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Water: From Comets to the Sea?

by David Jewitt

Comet Hyakutake above Waimanalo Bay. © R. J. Wainscoat

That Earth is wet is obvious. In Hawaii, we live surrounded by a deep ocean of liquid water that extends for thousands of miles in all directions. But where did all this water come from? One idea is that steam rose from the hot young Earth much as we see steam rising from the volcanoes on the Big Island today. However, studies of the composition of magmas suggest that when Earth formed it was too hot for much water to be included in its rocks. Instead, many astronomers suspect that some of the water was delivered to Earth by impacting bodies sometime after Earth's formation.

Astronomers at the Institute for Astronomy are undertaking studies that relate to the origin of Earth's oceans in surprising ways. One hypothesis is that comets, known to be bodies rich in water ice, sometimes collide with Earth to deliver their water. IfA astronomers Tobias Owen, Roland Meier (now at Xerox Corp. in Switzerland), and I are among those who have examined this possibility by making detailed measurements of the isotopes of hydrogen in cometary water. We and others have found that comets have about two times more deuterium (hydrogen with an extra neutron) than ocean water. This discrepancy, although based on measurements of only three comets, seems to argue that these comets cannot be the dominant source of Earth's oceans.

A second problem is that typical comets, like those visible to the naked eye every few years, are too small to do the job: these objects are a few miles across and carry only a few billion tons of water each. Roughly a billion such comets would be needed to deliver Earth's water. The oceans contain about a million trillion tons of water, corresponding to a comet roughly 600 miles in diameter (a sphere roughly ten times the size of the Big Island). No such large comets are presently known. Are there places in the solar system where these megacomets might be found?

The answer is provided by other research ongoing at the IfA, and it is a definite "yes." The Kuiper Belt, discovered in 1992 by Jane Luu and me with the UH 2.2-meter telescope, is the region of the solar system starting at the orbit of Neptune and extending out to a few thousand astronomical units from the Sun. Previously thought to be empty, this region is now known to be occupied by vast numbers of objects that are within reach of the telescopes on Mauna Kea. Included are perhaps 50,000 to 100,000 ice-rich objects larger than 60 miles in diameter, and dozens as big as 600 miles. Dynamical calculations show that Kuiper Belt objects are occasionally dislodged from their trans-Neptunian orbits, and spiral in toward the Sun, where they begin to sublimate and are relabeled as comets by Earth-based observers. On their way, a small fraction may collide with Earth and other planets. The calculations also show that the modern-day Kuiper Belt is but a shadow of its former self. The number of Kuiper Belt objects originally present was 100 times larger than now, making it an even better source of water-carrying comets in the early days of the solar system.

A Kuiper Belt source of water might explain the discrepancy between the measured cometary and oceanic deuterium abundances. The three measured comets were formed at a different place and at a higher temperature than the comets in the Kuiper Belt. For these reasons, their deuterium values may not be representative of the lower temperatures that prevail in the more distant Kuiper Belt. How can we tell? What are needed are accurate measurements of the deuterium abundance in the so-called Jupiter-family comets, whose origin lies in the Kuiper Belt. We hope to take such measurements using the James Clerk Maxwell and Submillimeter Array telescopes on Mauna Kea when a suitably bright Jupiter-family comet appears. Unfortunately, bright Jupiter-family comets are exceedingly rare, so we could be in for a long wait.

A second route relies on the study of the inert "noble" gases, which might also be supplied, in part, by comets. However, the noble gases have very subtle observational signatures, and their study must probably await the investigation of comets and Kuiper Belt objects by spacecraft.

Meanwhile, next time you drink water or go for a swim, keep in mind that we don't know where that water came from. NASA has just awarded a $5 million grant to scientists at UH so they can try to find out.

Astronomical unit: A unit of length equal to the average distance of Earth from the Sun, about 93 million miles.

Jupiter-family comet: A periodic comet whose orbit has been altered by and is gravitationally controlled by Jupiter. These objects are most likely to have recently arrived in the inner solar system from deep-freeze storage in the Kuiper Belt.

Noble gases: Helium, neon, argon, krypton, xenon, and radon, which are chemically inert elements, that is, they do not easily combine with other elements to form compounds. Except for argon, only trace amounts of these gases are found in Earth's atmosphere.