What has the Curiosity Mars Rover been doing this past year?
The Mars Science Laboratory (MSL), affectionately known as Curiosity, has been on Mars for over a year now. It made a dramatic landing, it captured our imagination by being a huge nuclear-powered laser tank, and it has returned some of the most stunning high-resolution imagery of Mars ever recorded. But NASA don't spend billions of dollars on projects just because they're "Cool". They have definite scientific goals in mind; to advance our knowledge of worlds beyond Earth and answer basic questions like "Are we alone?". So what has Curiosity actually done to meet those goals?
The basic goal of Curiosity was this: Find out if Mars has ever, at any point in its history, been capable of supporting life. It's a simple question, but quite difficult to answer, so the MSL mission planners chose to look primarily at the planet's geology. Search for minerals that are formed in water, that contain large amounts of oxygen or water, and various other indicators of conditions suitable for Earth-like life. Curiosity's schedule was carefully planned to allow a full year to answer that primary question, so everybody was really surprised when it answered the question only a few months in with a resounding "Yes, Mars was once a warm and wet world with a thick atmosphere, capable of supporting the same sort of life as we find on Earth."
In the months since, Curiosity has allowed scientists to learn an astonishing amount about our neighbour, and in December last year they released a batch of science papers summarizing the most important discoveries since the mission began. They are accurate measurements of the ages of surface rocks, the proof that microbial (and possibly more complex) life could definitely have lived on Mars in the past, measurements of radiation levels at the surface, and ways in which erosion and weathering can reveal direct evidence of organisms that may once have lived on the surface.
The first important discovery was made when Curiosity analysed a rock named "Cumberland" by a science team led by Kenneth Farley of the California Institute of Technology (Caltech) in Pasadena. The rover first drilled a hole into the rock, then collected the resulting mineral powder and analyzed it in Curiosity's on-board geology laboratory using the same radioisotope dating methods used on Earth. They found that its age was about 4.1 billion years (give or take about 10%), which closely matched ages of rocks in the region which had previously been calculated by other methods. "The age is not surprising, but what is surprising is that this method worked using measurements performed on Mars," said Farley. "When you're confirming a new methodology, you don't want the first result to be something unexpected. Our understanding of the antiquity of the Martian surface seems to be right."
This means that Curiosity proved that its equipment for dating rocks by measuring the ratios of stable and unstable isotopes of Argon in the minerals is reliable, by comparing the results to figures that were already confirmed by less precise, but well-understood methods. Previously, regions of the martian surface had been dated by counting the craters formed by meteoroids smashing into the surface, noting how badly they were eroded, and comparing to the ages of similar regions on the Moon. Those lunar ages are well known because rock samples returned by Apollo astronauts have been carefully analysed in geology laboratories on Earth.
The second discovery, made by the same team, was a measure of how long those rocks had been on the surface, exposed to Mars's thin atmosphere. Some of the measurements were made by studying weathering and erosion from the seasonal winds and dust storms which plague Mars, but another important data set came from examining the effects of cosmic radiation. Unlike Earth, Mars has a negligible magnetic field and atmosphere, so objects on its surface have practically no shielding from high energy particles and other forms of space radiation emanating from the Sun, distant supermassive black holes, and the violent destruction of supernova explosions across the universe. These high-energy particles collide with the molecules comprising the various minerals that make up the rock, and break them down, allowing various gases to be emitted over time. Gas formed just beneath the outer surface of a rock remains trapped within its crystal structure, and can be detected when Curiosity uses its drill or laser. Other gases trapped deeper in the rock will have different compositions from those on the surface, because they are better shielded from the cosmic radiation, and mission scientists can calculate roughly how long the rock has been exposed to the radiation (and therefore how long the rock has been on the surface) by comparing the different gas readings. The time that the rocks in Gale Crater have been on the surface? Somewhere between 60 and 100 million years, which in geological terms is very young. That suggests that wind erosion on Mars can wear down mountains and hills to expose the rocks within quite quickly.
A separate team, led by Doug Ming, of NASA's Johnson Space Center, has been looking for organic compounds. These are a group of chemicals based around carbon atoms, and which are associated with life because they're often produced in large amounts by living organisms. Ming's team found that the region of Gale Crater named Yellowknife Bay was once a lakebed, and that the ground beneath Curiosity's wheels was a type of clay formed in an environment that would have been very friendly towards life. It is rich in minerals like sulphur and iron, which many microbes on Earth consume as an energy source, and was formed in water with a relatively benign pH - neither too acidic nor salty for life. All these discoveries so far show that not only were conditions on Mars once suitable for life, and that these conditions began at least four billion years ago.
Scott McLennan of Stony Brook University in Stony Brook, N.Y., and colleagues, answered the next question: Did these wet and mild conditions persist long enough for life to get started, and how long ago did they end? McLennan's team discovered a rived bed, full of pebbles and clays that were carried by a water flow from the place where they'd formed upstream. It's understood that as Mars dried, surface water would have become saltier and more acidic, and poisonous to life. To find out how long ago this happened, McLennan's team studied the weathering and mineral composition of the materials in the river bed. Certain elements, like calcium and sodium, leach out more quickly than others, and if the rocks had been weathered into clay near the river's source, then this leaching would be obvious. Instead, those minerals were still present, indicating that the weathering happened after the rocks had already been transported.
At the same time, a team led by David Vaniman of the Planetary Science Institute in Tucson, Arizona, ran their own analysis of sedimentary rocks in the region and found them to have formed from clays with low amounts of the mineral olivine but much magnetite. To a geologist, this supports McLennan's conclusion that the clay formed after the original pebbles had washed downstream. They also found smectite in the original clay, which reveals something about the conditions in which the clay formed: "Smectite is the typical clay mineral in lake deposits," Vaniman said. "It is commonly called a swelling clay -- the kind that sticks to your boot when you step in it. You find biologically rich environments where you find smectites on Earth."
All this information about life-friendly conditions is closed off by the observations of yet another team, led by John Grotzinger of Caltech. They looked at the physical layering of rock formations in Yellowknife Bay, and came to the conclusion that the habitable conditions which so many researchers have confirmed, existed about four billion years ago, when parts of Mars were already starting to dry out. "This habitable environment existed later than many people thought there would be one," Grotzinger said. "This has global implications. It's from a time when there were deltas, alluvial fans and other signs of surface water at many places on Mars, but those were considered too young, or too short-lived, to have formed clay minerals. The thinking was, if they had clay minerals, those must have washed in from older deposits. Now, we know the clay minerals could be produced later, and that gives us many locations that may have had habitable environments, too."
So the evidence suggests that, in the region around Gale Crater at least, Mars was wet and muddy for a period of tens of millions of years. Lakes and rivers would have formed, dried, and re-formed, but the soil beneath the surface would have stayed damp, providing a consistently safe environment for microbes to flourish. All that remains is to find evidence that those microbes ever appeared in the first place.
The final bit of research has little bearing on Curiosities primary mission, but is nevertheless vital to the various planned missions to send humans to Mars. Curiosity is equipped with a suite of sensors to measure various forms of radiation, and these were the very first instruments to be turned on, while the rover was still in deep space between the planets. The purpose of these sensors is to determine just how much radiation explorers on Mars will have to cope with, and whether they will even survive the trip from Earth without suffering deadly doses of radiation poisoning. According to a report by Don Hassler of Southwest Research Institute in Boulder, Colo., and co-authors, the news is promising, in that barring any unexpected Solar storms, an astronaut making a round trip to Mars, not including the time actually spent on the surface of the red planet, would receive a radiation dose of 1000 millisieverts. In the world of radiation safety, this is an unacceptably high dose, as it means that one's chances of developing cancer at some point in their life increases by about 5%. The somewhat lower dosage received while living and working on the surface of Mars will add to this. But the good news is that people can be protected from radiation with simple shielding measures - habitats on Mars can be built underground, and the water supply on a space-craft could be stored in its walls (a few centimeters of water or soil are enough to absorb most incoming space radiation). So while a trip to Mars involves putting people in harsh and dangerous environments for long periods of time, it's not so bad that we can't do something to ensure that everybody comes out alive and healthy. Of course, Hassler is equally interested in the effects of radiation on any native martian life. "Our measurements provide crucial information for human missions to Mars," he says. "We're continuing to monitor the radiation environment and seeing the effects of major solar storms on the surface at different times in the solar cycle will give additional important data. Our measurements also tie into Curiosity's investigations about habitability. The radiation sources that are concerns for human health also affect microbial survival as well as preservation of organic chemicals.