July 25, 2010 | David F. Coppedge

Dating of Impacts and Impacts of Dating

Earth and Neptune were both on stage this week with stories of impacts.  How do scientists know when they occurred?

  1. Neptune:  A comet struck Neptune 200 years ago.  That’s what planetary scientists are claiming, according to National Geographic.  The data only “suggests” this explanation, according to Space.com.  Since nobody witnessed the impact in 1810 (Neptune had not even been discovered yet), how do they know?  The data consists of elevated carbon monoxide levels in the outer atmospheric layers of Neptune compared with the lower layers, as measured by the Herschel spacecraft.  According to one of the authors of a paper on the hypothesis, “The higher concentration of carbon monoxide in the stratosphere can only be explained by an external origin,”  Another author added, “From the distribution of carbon monoxide we can therefore derive the approximate time, when the impact took place.”
        According to the articles, similar techniques were used on Saturn to suspect an impact about 300 years ago.  The only impacts on gas giants witnessed by humans have been on Jupiter.  Scientists estimate the one that hit Neptune was twice as big as the first fragment of Comet Shoemaker-Levy 9 that struck Jupiter in 1994.
  2. Earth:  A new impact crater was found in the deserts of Egypt, according to Space.com – one of the most pristine ever found.  National Geographic has a good picture of it.  Because of its lack of erosion, they estimated the crater formed within the last 2,000 years.  Called Kamil Crater, it is 147 feet in diameter and 52 feet deep.  This leads astrophysicists to estimate the characteristics of the impactor: “Based on their calculations, the team thinks that a 4.2-foot-wide (1.3-meter-wide) solid iron meteor weighing 11,023 to 22,046 pounds (5,000 to 10,000 kilograms) smashed into the desert—nearly intact—at speeds exceeding 2.1 miles (3.5 kilometers) a second.”
        Based on estimates of the number of impactors orbiting our region of the solar system, the scientists estimate that 1,000 to 10,000 such impactors should strike earth each million years.  Why are more not found?  An Italian scientist explained, “The reason why they are rare, however, is that, on Earth, weathering rates are high—small craters are usually easily eroded or buried.”

For more on crater count dating methods, see this list of search bar results.

Which is easier: (1) to make up a story about something in the past that was not observed, or (2) to predict when something will happen?  If planetary scientists can tell us when and where a meteor will strike and form a crater, that would be very impressive.  “Too many variables!” they would rightly complain.  But those same variables are time-independent.  When you see them predicting 1,000 to 10,000 impacts each million years, that would be 4.5 to 45 million craters over the assumed age of the earth.  It smells like a theory-rescuing device to say they were all eroded and weathered away.  Not all portions of the earth erode at the same rate.  Geologists tell us there are some rock outcrops 3.8 billion years old.  Surely some evidence, direct or indirect, of 45 million craters should be detectable beyond the 176 National Geographic said have been discovered.
    It appears there is some potential for testing deep time here.  Take the assumed flux of material, the assumed age of the earth, the compositional content of the impacting material, the geological column, reasonable erosion rates, and research the question: is there evidence for this much meteoritic or cometary material in the rock record of our planet?  Let’s not just take the secular geologists’ word for it.  They are wedded to deep time.  It would never occur to them to doubt their spouse: besides, it would be in bad taste.  It’s up to the untied to ask such questions.

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Categories: Solar System

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