I am currently studying what we think is a thick (10s
of meters), extensive layer of subsurface ice in Arcadia Planitia,
Mars. We are probing this layer by combining two data sets: imagery of
terraced craters and SHARAD radar data.
Terraced craters indicate
layered target material while SHARAD radar reveals reflections in the
subsurface. Combining the data will allow us to constrain the
properties, and map the variations, of this layer across the region.
HiRISE Image ESP_018522_2270
Crater 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 layers that existed in the target material when the
impact occurred. 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)).
is an example of a SHARAD radargram in Arcadia Planitia showing
reflections due to the interface with the surface and a subsurface
interface. 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 subsurface interface
is what we are interested in studying, as it appears to be related to a
widespread subsurface layer in the region. The horizontal axis is
distance along the track while the vertical axis is time.
This layer of ice....
... is puzzling. Mars' obliquity goes through extreme variations compared to
Earth's due to the fact Mars doesn't have a large, stabilizing moon
like Earth does. These obliquity variations can drive large-scale
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 disappearing
during the recent low obliquity times?