Just a streambed

Ken Tapping, October 24, 2012

In the sky this week…

  • Mars is very low in the Southwest after sunset.
  • Jupiter rises around 9 pm and Venus around 5 am.
  • The Moon will be Full on the 29th.

At first, the image did not look very impressive. It showed a piece of rock sticking out of sandy, gravelly soil. The rock itself, reddish, like its surroundings, consisted of rounded pebbles cemented together by some much finer stuff. This kind of rock is called conglomerate, but quarrymen often refer to it as “pudding stone”, because it looks like a plum pudding. This particular specimen was particularly generous with the plums.

Things get more intriguing as we look more closely. The pebbles are all nicely rounded, like the ones we find at the bottom of streams and on old stream beds. If you bash two pieces of rock together, you’ll get some dust and a jagged fragment or two. If you want to try this, wear gloves and eye protection.  Proper footwear would not hurt as well. Dropped rocks have a magical property that makes them home onto feet.

Rock fragments start off as jagged bits. They get turned into rounded pebbles by flowing water. They get rolled along, abraded by sand and collide with other rocks. The result is nicely rounded and often polished pebbles. Looking even more closely at the image shows not all the pebbles are the same kind or rock. There are contributions from a number of different rock outcrops. They were all picked up, rounded and rolled to a new location kilometres away by flowing water.  We see this in almost any stream bed.  Then, as the layer of pebbles builds up, silt settles in the cracks, where the current cannot wash it way, until over millions of years it consolidates into the pudding stone we see today.

So our rock outcrop says that long ago, a fairly brisk stream flowed through that location, possibly billions of years ago, and what we see today is a reminder of that ancient stream. What sort of terrain did it flow through? Were there living creatures in it? What makes this particular stream interesting is that it flowed on Mars, and the image was sent back by the Curiosity rover.

Pictures taken from Mars Reconnaissance Orbiter, a spacecraft surveying the Red Planet from above, show that Curiosity landed near a stream that once flowed down from nearby high ground. As it reached the bottom of the grade and slowed, it made a huge fan of dropped material.

Some knowledge of geology and physical geography help us to understand the landscape around us, and the forces that made it as we see it today. The features we see might individually seem commonplace and everyday, but the story they tell us is anything but that.

Now we are seeing that although Mars has been a frozen, almost airless and unbelievably dry place for many millions of years, once, long ago, it was not like that at all. There was a denser atmosphere, it was warmer, due to the greenhouse effect, and there were lakes and rivers, even seas, which meant it must also have rained. Were there living things on Mars’ surface back then? Why did it all go wrong on Mars, but not for us here on Earth? It would be very useful to know that!

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, Penticton, BC, V2A 6J9.

Telephone: 250-497-2300
Fax: 250-497-2355
E-mail: ken.tapping@nrc-cnrc.gc.ca

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