Sky & Telescope - July 2021 - 9

SOLAR SYSTEM

Is an Ocean of Water Trapped in the Martian Crust?

Bits of Theia Might Be
in Earth's Mantle

WATER LOSS DIAG R A M: G REGG DINDER M A N / S&T; SOURCE:
SCHELLER E T A L. / LPSC; LLSV P M ASSES: SA NNE.COT TA A R /
WIKIMEDIA COMMONS / CC BY-SA 4.0

MARS ONCE HAD ENOUGH water to

ﬁll a global ocean 100 to 1,500 meters
(300 to 5,000 feet) deep. But scientists
have had trouble explaining where all
of it went. In the April 2nd Science,
graduate student Eva Scheller and
advisor Bethany Ehlmann (both at
Caltech) used a model of the Martian
water cycle to show that most of it was
trapped in water-loving minerals in
the Martian crust.
Water (H2O) rarely contains a
heavier form of hydrogen called deuterium, which has an extra neutron.
Water on Mars has become heavier
over the eons, and previous studies
assumed this happened as water was
lost to space. After ultraviolet light
from the Sun broke apart water molecules, normal hydrogen atoms were
more likely to escape the atmosphere
than heavier deuterium.
However, escape hasn't explained
all of the water loss. So Scheller,
Ehlmann, and their colleagues built a
model that takes into account watertrapping chemistry on the surface,
and they found it plays a signiﬁcant
role. " More than half of Mars's initial

water was sequestered in the crust by
3 billion years ago, " Scheller said at
the 52nd Lunar and Planetary Science
Conference in March.
Graduate student Liza Wernicke
and Bruce Jakosky (both at University
of Colorado, Boulder) independently
used data from the Mars Odyssey and
Mars Express orbiters to gauge the
amount of water locked into hydrated
minerals. Their estimates, published
in the March Journal of Geophysical
Research: Planets, are consistent with
the results from Scheller's team. However, Jakosky cautions that there's still
a lot we don't know about the planet's
history. Those unknowns complicate
the connection between the current
presence of hydrated minerals and the
long-term evolution of the water cycle.
NASA's Perseverance rover will be
key to testing and building on the
model of Mars's water cycle, Ehlmann
says. The rocks in and around Jezero
Crater are the oldest to be explored
by a rover, and lab tests of eventual
sample returns could distinguish
between evolution scenarios.
MONICA YOUNG

Initial water volume from
comets and asteroids

Water from
volcanism

Water loss to crust

Water freezes
to ice

Hydrated crust
Unhydrated crust

4 billion

3 billion

Present

Time (years ago)

S This diagram shows water sources and sinks on Mars. Comets and asteroids initially delivered water to the planet, and volcanic outgassing continued to add more over time. Water was
lost to the planet and to space, but in the end, crustal chemistry claimed more than space.

EVIDENCE FOR the impact that created
the Moon might lie far beneath our feet.
The leading theory for the Moon's
formation is that a roughly Mars-size
object, dubbed Theia, struck Earth
around 4.5 billion years ago. At the virtual 52nd Lunar and Planetary Science
Conference, Qian Yuan (Arizona State
University) and his colleagues suggested
that bits of Theia might remain as two
dense masses deep in our
planet's mantle. Their
study will appear
in Geophysical
Research Letters.
The two
continent-size
masses are
known as large,
low-shear-velocity
provinces (LLSVPs).
Seismic waves traS This depiction
of the large, lowversing our planet's
shear-velocity
interior have revealed
provinces (LLSthese to be roughly
VPs) is based
1,000 kilometers tall
on a 2016 study
and several thousand
using seismic
kilometers wide, stuck tomography.
on either side of the
Earth's core like misshapen earmuffs.
Inspired by recent results dating the
LLSVPs to 4.45 billion years old, Yuan
and his colleagues simulated the evolution of Theia's remains following the
giant impact. The colliding objects' cores
merged right away, but the researchers
found that as long as Theia's mantle
was dense enough, it would have stayed
separate from Earth's mantle, sinking to
the bottom instead of mixing into it.
However, Jennifer Jenkins (Durham
University, UK), who was not involved
in the study, says we still don't understand the LLSVPs' exact nature, as studies of them have used low-frequency
seismic waves that paint a fuzzy picture.
Lunar sample returns from the South
Pole-Aitken Basin, where a different ancient impact exposed the lunar
mantle, would help settle the matter.
DAVID DICKINSON
s k y a n d t e l e s c o p e . o r g * J U LY 2 0 2 1

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Sky & Telescope - July 2021

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