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Stop 2 at Hellas

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Alluvial fan at the base of a mountain in Death Valley, CA
Alluvial fan at the base of a mountain in Death Valley, CA
Photo: M. Miller, University of Oregon
Location: Death Valley, California, USA

Several impact craters that are many tens of kilometers across in the northern portion of the Hellas region have geologic features at the base of the interior crater walls called alluvial fans.

What is an alluvial fan, you may ask? An alluvial fan is an outspreading, gently sloping mass of water and detritus that is deposited onto a flat plain or valley floor below the mouth of a stream. Because the materials deposited onto the floor are not obstructed, it spreads out radially in all directions forming a fan-like shape. Over time, the fan may grow in size as repeated deposits of material spill onto and over the first set of deposits. Individual distributary channels may develop on the fan surface that allows the water and detritus to be carried further to the edges of the fan. Alluvial fans are common features at the base of narrow canyons in mountainous arid locations like Death Valley, California, USA (left image). Flash flooding carries material downslope quickly as there is little vegetation to soak up the rainwater.

Alluvial fans in an un-named impact crater in the northern Hellas region
Alluvial fans in an un-named impact crater in the northern Hellas region
Image: modified from figure 5 in Moore and Howard, 2005, Journal of Geophysical Research
Location: Crater center at approximately 74.3° E, 23.5° S
Scale: Diamter of the crater is approximately 66 kilometers

The image to the left shows a un-named impact crater in the northern portion of the Hellas region with as many as seven alluvial fans along the perimeter of interior crater walls. If you look closely, you can see source points where material is being deposited onto the crater floor. Distributary channels are also apparent on the fan surfaces.

Below, we will take a closer look at one of the alluvial fans highlighted by the black box.

Close-up of an alluvial fan in the northern Hellas region
Close-up of an alluvial fan in the northern Hellas region
Image: Portion of Context Imager image P03_002084_1567 (shortened ID)
Location: Within an un-named crater at approximately 74.3° E, 23.5° S
Scale: Diamter of the crater is approximately 66 kilometers

This image was taken by a camera onboard the MRO spacecraft called Context Imager (CTX), which can capture the surface in high detail at up to 6 meters per pixel. Thus, something about the size of a large automobile should be distinguishable. Much like recent digital cameras can take far more detailed pictures than the first digital cameras ever produced, cameras like CTX are much improved compared to their predecessors from previous Mars missions.

In the image, we can see that the fan shape is roughly defined by the many distributary channels that run across the surface. Normally, channels are negative-relief features, meaning that their surfaces are lower than the surfaces outside the channel. But if you look closely at the image, these channels no longer have negative relief, but in fact have positive-relief (sun illumination from the left).

How did the channels change? These channels, now ridges, are an example of inverted topography. On Earth, this style can develop when the channel floor or leftover sediment in the channel is more resistant to erosion compared to the surrounding walls. Over time, erosion strips away the walls leaving the former channel as a high-standing ridge. So for this alluvial fan on Mars, it appears that the fan material has been eroded and redistributed, leaving behind relict raised distributary channels.

The presence of alluvial fan features provide evidence that liquid, perhaps water, once flowed on the surface in this location. Exciting stuff!

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