[extropy-chat] Water Marks the Asthenosphere

Amara Graps amara at amara.com
Sat Jan 27 16:44:29 UTC 2007

Technotranscendence (neptune at superlink.net):

>Reading this,

Thanks! For me, it gives another data point, together with data/models
of other interior regions of the Earth, for the total amount of water
stored in the Earth, which is not known. At least not precisely. Numbers
from different methods are converging to ~10 ocean masses (1 ocean mass
= 1.5E21 kg) or less, of water stored inside of the Earth.

That 0.07% by weight in the article is similar to the 0.05% by weight of
water stored in ocean island ridge basalts, which are usually thought to
come from reservoirs located deep in the mantle.

>I'm wondering if a lot of water on other worlds might be
>integrated into the mantle or other strata...

Did you read the last sentence in the author's paper ? It provides
a constraint:

"In any case, however, our results imply that the existence of an
asthenosphere-and therefore of plate tectonics as we know it-is possible
only in a planet with a water-bearing mantle."

Mars doesn't have plate tectonics. Neither does the Moon. I'm not sure
if one can say that if you don't have plate tectonics that you don't
have a water-bearing mantle, but there is some important geophysics in
this result (I must go ask a geophysicist!).

>Perhaps Mars has lots of water, but it's all part of a mineral matrix
>deep below the surface.

But how deep? The outer layers didn't change very much in Mars' history.
How does one know? The oxygen isotope information in the Martian
meteorites, which span large time periods, all have similar numbers,
which means that the Martian lithosphere became well homogenized early
in its history.

See: McKeegan, Kevin D.  and Leshin, Laurie A., (2001), Stable
Isotope Variations in Extraterrestrial Materials,
{Rev. Mineral Geochem}, 43, 279.


The Origin of Water on Mars

Amara L. Graps,a,f Jonathan I. Lunine,b, John Chambers,c Alessandro
Morbidelli,d and Laurie A. Leshin, e, David P. O'Brien,f

a INAF Istituto di Fisica dello Spazio Interplanetario, Rome, Italy
b Lunar and Planetary Laboratory, The University of Arizona, Tucson, AZ
85721, USA
c DTM, Carnegie Institution of Washington, Washington, DC 20015, USA
d Observatoire de la Côte d'Azur, BP 4229, F-06304 Nice cedex 4, France
e Department of Geological Sciences, Arizona State University, Tempe, AZ
85287, USA
f Planetary Science Institute, Tucson, Arizona, USA  85719


Here we consider the origin of water on Mars, in the context of a
dynamical model that accounts for most of the Earth's water as a product
of collisions between the growing Earth and planet-sized "embryos" from
the asteroid belt. Mars' history is found to be different; to explain
the present mass of Mars, its core formation times (Hf-W), and the broad
age range and homogeneity of SNCs (O isotopes), it requires that it
suffer essentially no giant collisions, was homogenized early in its
history, and the bulk of its growth was through the addition of smaller
bodies. Essentially Mars is itself an embryo. Nitrogen isotope ratios,
the Fe-O magma content, and new dynamical simulations together suggest
that parent bodies from the inner asteroid belt could have provided
Mars' interior component as well as providing some part of Mars' water.
This water can be supplemented with small bodies beyond 2.5~A.U. and
cometary material. The percentage of each contribution, along with
the material's D/H ratio will be given in the presentation.


(To be presented April 2007 at the AGU, Vienna. )

This work was previously published in the journal Icarus in 2003 by
Lunine et al. This April, I will give updates on their 2003 results.
Mostly I'm representing Lunine and Morbidelli for Mars, while we work on
the same topic for Earth.

>What about Luna?

It's too dry, from its origin. Sample returns from the Apollo missions
showed that the Moon had an "anorthositic crust", which indicates a very
hot magma ocean beginning.

Here I quote from Lunine (2006), because the words are concise:


Lunine, J. "Physical Conditions on the Early Earth" Phil. Trans. R. Soc.
B (2006) 361, 1721-1731.

{begin quote}

The origin of the Earth's Moon lies in one giant collision, and one with
very specific properties initially constrained by geochemistry (Hartmann
and Davis, 1975) and later by detailed dynamical simulations (Benz et
al., 1986; Canup, 2004).

The Moon-forming impact occurred toward the middle to late time in the
growth of the Earth, principally based not only on isotopic data
(Halliday et al., 2004a), but also on the need for the Earth to have
been well-differentiated at the time of the collision. The event
involved a colliding body the mass of Mars or a few tens of per cent
larger, coming in obliquely so as to produce material that would collide
with the Earth a second time so that significant material would remain
in orbit around the Earth, rather than ejected into solar orbit
(Canup, 2004).

{end quote}

Benz, W., Slattery, W.L. and Cameron, A.G.W. (1986). "The origin of the
Moon and the single impact hypothesis I", Icarus 66, 515-535.

Canup, R.M. (2004), "Simulation of a Late Lunar-Forming Impact", Icarus
168, 433-456.

Halliday, A.N. 2004a, "The origin and earliest history of the Earth",
pp. 509-557. In: Meteorites, Comets and Planets (ed. A.M. Davis) Vol. 1
Treatise on Geochemistry (eds. H.D. Holland and K.K. Turekian),
Elsevier-Pergamon, Oxford.

Hartmann, W.K. and Davis, D.R. (1975). "Satellite-sized planetesimals
and lunar origin", Icarus 24, 504-515.


The Giant Impact model explains some important observational points. It
explains why the Moon has a lower density -- because the outer part of
the Earth would have lower density material since it already
differentiated, with iron  already being formed in its (Earth's) core.

It explains why so much of the angular momentum of the Earth-Moon system
is in the Moon's rotation.

It also explains why the Moon is is 'bone dry' -- because much of the
material that shot into orbit was vaporized, with only the least
volatile material remaining and condensing into solids.

There is an observational 'sticking point' for the Giant Impact
hypothesis. The oxygen isotopes from the Earth and the Moon are
_identical_. About 80% of the Moon-forming material is estimated from
theoretical work to originate from the Mars-sized impactor. Because of
the stochastic collisions between embryos born in different parts of the
inner solar system, one needs a consistent explanation for the oxygen
isotope similarities between the Moon and Earth. The answer might be

Pahlevan K., and Stevenson D. J. (2005) "The Oxygen Isotope Similarity
of the Earth and Moon: Source Region or Formation Process?", Lunar
Planet. Sci., XXXVI, #2382

.. which says that an oxygen isotopic exchange occurred between the
Earth's mantle and the circumterrestrial vapor-melt silicate disk that
produced the Moon. In other words, that material that formed the Earth's
mantle and the Moon was very hot and mixed.

Hope that this was useful.



Amara Graps, PhD      www.amara.com
INAF Istituto di Fisica dello Spazio Interplanetario (IFSI), Roma, ITALIA
Associate Research Scientist, Planetary Science Institute (PSI), Tucson

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