Meteors strike the earth with some frequency. Indeed, when one looks at the surface of the moon or Mars, the landscape is dominated by impact craters. However, the earth's surface changes so rapidly that craters have a tendency to disappear. This makes studying, or even identifying, past meteor impacts very difficult.
A study in today's issue of Science uses the Osmium isotopic composition of ocean sediments to identify and measure the size of two meteor impacts, including the famous K-T boundary impact that may have been responsible for the extinction of the dinosaurs. The size of the K-T boundary impact event has important implications for whether or not it could be the primary cause of the extinction event.
They determine the size of a meteor impact using a simple mass balance approach. If you know how much Osmium was in the ocean before the impact, and you know how much Osmium was in the ocean after the impact, then you can calculate how much was added by the vaporization of the meteor on impact. Because most meteors have a very similar chemical composition, we can infer how big the rest of the meteor had to be add this much Osmium to the ocean. Osmium is very rare in the earth's crust, but relatively common in meteors, making it an excellent measurement for studying meteor impacts.
Unfortunately, we have no direct measure of any of these values. The main measurement that they use is the ratio of two isotopes of Osmium, 187Os/188Os. By measuring the change in this ratio as a function of depth (and therefore time) in an ocean sediment core, they can measure the relative effect of the meteor on the ocean Osmium content. By then assuming the pre-impact ocean Osmium concentration is similar to today's values, they can calculate how much Osmium was added to the ocean. Finally, by assuming a reasonable Osmium concentration and isotopic ratio in the impacting meteor, they can calculate how big the meteor must have been to add the amount of Osmium they calculated.
Their results indicate that the meteor that struck the earth at the K-T boundary may have been substantially smaller than is generally accepted. Their approach suggests that the meteor was about 4.1 to 4.4 kilometers in diameter, while studies based on measuring the size of the impact crater have estimated the meteor diameter to be 10-15 kilometers in diameter.
Because of the assumptions involved, their estimate is probably a minimum value. First, they assume that the entire meteor vaporized and was dissolved in the ocean. Second, they assume that they actually measured the most extreme 187Os/187Os ratio. The data they present for the K-T boundary is a lower resolution core, and their diameter estimate is based largely on a single data point. It is unlikely that this data point represents the most extreme ratio, and a more extreme ratio would drastically increase the size of the meteor predicted by their method. As a result, it is not clear that a definitive statement about the K-T impact can be made from the current paper.
Ultimately, this paper presents an interesting method of identifying and measuring meteor impact sizes. Studies have estimated that there should be between five and 27 impact events in the past 65 million years, but very few of these have been identified. This new method may allow us to identify more meteor impacts as well as their sizes, which will give us a better understanding of how often the earth gets hit by meteors.
Science, 2008. DOI: 10.1126/science.1152860