Since August 6, 2012, NASA’s Mars Science Laboratory rover, named Curiosity, has been exploring the surface of Mars within a region known as Gale crater. In that time the rover has been navigating a path from its landing site toward—and up—a central mountain, Mt. Sharp, that rises approximately 5.5 km above the crater floor. Curiosity’s prime mission is to evaluate the potential habitability of Mars in the planet’s past. By observing and analyzing the landscape and rocks on its planned path across the crater floor and up Mt. Sharp, the Curiosity rover is investigating a crucial slice of the history of Mars.
JGR-Planets recently published a special issue of papers detailing the results of an extended stop at a site called the Kimberley. During nearly 5 months at the Kimberley, the Curiosity rover investigated the rocks of the site with a full range of tools to characterize the environmental conditions that would have been responsible for formation of the rocks and to search for signs that the environment was habitable. Melissa Rice, one of the organizers of the special issue and lead author of its introduction, describes why this site is important for understanding Mars and its potential for being habitable in the past.
The Kimberley waypoint was chosen as a site for investigation for its unique layers of rock (“stratigraphy”). What are these layers made up of, and why is this important?
Soon after Curiosity landed, when the science team was studying the orbital images along the rover’s future path towards Mt. Sharp, it became clear that the Kimberley would be a unique area to explore. The stratigraphy includes a rock layer that looked different from anything the rover had previously encountered, or would later encounter at Mt. Sharp. In the images from orbit, this layer (which we now call the “Orbital Striated Outcrop”) appears to have dark stripes everywhere the rock is exposed, and the direction of these “striations” (northeast-southwest) is remarkably consistent over a large area. When Curiosity arrived at the Kimberley, we were surprised to find that this rock layer is actually made up of many individual sandstone beds that are dipping to the southeast. The dark stripes seen from orbit turned out to be the shadows cast by each tilted sandstone slab.
Above the sandstone is a horizontal rock layer of much finer sand, and below it is a conglomerate layer of larger pebbles. Taken together, we interpret this layered stratigraphy as having been deposited in streams and deltas, where rivers met ancient lakes within Gale crater. The dip direction of the sandstone beds tells us the direction that the sediment was moving, which was generally from the north rim of Gale crater towards the south. Today the topography south of the Kimberley slopes upwards towards Mt. Sharp – but water does not flow uphill, so the ancient landscape must have been much different when the rocks at the Kimberley were emplaced. There must have once been lakes where there is today a mountain.
The team was surprised by the levels of potassium oxide (K2O) in the sediment at the Kimberley waypoint. What’s significant about K2O being found at the levels that it was?
From satellite data, Mars was long thought to be a simple “basaltic planet” made of mafic rock. This kind of rock is produced by quickly cooling lava. However, rock produced by slowly cooling magma has a different composition, including minerals called “alkali feldspars” that can have high levels of potassium. The levels of potassium measured in the rocks the Kimberley were significantly higher than anything previously measured by Curiosity, and the amount of K2O compared to other elements is more than five times higher than that of the average Martian crust.
We know that the Kimberley rocks are composed of sediments that had been transported via rivers from the north, so there must be a source of potassium-rich alkali feldspar rocks along the north rim of Gale crater. The rocks that Curiosity previously studied along its path had also been transported from the north, but those did not have high K2O abundances. This all means that there is sediment from multiple igneous sources, which implies that the northern rim of Gale crater is more diverse than we had initially thought, and, at least as diverse as geographical regions of similar ages on Earth.
How did results from analyzing sediments in the Kimberley advance the “water on Mars” hypothesis?
We’ve been able to say there was once liquid water on Mars for quite some time now, from an abundance of evidence from orbital spacecraft over the past decades and from the discoveries of the Mars Exploration Rovers. Curiosity’s major contribution to the “water on Mars” story has been the confirmation that the water existed in “habitable” environments, meaning that life (as we know it on Earth at least) could have survived in Martian lakes. In a region of Mars known as Yellowknife Bay (YKB), the geochemistry of the rocks told us that the lake waters might have been good enough to drink, in that they had low salinity and neutral pH, and that the key elemental ingredients for life (carbon, hydrogen, oxygen, sulfur, nitrogen, and phosphorus) were available [Grotzinger et al., 2014, Science].
After Curiosity spent time in YKB, it traveled to the Kimberley where it studied rocks that formed in southward-advancing deltas at the margin of another large lake. It appears that the lake levels fluctuated over the period in which the rocks were deposited, and the Kimberley was at times covered by up to tens of meters of water, and at times dry. Importantly, this tells us that the habitable lake at YKB was not a one-off. The rocks at YKB, at the Kimberley and those further along Curiosity’s traverse collectively tell us that lakes came and went within Gale crater over a period of tens of thousands to many millions of years. Each individual lake may have been around for hundreds to tens of thousands of years, which is perhaps long enough for life (had it ever emerged on Mars) to have taken hold.
How did the Kimberley sediment analysis add to what we know about the possibility of organic compounds on Mars?
A major goal of the Kimberley campaign was to analyze a rock sample with a high potential for containing organic compounds. When searching for good sampling locations, we consider both the rock’s potential for concentrating organics when the sediments are deposited and for preserving those organics for several billion years (by protecting them from destruction by cosmic rays, for example). The Windjana site was selected for sampling because of its location at the base of a hill called “Mt. Remarkable” that appeared to be actively eroding, suggesting that Windjana might have been recently-exposed from underneath the Mt. Remarkable rocks (where it would have been protected from radiation damage for most of its history).
Although organic compounds have not been confirmed within the Windjana sample, their absence is perhaps not surprising given Windjana was taken from a fine-grained sandstone outcrop. Organic matter is much more likely to be concentrated in lake mudstones, such as where organics were previously detected at YKB, and there are no mudstones at the Kimberley. However, Curiosity was able to estimate how long the Windjana sample had been exposed at the surface using a cosmogenic dating technique and confirmed that the rock was buried beneath a few meters of Mt. Remarkable until roughly 46 million years ago. This is a “young” exposure age for rocks on the surface of Mars, and it suggests that, had organics been concentrated during the deposition of rocks at the Kimberley, they would have been shielded from cosmic ray degradation and Curiosity would’ve had a good chance of detecting them.
—Melissa Rice, Department of Geology, Western Washington University; email: [email protected]