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The GPS data we use come primarily from the PBO and
are publicly available from UNAVCO through the Geodesy
Advancing Geosciences and EarthScope Facility, which is
supported by the NSF and NASA under NSF cooperative
agreement no. EAR-1261833. Meteorological data are
publicly available from the National Climatic Data Center’s
Global Historical Climatology Network. The software used for
load computation (SPOTL) is publicly available, and the
processing software may be obtained from us. We acknowledge
the efforts of many at UNAVCO to produce the exceptional
PBO GPS data set, especially the station installation efforts
of C. Walls, K. Austin, and K. Feaux. We thank M. Dettinger
for comments. This work was supported by USGS grant
Materials and Methods
Figs. S1 to S9
20 June 2014; accepted 8 August 2014
Published online 21 August 2014;
EARLY SOLAR SYSTEM
The ancient heritage of water ice in
the solar system
L. Ilsedore Cleeves,1 Edwin A. Bergin,1 Conel M. O’D. Alexander,2 Fujun Du,1
Dawn Graninger,3 Karin I. Öberg,3 Tim J. Harries4
Identifying the source of Earth’s water is central to understanding the origins of
life-fostering environments and to assessing the prevalence of such environments in
space. Water throughout the solar system exhibits deuterium-to-hydrogen enrichments,
a fossil relic of low-temperature, ion-derived chemistry within either (i) the parent
molecular cloud or (ii) the solar nebula protoplanetary disk. Using a comprehensive
treatment of disk ionization, we find that ion-driven deuterium pathways are inefficient,
which curtails the disk’s deuterated water formation and its viability as the sole source for
the solar system’s water. This finding implies that, if the solar system’s formation was
typical, abundant interstellar ices are available to all nascent planetary systems.
Water is ubiquitous across the solar sys- tem, in cometary ices, terrestrial oceans, the icy moons of the giant planets, and the shadowed basins of Mercury (1, 2). Water has left its mark in hydrated min-
erals in meteorites, in lunar basalts (3), and in
martian melt inclusions (4). The presence of
liquid water facilitated the emergence of life
on Earth; thus, understanding the origin(s) of
water throughout the solar system is a key goal
of astrobiology. Comets and asteroids (traced by
meteorites) remain the most primitive objects,
providing a natural “time capsule” of the condi-
tions present during the epoch of planet forma-
tion. Their compositions reflect those of the gas,
Sun at its birth, i.e., the solar nebula protoplan-
etary disk. There remain open questions, however,
as to when and where these ices formed, whether
they (i) originated in the dense interstellar medium
(ISM) in the cold molecular cloud core before the
Sun’s formation or (ii) are products of reprocessing
within the solar nebula (5–7). Scenario (i) would
imply that abundant interstellar ices, including
water and presolar organic material, are incorpo-
rated into all planet-forming disks. By contrast,
local formation within the solar nebula in scena-
rio (ii) would potentially result in large water abun-
dance variations from stellar system to system,
dependent on the properties of the star and disk.
In this work, we aim to constrain the for-
mation environment of the solar system’s water,
using deuterium fractionation as our chemical
tracer. Water is enriched in deuterium relative to
hydrogen (D/H) compared with the initial bulk
solar composition across a wide range of solar
system bodies, including comets, (8, 9), terres-
trial and ancient martian water (4), and hydrated
minerals in meteorites (10). The amount of deu-
terium relative to hydrogen of a molecule de-
pends on its formation environment; thus, the
D/H fraction in water, ½D=H;H2 O, can be used to
differentiate between the proposed source en-
vironments. Interstellar ices, as revealed by sub-
limation in close proximity to forming young
stars, also exhibit high degrees of deuterium
enrichment, ~2 to 30 times that of terrestrial
water (11–14). It is not known to what extent
these extremely deuterated interstellar ices are
incorporated into planetesimals or if, instead,
the interstellar chemical record is erased by
reprocessing during the formation of the disk
(15, 16). Owing to water’s high binding energy to
grain surfaces, theoretical models predict that
water is delivered from the dense molecular cloud
to the disk primarily as ice, with some fraction
sublimated at the accretion shock in the inner
tens of astronomical units (AU) (15). If a substan-
tial fraction of the interstellar water is thermally
reprocessed, the interstellar deuterated record
could then be erased. In this instance, the disk is left
as primary source for (re-)creating the deuterium-
enriched water present throughout our solar system.
The key ingredients necessary to form water
with high D/H are cold temperatures, oxygen,
and a molecular hydrogen (H2) ionization source.
The two primary chemical pathways for making
deuterated water are (i) gas-phase ion-neutral
reactions, primarily through H2D+ and (ii) grain-
surface formation (ices) from ionization-generated
hydrogen and deuterium atoms from H2. Both
reaction pathways depend critically on the forma-
tion of H2D+. In particular, the gas-phase channel
(i) involves the reaction of H2D+ ions with atomic
oxygen or OH through a sequence of steps to
form H2DO+, which recombines to form a water
molecule. The grain-surface channel (ii) is pow-
ered by H2D+ recombination with electrons or
grains, which liberates hydrogen and deuterium
atoms that react with oxygen atoms on cold dust
grains. H2D+ becomes enriched relative to Hþ 3
because the deuterated isotopologue is ener-
getically favored at low temperatures. There is
1590 26 SEPTEMBER 2014 • VOL 345 ISSUE 6204
1Department of Astronomy, University of Michigan, 311 West
Hall, 1085 South University Avenue, Ann Arbor, MI 48109,
USA. 2Department of Terrestrial Magnetism, Carnegie
Institution of Washington, Washington, DC 20015, USA.
3Harvard-Smithsonian Center for Astrophysics, Harvard
University, Cambridge, MA 02138, USA. 4Department of
Physics and Astronomy, University of Exeter, Stocker Road,
Exeter EX4 4QL, UK.
*Corresponding author. E-mail: firstname.lastname@example.org.