Anita Cochran, assistant director of the McDonald Observatory
As part of your research into the nature of comets, you’ve looked at the question of whether comets are responsible for the water in the Earth’s oceans. Why have astronomers thought this?
We know comets hit the Earth. They’re about 40 percent frozen water, so it made sense to suppose that they were a big contributor to the formation of the oceans. Over the last 20 years or so, however, we’ve come to believe that comets were responsible for no more than 10 percent of the oceans’ water.
Why the change in perspective?
Our instruments—which measure the spectral “glow” that results when the sun’s heat turns the frozen water in comets to gas—became precise and powerful enough for us to be able to accurately measure the chemical composition of three or four comets that orbited close to the Earth. If comets were the source of most of the oceans, then the ratio of deuterium to hydrogen in the frozen water in the comets should have been roughly the same as the ratio in the water in the oceans. Or there should have been less deuterium relative to the hydrogen. What we’ve found, instead, is that there is more deuterium relative to hydrogen in the comets than in the Earth’s oceans.
So is that the end for the comet hypothesis?
There’s another possibility. If we look at comets spectrally, we find that there are basically two kinds. About 70 percent look the same, and roughly 30 percent are depleted in certain molecules. The comets in which we’ve been able to measure the deuterium are all of the majority type. It’s possible that the comets in the minority group collided with the Earth in sufficient quantity to seed the oceans.
Are there any data to support that idea?
None. The problem is that these comets, which are known as Jupiter family comets, are typically much fainter, so we haven’t gotten the data yet to be able to look at the deuterium to hydrogen ratio in them. Right now there is a European probe, called Rosetta, that’s supposed to land on a comet that may be one of these other kinds. If the spacecraft is successful, then we may find that the comet has a deuterium to hydrogen ratio similar to that of the Earth’s oceans. This would give support to the alternative theory.
What are the other possible explanations?
One hypothesis is that asteroids are responsible. Because they’re so much closer to the sun, asteroids don’t have the water in the quantities that comets do, but there’s evidence of active asteroids which seem to have icy materials, and of course the solar system changes with time, so they could have had much more ice in the past. It’s also possible that water was caught up in the rocky body of the Earth as it formed. We just don’t know, though. These are just fundamentally extremely hard observations to make.
This article also appeared in the Spring 2009 issue of Focus magazine.
Comments