October 20, 2002 | David F. Coppedge

Planets and Moons Suddenly Got Much Younger

A planet or moon covered with craters just looks old, doesn’t it?  Planetary geologists have long relied on crater counts to estimate the absolute ages of surfaces, such as on the moon, Mars, Europa, and every other solid body.  Lots of craters meant old.  Few craters meant young.  Presumably, impacting bodies came in like clockwork and left their marks over the eons.  An uncomfortable fact has come to light that disturbs this simple picture like a bolide: most of the craters are secondary impacts.
    Picture a big meteor hitting Mars.  Did you know that it could toss up enough debris to create 10 million more craters – all from a single event?  That’s one of several shocking facts presented by Clark R. Chapman and two colleagues in a Letter to Nature.1 (see also summary on Space.com).  Believe it or not, they calculate that some 95% of small craters (1 km in diameter and under) are secondaries, and many of the moderate size craters probably are, too.  This means that only a few impactors could quickly saturate a body with craters.  It also means that estimating surface ages via crater counts is a lost art, because it just lost its credibility:

Surface ages can be derived from the spatial density of craters, but this association presumes that the craters are made by interplanetary impactors, arriving randomly in time and location across the surface.  Secondary craters cause confusion because they contaminate the primary cratering record by emplacing large numbers of craters, episodically, in random and non-random locations on the surface.  The number and spatial extent of secondary craters generated by a primary impact has been a significant research issue.  If many or most small craters on a surface are secondaries, but are mistakenly identified as primaries, derived surface ages or characteristics of the impacting population size-frequency distribution (SFD) will be in error. (Emphasis added in all quotes.)

Their mathematical analysis yielded the 95% figure for secondaries.  The production of secondary craters on Europa, they found, was “unexpectedly efficient.”  Although secondary crater formation on icy bodies was so, they feel that similar secondary crater production occurs on rocky bodies like the Moon and Mars, and granted that, has a ripple effect casting the entire method into doubt:

Our work raises doubts regarding methods that use the lunar small-crater distribution to calibrate other inner Solar System surface ages (for example, Mars).  If, as on Europa, lunar and martian secondaries are 95% of the small crater (less than a few kilometres) population, the error bars (and thus derived surface ages) on any residual primary crater population become large (uncertainties are 20 times the measured density value).  This uncertainty applies to both the measured population on a martian surface unit and the lunar SFD that supposedly represents absolute age.  We emphasize that traditional age-dating analyses still derive robust ages when using large craters (greater than a few kilometres diameter), which are less likely to be secondaries.  However, the technique becomes increasingly unreliable when applied to dating tiny geographical units using small craters, which may be mostly secondaries.

As a result, they conclude that “any attempt” to age-date surfaces or characterize the population of impactors may suffer “a significant and perhaps uncorrectable bias” due to the contribution from secondaries.  They ended with that case of the single Martian impact that generated 10 million secondaries from 10 to 100 meters in diameter.
    Speaking of Mars, the Mars Global Surveyor recently took a sharp image captioned “secondary craters.”  Click here for a look.

1Bierhaus, Chapman and Merline, “Secondary craters on Europa and implications for cratered surfaces,” Nature 437, 1125-1127 (20 October 2005) | doi: 10.1038/nature04069.

Things are not always what they seem.  This announcement must hurt like a rock to Darwinists, who really need those long periods of uniformitarian processes.  Imagine ten million craters forming in one day!  If crater counts say nothing about age, why could not all the observed cratering have occurred quickly, in relatively recent times?

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