Nadine Barlow of Northern Arizona University reported a discovery -- based on photos from both Mars orbiters -- that may possibly require reinterpretation of Mars' geological history.

The age of surfaces on the planet -- that is, the time since some geological or weather process such as lava flows, water or wind last resurfaced them -- is established, as with the Moon and most other worlds in the Solar System, by counting the number of craters on them.

(The fact that we actually have returned lunar samples that have been radioactively age-dated -- to determine the time at which they last solidified out of lava -- has enabled us to compare this with the number of craters on those parts of the Moon and thus estimate the rate of meteoroid impacts at various points in the Solar System's previous history. This in turn allows us to calibrate age estimates of other worlds' surfaces by cratering.)

But such crater-counting age estimates for Mars have been based on the assumption that it has the same ratio of primary craters to "secondary" ones (the small craters produced by chunks of rock thrown out by an initial bigger primary impact) as the Moon does. Barlow's survey of Martian primary craters and the number of smaller secondaries scattered around each one suggests that this may be wrong.

It's almost impossible to distinguish secondary craters from the smaller primary ones mixed in with them on any densely cratered surface; you have to locate some surface which is recent enough that it has few primary craters, enabling you to identify the small secondaries produced by each primary impact by the way in which they trail off in number as you get further from the primary impact crater.

After doing this, Barlow reports that -- even on relatively recent Martian surfaces with shallow regolith layers -- Martian impacts actually produce far fewer secondaries than lunar impacts do, because the ground ice mixed with its regolith causes it to act in a radically different mechanical way. The heat of the impact melts and boils the ice, producing a wave of liquified mud that "splashes" down around the crater to produce a local sheet of lobe-like raised surface, like a pebble thrown into a mud puddle -- something already noted in photos of many Mars craters -- and apparently this tends to prevent individual pieces of rocky shrapnel from being thrown longer distances to gouge out separate craters, as on the Moon.

If so, the consequences for age estimates of Martian surfaces are enormous: a lot of their small craters assumed to be secondaries have really been produced by primary impacts by small meteoroids -- which means that those surfaces have been hit by a lot more such small meteoroids than had been thought, which in turn means that they must be considerably older than thought.

This, in turn, means that some of the freshest lava flows on Mars, around the Tharsis volcanoes and in the Cerberus region -- whose crater counts indicate that they may have been laid down by Martian volcanic eruptions less than 10 million years ago -- may actually be considerably older.

However, the University of Arizona's Al McEwen has recently reported elsewhere that THEMIS' photos reveal a very fresh 10-km wide crater in the Cerberus region, which hit in an area where there was relatively little ground ice to splash -- and which a a result has produced literally millions of tiny secondary craters spread out in crater rays from it all over the region. Fully three-quarters of the craters in the Athabasca Valley portion of Cerberus (which came fairly close to being picked as a site for one of the 2003 rovers) now turn out to be secondary -- far more than expected. He thus thinks that the count of primary craters on the Cerberus lava flows is SMALLER than had been thought, and that these lava plains may actually be a lot younger than the previous estimate that they were laid down by big Martian eruptions only a million years ago.

As he points out, if the number of secondary craters produced by Martian impacts varies so wildly depending on how thick the layer of ground ice is -- which is very hard to judge from orbit -- then age-dating virtually any part of Mars by counting craters may actually be almost impossible. This would make it still more important to do radioactive age-dating of different regions' rocks (which may require returning samples).

By the way, this new crater in Cerberus may well be the freshest crater that big anywhere on Mars -- on the average, only about one crater 10 km wide or bigger should be produced every million years, and it may well be younger than that. And it may well be the source for one Martian meteorite (EETA 79001), which is known by cosmic-ray dating to have been kicked off the planet only 700,000 years ago. We may now know exactly where one piece of Mars already in our possession came from (although the actual lava making it up apparently came from an earlier buried flow laid down in the same place 180 million years ago).

Click for Part Six

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