New Impact Craters on Mars

The team operating the Context Camera (CTX) aboard the Mars Reconnaissance Orbiter (MRO) frequently discovers new dark spots on Mars that, upon closer examination, turn out to be brand new impact craters. Sometimes only a single crater is present, but often there is a cluster of several craters. Depending on the patterns and size, crater clusters are interpreted as indicating that the incoming meteorite broke apart before it hit the surface. In some cases, clusters could be formed by ejecta from other, larger impact craters.

Shown here are sub-frames of two CTX images of terrain in western Arcadia Planitia (near 46.7°N, 183.2°W). The first was taken on 4 June 2008, the second on 10 August 2008. In between the time the two images were acquired, a cluster of dark spots—and dark rays radial to some of the dark spots—were formed. The CTX team immediately considered this to be a candidate new meteor impact site, but CTX images have a resolution of 6 meters (19.7 feet) per pixel. This means that features, including impact craters, smaller than about 20–25 meters (65–82 feet) across cannot usually be resolved. Thus, most of the dark spots (like these) that the CTX team finds and suspects to be new impact craters have to be imaged at higher resolution to find out whether small impact craters are actually present.

To confirm that the features were craters, the CTX team created an opportunity for the MRO High Resolution Imaging Science Experiment (HiRISE) team to take a picture at about 30 centimeters (~12 inches) per pixel resolution. Such images can be used to identify objects as small as about a meter (3–4 feet) across. The CTX team targeted the CTX camera to take a new picture centered on the candidate impact site and made a request to the HiRISE team to take a picture at the same time.

Meanwhile, the CTX team also did some homework: before asking HiRISE to take a look at candidate new impact sites, like the dark spots shown here, they make the best determination possible as to when the features formed. If they result from a meteor impact, then the team can state, approximately, when the impact occurred. In the case shown here, the impact occurred sometime between 4 June and 10 August 2008. At the time it was found, this candidate impact site was so new that the CTX team very quickly suggested that HiRISE take a look so that a good example of a very, very new impact site could be examined.

That interest—and the CTX team’s quick work—paid off. The HiRISE team obtained a picture of the site on 12 September 2008, just a month after the 10 August CTX image was received on Earth and evaluated by the CTX team. That HiRISE image, identified as PSP_009978_2265, shows a nice cluster of new impact craters. Some of the craters exposed a light-toned material that appeared somewhat bluish in the HiRISE color data. Immediately suspected to be an exposure of subsurface ice, the HiRISE team began a campaign to monitor this site to see if the bright material changed. It did, suggesting that perhaps the material was going away, just as ice might do at that time of year (it was northern summer). The CTX team worked with the HiRISE team over the next several months to identify more such craters and, in the end, found a total of 5 brand new impact crater sites at northern middle latitudes that seemed to have exposed buried water ice. At the same time, the HiRISE and CTX teams encouraged the MRO Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) team to obtain multispectral infrared observations. Most of the craters and bright patches were too small for CRISM to resolve, but one of them was large enough that the CRISM team did, in fact, confirm that the light-toned material was water ice.

Details and scientific interpretation of these new impact craters, and their use to examine subsurface ice at northern mid-latitudes on Mars is captured in a paper published this week in Science by Shane Byrne and 17 colleagues on the CTX, HiRISE and CRISM teams. All of that effort began with a single pair of CTX images, the two shown here. If the CTX team had not found these first two images and had not immediately attempted to re-image the suspected crater cluster and provide an opportunity for HiRISE to “ride along,” then the most important aspect of the observations reported in Science—the presence of excavated ground ice—would have been lost because the ice began to disappear or darken immediately after the impacts occurred.

The two pictures are map-projected sub-frames of CTX images P20_008699_2247_XN_44N182W and P22_009556_2263_XI_46N183W. North is up and sunlight illuminates the scenes from the lower left. Both images are archived with the NASA Planetary Data System Imaging Subnode.

To see the above two images without annotations: 4 June 2008 and 10 August 2008.

Malin Space Science Systems, Inc., San Diego, California, U.S.A.