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Earth’s Oceans: A Cometary Source After All?

Getting water into the inner Solar System is an interesting exercise. There has to be a mechanism for it, because the early Earth formed at temperatures that would have caused any available water to have evaporated. Scientists have long speculated that water must have been delivered either through comets or asteroids once the Earth had cooled enough to allow liquid water to exist. The former was preferred because the water content in comets is so much higher than in asteroids.

But the theory had problems, not the least of which was that comets studied in this regard showed deuterium levels twice that of Earth’s oceans. The ratio of deuterium and hydrogen, both made just after the Big Bang, can vary in water depending on its location because local conditions can affect the chemical reactions that go into making ice in space. A comparison of the deuterium to hydrogen ratio in extraterrestrial objects can be compared to water found in Earth’s oceans to identify the source of our water. Now comet Hartley 2 swings into the picture, for researchers have announced that its hydrogen/deuterium ratio is similar to Earth’s oceans.

Image: This illustration shows the orbit of comet Hartley 2 in relation to those of the five innermost planets of the Solar System. The comet made its latest close pass of Earth on 20 October last year, coming to 19.45 million km. On this occasion, Herschel observed the comet. The inset on the right side shows the image obtained with Herschel’s PACS instrument. The two lines are the water data from HIFI instrument. Credit: ESA/AOES Medialab; Herschel/HssO Consortium.

So how do you measure the hydrogen/deuterium ratio in the water of a comet? The answer is an instrument called HIFI, which operates aboard the European Space Agency’s Herschel infrared space observatory. HIFI (Heterodyne Instrument for the Far Infrared) is a high-resolution heterodyne spectrometer developed in The Netherlands that covers two bands from 480-1250 gigaHertz and 1410–1910 gigaHertz. Herschel was examining the comet’s coma, which develops as frozen materials inside vaporize when the comet moves closer to the Sun.

Remember, previous comet studies had found hydrogen/deuterium ratios different from our oceans. The difference between these comets and Hartley 2 may be that Hartley 2 was formed in the Kuiper Belt, whereas other comets studied in this regard are thought to have first formed near Jupiter and Saturn before being flung out by the gravitational effects of the gas giants, returning millions of years later for their pass around the Sun. The hydrogen/deuterium ratio we see in water ice may well have been different in the Kuiper Belt than in ice that first formed in the inner system, where conditions are much warmer. Further comets studies may confirm the idea.

Says Dariusz Lis (Caltech):

“Our results with Herschel suggest that comets could have played a major role in bringing vast amounts of water to an early Earth.This finding substantially expands the reservoir of Earth ocean-like water in the solar system to now include icy bodies originating in the Kuiper Belt.”

Surely the early oceans were the result of both comet and asteroid impacts, but the new findings point back to comets as major players. Even so, we have plenty of work to do to understand the role of the lightest elements and their isotopes in the early Solar System. Six comets besides Hartley 2 have been examined for hydrogen/deuterium levels, all with deuterium levels approximately twice that found in Earth water. Kuiper Belt comets were once thought to have even higher deuterium levels than Oort Cloud comets, an idea the Hartley 2 results have now refuted.

The team led by Paul Hartogh (Max Planck Institute for Solar System Research) has also used the Herschel Observatory to measure the hydrogen/deuterium ratio in comet 45P/Honda-Mrkos-Pajdusakova, another Kuiper Belt comet whose data is now under analysis, so we may soon have new data to add to this story. The paper is Hartogh, “Ocean-like water in the Jupiter-family comet 103P/Hartley 2,” published online in Nature 5 October 2011 (abstract).

And there is further news out of the joint meeting in Nantes, France, of the European Planetary Science Congress and the American Astronomical Society’s Division for Planetary Sciences, where this work was announced. As noted in this article in Nature, a new study of the Sun-like star Eta Corvi, which is roughly the same age our Sun was during the Late Heavy Bombardment (when most water is thought to have been delivered to the Earth), shows that the star has an inner ring of warm dust that is rich in carbon and water. Team leader Carey Lisse (JHU/APL) thinks we’re seeing the traces of one or more Kuiper Belt-class comets being flung into the inner system, colliding with a planet there to form the ensuing ring of material.

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  • jkittle October 6, 2011, 10:08

    One Week Out
    We are reminded that the region of space just outside of the major known planets may have played a very large role in the formation of life on earth. Because of the enormous volume of the region and the weakness of light reaching deep space from the sun, astronomers have relatively little knowledge of the region much past 100 AU.
    This limitation is rapidly changing, though. Observations from large survey scopes such as Panstarrs, the Catalina sky survey, VISTA are poised to discover new objects at greater distances, where the obit may only progress across the sky at only a few seconds of arc per year. At these distances even large , interesting objects would appear as very faint images in a sea of stars, galaxies and gas clouds. These sky surveys thus take time to mature and identify new discoveries. The planned LSST survey scope and Gaia space telescope will intensify the search. Over the next decade we will be getting a lot more information.
    One AU is about 8 light minutes.. so a light week is about 1,200 AU. This is still well within our solar systems gravitational field. If the past is any guide to the present, as astronomers pour though the new data, they will find more objects full of surprises and wonderment. This distant region is the birthplace of comets and maybe intimately involved in the formation of a solar system with ( at least one ) life sustaining planet, as the Deuterium data may indicate. On these posts we are all interested in the means to travel interstellar distances. We want to plan travel to distances 10 to 20 light years from earth. The speeds required are substantial fractions of the speed of light. To send a probe to this outer region of the solar system presents some of the same challenges, scaled back. IF we could consider a journey one light week out- and achievable with a travel time of three to five years.. that may be a good test of the technologies that might eventually carry us to the stars. 5 years to plan.. 5 years to build, 5 years to fly.. seems do- able to me , and do-able in my lifetime perhaps.
    The astronomers will provide the targets. Will we be ready?
    The new horizons mission is a warm up.
    A light year or a week? we still have promises to keep.. and many miles before we sleep.

  • Ronald October 6, 2011, 11:23

    @jkittle:
    if we are able to send a craft to 1200 AU in 3 to 5 years, we could also easily send a solar gravitational telescope (such as Maccone’s FOCAL) to the sun’s gravitational focal point, ‘only’ about 3 to 3.5 lightdays (500-600 AU) out.

  • Ronald October 6, 2011, 11:28

    With regard to water on earth: the amount of water in the earth mantle has been estimated at between 5 and 20 times all the ocean’s water.
    Did (part of this) this water also come from comets? That would have been a lot!
    On the other hand, if it did not and mantle water is ‘original’ earth water, located deep enough to have been prevented from evaporation and photo-dissociation, then why is Venus so dry and consequently without plate tectonics?

  • Martin J Sallberg October 6, 2011, 11:49

    Earth may have got its water from multiple sources, and then mixed it together to its current hydrogen/deuterium ratio. It is also possible that more hydrogen than deuterium have escaped.

  • Bounty October 6, 2011, 14:58

    I’m assuming the small amount of water on the moon should have similar deuterium levels…. whether it brought it in with it, or was also impacted by these comets. Not sure I’ve seen the gasses/ices modeled in Earth-Moon collisions. I’ve read somewhere that “a significant delivery of cometary water to the Earth–Moon system occurred shortly after the Moon-forming impact.” I wonder if some of it could be ice raining down after exploding off of the pre-moon object during impact.

  • Bob October 6, 2011, 16:48

    “We are reminded that the region of space just outside of the major known planets may have played a very large role in the formation of life on earth. ”

    I wish the scientific community at large would admit that we really don’t know where life first originated, in comets, on earth or somewhere else and that the idea that life must have independently originated on earth is based on a dearth of knowledge and is at this point a dogmatic assumption. Any thoughts?

  • jkittle October 7, 2011, 6:00

    Martin You are right. there is a HUGE kinetic isotope effect in the escape of Hydrogen from a gravity field. For a terrestrial planet, first there is the the breakdown of methane , ammonia and water by ionizing radiation ( mostly UV) and then the subsequent escape of hydrogen atoms in a atmosphere. The kinetic motion of of hydrogen is much higher than deuterium and so it escapes more frequently. thus deuterium may become enriched over time

  • ljk December 10, 2012, 11:17

    http://arxiv.org/abs/1212.0872

    On the photosynthetic potential in the very Early Archean oceans

    Authors: Daile Avila, Rolando Cardenas, Osmel Martin

    (Submitted on 4 Dec 2012)

    Abstract: In this work we apply a mathematical model of photosynthesis to quantify the potential for photosynthetic life in the very Early Archean oceans. We assume the presence of oceanic blockers of ultraviolet radiation, specifically ferrous ions. For this scenario, our results suggest a potential for photosynthetic life greater than or similar to that in later eras/eons, such as the Late Archean and the current Phanerozoic eon.

    Comments: Accepted for publication in Origins of Life and Evolution of Biospheres

    Subjects: Populations and Evolution (q-bio.PE); Biological Physics (physics.bio-ph); Subcellular Processes (q-bio.SC)

    Cite as: arXiv:1212.0872 [q-bio.PE]
    (or arXiv:1212.0872v1 [q-bio.PE] for this version)

    Submission history

    From: Daile Avila [view email]

    [v1] Tue, 4 Dec 2012 21:07:53 GMT (157kb)

    http://arxiv.org/ftp/arxiv/papers/1212/1212.0872.pdf