Even now that we know comets to be lumps of ice and rock (‘dirty snowballs’) flying through space, countless movies portray them as harbingers of doom and destroyers of mankind. One such film, Night of the Comet, deserves a special mention, if only for being brave enough to mix apocalyptic horror, big 1980s hair, and a soundtrack featuring Cyndi Lauper.
Our fear of comets (and of asteroids, their less icy cousins) is perhaps not completely irrational. After all, an impact is believed to have wiped out the dinosaurs 65 million years ago, and the Earth will probably encounter another deadly near-Earth object sometime in its future (at which point humans may or may not still be around).
But comets also deserve some praise, because we probably wouldn’t be here without them. To find out why, we need to travel back in time to the beginning of the Solar System…
Oceans from outer space?
Around 4.5 billion years ago, the Sun was a lonely star, encircled by a rotating disk of gas and dust. As this disk cooled, the planets began to form, as well as smaller bodies such as comets and asteroids.
The early Earth was a fiery, hellish place. Any liquid water would have instantly evaporated, raising the question of how exactly our planet came to be covered in oceans.
Now, thanks to two studies published this month, there are new hints that the Earth’s water arrived from outer space, hitching a ride on earthbound, ice-covered comets or asteroids.
These astronomers found that the comet’s ice has a strikingly similar chemical composition to the water in Earth’s oceans, measured by the ratio of ‘heavy’ water to ‘regular’ water. (In heavy water, one of the two hydrogen atoms has been replaced by the hydrogen isotope known as deuterium.)
Previous studies analysed the ice content of the Halley, Hyakutake and Hale-Bopp comets, but found that these particular comets contained too many deuterium atoms to be compatible with the Earth’s water.
The matching water signature found by Hartogh et al. for the Hartley 2 comet is a breakthrough – a clue that our oceans may have come from the Kuiper belt comets in the outer Solar System.
A distant, icy reservoir
A second piece of evidence for the ‘oceans-from-space’ theory emerged this week in a study published in Science. Michiel Hogerheijde and colleagues used the same space observatory (HSO) to investigate a developing disk of gas and dust around the young star TW Hydrae, situated approximately 175 light-years away from Earth.
The researchers detected cold water vapour in the outer regions of the disk, produced when ultraviolet light from the star hits ice grains near the disk surface. Using a computer simulation, Hogerheijde et al. estimated that this water vapour corresponds to a sprawling reservoir of ice crystals, containing enough water to fill several thousand Earth oceans.
Because our Solar System formed from a similar disk, this TW Hydrae system can provide hints about how the Earth came into being – the astronomical equivalent of looking back in time.
If the early Solar System harboured a similarly vast ice reservoir, then a large proportion of comets and asteroids would have carried water, thus increasing the likelihood of our Earth being hit by a watery projectile.
So it looks like our seas may have come from the skies, in which case comets will have given us a lot more than just a few pretty flybys and apocalyptic B movies…
Hartogh P, Lis DC, Bockelée-Morvan D, de Val-Borro M, Biver N, Küppers M, Emprechtinger M, Bergin EA, Crovisier J, Rengel M, Moreno R, Szutowicz S, & Blake GA (2011). Ocean-like water in the Jupiter-family comet 103P/Hartley 2. Nature, 478 (7368), 218-20 PMID: 21976024
Michiel R. Hogerheijde, Edwin A. Bergin, Christian Brinch, L. Ilsedore Cleeves, Jeffrey K. J. Fogel, Geoffrey A. Blake, Carsten Dominik, Dariusz C. Lis, Gary Melnick, David Neufeld, Olja Panic, John C. Pearson, Lars Kristensen, Umut A. Yildiz, & Ewine F. van Dishoeck (2011). Detection of the Water Reservoir in a Forming Planetary System Science 6054 (2011), 338 arXiv: 1110.4600v1