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Sizing Up Impacts and Their Effects

Do we have a good idea how many impact events have affected life on Earth? New work on ocean sediments offers the chance to expand our knowledge, helping to flag the distinctive signature of an impact and even to tell us how large the incoming object was. We may find more historical impacts than have previously been identified, reminding us yet again that our habitable zone is an active and sometimes dangerous place to be.

True, the issues involved in mass extinctions are complicated, but major impacts clearly played a role in some, including the death of the dinosaurs. François Paquay and team estimate the impactor that struck 65 million years ago at the Cretaceous-Tertiary (K-T) boundary was between four and six kilometers in diameter. While other factors, including volcanism, can’t be ruled out, the meteorite certainly didn’t help matters.

Paquay (University of Hawaii at Manoa) analyzed samples of ocean sediments to study osmium levels therein. The element is useful because, as this news release explains, meteorites carry a distinct osmium isotope ratio that differs from that normally found in Earth’s oceans. Read properly, the osmium levels can provide a record of ancient impacts:

“The vaporization of meteorites carries a pulse of this rare element into the area where they landed,” says Rodey Batiza of the National Science Foundation (NSF)’s Division of Ocean Sciences, which funded the research along with NSF’s Division of Earth Sciences. “The osmium mixes throughout the ocean quickly. Records of these impact-induced changes in ocean chemistry are then preserved in deep-sea sediments.”

The critical issue is to verify how the impact changes the osmium levels, which is why Paquay has been focused on samples from the late Eocene, itself the time of the extinction event called the Grande Coupure, which may have been affected by impacts in Siberia and Chesapeake Bay. In any case, the period is known to have been marked by a number of impacts and thus offers the opportunity to observe the osmium signature related to these. Such studies led to the estimates of the K-T boundary event and may uncover the signs of previously unknown strikes.

The paper is Paquay et al., “Determining Chondritic Impactor Size from the Marine Osmium Isotope Record,” Science Vol. 320, No. 5873, pp. 214-218 (April 11, 2008). Abstract available.

Comments on this entry are closed.

  • Edg Duveyoung April 14, 2008, 13:22

    Anyone care to guess how much larger a planet would have to be before “most” impacts would be too small to be an extinction level event? Maybe twice our size and most impacts don’t affect the entire globe. Assuming in this scenario that this planet also has a Jupiter and its own way-big moon like we do.


  • Adam April 14, 2008, 15:37

    Hi Edg

    Unfortunately a large planet increases the impactor’s energy via its own gravity well. Higher masses lead to more core compression and a higher density, increasing the escape velocity. Relative Radius is ~0.27 power of the relative mass, thus escape energy scales as a 0.73 power of the mass. Also higher gravity means the atmosphere is shallower and possibly more impactors will hit the ground. Secondary impactors (debris flung out by the first impactor) will fall to ground closer to the impact though, so there is some benefit. Mercury shows lots of small secondary craters next to the main craters, unlike the Moon with ~1/2 Mercury’s gravity.

    Imagine a habitable moon, gravity ~0.25 gee, thus the atmosphere declines 4 times slower than Earth. That’s a lot of air for a impactor to get through. When it does the debris will stay up longer and it might travel further, so it’s hard to compare against a big planet with x2 Earth’s gravity where impacts are up to x3 more energetic, but debris falls much closer.

    Red dwarf planets will suffer some high energy impacts too – a red dwarf with 1/16th solar luminosity means the star is 1/2 sol’s mass, but the planet is x4 closer, thus impacts are twice as energetic. Things get worse the closer you get to the star. At 0.1 solar masses the luminosity is 1/1200th, the impactor energy 3.46 times higher.