A new set of images from the ground of Mars has been a sensation: they show an outcrop of gravelly mudstone, that is, conglomerate. This is really not a surprise; it's what we meant to find when Curiosity's itinerary was planned.
News stories have called this image "definitive proof." It is not definitive proof. But I'm still OK with the scientists' conclusion that it represents streambed deposits, much like those we can find in any desert wash.
Let's look at the picture like geologists do in the field. We see rock beds, with the look of consistent layers, that are tilted out of the ground. We see that the rock is full of clasts—fragments of older rocks—with a wide range of sizes. They are not jagged pieces, like the crushed stone used in roadbeds, but are rounded. To be more precise, they are subrounded rather than the fully rounded "river rock" sold in landscapers' yards. On Earth, there are three basic settings that can create such a rock: streambeds (conglomerates), underwater landslides (turbidites) and volcanoes (pyroclastic deposits). How to choose the most likely of these?
To me, the most telling detail is in the gravel lying below the outcrop, which I will assume is stuff that has weathered out of the rock. It's very nice gravel, by which I mean that it has a pretty consistent grain size. It is sorted. This is where I rule out pyroclastic flows, which don't sort out clasts by size to this extent. Pyroclastic flows carry everything with them regardless of their density.
The choice between streambed and turbidite is less clear at the outcrop. We don't see any bedding features or sedimentary structures beyond the crude layering. The Curiosity science team looked to the regional setting and the evidence from other outcrops to conclude that this represents an ancient river channel where water (salt or fresh? we can't yet say) once ran around knee deep. I think it's time to accept a working consensus that a genuine river was here and not something else.
NASA/JPL-Caltech/MSSS image
Comments
There’s a lot of interpretation that could be applied to this photo. One thing I notice is that the loose gravel on the left side of the image appears to be better sorted than the presumed source conglomerate. Could this be a result of secondary sorting that has nothing to do with water? I’m thinking that it may be a deflation deposit, aided by Mars’ famous windstorms: any grains or granules below a certain size are removed by wind, leaving a better-sorted lag of pebbles.
Another question: what’s cementing the conglomerate? That speaks to the presence of water at least as loudly as the rounded/sorted clasts. It’s hard to imagine what sort of cement could be deposited in the absence of water.
Another question: the attitude of this scene is not obvious; are we looking at a slope? A horizontal surface? The conglomerate is sorted poorly enough (and showing no obvious bedding or clast imbrication) that it could be part of a debris-flow deposit, which would require water, but not as much as a clear-running stream. Perhaps a slurry of melted permafrost running down a slope could create a deposit like this (and contribute some degree of rounding, depending on the distance and the hardness of the clasts).
Lots of questions!
Follow the link at the bottom of my post to see more images.
About deflation, it’s my understanding that Mars’s thin atmosphere can’t lift anything larger than fine silt. If that is a lag deposit, it would have to have been winnowed by water. But what caught my eye was the upper size limit rather than the lower limit. That says to me that the gravel is not just rounded, but sorted.
Yes, who knows what the cement is? Sulfates seem to have prominent roles on Mars as well as clays.
I liked the observations made by Howard. Very astute.
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Thanks for your kind comment, Mr. list, but I don’t believe a word of it.