Published online today in JGR-Planets:

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Carr, Michael H.; Head, James W.
Oceans on Mars: An assessment of the observational evidence and possible fate
J. Geophys. Res. Vol. 108 No. E5
10.1029/2002JE001963
20 May 2003

Abstract

If the large Late Hesperian outflow channels were eroded by
extensive floods, as appears likely, then large bodies of water must
have once occupied the northern plains during that period. Previous
estimates of the sizes of bodies of water in the northern lowlands
range up to 3 × 108 km3. Several contacts have been previously
mapped around the edges of the northern plains and interpreted to be
shorelines remaining from these former standing bodies of water. We
examine the elevations and geologic relations along these contacts
in detail and find little support for their interpretation as
shorelines. Some contacts are clearly of volcanic origin, and all
have significant variations in elevation. Better support for the
former presence of water over large parts of the northern plains is
provided by the Vastitas Borealis Formation (VBF). Most of the post-
Noachian fill within the northern basin is ridged plains of Lower
Hesperian age, interpreted to be volcanic in origin. Overlying the
ridged plains is the VBF, a thin veneer of material of Upper
Hesperian age. The VBF may have been deposited from large floods.
Support for this interpretation is the similarity in age between the
outflow channels and the VBF, the presence of the VBF at the lower
ends of the outflow channels, and identification of numerous
features in the outcrop areas of the VBF that are suggestive of
basal melting of an ice sheet. To cover all the area over which the
VBF is exposed would require ~2.3 × 107 km3 of water. Spread
over the entire surface of Mars, this volume is equal to a global
layer (global equivalent layer, or GEL) ~156 m deep. We find no
support for the larger estimates of ocean volumes that range up to 3
× 108 km3 and which imply comparable amounts of water per unit
area as are currently present on the surface of the Earth. Under
present climatic conditions on Mars an ocean would freeze in a
geologically short time period (~104 years), then would sublimate
away at rates strongly dependent on the presence or absence of
debris on the ice surface. The present VBF is interpreted as a
sublimation residue from the ponded outflow channel effluents. The
fate of a volume of water thought to have been emplaced by the
outflow channels (~2.3 × 107 km3) is largely accounted for by
the presence of other existing reservoirs on the planet. An
approximately 20–30 m GEL of water is estimated to be in the
present polar caps, and a 50 m GEL may have escaped to space since
the Hesperian, leaving ~80 m GEL unaccounted for. This amount may be
partly trapped in other volatile-rich deposits on the surface, and a
significant amount could have reentered the groundwater system by
south polar basal melting and been progressively cold-trapped at the
base of a growing cryosphere. On the basis of our assessment of the
Hesperian-aged deposits, we predict that testing of the Clifford and
Parker [2001] hypothesis that a Noachian-aged ocean covered up to
one third of the surface of Mars will be made very difficult by the
enhanced degradation rates in the Noachian and subsequent geological
events in the northern lowlands.

Received 17 July 2002; revised 5 December 2002; accepted 23 January
2003; published 20 May 2003.

Keywords: Mars, oceans, channels, shorelines, Hesperian.

Index Terms: 6225 Planetology: Solar System Objects: Mars; 4267
Oceanography: General: Paleoceanography; 5407 Planetary Sciences:
Atmospheres—evolution; 5470 Planetary Sciences: Surface materials
and properties; 1635 Global Change: Oceans (4203).

Citation: Carr, M. H., and J. W. Head III, Oceans on Mars: An
assessment of the observational evidence and possible fate, J.
Geophys. Res., 108(E5), 5042, doi:10.1029/2002JE001963, 2003.