The MOC “Book”: Subsurface Patterns and Properties
The MOC paper saga continues. If you’re just tuning in, I’ve been writing a series of posts detailing a slow and detailed reading of the classic 2001 paper summarizing the results from the Mars Orbital Camera (MOC), the first high-resolution camera in orbit around Mars. Check out the previous posts here and here. Also, a reader pointed out to me that the full PDF of the paper is freely available at the author’s website! So if you’re interested, I encourage you to download it and read along with us.
On friday we talked about section 3.6 of the paper: subsurface patterns and properties. The real take-away message of this section is that Mars is layered. Everywhere that bedrock is exposed, it has layers. From the paper:
“On Earth, the observation of layers would not be a surprise, but the prevailing consensus … prior to the MGS mission held that much of the Martian crust, particularly in the ancient, heavily cratered highlands, should be something like that of the lunar highlands: an upper kilometer or two of interbedded crater ejecta, lava flows, and perhaps sediments and soils underlain by tens of kilometers of megabrecciated primordial crust…”
A great example of layering on Mars is the walls of Valles Marineris. The kilometers-high walls of this giant canyon show layers as much as 10 km below the surface! The authors point out that they had suspected that Mars was layered but they were really surprised by how deep the layers go!
One especially interesting part of the Valles Marineris section was the observation that there aren’t very many boulders at the bottom of the cliffs. This implies that the rocks the walls are made of is breakable enough that large blocks can’t survive the violent tumble down the canyon walls. Thick volcanic rocks could easily survive such a fall, so the authors deduced that many of the layered outcrops are made of sedimentary rather than igneous rocks! That’s a pretty important conclusion based on an observation I wouldn’t even have thought of!
The paper points to other examples of sedimentary rocks elsewhere on the planet. In some cases, locations that were thought to be layered based on low-resolution images turned out to indeed be layered, but at a much finer scale. The large apparent layers were actually accumulations of dark sand, but where there was no sand, MOC revealed many small layers.
Based on the discovery of layers all over the planet, at depths up to ten kilometers, Malin and Edgett proposed a new model for the crust of Mars, depicted in this cartoon:
This model shows the surface of Mars as a very complicated place, constantly being buried and then uncovered, with craters interspersed throughout the sequence of layers. It’s a big contrast from the view of Mars as being essentially the same as the moon, with a cratered surface and kilometers of crumbled up debris underneath. To support their new model, Malin and Edgett give some great examples:
The authors point out that the constant burial and exposure of surfaces on Mars makes it very difficult to reliably tell the age of the surface by crater counting. A heavily cratered surface is certainly old, but it may have only been uncovered relatively recently. And how do you deal with a surface that accumulated some craters, was buried for a billion years, and then exposed, and hit by more impacts? Obviously, the new “cratered volume” idea of the martian surface in this paper poses some difficulties…
Michael C. Malin, Kenneth S. Edgett (2001). Mars Global Surveyor Mars Orbiter Camera: Interplanetary cruise through primary mission Journal of Geophysical Research, 106 (E10), 23429-23570 DOI: 10.1029/2000JE001455