Update:  Crater-Count Dating Squabble Unresolved

Remember the revelation last year that many craters on Mars used to infer ages may have been secondary impacts from fallback debris? (see 10/20/2005 entry).  Well, a microsymposium on this subject was held in Houston in March, and Richard Kerr in Science¹ said that “125 planetary scientists deadlocked over how to apply crater-dating techniques to recent Mars history.”
    Some are adamant that most craters are secondaries, and that they are difficult to distinguish from primaries, while others feel their methods produce consistent dating results.  If the former are right, however, then crater counts “have little to do with telling time,” Kerr wrote.  Inferences about ages of features in geological history built on these dating methods could be “off by orders of magnitude.”
    Support for the secondary-crater explanation comes from ground level.  It shows that the process occurs even at small scales:

Close-up images from the Mars rover Spirit also suggest that secondaries dominate small craters on Mars, too.  Geologist Matthew Golombek of the Jet Propulsion Laboratory in Pasadena, California, a Spirit team member, reported on a survey of impact craters measuring from 10 centimeters to a couple of hundred meters across.  Spirit found that they are all far shallower and less bowllike than primary craters tend to be.  The high speed of impactors falling in from the asteroid belt makes for relatively deeper craters than those made by slower ejecta blocks of the same size.  “Almost every crater you see looks like a secondary,” said Golombek.

¹Richard A. Kerr, “Planetary Science: Who Can Read the Martian Clock?”, Science, 26 May 2006: Vol. 312. no. 5777, pp. 1132 – 1133, DOI: 10.1126/science.312.5777.1132.

Before this realization came to light, the assumption was that meteoroids produced a steady rain of impactors on a planet or moon, allowing scientists to estimate the age of the surface.  “The concept sounds simple enough,” Kerr said, nostalgically; “To decipher the geologic history of other bodies in the solar system, count craters formed by the slow rain of bombarding rocks.  The more craters on a lava flow, glacial debris, or a flood deposit, the farther back in time a volcano erupted, ice flowed, or water gushed.”  Now, we know that “In practice, however, telling geologic time beyond Earth has proved tricky.”  A sidebar shows that there is even debate on how the impacts took place, and whether there was a catastrophic period of bombardment, or a slow decrease in events in the first million years of the solar system.
    But wait; about that first million years – it is inferred from studies of the moon, which were dated by crater counts.  And the moon is also dated by extrapolation of dating methods from earth which were largely accepted due to their allowance of time for Darwinian evolution.  What if evolution is not true?  And what if the dating methods on Earth are flawed?  Can secular scientists really know anything about ages in the solar system?  Don’t count on it.  Assumption is the mother of confusion.