Post contributed by A.M. Bramson, Lunar and Planetary Laboratory, University of Arizona

When an object impacts into layered material, it can form a crater with terraces in the crater’s walls at the layer boundaries, rather than the simple bowl-shape that is expected. The shock wave generated by the impact can more easily move the weaker material and so the crater is essentially wider in that layer, and smaller in the underlying stronger material. From overhead, these concentric terraces give the appearance of a bullseye. Craters with this morphology were noticed on the moon back in the 1960s with the terracing attributed to a surface regolith layer. More recently, numerous terraced craters have been found across a region of Mars called Arcadia Planitia that we think is due to a widespread buried ice sheet.

Image 1: A terraced crater with diameter of 734 meters located at 46.58°N, 194.85°E, in the Arcadia Planitia region of Mars. This 3D perspective was made by Ali Bramson with HiRISE Digital Terrain Model DTEEC_018522_2270_019010_2270_A01. Using this 3D model, we were able to measure the depth to the terraces, and therefore the thicknesses of the subsurface layers that cause the terracing.

One of the first of these “terraced craters” found on Mars was initially called “bullseye” crater because it was also thought that it formed from a very lucky double impact- a crater within a crater. However, using the Context Camera (CTX) and High Resolution Imaging Science Experiment (HiRISE) camera, both onboard the Mars Reconnaissance Orbiter (MRO), we have now have imaged over 100 of these terraced craters across the northern mid-latitude region of Arcadia Planitia. The probability of having this many lucky double impacts is nearly zero. A much more likely explanation is that there is widespread layering across this region of Mars. Sure enough, the Shallow Radar (SHARAD) sounder on MRO also detects a subsurface interface in this region.

Image 2: An overhead view of the terraced crater in Image 1 taken by the HiRISE camera (Image ESP_018522_2270). This is one of the two images that formed the stereo pairs used to create the 3D model of this crater.

We made 3D models of the craters using stereo image pairs taken by the HiRISE camera; this allowed us to measure the depth to the terraces, and thus the thickness of the layer to be ~40-50 meters. Comparing the layer thickness with measurements of the time it takes for the radar signal to pass through the layer, we were able to constrain the dielectric constant of the layer, a property dependent on the composition of the material. The value that we measured (2.5) suggests the widespread layering across Arcadia Planitia is due to a nearly pure water ice layer atop underlying stronger rocky material. This ice sheet covers an area up to ~106 km2, the size of California and Texas combined. It extends to 38°N latitude, where conditions are more favorable for human explorers than at the polar ice caps. A couple dozen of these craters also exhibit a shallower, subtler terrace, suggesting multiple layers exist across the region, possibly due to layering within the ice.

Further Reading

Bramson, A.M., et al. (2015), Widespread excess ice in Arcadia Planitia, Mars. Geophysical Research Letters, 42, doi:10.1002/2015GL064844.

Ormö, J., A.P. Rossi & K.R. Housen (2013), A new method to determine the direction of impact: Asymmetry of concentric impact craters as observed in the field (Lockne), on Mars, in experiments, and simulations. Meteoritics & Planetary Science, 48, 3, 403-419, doi:10.1111/maps.12065.

Quaide, W.L., & V.R. Oberbeck (1968), Thickness determinations of the lunar surface layer from lunar impact craters. Journal of Geophysical Research, 73, 16, 5247-5270, doi:10.1029/JB073i016p05247.