27 March 2012
Well folks, I’m back from another successful LPSC! I am going to approach my recap differently this time: instead of an attempt to exhaustively list talks that I found interesting, I’m just going to do a few posts about key highlights, starting with the Masursky lecture by Jim Head about the history of the Martian climate. You can watch the lecture online here (if you don’t want to watch the announcements and student awards before the talk, skip ahead to ~14 minutes).
I thought Jim’s talk was a great summary of the state of our understanding of the evolution of the climate on Mars, and I will do my best to summarize it here. He started with our observations of recent Mars and worked his way back through the three major geological time periods, from the Amazonian (0 to 3 billion years ago) to the Hesperian (3 to 3.7 billion years ago) to the Noachian (pre-3.7 billion years ago).
For the Amazonian period, he emphasized the effects of the planet’s tilt (obliquity) on the distribution of ice. In short, when Mars is tilted more, the ice migrates from the poles to lower latitudes. And, based on orbital dynamics, we think that Mars is currently at an unusually low tilt. Unlike the Earth, which has a nice big moon to stabilize its tilt, Mars behaves chaotically, with large changes in obliquity.
Head also showed that even though the tilt of Mars is chaotic, observations can help to constrain which of the myriad paths of orbital evolution the planet took going back 3 billion years to the early Amazonian. By counting craters on ice-related features such as pedestal craters and lobate debris aprons (thought to be glaciers of some sort), you can get their approximate age and infer what the average obliquity was at that time.
The evidence points to a very cold and dry Amazonian period with ice movement controlled by the plant’s tilt.
So, what about the Hesperian? In this time period, the massive outflow channels that scar the planet formed when huge amounts of water were catastrophically released. Despite all this water, however, Head cited some papers that showed that the instead of forming an ocean, you just freeze the water and then sublimate the ice.
The other major feature of the Hesperian that Head talked about was the “Hesperian ridged plains” which are thought to be huge floods of basaltic lava that formed early in the Hesperian. Flood basalts on earth can form very rapidly, producing huge amounts of gas and ash. One of the major points was that these were very large, discrete events. As Head said: “You can’t take the average of this over a long period of time and have a realistic view of what’s going on.” The sulfur dioxide released from these eruptions initially acts as a greenhouse gas, warming the planet by up to 25 degrees, but then gradually the SO2 gets turned into aerosols, which cause global cooling. So the result of the sudden warming and then cooling is a few hundred years of unusually warm temperatures.
Head suggested a somewhat unorthodox explanation for the sulfates seen at Meridiani, where instead of forming by groundwater upwelling, they are deposited when the water from one of these brief warm periods gradually evaporates.
In the Noachian, of course, the presence of valley networks, possible lakes, and clay minerals (which form in the presence of lots of water) are all important constraints on what the climate was like. But Head points to a recent paper suggesting that many of the phyllosilicate deposits are likely hydrothermal and didn’t form at the surface. Head also pointed out that there are huge polar deposits that are Noachian-aged, which is confusing if the Noachian was warm and wet.
Recent climate models show that for early Mars with a thicker atmosphere, you don’t get temperatures above freezing, but you get colder temperatures at high altitudes (just like on Earth) so you end up with ice collecting in the southern highlands, right where the Noachian-aged polar deposits are.
So how does Head explain the valley networks in the Noachian? Well, remember those brief warm periods caused by sudden volcanic outgassing? Those might provide all the melting necessary. In Head’s words, “punctuated volcanism leads to punctuated climate change”.
This a picture of Mars with no long-lived warm conditions or large oceans, making it disappointing for those who want to believe that Mars was once a nice place. I’m sure many people will argue against this hypothesis of a cold planet with brief volcanic warm periods, and as Jim Head says at the end of his talk, that’s a good thing. This hypothesis should be tested, and Jim advocated that everyone contact their representatives to make sure that the NASA budget supports continued missions to help test it.