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Investigating Underground Martian Ice with Impact Craters
By Ali Bramson


Imagine if there was a layer of ice as tall as a 13-story building underneath the entire state of Texas. We have found a layer of ice that big under a region of Mars called Arcadia Planitia.

I am a graduate student in the Department of Planetary Sciences at the University of Arizona and I study ice under the surface of Mars. The structure of Martian impact craters, it turns out, can tell us much about what's going on underground. I am using impact crater measurements to learn more about this Texas-size layer of ice under Arcadia Planitia. I am also studying the radar signals that bounce off the ice and go back to the Mars Reconnaissance Orbiter (MRO) spacecraft's SHARAD (Shallow Radar) instrument. The radar measurements are telling us about the composition of the ice.

Craters that form where there are layers in the subsurface have terraces within their walls, rather than being simple bowl shapes. This is because the shock wave generated by the impact moves differently through the different materials, leaving a terrace at the boundary between two layers. Many of the craters I see in Arcadia Planitia have two terraces: a wide "floor terrace" at the bottom where the ice meets the underlying rock, and a smaller "wall" terrace which likely indicates layering within the ice. This double-terraced structure gives the craters a concentric, "bullseye" look to them.

Bramson Blog Crater .jpg
Examples of crater profiles for a subsurface without (top) and with (bottom) layers.

Measuring how deep these terraces are tells us how deep the ice goes. To measure these depths, we use the HiRISE (High Resolution Imaging Science Experiment) camera to take pairs of stereo images. Like how you and I can see in 3D by having 2 eyes that look at the world from slightly different angles, the HiRISE stereo pairs are two images of the same feature, taken from different angles, to see Mars in 3D.
Flyover of terraced crater DTM
Flyover of terraced crater DTM
The 3-dimensional product we make using these stereo pairs is called a Digital Terrain Model (DTM). We can represent depths with colors: for the examples below, red is the elevation at the surface and purple is the deeper elevation of the terrace. Using these DTMs, I have measured these terraces to be about 40 meters (about 130 feet) deep.
digital-terrain-models-elevation-map.jpg
Two examples of Digital Terrain Models I have made of terraced craters.

When we combine the depth of the ice with information about the time it takes the radar signal to bounce to the bottom of the ice layer and back, we can measure additional information about how the radar wave moves through the layer; the way the radar wave moves through this layer suggests the layer is made of water ice.

sharad-radar-reflections.jpg
An example of what the SHARAD radar instrument on MRO returns: a "radargram" showing the strength of the radar wave at different positions (x-axis) and times (y-axis). The top bright radar signal is from the radar wave bouncing off the surface. The bottom subsurface signal comes from the bottom of the ice layer.

The radar reflections and terraced craters across the whole region of Arcadia Planitia (180-225°E, 38-50°N) tell us this layer exists across an area as big as Texas.

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