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!

moc_fig24

a) Layers in Valles Marineris; b) the box shows the extent of a, the arrows point to outcrops of light-toned layered material; c) & d) show other light-toned outcrops, and e) provides their context.

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.

A spectacular example of layered sedimentary rocks in Arabia Terra on Mars.

A spectacular example of layered sedimentary rocks in Arabia Terra on Mars.

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:

The new model for the martian surface as a "cratered" volume in which surfaces are constantly buried and re-exhumed while also undergoing impacts.

The new model for the martian surface as a "cratered volume" in which surfaces are constantly buried and re-exhumed while also undergoing impacts.

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:

Reull Vallis cuts right through the rim of a large impact crater. If the river had been flowing over a flat plain and encountered the crater, it should have been diverted. However, if the crater was buried, the river's course could pass right over it. Then as the landscape was eroded, the river could carve through the crater walls as they began to emerge. This is often seen on earth (e.g. the Susquehanna river cutting through the Appalachians) where mountains are lifted up and pre-existing rivers carve through them.

Reull Vallis cuts right through the rim of a large impact crater. If the river had been flowing over a flat plain and encountered the crater's rim sticking up, it should have been diverted. However, if the crater was buried, the river would not "see" it, and could pass right over it. Then, as the landscape was eroded, the river could carve through the crater walls as they began to emerge. This is often seen on earth (e.g. the Susquehanna river cutting through the Appalachians) where mountains are lifted up but pre-existing rivers maintain their original course by carving through them.

moc_fig33

Buried craters, ranging from completely filled (a), to partially exhumed (b,c) to almost completely uncovered, with only a small remnant of the fill remaining (d).

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…

a) Context image showing a valley; b) a view of the valley wall. Notice that there are two craters that appear to be partially underneath the valley's layered wall. It is not obvious whether these are ancient craters that are being uncovered as teh valley erodes away the overyling layers, or whether they formed on teh valley floor and have been partially covered by debris falling off of the wall. These are shown in more detail in e. c and d show views of the upland and valley floor. It is impossible to tell whether the craters on the valley floor are old or young.

a) Context image showing a valley; b) a view of the valley wall. Notice that there are two craters that appear to be partially underneath the valley's layered wall. It is not obvious whether these are ancient craters that are being uncovered as teh valley erodes away the overyling layers, or whether they formed on teh valley floor and have been partially covered by debris falling off of the wall. These are shown in more detail in e. c and d show views of the upland and valley floor. It is impossible to tell whether the craters on the valley floor are old or young.

ResearchBlogging.orgMichael 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

Explore posts in the same categories: Geology, MGS, MOC

3 Comments on “The MOC “Book”: Subsurface Patterns and Properties”


  1. Doesn’t the Reull Vallis photo indicate the river activity must predate, postdate, and consistently out-erode the crater uplift? That would seem to require hundreds of thousands of years or more of at-least periodic flows, and something that would be difficult to ascribe to rare warm periods from volcanic eruptions and giant impacts.

  2. Ryan Says:

    That’s a good point. Reull Vallis must have been active for quite some time to me able to carve down through the crater rim. That doesn’t mean that it had water flowing all the time, rivers do most of their erosion during the flood stage, but it does imply that there were at least intermittent floods often enough to keep up with the uplift/exhumation of the crater.


  3. Thinking some more, I suppose that if the uplift came and ended first, then the river came along and only after the river got started that exhumation of the crater began, then a few massive flood events might have done it without need to keep pace with a consistent uplift.


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