The Ignimbrites of Mars

impact sinterI’m here to propose a new type of rock. Martian ignimbrite. Or call it impact sinter. Same thing.

Huh? You say.

The Mars rover Curiosity has passed by some pretty interesting outcrops of rocks as it rolls along, and nobody seems to quite know what they are, so I’ll go ahead and give it a try. After all, I got a B in second-year geology for majors at the University of Washington a couple decades ago. That ought to qualify me.

Bungled call?

Look at the images above. Click them for better views. Curiosity snapped them a week or two ago while just passing by. And Curiosity had already passed by a couple more of these low outcrops along her way to this one. The scientists in charge suggested they were “conglomerate” rocks, which here on earth are normally found where rivers once laid down a mix of rocks and sand that later hardened into a type of stone that is itself made up of stones. Cool stuff.

When they saw these layers of lumpy stone on Mars, they cried, “Aha! Conglomerate!” and went on to postulate that ancient rivers and floods on a warmer and wetter Mars had delivered these fields of stones into the bottom of Gale Crater, where they solidified into conglomerates over a billion years or so.

Well, I don’t think so. I think the lumpy stone is not a conglomerate, at least not like anything here on earth.

Heat-fused stoneHow about this for an alternative: impact sinter. Suppose a major asteroid impact hit somewhere in the area of Gale Crater, long after Gale had formed. Suppose material thrown out by the new impact blanketed the floor of Gale Crater. What would you expect to see. Rocks and boulders? Sure. They would be tossed out by the megaton and land in Gale Crater. Sand and dust? Sure. Those too would be blown out of the impact site, the result of the impacting meteor pulverizing much of the rock it collided with. But what else?

How about gravel-sized chunks, say, the size of the lumps and bumps seen in the rover’s photo of the “conglomerate”? A lot of gravel ought to have landed in Gale, along with the boulders and dust. But is it that simple? Should we just look for gravel? Heck, there’s plenty of it there, all around Curiosity.

But a large-scale meteor would generate more than boulders, gravel, sand, and dust when it slammed into Mars. It would generate heat, and a lot of it. Scientific calculations of good-sized impacts suggest stupendous explosions at the impact point, generating white-hot heat on the magnitude of an atom bomb blast. Wow!

So, as that gravel was flying out of the new hole in Mars, and traveling to lay down a blanket of rubble across Gale Crater, it was also being heated hot enough to melt the rock, and even vaporize some of the solid stone. What landed in Gale, then, was an incandescent, glowing layer of rocks, sand, and dust with a few boulders in the mix.

After some time, the heat would dissipate and the rocks would cool. But the end result of the process would be a blanket of rocks that had stuck to one another by via their surface coatings of melted stone, in a matrix of melted-together sand and dust particles. And that would yield the conglomerate-like rocks Curiosity has been seeing.

Earthly ignimbriteSo. No water needed. Sorry rover scientists. I think you missed that call. The layers of chunky stone you have been seeing are the product of white-hot heat, not flowing water. It’s understandable, your missing this one. There are no such rocks on earth. They’ve long since been weathered away or destroyed by other geological processes. The only sintered rocks we see these days are hardened volcanic ash layers, like the one at right. Rather meek and mild-mannered, compared to the gnarly rocks Curiosity has been photographing.

Now, I know I’m not a planetary scientist, just a fellow with a pretty good record of scientific experiments and observations. So, maybe I’m not qualified to prove anybody’s geological theory right or wrong. But give this one some more thought, won’t you, rocket scientists?

About Tom Hopp

Thomas P Hopp is a scientist and author living in Seattle. He writes medical thrillers, natural disaster novels, and the Dinosaur Wars science fiction series.
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