28 March 2012
On the second day of this year’s Lunar and Planetary Science Conference, two of the most interesting talks that I saw were back to back in the morning session about planetary hydrology.
The first was by Jeff Andrews-Hanna, about the latest results from his groundwater modeling. You can read the abstract here. For years, Andrews-Hanna has been developing global models of groundwater activity on Mars and using them to explain some of the geologic features that we see. At this meeting, he presented the result of adding a “realistic” climate model to predict where it would rain on early Mars, rather than just assuming a uniform distribution of rain in the mid latitudes. I put “realistic” in quotes because the climate model that he used was developed for Earth and essentially treats Mars as though it is the Earth minus oceans, with Mars topography. Maybe early Mars was not as warm and wet as this model assumes, but it does give some interesting results.
By predicting where rain would fall on Early Mars, Andrews-Hanna was then able to simulate how that precipitation would make its way into the groundwater table and then upwell in low-lying areas. These areas of groundwater upwelling would be places where sediment would accumulate and be cemented by the salts in the groundwater, forming sedimentary sequences. Lo and behold, this model predicts large sequences of sediment to be deposited in Arabia Terra, Meridiani, and in Gale Crater! The Arabia deposits are consistend with the remnants of eroded sedimentary outcrops seen from orbit, and at Meridiani the evidence from the Opportunity rover suggests that the rocks are sandstones cemented by sulfates, likely the result of groundwater upwelling.
As for Gale crater, the lower unit of the mound is a couple of kilometers high, which is about the thickness of the sediment predicted by Andrews-Hanna’s model. As the crater filled in, the amount of groundwater upwelling would have decreased, providing a natural limit on how thick the deposit could be. The upper portion of the mound looks geomorphically and mineralogically different, and could have been deposited later through some other process.
The second talk that I found particularly interesting was by Itay Halevy, and it expanded on some of the ideas discussed by Jim Head in his Masursky lecture. I met Itay at the Agouron field trip last summer and am always impressed by his work, which combines climate modeling, chemistry and geology. Itay pointed out that more than 30 percent of the martian surface is covered in lava flows that formed in the late Noachian to early Hesperian, which implies a whole lof of volcanic outgassing associated with such huge eruptions. Even for the relatively small columbia river flood basalt on Earth, the amount of outgassing was enormous: ~450 times as much as what was released in the 1991 eruption of Mt. Pinatubo.
There has been some debate among martian climate experts over whether the sulfur dioxide gas produced by volcanic eruptions would cause warming (SO2 is known to be a greenhouse gas), or whether it would rapidly turn into aerosol paricles of sulfuric acid, which can cause cooling. The key is the timescales: the warming from release of SO2 would start immediately, while it would take a while for the aerosols to form, so for short-lived eruptions, there would just be a spike of warming. For longer eruptions, the warming would eventually be overpowered by the aerosol cooling, and the planet would cool. In either case, the warming period would last at most a few hundred years before the planet returned to its normal cold, icy state.
Itay suggested that the presence of sulfates in younger rocks than the phyllosilicates in most places on Mars is not because sulfates weren’t produced earlier on. He argued that it is just that the earlier sulfate deposits were eroded in subsequent phases of warming and melting, so we only see those deposits that came late enough that they weren’t eroded away.
The real take-away from Itay’s talk (and Jim Head’s lecture) was that early Mars could have been cold most of the time, with sporadic periods of volcanic activity leading to brief periods of warming and melting.