Did distant comets quench a thirsty Earth?

A striking active prominence issues from the big sunspot group 1305 as it approached the western limb of the sun last Thursday. Prominences are towering flames of incandescent hydrogen gas many thousands of miles long often associated with sunspot groups. Credit: John Chumack

A beautiful, bright sun shines outside my window this morning. It was preceded by the setting moon and the onset of dawn, both of which I saw while hunting for Comet Elenin. In the briefest of intervals, I watched the sky go dark around 5:45 a.m. and then the lights slowly come back up with twilight after six. Try as I might however, I couldn’t convince myself of seeing the comet through the big 15-inch scope. Maybe there was a bit of fuzz there, maybe not. I used low power, medium power and even the power of imagination but nothing I tried convinced me Elenin was there. I’ll be out again around the 23rd or 24th when conditions will be much better. Until then, happy trails Comet E!

Comet Hartley 2 photographed by the Deep Space/EPOXI spacecraft in 2010. Credit: NASA

Last week European astronomers using the Herschel Space Observatory announced they’d found water in Comet Hartley 2 with almost the same composition of water found in the Earth’s oceans. You might remember Hartley 2 which passed near Earth in October 2010 and became faintly visible with the naked eye for a time. It was also the target of a close flyby by the Deep Impact mission a month later.

The question of where our planet got all its water has been hotly debated. The Earth grew over a period of 1-200 million years 4.3-4.5 billion years from the accumulation of millions of meteorites and asteroids. Heat arising from all that impacting material and the decay of radioactive elements (which release heat energy) melted our little globe, causing the heavier metals like iron and nickel to sink to the core with lighter rocks floating to the top to form the planet’s crust. Our world was once a glowing ball of hot magma – not exactly the kind of place you’d find water sloshing about.

That’s why scientists believe the water that’s now so plentiful and makes our world so distinctive had to be ‘delivered’ later after things cooled down. Since comets are composed largely of water ice and there are billions of them – especially in the early solar system when there was so much more ‘junk’ around – they seemed the perfect choice as a delivery mechanism to quench our planet’s thirst.

A neutron (in blue) joins hydrogen's single proton to make deuterium, represented by the chemical formula 2H.

One problem: almost all comets astronomers have studied so far contain twice as much deuterium in the hydrogen that makes the H in their H2O. Deuterium? The most common form of hydrogen has only one proton in its nucleus circled by a single electron, but 1 out of 6,420 hydrogen atoms in the Earth’s oceans also has a neutron paired up with that proton. The addition of the neutron doubles the mass of the hydrogen which is why D2O (D for deuterium) is nicknamed ‘heavy water’. If comets really are responsible for bringing water to Earth, why don’t our oceans have more heavy water? Enter the European Space Agency’s Herschel Space Observatory, the largest space-based scope currently in orbit. Its 138-inch (3.5 meter) diameter mirror and optical system are optimized to study the sky in infrared light.

This illustration shows the orbit of Comet Hartley 2 in relation to those of the five innermost planets. The comet made its latest close pass of Earth in October 2010, at which time Herschel observed the comet. Water shows as a big blue bump in the comet's light spectrum (right). Credits: ESA/AOES Medialab; Herschel/HssO Consortium

Herschel studied Comet Hartley 2 and discovered that its water has almost the same composition – regular vs. heavy water – as the Earth’s oceans. Most of the comets previously studied are thought to have formed closer to Jupiter and Saturn and then booted into the outer solar system’s Kuiper Belt through gravitational interactions with those planets. Comet Hartley 2 is different. Its birthplace was the frigid Kuiper Belt, where the deuterium to hydrogen ratio may have been very different from the one in comets formed closer to the sun.

Beyond the orbit of the most distant planet Neptune lies a vast region populated by icy asteroids and comets called the Kuiper Belt. Comets like Hartley 2 are believed to have formed here 4.5 billion years ago. Later some of them migrated inwards and may have collided with Earth, delivering water to the early planet's parched surface. Credits: ESA/AOES Medialab

So the water that makes our planet the wettest, wildest place in the solar system may still have come from comets, but from ones formed in the far reaches of the solar system. Astronomers using the Herschel scope will now be looking at other comets to confirm their hypothesis.

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About astrobob

My name is Bob King and I work at the Duluth News Tribune in Duluth, Minn. as a photographer and photo editor. I'm also an amateur astronomer and have been keen on the sky since age 11. My modest credentials include membership in the American Association of Variable Star Observers (AAVSO) where I'm a regular contributor, International Meteorite Collectors Assn. and Arrowhead Astronomical Society. I also teach community education astronomy classes at our local planetarium.

7 thoughts on “Did distant comets quench a thirsty Earth?

  1. hi bob. interesting article. but i’m a skeptic. find it hard to accept the view that comets alone could have “delivered” oceans full of water. even taking into account the probability of a comet rich early solar system. what do you think?

    • Thomas,
      I understand your skepticism. Although it’s not my area of expertise, I have to think that at least some of the water arrived from volcanoes belching gas and water vapor trapped inside the Earth’s crust. Maybe it was really a combination of delivery by both comets and meteorites (certain carbonaceous ones are water-rich) as well as in-situ through volcanic activity and earthquakes.

    • Les,
      Vesta revolves in its own orbit far away in the asteroid belt between Mars and Jupiter. No threat to Earth. The Dawn spacecraft is currently orbiting Vesta, taking pictures and studying the minerals on its surface. Google ‘Dawn and Vesta’ to find out more.

  2. hi , I just have a question , I new they tried film the comet with the faulk telescop I think it was yesterday but they was not able to see the comet , shouldnt they see it with that great telescop? or is it because the coma is gone? its really hard to discover then maybe?

    • Hi Peder,
      First, it means that the fragments are very, very faint. Too faint to show in even a very large telescope. But it doesn’t negate the amateur observation by J.J. Gonzalez. He observed under optimum conditions using a telescope with a much wider field of view. This allowed him to see the dim, low contrast coma or gas cloud against a dark sky background. The Faulkes telescope has narrow field of view (in the photo taken it was on 10×10 minutes of arc) not as well suited to showing a large, dim coma. The Faulkes observation was made on October 10 and likely was sullied by a small amount by twilight and moon glow near the horizon. Gonzalez observation was made the previous morning without interference from twilight or moonlight.

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