| Submit your comments on this article |
| -Short Attention Span Theater- |
| Crater count led Mars historians astray |
| 2005-03-29 |
| The method used by planetary scientists to estimate the ages of various regions of Mars is flawed. Something I can agree with... "This really changes things," says Nadine Barlow of Northern Arizona University in Flagstaff, US. For instance, the findings will significantly change our understanding of when Mars may have been volcanically active. To estimate the age of any region on Mars, geologists count the number of meteor craters they can see in images of the area. The idea is that the older a surface, the more craters should have accumulated over time. Crater counts give an indication of the relative age of different Martian regions. To determine absolute ages, these counts are then compared with crater counts from regions of the moon, some of which have been precisely dated thanks to the rocks brought back by the Apollo astronauts. Assuming that the rate of meteor impacts at any given time was roughly uniform throughout the inner solar system, those ages should translate directly to Mars. Now it seems there is a fatal flaw in this method, at least when it is based on counts of very small craters. According to Barlow, recent infrared images taken by NASA's Mars Odyssey spacecraft have unexpectedly revealed very long ray patterns around even relatively small craters, showing that much more material was ejected by meteor impacts than had been calculated. Massive plumes of ejected rock would have rained down to produce, in some cases, millions of secondary craters. It means the vast majority of craters smaller than 2 kilometres in diameter may be secondary craters, making them virtually useless for dating surfaces, says Al McEwen of the University of Arizona in Tucson, US. For example, McEwen studied a 10-kilometre Martian crater called Zunil. The impact of the meteor that produced it could have created millions of secondary craters thousands of kilometres away from the main impact. William Bottke of the Southwest Research Institute in Boulder, Colorado, US, who is an expert on the dynamics of the asteroid belt, also concludes that small craters are predominantly secondaries. He says there are just not enough small objects in the asteroid belt to account for the observed rates of small impacts on Mars. "Small craters may not be telling you much," Bottke says. Some crater-free areas on Mars have been estimated to be only about 150,000 years old. But based on the new findings, McEwen concludes that the age of such regions can only be pinned down to within 10 million years. And such uncertainty will remain until actual rock samples from Mars are dated. Bottke agrees: "We probably can only set rough limits." I hate to argue here, but since the small craters are apparently secondaries and thus likely fall into the same time as the biggies and that estimatimg their age is uncertain, it may as well mean that they could have happened withing the last 10,000 years. Or even between ~ 702BCE and 160CE. |
| Posted by:Sobiesky |
| #2 Long story, Phil. I'll take a raincheck on that one. |
| Posted by: Sobiesky 2005-03-29 10:56:19 PM |
| #1 What's the significance of the dates? |
| Posted by: phil_b 2005-03-29 8:30:41 PM |