Impacts, Occam’s Razor, and Layers

The final Mars talk that I saw yesterday came at the end of a session rich with discussion of geochemistry, aqueous alteration, hydrothermal systems, and reference to the ubiquitous layers seen by both Mars rovers as being emplaced, or at least altered, by water. So I was interested to hear that the final presenter, Don Burt, has an alternative hypothesis.

Burt suggested that since layers are so ubiquitious, and that they are seen at both landing sites, we should not invoke multiple unique, complicated scenarios to explain them, when there is a simple one that could do the job. He think that flows of debris from impact craters, called “base surge”, could explain everything that is seen. He says that the cross-bedding (slanted layers that intersect each other) that is observed was also commonly seen in sand around nuclear test sites, and is seen in the remains of volcanic explosions. He also pointed out that it is easy to form spherical particles similar to the famous “blueberries” at Meridiani in a turbulent cloud of impact surge.

His hypothesis is an unpopular one, and has some serious issues. It doesn’t explain the chemistry of the rocks, and the sorts of spherules formed in “base surge” should be randomly distributed, not evenly spaced like the blueberries. Also, he said that the blueberries are blue-grey, while similar water-formed concretions on Earth are reddish. The blueberries are not blue. They are jus tless red than the rest of the dusty martian surface, so they show up as blue in false color images.

Burt claimed that his impact layer hypothesis is simplest, and therefor the best. This line of reasoning is often called “Occam’s Razor”. Unfortunately, I think Burt falls into the trap of making things too simple. There is a famous quote (or paraphrased quote) from Einstein that sums this up: “Everything should be made as simple as possible, but not simpler.”

Despite the many problems of the impact hypothesis, I was glad to see Burt’s presentation. What good is science if we all just get up and agree with each other? He also had some very good points, particularly that it is dangerous to assume that Mars was or is earth-like. That sort of assumption can lead to unintended biases and clouded judgment. Obviously, we have to assume that processes on other planets work similar to those seen on our own: that’s basic comparative planetology. But we have to be careful not to “force” Mars into an earth-like mold.

Burt’s talk was not popular, and his hypothesis will probably not be picked up by many people, but I think it is a good sign that there are the occasional voices going against the crowd. Sometimes those voices are correct and everyone else is wrong, and that’s how progress is made.

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2 Comments on “Impacts, Occam’s Razor, and Layers”

  1. dburt Says:

    Thanks for your thoughtful impressions of my final talk of the session. Right afterwards I caught Tim Glotch and Steve Ruff together with JSC’s Dick Morris, and put them all on the spot about true spherule color. Dick pointed out that the only available spherule spectra were from the MiniTES instrument on site, and were inevitably contaminated with red dust, thus explaining the minor red in the spectrum. Glotch, who had been so vehement in public (blindsiding me, as you probably observed), had no comeback. The best guess for the spherule color, minus the dust, still is gray, as I stated (rendered somewhat blue in false color; terrestrial gray hematite, typically formed in super-hot steam, commonly appears bluish too). No terrestrial sedimentary concretions have this gray color or appearance, as I stated.

    The impact hypothesis, BTW, DOES account for every feature seen at both rover sites, AFAIK, if impact into an Fe-rich target (e.g., a magmatic sulfide deposit as first hypothesized for Mars by the late Roger Burns) is initially assumed for Meridiani. The extremely specialized alternative hypotheses do not, despite their astonishing complexity (first this had to happen, then that, then this, and then…). Read my detailed LPSC abstract, and our earlier work, for more details (see also a now-closed Mars thread at unmannedspaceflight.com). Most people do prefer the official hypotheses, possibly because they make Mars seem more Earth-like (or possibly because they represent many more blind men examining different parts of the elephant). And thanks again for your commentary.

  2. dburt Says:

    Blame it on evening jetlag or subconscious ASU vs. Cornell prejudice, but yesterday I should have said that the only instrument on site for taking visible spectra was Cornell’s amazing Pancam with its filter wheels, not ASU’s MiniTES. Sorry.


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