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Did Asteroid Impacts Make Earth Habitable?

Scientists suggest asteroid impacts may have made life on Earth possible. If so, this is another fascinating example of our existence hinging on a delicate balance of the nurturing and annihilating forces of nature.

Asteroid Eros
    Look who's coming to dinner!

Asteroids’ devastating impacts are being credited with providing Earth with water, organic compounds and other vital materials. Such impacts may have stimulated the evolution of complex life on Earth, and may also have created the Moon, which stabilizes our seasons.
In a new report, Rebecca G. Martin and Mario Livio from the Hubble Space Telescope Science Institute, suggest that life may be possible only in planetary systems that have asteroid belts with the right stuff. They say this may be quite rare, making our solar system even more special than previously appreciated.
Martin and Livio focus on the “snow line”, the distance from a star at which water freezes. Water freezes at 32F on Earth’s surface, but at about –154F in outer space. Inside the snow line, planets are small and dry, like Earth, while the outside planets are gas giants, like Jupiter. In our solar system, the snow line is at 2.7 AU (Earth’s distance from the Sun is 1 AU). These astronomers calculate that around other stars, the snow line would be at about the same distance, as it varies only with the cube root of the star’s mass.
When Earth formed, it was molten. Heavy elements, including many essential to life, such as iron, sank to Earth’s core while water escaped into space. Even after cooling, Earth would have been uninhabitable if not for asteroids.

Our story now shifts to Jupiter. With over 300 times Earth’s mass, Jupiter’s gravity disrupts planet formation in its vicinity. Thus the material that might have formed a planet near the snow line instead became the asteroid belt. Famed UCB astronomer Geoff Marcy estimates that only 20% of stars have a gas giant planet that might produce a similar asteroid belt. Our asteroid belt may have originally contained as much mass as Earth, 1000 times more than it has now. Had that situation continued asteroids would now bombard us 1000 times more often, probably precluding any advanced life on Earth. Having an asteroid wipe out the dinosaurs 65 million years ago, allowing the ascension of mammals, was a good thing, but having an asteroid devastate Earth every 65,000 years would be over-kill, so to speak.
But, asteroid belts can change dramatically when gas giants migrate after forming. Since it formed, Jupiter has probably moved 5% closer to the Sun. That seems modest, but computer simulations show it was enough to eject 99.9% of the original asteroids, many of which rained down on young Earth. This torrent probably delivered most of Earth’s water, seeded the surface with heavy elements like iron, and may even have delivered the building blocks of life itself.
Most astronomers believe our Moon formed from the debris of a cataclysmic impact by an asteroid named “Theia” that ripped off much of Earth’s crust. Some suggest one of the first consequences of Jupiter’s movement toward the Sun was pushing Theia our way.

Asteroid Theia hitting and forming our Moon
  Asteroid Theia hitting and forming our Moon

Jupiter’s 5% movement may lie in a “Goldilocks Zone” — less movement would have left too many asteroids bombarding us today, and more movement would have disrupted the asteroid belt and destabilized the orbits of the inner planets.
The asteroid belt that has enabled life on Earth is illustrated below.

The next image shows what Armitage et. al. say happens to 98% of Jupiter-like gas giants: they migrate much closer to their star, disrupting the asteroid belt, resulting in frequent bombardment of the inner planets lasting billions of years, with dire consequences for life.

Martin and Livio say only 4% of gas giants in other star systems remain outside the snow line and that includes planets that formed much farther out, as did our system’s other gas giants: Saturn, Uranus, and Neptune.
Livio suggests scientists focus the search for habitable exoplanets on planetary systems with: a gas giant just beyond the snowline; and the infrared signature of a “warm” asteroid belt.
Separately, astronomers estimate that only 1% of stars are comparable to the Sun in mass, heavy element abundance, and absence of stellar companions. Now factor in the estimates that only 20% of such stars have appropriate gas giants, and less than 2% of those are in the migration Goldilocks Zone. So far, our type of solar system is 1 in 24,000. With 300 billion or so stars in our galaxy, we could expect 12 million solar systems similar to ours. But how many of those have habitable planets? No one knows for sure. In my books, I list eight remarkable properties of Earth, beyond those mentioned above. If the odds of satisfying each of the additional requirements were 1 in 10, we would likely be alone in the Milky Way galaxy. And, I think most of Earth’s additional properties are actually much rarer than that  — just consider the other planets in our solar system.
All this highlights just how special is the place we share in the universe.
So, be Thankful and have a Happy Holiday!
Nov 19, 2012
Dr Robert Piccioni

Dr Robert Piccioni,
Author of "Everyone's Guide to Atoms, Einstein, and the Universe",
Can Life Be Merely An Accident?"
"A World Without Einstein"

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