This mosaic view from the Mast Camera (Mastcam) on NASA's Mars rover Curiosity shows textural characteristics and shapes of an outcrop called "Point Lake." The outcrop is about 20 inches (half a meter) high and pockmarked with holes. Curiosity recorded the 20 component images for this mosaic on the mission's 302nd Martian day, or sol, (June 12, 2013), during a second approach to Point Lake. The rover used the Mastcam's right-eye camera, which has a telephoto lens.
Point Lake first caught the interest of Curiosity's science team in October and November of 2012, when the outcrop stood out in images taken during the rover's trek eastward to "Yellowknife Bay." Point Lake is conspicuous in the right third of a scene from that time period at http://photojournal.jpl.nasa.gov/catalog/PIA16453. It consists of a relatively horizontal surface that ends in a steeper slope, shadowed in that 2012 view. The camera perspective made it look as if there are two steps, but they are actually at the same elevation as each other.
Point Lake stood out for two reasons. First, it forms a small cliff. Geologists love cliffs because they offer a sense of how a rock unit differs from bottom to top. Second, as Curiosity drove closer to Point Lake on the route to Yellowknife Bay, images revealed that the outcrop is full of holes. Holes form in rocks by diverse mechanisms. Identifying which mechanism can provide understanding about the rock and its history. Curiosity parked near Point Lake in November and gained a good view of the top part (http://photojournal.jpl.nasa.gov/catalog/PIA16447), but not of the vertical face. Months later, while at the "John Klein" rock-drilling site in Yellowknife Bay, the rover recorded a face-on view of Point Lake (http://photojournal.jpl.nasa.gov/catalog/PIA17071). Still, the holes remained puzzling, so the science team decided to get a closer look at Point Lake after leaving Yellowknife Bay. The Sol 302 image is one result.
This image shows that the upper and lower parts of Point Lake differ. The upper part has more holes and is more resistant to weathering. The holes range from smaller than pea size to larger than golf-ball size. They are circular to elliptical in shape. Some of the larger holes have raised rims, as if the material immediately around a hole is slightly more resistant than material farther from the hole. At the right-hand end of the outcrop are a few stones that look as if they could have fallen out of holes in the rock face. At least one of these looks like a thin, curved lining that could have coated the interior of a hole. Embedded nearby in the rock face is a larger rounded rock that has a rock lining around it.
Curiosity's science team is considering diverse geological processes -- both igneous and sedimentary -- as explanations for the holes and other characteristics of Point Lake.
Igneous rocks commonly have holes called vesicles, which are frozen gas bubbles left over from when the rock was molten or fluidized. However, it is also possible to create holes in sedimentary rocks. The easiest way is for pebbles or cobbles in the rock to fall out as the rock erodes, leaving holes in the remaining rock. This is more likely to occur if the pebbles or cobbles are much harder than the surrounding rock.
Holes in either igneous of sedimentary rock can later be partly or wholly filled with secondary minerals delivered by fluids or gases. The secondary minerals that fill the holes are sometimes harder than the host rock, so that when the entire assemblage starts to erode, they remain behind as round nodules. Geodes are an example of this process.
This view is presented in raw color, which shows the scene's colors under Mars lighting conditions as they would look in a typical smart-phone camera photo. Views with white-balanced color, which shows what the rocks would look if they were on Earth, are also available with (Fig. 1) and without (Fig. 2) scale bars for two different parts of the scene.