This HiRISE image shows a fan-shaped deposit at the distal end of a valley. The fan is approximately 3.5 x 3.7 kilometers in size.

While other similar fans on Mars display stair-step terracing along their edges, this particular fan does not show any terraces. There is a valley to the upper left that is the source of material that now composes much of the fan.

Martian fans are thought to be either alluvial or deltaic in origin. On Earth, alluvial fans form when material upslope is eroded and transported by water down a confined valley until reaching a flatter, broader surface downslope where the material is deposited to produce a fan-shape.

Deltaic fans form when rivers transport sediment downstream until an unconfined and flatter surface is reached under water, at which time the sediment is deposited in a fan-shape. Whether the Martian fan formed by alluvial or deltaic processes in unknown, but both processes require a fluid (most likely water) that carved the valley and transported the sediment downstream.

Written by: Cathy Weitz

Who wants color? My preferred HiRISE color product uses all three color bands (i.e, wavelengths) that HiRISE can image -- IRB, which refers to Infrared-Red-Blue/Green. Ever heard of R-G-B (Red-Blue-Green)? These three primary colors essentially combine to make all the perceived colors that we know of.

By substituting a wavelength that is normally invisible to the human eye for one that is, like infrared for red, we are able to create a "false"-color image. The infrared is useful because its sensitive to iron-bearing minerals and their oxidation state (degree of "rusting").

Ferric iron, the more oxidized variety, is what makes Mars so reddish. Basically, most of the materials on Mars are pretty oxidized/rusted, and therefore altered from the more original ferrous iron state (the less oxidized iron common to volcanic minerals such as olivine and pyroxene). So in general, the bluer the materials in our IRB images the less oxidized (altered) and the redder or yellower the materials, the more oxidized, or altered.

The gorgeous, degraded, but well-preserved crater in this image has likely been sandblasted to reveal some of its exquisite bedrock exposures within the crater wall and sporadically on its ejecta blanket. This HiRISE acquisition is a particularly nice image because this an approximately 4 to 5 kilometer diameter crater almost fits perfectly in our 5 to 6 kilometer image swath (width; fixed by our camera's characteristics and the orbit of MRO).

This crater also exposes some exquisite bedrock layers from beneath the Martian cratered plains. Given the context of the geographic location of this crater, the light-toned layers exposed by the craters are likely clay-rich materials for which the Mawrth Vallis region on Mars is well known for.

Written by: Livio L. Tornabene

The compositional diversity of Mars was poorly known until a fleet of missions began mapping the surface with thermal and near-infrared spectrometers a dozen years ago.

CRISM on MRO is the latest of these instruments and targeted this particular region because exotic minerals had been detected nearby. Both sulfates and hydrated silica have been identified in this area by their near-infrared spectral signatures. The minerals were discovered near the rim of the giant Valles Marineris canyon system, in light toned layered deposits thought to be Hesperian in age (around 3 billion years old). The sulfates are spectrally similar to jarosite, a potassium iron sulphate hydroxide. The silica appears to lack a clear crystal structure and is thought to be made up of amorphous opal, which can contain up to 20 percent water.

If these interpretations are correct, the mineral assemblage suggests precipitation of the minerals from low temperature acidic fluids or formation by low temperature aqueous alteration of basalt. HiRISE images of light toned layered deposits elsewhere near the Valles Marineris show inverted channels and other morphological indications that the sediments were once saturated with water.

This HiRISE image gives some clues to how the minerals are expressed on the surface. The banded bedrock is visible beneath a partial cover of much younger dunes. Bright, relatively white bands can be seen to alternate with slightly redder layers. These bright bands could be concentrations of opal. The darker bands might correspond to concentrations of brown jarosite. The minerals may have segregated because of gradients in the temperature or acidity of the fluid.

These Martian deposits are not likely to be made up of pure opal of gemstone quality. However, opals and cherts on Earth are well known to preserve fossils and other biological evidence. Even a small sample of one of the bright bands in this scene would be priceless.

Written by: Paul Geissler

This observation is a candidate landing site for the Mars Science Laboratory (MSL) rover—named Curiosity—to be launched in 2011.

One of the main science objectives for MSL at this location is to examine deposits containing different types of clay minerals along the banks of Mawrth Vallis.

Written by: Sharon Wilson (23 June 2010)

HiRISE observations can be used to aid in the classification and volume estimates of dunes for the USGS global dune database--another way how the HiRISE camera helps other science agencies.

Sand dunes are among the most widespread aeolian features present on Mars. Their spatial distribution and morphology, sensitive to subtle shifts in wind circulation patterns and wind strengths, can relate to patterns of erosion and deposition, and give clues to the sedimentary history of the surrounding terrain.

Dunes are particularly suited to comprehensive planetary studies because they are abundant over a wide range of elevations and terrain types. Thus a global scale study of Martian dunes serves a dual purpose in furthering understanding of both climatic and sedimentary processes, two fundamental topics currently driving Martian science.

This caption is based on the original science rationale.

Written by: HiRISE Science Team

This "fresh" (very well-preserved) impact crater has created a radial pattern of dark rays. The image was suggested to address the question of why the rays are dark.

Is the crater so fresh and recent that there hasn't been time for bright dust to settle on the rays? That doesn't seem likely, as we can see windblown deposits inside the crater, which requires at least thousands of years to form after the impact event. Also, fresh craters with dark ejecta are common in Meridiani, and they can't all be extremely recent.

Did the crater eject a subsurface layer of dark material? Maybe, but all of the bedrock exposures in the surrounding region are relatively bright. The surface layer is darker than the bedrock because dark materials like hematite concretions ("blueberries" found by Opportunity rover) are resistant to wind erosion and get left as a lag deposit. At HiRISE scale the rays are seen to be a thin deposit, perhaps less than 1 meter thick.

The more distant ray segments contain many small secondary craters created by impact of rocks ejected from the primary crater. Maybe these are lag deposits from the original rays. In other words, a mix of broken-up target material was deposited, but the relatively bright materials have blown away since the crater formed. The darker sand or granule-sized materials might eventually be moved by the wind and trapped inside craters, as commonly seen over Meridiani Planum, but there hasn't been sufficient time since crater formation for this process to remove the rays.

The coarser particles might not be movable by wind in the current climate regime of Mars, but that changes over thousands and millions of years as Mars experiences periodic changes in orbital parameters such as tilt of the rotational axis. There has been some sand movement since the crater formed, since we see deposits inside the crater, but not enough to remove the rays.

Written by: Alfred McEwen

This image, and the associated subimage, show changing dunes and incised gullies in the southern Matara Crater.

Repeat imaging of the dunes in this crater shows that material towards the top of the gullies has moved downslope (towards the left in subimage), and the channel beds may have widened over time. Because this activity occurs during Martian Southern hemisphere winter, it is believed to be related to carbon dioxide frost that forms as the area grows colder. Scientists continue to monitor this region for changes in the gullies and the dunes themselves.

Written by: Kristin Block

This HiRISE image shows a large ice mound located in Louth Crater. At 70 degrees North, this is the lowest latitude permanent deposit of water ice on Mars.

The HiRISE image, taken in early summer, shows details of the mound and non-ice portions of the crater floor. The mound is characterized by rough textures and layering similar to features seen on the north polar layered deposits near the Martian North Pole. Zooming in to
an area in the southeast part of the mound, dark sinuous ridges are apparent.

These may be the crests of partially defrosted dark sand dunes or perhaps some other feature that we do not understand. This is the only area on Louth where these enigmatic ridges are found.

Written by: Nathan Bridges

This observation shows a crater with a surrounding bench. The crater's original rim appears to have been eroded, but mainly above a resistant layer. It's this sort of layering that gives the crater such a distinctive, "double-ring" appearance.

The image shows fine layering in both the bench and the inner crater walls. The interior of the crater--itself ringed by darker material--appears relatively featureless except for the ripples caused by wind.

Written by: HiRISE Science Team