HiRISE Captioned Images 2019

[bystander]

Week of 21 January 2019

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Multi-Elevation Gullies (ESP_057450_1410) (HiClip)

Gullies probably formed along the bouldery layers in the upper slopes of this unnamed crater within the last few million years. Gullies eroded these crater slopes and transported sediment downslope forming debris aprons multiple times.

These older apron surfaces were cut by numerous fractures running perpendicular to the slope. Subsequent episodes of gully activity eroded through these fractures and deposited new aprons.

On the floor of the crater are ridges with bouldery layers. These ridges may mark the furthest extent of glaciers that predate much of the original gully activity. Bright flows continue to form in these gullies seasonally.

In the upper gully regions, long shadows cast by jagged outcrops allow scientists to determine the heights and depths of landforms by measuring the length of the shadows cast by the ridges onto the gully floor.

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Impact Near the South Pole (ESP_057152_0985) (HiClip)

This image shows a new impact crater that formed between July and September 2018. It’s notable because it occurred in the seasonal southern ice cap, and has apparently punched through it, creating a two-toned blast pattern.

The impact hit on the ice layer, and the tones of the blast pattern tell us the sequence. When an impactor hits the ground, there is a tremendous amount of force like an explosion. The larger, lighter-colored blast pattern could be the result of scouring by winds from the impact shockwave. The darker-colored inner blast pattern is because the impactor penetrated the thin ice layer, excavated the dark sand underneath, and threw it out in all directions on top of the layer.

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Cross-Section of a Complex Crater (ESP_058057_1465) (HiClip)

This image shows a cross-section of a complex crater in Terra Cimmeria.

Starting in the center, we see a series of peaks with exposed bedrock. These peaks formed during the impact event when material that was originally several kilometers below the surface was uplifted and exposed. The impact also melted the rocks. This eventually cooled, forming the pitted materials that coat the crater floor around the uplift.

The rim of the crater was unstable, and collapsed inwards to form terraces, and we see additional pitted materials between the terraces and the rim. Just outside the crater we can see dark-toned material that was excavated and thrown out after the impact.

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A First Look at Dunes (ESP_057903_1390) (HiClip)

This image shows us a cross-section of a dune field. Dune shape depends on several factors, including the amount of sand present and the local wind directions. This dune field displays several distinct dune morphologies.

We see both individual barchan-like dunes and more complex dune shapes. The dunes are arranged in a linear fashion at the northern extent of the field, first in areas with lots of sand, and then with relatively sand-free patches in between dune crests. HiRISE has observed dune activity in other similar fields, but this is our first image over this group of dunes.

A second image is needed to determine if these dunes are also evolving and moving.

Credit: NASA/JPL-Caltech/University of Arizona

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[bystander]

Week of 04 February 2019

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Exposing the Rock in Impact Craters (ESP_057866_1670) (HiClip)

In this complex crater (about 44-kilometers in diameter), we see bedrock in several locations from different depths in the crust. The central uplift exposes large fragments of green-toned bedrock that possibly originated from several kilometers beneath the surface.

To the south of the crater, we see more of this bedrock along with material that was excavated and thrown out after the impact. In craters of this size, the rim is unstable and collapses inwards forming terraces, which occasionally exposes more bedrock that would have originated from close to the surface than the rocks exposed within the uplift itself. Central uplifts have better exposures of bedrock, but in this example the terraces steal the show, displaying beautiful green- and light-toned bedrock at multiple locations.

This is a stereo pair with ESP_057932_1670.

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Layered History (ESP_057970_1645) (HiClip)

The geologic history of a planet is written in its layers. Erosion of the surface reveals several shades of light toned layers, likely sedimentary deposits.

The most recent geologic features are the narrow sand dunes snaking across the top of all the rock.

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A Dune Field near Nili Patera (ESP_057071_1890) (HiClip)

In this image many sand dunes are visible. They have an elongated crescent form and are called “barchan dunes.” They are formed by the continuous action of the wind, blowing in the same direction, giving this particular shape.

The orientation of these dunes tell us that the prevailing wind blows from the right to the left (east to west). The wind is continuously moving sand grains up the longer dune slope, towards the top. The small ripples on the slope are caused by this movement. When the sand grains arrive at the top, they fall down the steeper and shorter slope, which as a consequence, has no ripples. It is this gradual sand movement that causes the dunes to slowly move over time.

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Wind Flow (ESP_057930_1720) (HiClip)

The atmospheric pressure on Earth at sea level is about 1 bar. On Mars, the pressure is 6 to 10 millibars, or 1/100th that of our planet. But even in this atmosphere, wind still flows around obstacles.

In this image the ripples in the sand tell us which way the wind was moving and how it was diverted around these rock formations.

This is a stereo pair with ESP_057864_1720.

Credit: NASA/JPL-Caltech/University of Arizona