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.