Sky & Telescope - March 2023 - 36

The Blue Marble
Foreign Water
Based on its ocean-covered surface, Earth looks like a planet
overflowing with water. Even more hides below the crust: The
planet may hold enough hydrogen and oxygen bound in its
minerals to fill its oceans a half dozen times or more (estimates
vary wildly). But as a whole, Earth is 99.8% rock.
The neighboring planets are equally dry. While Venus and
Mars might have had some liquid water on their surfaces at
some point in the past, overall they were never much more
hydrated than Earth is.
This water paucity is due to how the terrestrial planets
were forged. The solar system formed when a giant cloud of
gas and dust collapsed to give birth to the Sun. What was
left of the cloud, known as the solar nebula, flattened into a
disk where the planets coalesced (S&T: May 2020, p. 34). The
solar nebula had plenty of hydrogen and oxygen, which are
the first and third most abundant elements in the universe,
respectively. But close to the young Sun, in the zone where
the terrestrial planets formed, temperatures were in the range
of 2000K (3000°F), too hot for hydrogen and oxygen to
condense and become trapped in rocks. This means that the
building blocks of the inner planets remained dry.
Nevertheless, Earth does have water. This water carries a
distinctive chemical signature hidden among its hydrogen
atoms - the isotope deuterium.
Hydrogen is the simplest atom in the universe, with just
one proton and one neutron. Deuterium crams a neutron in
with the proton and thus is twice as heavy, even if chemically
it behaves like regular hydrogen. Deuterium is also rare: Only
25 out of every million hydrogen atoms in the universe are
deuterium, nearly the same ratio found in the Sun and the
long-gone solar nebula.
But by the 1980s, scientists had realized that Earth's
oceans have seven times more deuterium than the Sun. The
discrepancy suggested that Earth obtained much or all of
its water from something other than the material its bulk
formed from.
THAT'S NO METEOR Hayabusa 2's return capsule, carrying its
precious cargo, streaks through the nighttime sky near Coober
Pedy, Australia.
The handiest source to provide this water is the outer solar
system, which is rich in water and volatiles and also full of
deuterium. Computer simulations of the conditions in the
early solar system show that the deuterium-to-hydrogen
(D/H) ratio should increase with distance from the Sun,
as cold temperatures favor deuterium buildups. This is why
many researchers were excited when the European spacecraft
Giotto approached Comet Halley in 1986 and found that its
deuterium levels might match Earth's - although technical
limitations left ample room for error.
Scientists came up with an apparently sound theory: Comets
from the outer solar system delivered water to the inner
solar system after the planets formed, crashing into the rocky
PRECIOUS CARGO The sample cache from Ryugu descended by parachute to the Australian Outback (left) and broadcast its position with a radio
beacon. A helicopter team deployed and found the site soon after, carefully retrieving the wok-size cargo and bringing it home for researchers to pore
over in laboratories.
36 MARCH 2023 * SKY & TELESCOPE
JAXA (4)

Sky & Telescope - March 2023

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