How to Look for Hidden Mare

Red dots show the locations of basalt spectra on this map of the western limb of the Moon, while the background colouring gives information about the iron content of the surface. In general, mare are iron-rich (green to red), while the highlands are iron-poor (blue).  However, these data show that things can be more complex than expected.
Image Credit: Irene Antonenko

Well, it's summer. The kids are out of school and my home office is not the most peaceful place to work right now. In other words, it has been hard to find quiet time in which to write.  All this is my complex way of apologizing for not getting this blog post out earlier. But, on this very hot evening, I have a few calm hours to tell you more about my research on the Moon.

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.

The presence of basalt spectra (red dots) on the rim of this crater, along with higher iron compositions (green-yellow) than in the surroundings (blue), shows that basalt materials have been excavated from deep below the surface.
Image Credit: Irene Antonenko

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.

A cryptomare forms when ejecta from a large impact covers and obscures a mare deposit. Subsequent impacts into the region can penetrate the obscuring layer and excavate the underlying cryptomare material, exposing it on the surface.
Image Credit: Irene Antonenko

In my work, I search for craters that excavate hidden basalts. When I find them, I measure their diameter and use that to estimate their depth of excavation - that is the maximum depth from which this crater was able to excavate material. The smallest basalt-excavating craters in an area provide an estimate of the top of the cryptomare deposit, since smaller craters wouldn't penetrate through the overlying ejecta layer. The largest basalt-excavating craters in an area provide an estimate of the bottom of the cryptomare deposit, since larger craters would start to excavate the underlying highland crust and so obscure the basalt signature. The side boundaries of the cryptomare are estimated by where the basalt-excavating craters stop, since no craters of any size would excavate basalt where there is no hidden mare.  By using these techniques, I am able to estimate the size and shape of cryptomare deposits, allowing us to study them in more detail.

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

However, things aren't always quite so simple. In one study area, the Schickard crater on the south-western limb of the Moon, my research suggests that there may be two layers of cryptomare present. If my interpretation is correct, this means that shortly after Schickard crater formed in the highland crust, it was flooded with basalts. Then, these basalts were obscured by a thick ejecta deposit, forming a deep cryptomare. Later, more volcanic material was deposited on top of the ejecta layer. Another impact then obscured this second mare unit, creating a shallow cryptomare. Finally, a small amount of volcanic flooding formed two small mare patches that can be seen at the surface today.  This kind of work is rather exciting, because it suggests that areas of the Moon may be quite complex below the surface, resembling an intricate multi-layer cake!