Frost-Covered Asteroid Suggests Extraterrestrial Origin for Earth's Oceans

By Andrew Moseman | April 29, 2010 11:36 am

AsteroidThere are millions of asteroids in the asteroid belt between Mars and Jupiter, but yesterday attention focused on just one. According to a couple of studies in Nature, a large asteroid called 24 Themis is rife with water ice and organic molecules, and the researchers say that it could be more evidence that the water so precious to life on Earth came to our planet on board such rocks.

Two research teams took infrared images of 24 Themis, which is about 120 miles in diameter and was discovered in 1853. This asteroid has an extensive but thin frosty coating. It is likely replenished by an extensive reservoir of frozen water deep inside rock once thought to be dry and desolate [AP].

The team, led by Humberto Campins, says finding so much ice on the surface was a surprise; at the asteroid’s distance from the sun—3.2 astronomical units (AU), or just more than three times further than the Earth—exposed ice has a “relatively short lifetime,” the scientists write. As a result, the idea of a below-surface reservoir seems likely. (Icy comets aren’t nearly so close to the sun on average; Halley’s comet can come within .6 AU of the sun, but then retreats to a farthest distance of more than 35 AU.)

It might seem implausible that our planet’s water supply arrived incrementally as cargo on board comets or asteroids. But here’s how it may have happened: More than four billion years ago, after a massive collision between Earth and another large object created the moon, our planet was completely dessicated. Then, during the Late Heavy Bombardment period that followed, during which lots of asteroids hit Earth, the ice that the objects carried became our store of water []. The bombardment period, which occurred nearly 4 billion years ago, was largely responsible for our moon’s puckered appearance. A 2005 Nature study estimated that between 3 and 8 zettagrams of material slammed into the moon during that time (zetta means 10 to the 21st power, or a billion times a trillion), which implies that plenty of rocks slammed into the Earth, too.

Asteroids just keep getting more interesting. As we noted on Monday, the Japanese spacecraft that touched down on an asteroid is limping home to Earth, hoping to return its results (and maybe an asteroid sample) to the home world by June. And President Obama’s revised space exploration plan includes the idea for astronauts to visit an asteroid—a possibility that’s all the more scientifically enticing if they were the bringers of our water.

Related Content:
DISCOVER: Did An Early Pummeling of Asteroids Lead to Life on Earth?
80beats: Did An Asteroid Strike Billions of Years Ago Flip the Moon Around?
80beats: Our Alien Atmosphere? Earth’s Gases May Have Arrived Here Aboard Comets
80beats: Danger, President Obama! Visiting an Asteroid Is Exciting, But Difficult

Image: NASA

  • bigjohn756

    Well, naming these icy bodies is easy. Just call them iceteroids.

  • nick

    It’s kinda funny that this sorta thing is even up for debate. We already know that every atom on earth was birthed in the heart of a star. As Sagan liked to say, “We are made of star stuff.” Everything here except the elements we’ve made via fusion or fission is of extraterrestrial origin, and even the things we’ve made used extraterrestrial components. The only real question is “how recently extraterrestrial are they?”

  • Cory

    Well, the discussion is mostly upon WHEN the water came. If it came before the Earth “formed” as an entity, and was a part of its structure doing its early development, it is not “extraterrestrial”. If it came in bombardments after the Earth already was essentially formed, the water is indeed extraterrestrial.

  • Brian Schmidt

    NEAR Shoemaker was the first to land on an asteroid:

    Habayusa was the first to land and take off.

  • m


    how does the ice stay on the surface of the asteroid for them to see it? It has no atmosphere…and very little gravity.

    any water that seeps to the surface would surely sublimate right away.

    and how could an asteroid that small have extensive resevoirs of water? I mean…after millions and millions of years, surely any water it has retained would have seeped through and into space by now.

    it’s interesting….but there’s something bugging me about this that I cant put my finger on. Part of me thinks its not water they are seeing.

    Then again….haven’t had my coffee yet.

  • Eliza Strickland

    Brian Schmidt: Thanks for catching that. I’ll make the change.

    — Eliza, online news editor

  • Philip A.Morrissette

    This is no surprise to me. Read “Worlds in Collision” and “Earth in Upheaval” by Immanuel Velikovsky and see where in 1950 he predicted many of these things. You will find it fascinating.


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