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!
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.
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.
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.
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.