More Impacts on Crater Count Dating
Planetary scientists have relied on crater counts to estimate the surface age of a planet or moon. The more craters, the older the surface. This method has recently come under closer scrutiny (see 10/20/2005) because of the phenomenon of secondary cratering.
A simplistic look at a crater-scarred planet or moon might lead one to assume a ratio of one impactor to one crater. Planetary scientists have been realizing, however, that a big enough rock can produce many craters. Another paper on this subject was published in Icarus this month.1 A team of Russian and American planetologists announced three findings from studies of Mars:
- Small clusters: Some impactors break up in the atmosphere and produce small clusters of craters 100 to 300 meters wide, each pockmark a few tens of meters in diameter. The breakup of weak cometary bodies on entry through the Martian atmosphere could be responsible for some of these small clusters.
- Large clusters: The team identified a second population of larger clusters, distinct from the small clusters. Their models indicated that one giant impact can launch numerous fragments as big as hundreds of meters across. Some fragments can be ejected above escape velocity, but many will remain and fall back to the planet. On Mars, the weakened fragments tend to break up in the atmosphere into sizes 5 to 50 meters, which is why clusters with similar size distributions are not seen on our airless moon.
Secondaries can often be identified along rays from the primary crater, but not always. The fragments can travel long distances, flying through the atmosphere for more than an hour, “making it difficult to identify the parent crater.” The authors explained that for fragments ejected from a primary impact, “launch at 3 km/s can distribute fragments over much of a hemisphere, and launch at 4 km/s can distribute fragments over most of Mars….” Escape velocity on Mars is 5 km/s.
- Martian meteorites: The meteorites found on Earth that originated on Mars appear to require impactors 3-7 km across. An impactor hitting at an oblique angle can produce jets nearly parallel to the surface capable of accelerating surface rocks to escape velocity. This implies that many other fragments fail to escape. “Clearly, if fragmenting debris is lofted to escape velocity in order to produce martian meteorites … on Earth, then other debris is lofted to barely suborbital speeds and secondary impact pits must exist not just in near-primary swarms, but also in near-random positions scattered around Mars.”
The authors did not comment specifically on the implications of their findings on the crater-count dating method. They worked within the standard Martian timescale with its three periods, Noachian (3.8 to 3.5 billion years ago),2 Hesperian (3.5 to 1.8 billion years ago) and Amazonian (1.8 billion years to the present): e.g., “We conclude that most of the clusters discussed here probably formed in the last half of martian time, not during the Noachian era. For this reason, we suspect they formed under essentially present-day, low-pressure atmospheric conditions.” The degree of uncertainty in dating the clusters, however, was evident in the following paragraph:
Further constraints are possible, from crater formation rate information. Barlow and Osborne (2001) find most clusters on Noachian and Hesperian terrain, but some of our clusters, such as the one on Meridiani Planum (Fig. 4) and others on Olympus Mons, are found on geologically young surfaces. This implies cluster formation within the last few hundred Myr, possibly within the last 20 Myr in the case of the sparsely cratered surface of Meridiani Planum, which may have been exhumed within the last tens of Myr (based on paucity of small sharp craters; [Hartmann et al., 2001] and [Hartmann, 2005]). These results suggest clusters accumulating on surfaces throughout martian history. Note that it is plausible that many young clusters on Mars might be products of a single large impact.
Yet could these age estimates themselves be undermined by the findings of the paper? If crater-counting methods were used in establishing the commonly-accepted geological periods, how can they be considered reliable now, considering that a single large impact can produce secondaries at random locations across the whole planet?
The authors hinted at the only logical answer to these questions after discussing various models for the fragmentation of incoming bodies: “All these assumptions suffer from limitations.” Later, “Precise numbers of craters, their sizes and displacement are dependent on assumptions used,” they said. As a matter of fact, the words assume and assumption appeared 19 times in the paper.
1Popova, Hartmann, Nemtchinov, Richardson and Berman, “Crater clusters on Mars: Shedding light on martian ejecta launch conditions,” Icarus Volume 190, Issue 1, September 2007, Pages 50-73, doi:10.1016/j.icarus.2007.02.022.
2The Noachian epoch, after the prominent Martian region Noachis Terra (which means “Land of Noah”) was named because of the presumption that Mars was warm and wet early in its history. This assumption was called into question last week (see 09/24/2007, bullet 2).
Despite their rigorous and admirable work of calculating and estimating the physical effects of impacting bodies on Mars, the authors could not think outside the box. Their own work casts severe doubt on the ability to use crater counts as a dating method. In principle, if one impact can produce ten million secondaries scattered around the whole planet (see 10/20/2005), there is no way to know how old the Martian surface is. If anything, one would think the whole planet would have been saturated with craters in short order, hinting that the surface might be young. And think about it: we’ve already seen several global dust storms in the 33 years since the first Martian orbiter. How much weathering of craters would be expected in tens of millions of years? And why is bedrock still clean-swept in large areas on Mars? (06/01/2005).
Because these scientists began their modeling with undaunted faith in the Age of the Solar System (A.S.S.), a figure (4.5 billion years) that, according to the Law of the Needs of the Darwins cannot be altered, they had to make it all work within their mythical paradigm. Their crater cluster formation model must fit within the scheme of imaginary Noachian, Hesperian and Amazonian epochs, even though their own work undermines the assumptions that went into making the scheme in the first place (for other problems with the current scheme, see Astrobiology Magazine). What’s in a name? Would calling them the Washingtonian, Lincolnian and Clintonian epochs make Mars blink an eye? Let the discerning mind understand that the fancy charts of geological time scales on Mars and Earth are human impositions on the data – not inevitable products of the observations themselves.