My last blog post talked about how I use spectra to identify complexity on the Moon's surface. The fascinating thing is, this complexity often reveals that interesting things are happening below the surface as well.
In order to study what's going on below the Moon's surface, or the lunar stratigraphy, I look for basalt spectra in areas that are otherwise highland-like in their signature. These kinds of basalt spectra are often associated with impact craters. What's happening here is that impacts are excavating hidden basalt units and distributing them in an ejecta deposit around the resulting crater, thus exposing them on the surface.
So, what are these hidden basalt units and how did they form? The most established theory is that these units represent ancient hidden mare deposits.
Very early in the Moon's history (about 4.2-3.2 billion years ago), massive amounts of basalt flowed out onto the lunar surface. These solidified to form the mare deposits. However, during this time, occasional gigantic impacts were still hitting the Moon's surface. In some cases, the ejecta from such huge impacts was emplaced on top of an existing mare unit. Since these ejecta deposits could be quite thick (up 2 kilometers thick), they covered the underlying mare and hid it from view, forming a "cryptomare" (literally meaning hidden mare). Subsequent impacts, if they were large enough, were able to penetrate through the overlying ejecta layer and into the cryptomare, excavating the hidden basalt material and emplacing it around the newly formed crater.
|Mapping of basalt (greed dots) and highland (blue dots) spectra suggests that there may be more than one cryptomare deposit in the Schickard crater area. |
Image Credit: Irene Antonenko