NASA announced that Gale Crater will be the landing site for the Curiosity rover. The actual science target is a 5 kilometer high mountain of sedimentary material in the middle of this huge crater. Orbital images and spectra show clearly defined rock strata laid down over a long time span thought to cover perhaps a billion years. On Earth geologists look for places to examine sequences of rock layers to understand the geologic history, using features such as road cuts and channels or canyons produced by rivers. The Grand Canyon in the Southwestern United States is one of the most famous geological features displaying ~1.5 km of rock layers. The layers in Gale are three times as thick as those exposed in the Grand Canyon. Curiosity’s science team believes that studying the rock layers in the mountain, or “mound”, in Gale Crater will be similar in some ways to studying the long sequence of layers in the Grand Canyon.
The landing ellipse—the area within which the rover will land—fits nicely within the northwest corner of Gale (see image), where the terrain is relatively smooth, providing a safe landing area close to the mound. Engineers expect the rover to land near the center of this ellipse, but there is a slight possibility of landing anywhere within it. The ellipse is centered in the deepest part of Gale Crater which may have been a wet and possibly habitable place at some time in the past. The soil in this region is made of alluvial material eroded from the nearby crater walls.
After examining the materials surrounding the rover’s point of touchdown, Curiosity is expected to start driving towards the mound, taking advantage of whatever features are nearby—small craters, mesas, dunes, or inverted river channels—to study important clues along the way. Small craters are a natural window into the subsurface allowing the rover to see and study materials that are normally buried slightly below the soil. The landing ellipse includes several inverted channels—where small river or stream beds caused soil to become cemented together. Subsequent erosion of the region resulted in these stream beds being elevated relative to surrounding terrain due to its resistance to erosion.
The stratigraphic succession that composes the mound begins with layers rich in clay minerals. These define an important science target, in keeping with the mission’s goal of exploring habitability. As Curiosity starts up the mound it will encounter canyons and large numbers of layers in which the orbital data indicate the sedimentary rocks become richer in sulfate minerals.
Gale is situated near the equator, along the boundary between ancient highlands in the south and smoother lowlands in the north. There are several large craters in this region, some of which contain similar sedimentary material. Scientists want to understand how the sedimentary material was laid down, and how it was apparently removed from part of Gale Crater.
The ChemCam team is excited about the selection of Gale Crater as the Curiosity landing site. Its laser instrument is perfect for probing the composition of the many rock layers. During the drive to the mound ChemCam will be the main tool to be used for reconnaissance of any interesting features along the way, in addition to recording changes in the soil as the rover progresses. Once Curiosity arrives at the base of the mound, ChemCam can be used to create a “chemostratigraphic” record whereby changes in the chemical composition of layers are measured and recorded as a function of their stratigraphic position. Since layers lower in the mound are older than layers higher up in the mound, each successive layer may tell us about progressively younger times in the history of Mars’ environment. Different layers as small as a few millimeters thick can be accessed rapidly with laser scans, unraveling the secrets of Mars’ history.
The table below provides a quick summary of Gale Crater facts.