File:PSP 001666 1530 RED.browse.jpg

The original caption was : "This HiRISE image covers the southwest portion of the terraces and floor of Holden Crater situated in southwest Margaritifer Terra. This HiRISE sub-frame shows the most clearly-evident image of a megabreccia on Mars. Breccia is a rock typically consisting of rock fragments of various sizes and shapes that have been broken, tumbled and cemented together in sudden geologic event (e.g., a landslide, a flashflood or even an impact-cratering event). If it were not for the dark sandy dunes dispersed through out the sub-image, this image could easily fool an expert into thinking that this image is actually a photograph of a hand sample of an impact breccia. The prefix "mega" implies that the breccia in the sub-image consists of clasts, or rock fragments, that are typically larger than a large house or a building. The rectangular megaclast near the center of the image is a colossal 50 x 25 meters (~150 X 75 feet). As mentioned in the transition image caption for Holden crater (TRA_000861_1530), the crater likely experienced extensive modification by running water, which is supported by observations of drainage and deposition into the crater from a large channel (Uzboi Valles) breaching Holden's southwest rim. While it is possible that the megabreccia formed from a catastrophic release of water into the crater, a more likely possibility is that it formed from the impact that created the approx. 150 km-in-diameter Holden crater. Popigai Crater, a terrestrial crater of half the size of Holden, possesses a similar occurrence of megabreccia with a similar range in megaclast size to the Holden crater example. An impact-generated megabreccia deposit, as observed in terrestrial craters, typically lies beneath the crater floor, so the exhumation of the megabreccia may be the result of down-cutting and erosion of water that once flowed through Uzboi Valles.

Image PSP_001666_1530 was taken by the High Resolution Imaging Science Experiment (HiRISE) camera onboard the Mars Reconnaissance Orbiter spacecraft on December 4, 2006. The complete image is centered at -26.8 degrees latitude, 325.3 degrees East longitude. The range to the target site was 260.1 km (162.6 miles). At this distance the image scale is 26.0 cm/pixel (with 1 x 1 binning) so objects ~78 cm across are resolved. The image shown here [below] has been map-projected to 25 cm/pixel and north is up. The image was taken at a local Mars time of 3:41 PM and the scene is illuminated from the west with a solar incidence angle of 68 degrees, thus the sun was about 22 degrees above the horizon. At a solar longitude of 145.1 degrees, the season on Mars is Northern Summer."

HiRISE image of breccia in Holden crater. Source: [1] (higher resolution version available). See also Image:PSP_001666_1530_Holden.jpg.

See below.