I am currently studying a thick (10s of meters), extensive layer of subsurface ice in Arcadia Planitia, Mars. We are probing this layer with multiple data sets from the Mars Reconnaissance Orbiter: images and models (like the 3D model shown above) of terraced craters and SHARAD radar data.
Terraced craters indicate layered target material while SHARAD radar reveals reflections in the subsurface which also suggest layering in the subsurface. Combining these data has allowed us to constrain the properties, and map the variations, of this layer across the region. Our results are consistent with a widespread layer of decameters-thick excess (volumetric concentrations that exceed that available in the pore space of the regolith) water ice. The area covered by the layer is on the order of a million square kilometers, about the size of California and Texas combined!
HiRISE Image ESP_018522_2270
"Badger" Crater (as I informally refer to it) is an example of a terraced crater with a diameter of ~710 m. This crater has two terraces: a shallow, smaller terrace and a wider, deeper terrace above the "nested crater" which gives it the appearance of a bullseye. However, this crater did not form due to a lucky, second strike in the middle of a preexisting terraced crater. It got this shape because of the layers that exist in the target material upon impact. Note that the nested crater is slightly offset from center. This has been proposed to be due to an oblique impact (see J. Ormo, A.P. Rossi & K.R. Housen, Meteoritics & Planetary Science, 48, Nr 3, 403-419 (2013)).
I have been collaborating with Dr. Elena Martellato and Dr. Gabriele Cremonese to numerically model the formation of these craters in layered targets of ice. So far we have had good success in matching the models to actual profiles of Badger crater. These results have been presented at LPSC 2015 and DPS 2014.
This is an example of a SHARAD radargram in Arcadia Planitia showing reflections due to dielectric interfaces at the surface and in the subsurface. Above is an example of a SHARAD "radargram"; the horizontal axis is distance along the track while the vertical axis is delay time for the radar signal to bounce down and return to the instrument. This subsurface reflection was first documented by Plaut et al. in their 2009 LPSC abstract "A widespread radar-transparent layer detected by SHARAD in Arcadia Planitia, Mars." The shallow subsurface interface is what we have mapped, as it appears to be related to a widespread subsurface layer in the region that could also be responsible for the terracing in the craters across this region.
... is puzzling. Mars' obliquity goes through extreme variations compared to the Earth due to the fact Mars doesn't have a large, stabilizing moon like Earth does. These obliquity variations drive climate change, including water ice migration. At high obliquities, ice can move to lower latitudes. However, in recent times, the obliquity has been lower making ice unstable at the mid-latitudes. These recent periods of ice instability mean the ice should not even be here today and would have to be quite old (coming from those periods of high obliquity). What could have kept this ice from sublimating away during the recent low obliquity times?
I'm now writing a thermal model to look into conditions that would allow the ice to remain stable throughout the last 10s of millions of years.
This material is based upon work supported by the National Science Foundation Graduate Research Fellowship Program under Grant No. DGE-1143953. Any opinion, findings, and conclusions or recommendations expressed in this material are those of the authors(s) and do not necessarily reflect the views of the National Science Foundation.