Astronomy Picture of the Day

Today’s APotD reminded me of a book I want to recommend to you: Rare Earth: why complex life is uncommon in the Universe, by Peter D. Ward and Donald Brownlee (Copernicus, an imprint of Springer-Verlag, 2000). It’s full of interesting astronomical information (Brownlee is a professor of astronomy, Ward of geological sciences). Just last night I read this:

The scattering process that bestowed on Earth life-giving material from the outer solar system also has a dark side. We have noted earlier that the accretion process never really ended. The rate is many orders of magnitude less than it was 4.5 billion years ago, but, as in any solar system where planets form by accretion of solids, the process still goes on. The annual influx of outer solar system material falling to Earth is 40,000 tons per year. This is mostly in the form of small particles, but larger objects occasionally hit. The small particle flux is one 10-micron particle per square meter per day and one 100-micron particle per square meter per year.  (pp 48-49)

So the APotD was perfect today: some of the 40,000 tons per year! The “dark side” refers, of course, to the now much rarer larger bodies (1-, 10-, or 100-kilometers in diameter), whose impacts would have serious or dire consequences for plant and animal life.

Still cloudy and raining here.


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