Brown Bears and Roving on Mars
By Ralph E. Milliken
Assistant Professor (Ph.D. ’06)
It’s worth every penny. That was one of the many thoughts that went through my head a year ago in August 2012 when I saw the first images of Mt. Sharp returned from NASA’s $2.5 billion dollar Mars Science Laboratory (MSL) rover, known as Curiosity. The nerve wracking ride to the surface, dubbed the ‘seven minutes of terror’, utilized the new sky crane landing system that included a parachute, a rocket powered descent stage, and cables to lower the rover softly to the ground. Although it sounds crazy (and looks even crazier), physics and unparalleled engineering triumphed and the landing went without a hitch, as evidenced by the amazing descent video sent back by the rover. The first several months of rover operations required science team members to work at JPL, and in some ways it was like a reunion. There are a number of Brown graduates on the science and engineering teams, far too many to list here, and they are a part of the hundreds of people who continue to work tirelessly to ensure that Curiosity achieves its goals.
As for Curiosity’s field site, Gale Crater is a ~150 km diameter impact crater situated along the Martian dichotomy boundary that separates the ancient and heavily cratered southern highlands from the younger and smoother northern lowlands. A mountain, known colloquially as Mt. Sharp, rises more than five kilometers above the crater floor and appears to be composed of layer upon layer of sedimentary rock. Data from satellites orbiting Mars indicate that clay minerals, hydrated sulfates salts, iron oxides, and mafic minerals make up these layers, though their specific origins and the depositional environments they record await examination by Curiosity to be revealed. The evidence for a watery past, which includes hydrated minerals as well as fluvial features, is concentrated in the lower reaches of the mountain, whereas the younger overlying rocks lack fluvial features, hydrous minerals, and suggest a drier environment. These changes in lithology make Gale Crater and Mt. Sharp an interesting target for exploration because roving up the mountain is equivalent to roving through time, and the hope is that we can explore in detail how the Martian climate evolved during the time period in which the rocks were deposited.
One of the mission objectives is to characterize the geology and geochemistry of Mars and, by extension, to assess if any of the ancient environments preserved in the rock record at Gale could have been habitable. MSL is not a life detection mission, but identifying whether or not habitable environments ever existed on Mars is an important step in the greater search for Martian organics. However, we know from our geological experience on Earth that preservation of organic material is tricky business, and many post-depositional processes can erase the evidence of organics in what were once habitable environments, if organics were even there to begin with. Given these complexities, the hope is that Curiosity will encounter and be able to analyze a diversity of rock types that record equally diverse geological processes, thus increasing our changes of finding evidence of habitable environments. Fortunately, the rocks we have encountered so far in Gale are proving to be just as diverse and complex as those on Earth, and so far we have examined a variety of sedimentary rocks as well as some possible volcanic rocks. Indeed, the chemistry of the first rock that we touched with our analytical instruments, a piece of float whose source is unknown, turned out to be higher in alkalis than typical Martian basalts and is best classified as a mugearite (I’ll confess that I had to pull out my copy of Paul Hess’s petrology textbook for this one). More in line with the search for habitable environments, Curiosity has also encountered fluvial conglomerates, sandstones and, most recently, mudstones, all of which indicate aqueous activity.
Curiosity is the most capable and complex rover that has ever been sent to Mars, and the body of the rover houses the Sample Analysis at Mars (SAM) instrument suite that consists of a quadrupole mass spectrometer, a gas chromatograph, and a tunable laser spectrometer. These instruments allow for the detection of trace amounts of organics (if they happen to be present) and measurements of isotopic variations in soil, rock, and atmospheric samples. In addition, the CheMin X-ray diffraction instrument provides definitive mineralogical identification, a first for a Mars rover. The first sample passed to these instruments came from scooping a portion of a small drift of sand named Rocknest, and CheMin data showed it to be material of basaltic composition with tens of percent amorphous material. The latter may be the weathering product of the former under the cold, arid conditions that Mars has experienced for the past several billion years, and the results of this first time activity have provided detailed information about geological processes and water-rock interactions on ‘modern’ Mars.
Since the activities at Rocknest, Curiosity has traveled downsection (and away from Mt. Sharp) to a region known as Yellowknife Bay, one of the lowest points along the crater floor. The primary destination of the mission is still Mt. Sharp, but the diversion to this location and the associated costs in time were worth the wait. Simply put, the rocks in Yellowknife Bay are spectacular. The lower strata are fine-grained, relatively soft, and are best described as Martian mudstones. These rocks also exhibit concretions and other distinct diagenetic features, indicating a complex history of interaction with water, and they were an ideal location for the first time drilling activity. Earlier this year, Curiosity made history by drilling into a rock on Mars and analyzing the retrieved rock powder with its onboard analytical instruments. Even a casual glance in the night sky confirms that Mars is the red planet due to the abundance of iron oxide, but the science team was thrilled by the decidedly non-red color of the drill powder and rock. The CheMin instrument confirmed that these Martian mudstones contain abundant clay minerals, and they are crosscut by veins and fine fractures filled with calcium sulfate. These results, in combination with measurements by other instruments on the payload, have led to the conclusion that the rocks in Yellowknife Bay record evidence of a habitable environment, a huge step forward in our understanding of conditions on ancient Mars. However, there are still more questions than answers, and Curiosity has now departed Yellowknife Bay and begun her long trek to the base of Mt Sharp. The science team will continue to examine the vast amount of data that pour in, and with any luck we will be able to carefully and methodically piece together the history of Gale Crater as any field geologist would hope to do.
Being a part of the MSL mission is an unbelievable experience, and waking up each morning to view images and data from Mars as Curiosity roves the surface is inspiring, to say the least. Some current and future students in the geology program will also be able to participate in the mission, allowing the next generation of Brown graduates to experience firsthand the wonders of planetary exploration. No one on the team knows what will be around the next bend, behind the next rock, in the next rock, or what is really waiting for us at the base of Mt. Sharp. But if you hear about the exciting discoveries that Curiosity is sure to make over the coming months and years then be proud to know that those results are in part due to the efforts of a number of graduates from our very own geology department here at Brown.
Additional information, images, and videos about the MSL mission, team and Curiosity’s results can be found at: http://mars.jpl.nasa.gov/msl/