In Praise of the Comet

21 Oct

This post was chosen as an Editor's Selection for ResearchBlogging.orgComets get a lot of bad press. Before science came along, they were usually seen as bad omens from the gods – “tokens of impending doom” in the words of one Roman astrologer.

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.

Night of the, zombies and big hair

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.

In their Nature study, Paul Hartogh and colleagues used the Herschel Space Observatory (HSO) to study the composition of Hartley 2, a comet located in the Kuiper belt beyond the orbit of Neptune.

The Hartley 2 comet, looking very pretty (credit: NASA/JPL-Caltech/UMD)

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 (In heavy water, one of the two hydrogen atoms has been replaced by the hydrogen isotope known as deuterium.)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.

An artist's impression of the TW Hydrae star and its surrounding disk (credit: NASA/CXC/M.Weiss)

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…

Further reading:

ResearchBlogging.orgHartogh 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

ResearchBlogging.orgMichiel 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


6 Responses to “In Praise of the Comet”

  1. morganism October 23, 2011 at 9:27 pm #

    pretty brave considering you are using one single observation of each type to write this story.

    almost every comet studied that has broken up has shown NO water in the spectra. and this is the first disk that has shown ice in two different phases/sizes. could just be an error in reading some of the 3,000 lines that are attributed to water in spectra.

    Hartley 2 is also kind of a rouge anyway. it is outgassing (last i heard) out to 29 AU. that is way beyond what any theories have speculated as possible. most comets don’t even “light up” until they are in the 7-9 au range.

  2. thesoftanonymous October 24, 2011 at 3:00 am #

    Thanks for commenting. These two studies don’t, of course, provide conclusive proof for the ‘oceans from space’ theory – I tried to use phrasing that would show this uncertainty, but maybe I should’ve been more explicit.

    The studies do, however, offer the most convincing evidence yet, and there haven’t been any similarly high-profile, reputable studies to support alternative theories for the origin of Earth’s water.

    A study may someday be released which debunks this theory (like the analysis of Halley/Hale-Bopp seemed to), in which case this story would become out-of-date…that’s the nature of science I guess.

    I’d be interested to find out about the other comet studies you mentioned…do you have any links/references? The Hartogh et al. paper is the first in-depth analysis of a Kuiper belt comet, as far as I’m aware.

    For the error in the water spectra, I’d have thought that’d be a very basic mistake – I’ll put my trust in the expertise of the researchers/reviewers.

  3. Greg Laden October 25, 2011 at 3:48 pm #

    Congratulations on wining the prestigious Editors Selection Award!


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