THE HISTORY AND INVENTORY OF WATER ON MARS
B. M. Jakosky, Laboratory for Atmospheric and Space Physics, Univ. of Colorado, USA
(bruce.jakosky@lasp.colorado.edu) and L. J. Hallis, Univ. of Glasgow, UK (lydia.hallis@glasgow.ac.uk)

Introduction:
Key questions about Mars relate to the history
and abundance of water at the surface and the forms
that it can take (vapor, liquid, solid). Water interacts
with the surface, affecting the geological and geochemical history. And water is intimately connected
to the potential for life to exist or have existed.
We use measurements related to the enrichment
of D/H in H2O due to escape to space, surface morphology, and surface and subsurface mineralogy, to
derive an observational estimate for the total inventory of water, its current distribution on the planet, and
its history.
Measurements of D/H:
The ratio of D/H in H2O is fundamental to determining the history of water, due to its large enrichment relative to its initial value. D/H in atmospheric
water vapor today is enhanced by an average of ~6x
relative to its starting point. This value varies seasonally and spatially between ~2-10x, apparently due
to wintertime condensation and springtime sublimation of water. This large a global enrichment can be
produced only by escape of a large fraction of the H
to space, with H escaping more readily than D.
D/H has been measured in situ in weathering
products dated at about 3 b.y. by the SAM instrument on the Curiosity rover. It shows an enrichment
of ~3x.
The original value of D/H on Mars is very close
to terrestrial, based on ratios measured within hydrous primary igneous minerals in Martian meteorites. In addition, Martian meteorites show values in
weathering products that vary between ~2-6x; these
values occur in rocks dated throughout Martian history. Interpretation of Martian meteorite D/H is
complicated by multiple factors, including:
(i) Possible seasonal or spatial variations in the
source water at time of incorporation into the minerals, due to condensation and sublimation, as is seen
at the present epoch on Mars. Especially during earlier epochs, these variations could have been driven
by a seasonal water cycle analogous to that on Earth.
This effect can determine whether values measured
in a single rock are indicative of global abundances
at that time.
(ii) Potential modification on Mars at epochs later than when the weathering products originally
formed, due to exchange of water with the Martian
environment. For example, episodic or cyclic wetting
could result in the development of phyllosilicate

veins within surface/subsurface rocks with D/H ratios
representative of only the last wetting phase.
(iii) Possible changes in D/H resulting from impact. All meteorites have been shocked to some degree by the impact that blasted them from the Martian surface, and several meteorites show evidence of
more than one impact event (e.g., ALH 84001).
Changes in D/H due to shock pressure are not well
studied, but have been observed in laboratory studies
of certain hydrous minerals.
(iv) Potential contamination on Earth after arrival, due to exchange of water with the environment.
Exchange has been observed to occur rapidly (years)
in at least some instances.
Due to the large uncertainties in interpretation of
meteoritic D/H, we’ll carry out our analysis using the
initial D/H, the Curiosity measurement at about 3
b.y., and the present-day enrichment of ~6x. Our
basic results will not change significantly if we use
only the initial and present-day values, but we would
lose fidelity on the time resolution of the history.
D/H enrichment and exchange with nonatmospheric reservoirs:
H and D are able to escape directly to space by
thermal (Jeans) escape. The source of these is H2O,
initially in the lower atmosphere, broken apart primarily by solar UV photons. H is lighter and escapes
more readily than D, so the hydrogen remaining in
the atmosphere becomes enriched in D. The H and
D are reincorporated back into H2O by downward
mixing and chemical reactions in the middle and
lower atmosphere. The relative escape rates of D
and H have been calculated, but depend strongly on
assumed atmospheric behavior such as warming and
mixing during dust storms and the structure of the
upper atmosphere, all of which can vary with time.
Assuming these relative escape rates, the ~6x enrichment represents loss of between ~85-95 % of the
water in the system.
Atmospheric water exchanges with water not in
the atmospheric, and the fraction lost pertains to the
sum of water in the atmosphere plus in these exchangeable reservoirs. Exchangeable reservoirs include the polar ice deposits, ground ice identified at
high and middle latitudes, and possible low-latitude
ground ice. In addition, outgassing by volcanism can
affect the D/H ratio in surface water.

Fig. 1. Derived size of exchangeable reservoir (top)
and fraction of H2O remaining (middle) as a function
of time, as derived from D/H (bottom).

Estimates of the total volume of water in these
exchangeable reservoirs today are 20-30 m GEL
(global equivalent layer). We can use this value, the
current D/H, and the history of D/H to estimate the
volume of exchangeable reservoir through time, as
follows:
(i) Present-day enrichment of D/H by 6x, combined with the present-day exchangeable reservoir of
20-30 m H2O, yields an initial amount of exchangeable water of ~130-600 m GEL. This requires that
loss to space over time would have to have been 110570 m GEL.
(ii) Enrichment in D/H by 3x at 3 b.y.a. requires
an enrichment between then and now of 2x. In turn,
and including outgassing by volcanism that partially
resets the D/H, the reservoir at 3 b.y. would have to
have been 40-130 m GEL, and the amount lost to
space between then and now would have been 25210 m GEL.
Fig. 1 shows the inferred size of the exchangeable
reservoir through time. Note that the amounts required to have been lost to space are much larger
than would be derived by taking the present-day loss
rate as constant through time; this would have resulted in loss of only 3-30 m GEL.
Global inventory of water:
Table 1 shows the inferred inventory of water,
based on the present work and a number of recent
analyses. Amounts are as follows, all given as GEL
m H2O:
(i) Water lost to space, 110-570 m, with values
likely to be nearer to the high end based on our current understanding of relative escape rates of D and
H.
(ii) Water incorporated into minerals at the surface and in the subsurface. These have been identified from orbit and from the ground, and estimates of
their total volume have been made based on extrapolation to what cannot be directly observed. Best estimate is 130-260 m, but amounts as large as 500 m

cannot be ruled out.
(iii) Present-day exchangeable water ice, 20-30
m. This includes the polar caps and other ice deposits that have been identified and mapped.
(iv) Potential buried ice from an early ocean, 110
m. Although there is no universal agreement as to
whether it existed, its volume can be estimated if it
filled the northern-lowlands (and if it still resides
there as ice).
(v) Water inferred to be in the crust and that did
NOT debouch in floods, 400-1000m. This estimate
is based on the source region for the flood channels,
the volume of water necessary to carve the channels,
and the assumption that the rest of the global crust
also contained additional water that was not released
to the surface.
(vi) Water released by volcanic eruptions or intrusions. Estimates vary from 5-120 m, depending
on degassing efficiency and the role of intrusive volcanism. This water is counted as lost elsewhere, so
is not listed explicitly in the table.
There may be other potential sources or sinks of
water through time, but we have listed all for which
there is good direct or indirect evidence for their
existence. Note that only 370-970 m has been at the
surface subsequent to the earliest Noachian.

Table 1. Global water inventory based on volume of
the different sinks available.

Issues:
These estimates depend on a number of assumptions or issues that cannot be evaluated at this time.
These include:
(i) Is the atmospheric H2O well mixed with nonatmospheric ice deposits, such that they have a uniform or constant D/H?
(ii) What happens to the water cycle, dust-storm
behavior, and the H and D escape rate at other obliquities?
(iii) Are extrapolations of present-day behavior to
early epochs valid given the potential for a very different seasonal behavior and hydrologic cycle?
(iv) To what extent do the different D/H values in
meteorites reflect real spatial or temporal variations,
and how can we use them to determine global average values at each epoch?
(v) Is there free water or ice in the crust, has it

been at the surface subsequent to the Noachian, and
has it exchanged on any timescale with near-surface
water?
(vi) Have we accounted for all sources and sinks
of water and, especially, to what extent have they
equilibrated with each other in D/H throughout time?
Summary:
We provide an observation-based estimate of the
water inventory of Mars. The amount at the surface
has been as great as 370-970 m GEL H2O. Of this,
110-570 m has been lost to space, accounting for a
large fraction (~30-60 %) of the total water. Most of
the rest has been incorporated into surface and subsurface minerals or been locked up in the crust as ice.
A small amount (20-30 m) is accessible today at the
surface as ice in the polar regions or near the surface
as ground ice.