Sky & Telescope - March 2023 - 39

CARBONACEOUS CHONDRITE This slice of the famous Allende meteorite, which fell in Mexico in 1969,
shows the classic, spherical mineral grains called chondrules that appear in nearly all chondrites. The whitish,
irregular patches are calcium-aluminum inclusions (CAIs), thought to be fragments of the earliest rocks
formed in the solar system. For scale, the arrowhead-shaped CAI at lower left is about 6 mm long.
ASTEROID SOUVENIR Material from Chamber A of the sample catcher comes from Hayabusa 2's first
touchdown on Ryugu in February 2019. Slightly more than 1 gram appears here.
niques that can measure the composition of tiny mineral
grains embedded within meteorites, researchers have found
that, trapped in the minerals inside enstatite chondrites,
there's enough hydrogen to yield some 3 to 23 oceans' worth
of water to Earth during its formation. If Earth was entirely
made of this material, it would eliminate the need for a delivery
system from the outer solar system.
There is one caveat: Enstatite chondrites' D/H ratio is
lower than that of Earth's oceans.
That could be a good thing, though. Recently, a group
of researchers led by Karen Meech (University of Hawai'i,
Ma¯noa) found evidence supporting the idea that the ocean's
D/H ratio today might not be the same as the primordial
water that Earth originally acquired. Instead, reservoirs of
primordial water that are trapped deep in the mantle could be
a better match.
Oceans have gone through a lot during their history:
Volcanism, subduction, the escape of lighter isotopes from
the atmosphere, biological activity, giant impacts like the one
that formed the Moon . . . all of these can change the D/H
ratios over time. " How could the Earth's oceans possibly represent
the primordial water? " Meech wondered. " It just didn't
make sense. "
Geologists have long known that mantle plumes that
fuel hot-spot volcanism in places like Hawai'i or Greenland
emerge from deep reservoirs, located near the core-mantle
boundary. These plumes can bring to the surface material
that might have been buried since the early days of the
planet's formation. They carry a distinct isotopic signature in
their helium atoms: a lack of helium-4, which forms from the
radioactive decay of certain elements.
Meech realized that no one had measured the D/H ratios
of these rocks, because the technology to do so was new.
Along with Lydia Hallis (now University of Glasgow, UK)
and several collaborators, she measured the D/H ratios from
volcanic rocks with a deep origin from Baffin and Padloping
islands in northeastern Canada. These samples revealed D/H
ratios up to 25% lower than ocean values.
The finding shows that at least some rocks inside Earth
preserve a lower D/H ratio that is closer to the solar nebula
values and that could be compatible with an Earth built
from enstatite chondrites or a similar material. While the
result does not preclude the delivery of a late veneer, it might
change our estimations of the contribution comets and asteroids
made to the total water budget.
It could also mean that wet planets around other stars
could be more common than previously thought. If the building
blocks of planets that form close to their stars can preserve
enough water to form oceans, then a planetary system's
subsequent evolution might play a minor role in the final
outcome. It could be that building an Earth-like world doesn't
require the complex convergence of events we've experienced
in our solar system.
Back to the Skies
In 2010, after a trip full of technical difficulties, the first
Hayabusa spacecraft recovered a few grains of asteroid Itokawa
and brought them back to Earth. Several teams around
the world were allocated bits of this material to analyze,
among them Luke Daly (University of Glasgow) and colleagues.
In 2021, the team made a surprising find.
All airless surfaces exposed to space change physically and
sk yand tele scope .o r g * MARCH 2023 39
ALLENDE METEORITE: JAMES ST. JOHN / FLICKR / CC BY 2.0;
RYUGU SAMPLE: JAXA

Sky & Telescope - March 2023

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