riverbed. The robot spent a good deal of its first few months exploring an area known as Yellowknife Bay, taking pictures and drilling into the rocks. Scientists determined that the area once might have been a great place to find living organisms. The new findings, which appear today in Science, help put those initial results into context with the entire environment.
“Really what we’ve come to appreciate is that this is a habitable system of environments,” said geologist John Grotzinger of Caltech, Curiosity’s project scientist, during a press conference today, here at the 2013 American Geophysical Union conference. “The lake, associated streams, and the groundwater environment had a neutral in pH, that was low in salinity, and had minerals in different oxidation states, which would have been essential for a microbial metabolism.”
The team was excited to find a huge diversity of different rocks at Yellowknife, including fine-grained and medium-grained sandstones and crossbedded sandstones, types of rock that are known to be deposited by water on Earth.
“These rocks are telling us a story of deposition and flowing and standing of liquid water,” said geoscientist Joel Hurowitz of NASA's Jet Propulsion Laboratory, who also works on the Curiosity team.
Chemical analysis of the rocks suggests that they are a mixture of two types of volcanic rock. One is an every-day basalt seen all over the Martian surface while the other is high in potassium and is known as an alkaline-igneous rock. By looking at different levels of radioactive isotopes in the rocks, Curiosity was also able to determine their age: roughly 4.2 billion years old. This is the first time that scientists have ever measured the age of rocks on another planet and gives them an important data point for future investigations.
Looking at the mineralogy of Yellowknife Bay, the team found an abundance of clay minerals that could have only formed in water with a neutral pH. When Curiosity heated up the rocks in its internal oven and looked at their molecular composition, it found a huge amount of carbon dioxide and nitric oxide. These molecules show that the rocks at Yellowknife contain carbon and nitrogen, both essential components of living creatures on Earth.
Though Curiosity might have carried a small amount of the carbon and nitrogen to Mars when it launched, the team found far more of both molecules than could be possible if they were just contaminants. The researchers don’t know if this carbon and nitrogen came from organic or non-organic molecules, but it all adds up to “evidence of minerals that tell us that we had a good chance of preserving organics,” said biogeochemist Jennifer Eigenbrode during the conference.
The one wrench that could be thrown into Curiosity’s search for organic compounds is the large amount of radiation at the Martian surface. Because Mars has no protective magnetic field, six months on the Red Planet would give a person more than three times the radiation dose that an astronaut receives on the International Space Station (which is partially protected by Earth's magnetic field) over that same time period. Galactic cosmic rays and energetic ions from the sun could easily destroy any organic molecules sitting around in the Martian regolith.
But rocks buried deep underground get protected from this high-energy radiation bath. So the best place to search for organic molecules would be to drill deep down, while Curiosity’s drill only reaches a few centimeters into the ground. But there are also natural processes that expose deep rocks. Erosion from wind or water can eat away at the surface, revealing fresher material. Curiosity was able to analyze the rocks at Yellowknife Bay and determine that they had been exposed on the surface for a relatively short period of 80 million years.
This means that the dominant erosion process here is wind (there is no evidence that water existed on Mars this recently), which blasts rocks away as it sweeps over the ground. The wind hits the slopes of Mount Sharp, slowly chewing away at its lowest edges at a rate of around 1 meter per million years. The most recently exposed rocks would appear at the steep scarps at the base of large rock formations. Radiation would have had less time to destroy organic molecules here, suggesting that it is the best place to find signs of life.
Curiosity is currently driving toward Mount Sharp and expected to be halfway there in about two months. At the halfway point is a site where satellite pictures show a large rock that has scarps where rocks might have recently become exposed, perhaps in just the last million years or so. Curiosity will drill at this area and could find further clues of organic molecules.
