17 January 2013

The two-faced crust of Mars

Posted by Ryan

Hello loyal readers! After some deliberation, I have decided to try an experiment with this blog. Instead of trying to provide updates on current events in planetary science and space policy as I have done in the past, I am going to shift the focus to writing up summaries of peer-reviewed journal articles. As a scientist, one of my weaknesses has always been that I don’t make enough time to read current research, instead relying on occasional conferences to keep me up to date on what is going on in my field. So my thought is that if I can blog about at least one article per week, I can force myself to stay current, while providing all of you with condensed de-jargonified versions of the latest planetary research. I hope you enjoy it! And if you still need your fix of mission updates and space policy, I strongly suggest you follow the Planetary Society blog and Unmannedspaceflight. For real space policy wonks, Space Politics is also a good choice. Ok, now without further ado, let’s dive into the first paper!

I am always a sucker for research that uses very simple observations to come to profound conclusions, and that is definitely the case with “The dual nature of the martian crust: Young lavas and old clastic materials” by Josh Bandfield, Chris Edwards, David Montgomery, and Brittany Brand. This paper suggests that the martian crust has a dual nature, where the oldest rocks are actually softer and easier to erode, while more recently lava flows have led to much more durable terrain.

But before we get too far, we need to back up and look at past theories about the martian crust. For a long time, the thought was that Mars was not that different from the moon, and so its crust was composed of what’s called a “mega-regolith” which is just jargon for “a giant jumble of blocks broken up by impacts”. Older crust = more busted up = easier to erode. But then along came new orbiters with higher-resolution cameras, which revealed that Mars has outcrops of layered rocks, implying that it’s not just mega-regolith all the way down.

Given this background, Bandfield et al. took a look at some of the most prominent exposures of the martian crust: the walls of Valles Marineris, the canyon system that cuts across the face of Mars like a scar thousands of miles long. For the most part, the walls of the canyon are not sheer vertical cliffs, they have shallow slopes, which tend to indicate weak materials. Thermal inertia measurements tend to agree. Thermal inertia is a measure of how quickly something heats up or cools down. Generally, solid rock has a high thermal inertia, but fine sand or dust has a very low thermal inertia. The walls of Vallis Marineris have a low-ish thermal inertia that is more consistent with sand-sized particles than big solid blocks of lava rock.

Valles Marineris as seen by THEMIS on Mars Odyssey.

Another clever way of telling how strong the rocks in the walls of a canyon are is to look for boulders. Strong rocks break up into strong boulders that can tumble down for a long way before breaking up into pieces that are too small to see. Also, strong boulders can sit out exposed to erosion by the wind for much longer than boulders made of weak rock. Back in 2000, when Malin and Edgett were studying the first high-resolution images of the walls of Vallis Marineris, they noted that the boulders tend to survive only a few hundred meters down the slope, pointing to weak rocks that erode easily. There are some places in the canyon walls with strong rocks interpreted to be the result of big lava flows, but for the most mart the rocks appear to be weak.

Bandfield et al. also looked elsewhere on Mars, including in the giant outflow channels carved by ancient catastrophic floods. These channels tend to lack strong, blocky rocks in their walls, but have high thermal inertia on their floors. Bandfield et al. suggest that the floods may have carved through weaker materials until reaching stronger layers, which then ended up being the final floor of the channel. Also, they don’t mention it, but I know that at least some of the outflow channels are thought to have been flooded with lava flows after the water outflow was long-gone.

They also note that in the craters of the ancient southern highlands, most signs point to weak rocks. The craters tend to be heavily eroded and have low thermal inertia and few boulders, and the shape of the craters themselves (the ratio of their depth to their diameter) are more consistent with weak rocks.

The authors spend some more time comparing Mars craters to those on the Moon, which tend to be blocky (consistent with a megaregolith) and the surface of Venus, which shows in our very limited set of images from the surface shows particles ranging from gravel to blocks, consistent with a lava-covered planet. On Earth, Mt. St. Helens provides a good example of a soft-ashy material, in contrast to very strong blocky rocks on the nearby Columbia River basalt flow.

Figure 14 from the paper: An aerial view of the Columbia River basalts on Earth. These strong rocks erode to form sharp cliffs.

Figure 15: Mt.St. Helens ash is weak and erodes with much shallower slopes.

So what does it all mean? Well, Bandfield et al. suggest that “volcaniclastics” – that is, fine-grained ash and other small particles from volcanoes – can explain the weak early crust. There are lots of studies that have identified possible ash deposits on Mars, and one of the more prominent deposits, the “Medusae Fossae Formation” clearly has lava flows that lapped up against it, supporting the idea that early explosive ash-producing eruptions transitioned to “effusive” lava flow-producing eruptions later in martian history. This sort of makes sense: early on, the planet was wetter and the erupting magma itself may have contained more gases like water and CO2 that lead to explosive eruptions. As the climate dried and the gas-rich magmas were exhausted, you would end up with eruptions in the form of lava flows.

So, bottom line, the authors suggest that early Mars was probably dominated by explosive eruptions and ash deposits, leading to a soft, easy-to-erode ancient crust. As the planet dried out and the magma chambers for the major volcanoes ran out of gases, the eruptions switched over to lava flows. The end result is a planet with a weird dual nature to its crust, something to keep in mind when interpreting erosion on Mars, especially in the most ancient rocks.

 

 

 
ResearchBlogging.orgBandfield, J., Edwards, C., Montgomery, D., & Brand, B. (2013). The dual nature of the martian crust: Young lavas and old clastic materials Icarus, 222 (1), 188-199 DOI: 10.1016/j.icarus.2012.10.023