This HiRISE image covers a portion of an outcrop of the Medusae Fossae Formation, a series of light-toned terrains in the Martian mid-latitudes. The Medusae Fossae is characterized by wind-sculpted landforms, most notably eroded ridges known as yardangs. The composition of the Medusae Fossae is not known, but candidates include indurated (hardened) volcanic ash or remnants of dust-ice mixtures that formed in a different Martian climate. Three prominent yardangs are seen in this image, aligned with their long axes pointing NW-SE, with tapered ends on the NW, consistent with erosion from a southeasterly wind. One or more hard rocky layers within the yardangs are visible, with the layers commonly segregated into discreet boulders. Isolated rocks are seen on the slopes and at the base of the yardangs, indicating that some formed from breakup of the layers. The rocks may be similar in composition to the softer, non-rocky parts of the yardangs, but simply more indurated. Alternatively, they may be compositionally distinct, challenging current hypotheses for the origin of the Medusae Fossae. Light-toned ridges at center left have a gross morphology similar to that of barchanoid dunes formed from wind-blown sand. If these are dunes or ripples, their orientation is consistent with the presumed wind direction that carved the yardangs. However, zooming in to full resolution reveals flat tops, grooves, and smaller, darker ripple forms to the northwest of the ridges. Therefore if these are dunes, they seem indurated. The image scale is 27 cm/pixel (with 1 x 1 binning) so objects ~81 cm across are resolved. The image was taken at a local Mars time of 3:27 PM and the scene is illuminated from the west with a solar incidence angle of 55.2 degrees, thus the Sun was about 34.8 degrees above the horizon.
This HiRISE image shows geologic “contacts”, or boundaries, between light-toned and dark-toned material in Meridiani Planum, near the equator of Mars. Merdiani Planum is where the Mars Exploration Rover Opportunity is located, although this image covers an area that is more than 600 km to the east of the Opportunity site. The central portion of the image shows very smooth, dark plains that are typical of much of the Meridiani region. These plains are flanked by more rugged lighter-toned materials. The light-toned materials have been eroded to form pits, buttes and mesas. Based on the lengths of the shadows that they cast, some of the buttes and mesas are up to about 30 meters (~100 feet) tall. The light-toned material shows distinctive layering, suggesting that it may be composed of sedimentary rock. Scattered across the scene, especially in the light-toned materials where they are prominent in low spots and around some of the larger buttes and mesas, are dunes and other similar landforms created by martian winds. The image scale is 54 cm/pixel (with 2 x 2 binning) so objects ~162 cm across are resolved. The image shown here has been map-projected to 50 cm/pixel and north is up. The image was taken at a local Mars time of 3:28 PM and the scene is illuminated from the west with a solar incidence angle of 55.3 degrees, thus the sun was about 34.7 degrees above the horizon.
This HiRISE sub-image is located on the southern margin of Elysium Planitia, in the equatorial region of Mars. A scarp (cliff) winds through the scene, dividing the lower knobby terrain to the southwest from the higher terrain to the northeast. The scarp offers a glimpse into the material that underlies the higher terrain. No prominent layers are seen in the vertical face of the scarp, and boulders have not accumulated around its base. This suggests that the elevated northeastern terrain is not made of hard rock; however, it is also possible that rocks are present but buried under sediments. Several of the impact craters in the northern part of the sub-image are “pedestal craters,” which have fragmented material that was thrown out of the crater upon impact. But the ejecta was not always raised like this. Being more resistant to erosion, it was left high-standing after the surrounding material was removed, probably by wind. In addition to being raised, the ejecta around these craters is asymmetric – it is skewed towards the southeast. This might be because the craters formed when objects struck the surface of Mars at an angle, or perhaps erosion has preferentially removed the ejecta on the northwest sides of the impact craters. The knobby terrain southwest of the scarp is riddled with wind-blown dunes. The dunes radiate out around the bases of the knobs indicating that they are more strongly influenced by local topography than regional winds. Small boulders on the flanks of a few knobs reveal that they contain rocky material. The image scale is 59 cm/pixel (with 2 x 2 binning) so objects ~178 cm across are resolved. The image shown here has been map-projected to 50 cm/pixel and north is up. The image was taken at a local Mars time of 3:28 PM and the scene is illuminated from the west with a solar incidence angle of 54.9 degrees, thus the sun was about 35.1 degrees above the horizon.
Larger image of Holden crater. Holden crater is an impact crater that formed within an older, multi-ringed impact basin called Holden basin. Before an impact created Holden crater, large channels crossed and deposited sediments in Holden basin. Blocks as big as 50 meters across were blasted from Holden basin when Holden crater formed, then fell chaotically back to the surface and eventually formed “megabreccia,” a conglomeration of large, broken boulders mixed with smaller particles. HiRISE images show megabreccia outcrops in Holden crater walls. This megabreccia may be some of the oldest deposits exposed on the surface of Mars. At least 5 percent, by weight, of the fine sediments in the layer on top of the megabreccia consists of clay, according to another instrument on the Mars Reconnaissance Orbiter, the Compact Reconnaissance Imaging Spectrometer for Mars, or CRISM. Photo Credit: NASA/JPL/University of Arizona.