From: Amara Graps (amara@amara.com)
Date: Fri May 10 2002 - 04:22:07 MDT
Dear Extropes,
I heard a number of talks at the European Geophysical Society meeting
two weeks ago, addressing a puzzle about the origin of Earth's water.
When I returned to Heidelberg, a Science News issue was in my mailbox
talking about the same topic. So I gathered a bunch of references,
started reading and I outlined the main argument. Most of this note is
references, and it shows that there's much more to this topic. Now I'm
thinking what interplanetary dust measurements could add, if anything,
to aid in the investigations.
Amara
FROM WHERE DID EARTH'S WATER COME?
===================================
Earth has substantially more water than scientists would expect to
find at 1 A.U. Other compounds and elements also readily vaporize at
Earth's distance (see body of work by T. Owen et al). This mystery is
not new, but it was addressed by A. Delsemme, who said that Earth's
water came from comets. Delsemme's theory was not completely
satisfactory, but it provided a working hypothesis, until spectral
measurements of comets Halley, Hyakutake, and Hale-Bopp, during their
near-Earth passes in 1986, 1996 and 1997. The spectral analysis of the
three comets showed that the abundance of the deuterium isotope of
water is twice that found in Earth's water. Earth's water could
not have come all from comets.
So then: How did Earth get its water?
Old Hypothesis
--------------
Cometary bombardment, late in Earth's formation period supplied a
veneer of water to the previously dry planet. "Late-veneer Theory".
(A.H. Delsemme)
New Hypotheses
----------------
1) Some (up to half) of the water still came from comets, but, in
addition, asteroid-size planetesimals containing water from orbital
reservoirs following orbits inside Jupiter's path or crossing it, would
contribute. "Late-veneer Theory Modified." (Ozernoy et al.)
2) The Earth formed wet. The Earth must have formed from, and then
entirely depleted, an ancient supply of water-rich material located
near Earth's orbit, that now has no modern analog. "Wet-accretion
Theory". (Righter and Drake, et al.)
3) One Large Splash. Earth formed wet, but not from materials within a
narrow band located a specific distant from the Sun. One body from
between Mars and Jupiter, a chance encounter, brought the water and
volatiles to the Earth in one large splash, sparing Mercury, Venus,
Mars. The large body was derived from a relatively small number of
Moon-sized bodies that was bombarding the inner solar system.
"Stochastic Wet-accretion Theory". (Morbidelli et al.)
REFERENCES with Abstracts
==========================
Harder, Ben, "Water for the Rock: Did Earth's oceans come from the
heavens?", Science News, March 23, 2001, vol. 161, p.184-186.
Drake, M.J. and Righter, K., What is the Earth Made Of?, Lunar and
Planetary Institute Conference, 2002, Mar, vol 33.
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2002LPI....33.1239D&db_key=AST
Abstract
Measurements made on materials from Earth, Mars, comets, and
meteorites of Mg/Si, Al/Si, oxygen, Os, D/H, Ar/H2O, and Kr/Xe
ratios, all lead to the conclusion that no primitive material
similar to Earth mantle material is currently represented in our
meteorite collections.
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Drake, M. J. and Righter, K., Determining the composition of the
Earth, Nature, 2002, Mar, vol 416, 39--44.
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2002Natur.416...39D&db_key=AST
Abstract
A long-standing question in the planetary sciences asks what the
Earth is made of. For historical reasons, volatile-depleted
primitive materials similar to current chondritic meteorites were
long considered to provide the `building blocks' of the terrestrial
planets. But material from the Earth, Mars, comets and various
meteorites have Mg/Si and Al/Si ratios, oxygen-isotope ratios,
osmium-isotope ratios and D/H, Ar/H2O and Kr/Xe ratios such that no
primitive material similar to the Earth's mantle is currently
represented in our meteorite collections. The `building blocks' of
the Earth must instead be composed of unsampled `Earth chondrite' or
`Earth achondrite'.
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Drake, M. J. and Righter, K., Constraints on the Depth of an Early
Terrestrial Magma Ocean, Meteoritics & Planetary Science, vol. 36,
Supplement, p.A173, 2001, Sep.
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2001M%26PSA..36R.173R&db_key=AST
We have proposed that abundances of moderately siderophile elements in
the Earth's primitive upper mantle (PUM) were set by equilibrium
between peridotite melt and S- and C-bearing metallic liquid at 27 GPa
and 2000 degC [1,2]. Our calculations were based on metal-silicate
partition coefficients (D(M/S)) derived from ~ 100 experiments each
for Mo, W and P, >200 for Co and > 300 for Ni. Several recent
experimental efforts have concluded that the abundances of Ni, Co, V,
Mn and Cr in Earth's PUM were set by equilibration between metal and
peridotite melt at much higher pressures and temperatures (40 to 60
GPa, 3000 to 4000 deg C; [3,4,5]). Using our predictive expressions
for D(M/S), we show that the abundances of Ni, Co, Mo, W and P are
inconsistent with such a high pressure and temperature scenario.
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Shearer, C. K. and Righter, K., Hafnium and Tungsten Partitioning in
Silicates. A Key to Understanding the Earth Evolution of Both the Moon
and Mars, Lunar and Planetary Institute Conference, 2001, Mar, vol 32.
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2001LPI....32.1620S&db_key=AST
Abstract
We present data on the behavior of both Hf and W in a variety of
silicates and melt compositions and evaluate their role in generating
W isotopic signatures.
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Lunine, J. I. and Morbidelli, A. and Chambers, J. E., Origin of Water
on Mars, Lunar and Planetary Institute Conference, 2002, Mar, vol 33.
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2002LPI....33.1791L&db_key=AST
Abstract
Dynamical simulations suggest that the Earth's water budget was
delivered primarily from the asteroid belt, in the form of large
planetary "embryos". The same simulations present a very different
picture for Mars -- its water came from a mixture of cometary and
small asteroidal bodies.
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Ozernoy, L. M. and Ipatov, S. I., Origins of Water on Mars, Earth,
and Venus: Evaluating the Supply from the Outer Solar System, American
Astronomical Society Meeting, 2001, Dec.
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2001AAS...199.2806O&db_key=AST
Abstract
The goal of this project is to determine which particular kinds of
ice sources in the outer Solar system could have provided the
largest contribution of water onto the planets of the Earth group.
As is known, the contemporary influx of comets and asteroids is just
a tiny fraction of impact delivery occurring at the epoch of the
planet formation. Our approach has demonstrated that the amount of
water delivered to the Earth during this epoch has been
substantially underestimated. The following factors neglected in the
previous work turn out to be very important: 1) Previous studies
were based on symplectic integration and ignored the influence of
terrestrial planets. In our direct numerical integrations, which
take into account the terrestrial planets, the fraction of
Jupiter-crossers that reached the orbit of the Earth is greater by a
factor of 2 than that obtained by the symplectic method with the
integration step of 30 d. 2) While considering collisions of
planetesimals with the planets, we need to take into account that
the bodies move non-uniformly in their orbits. As a result, the mean
time between the collisions with Earth from Jupiter-crossing orbits
increases by a factor of 2. 3) Finally, a large factor that can
considerably (up to an order of magnitude) increase the amount of
water delivered to the Earth is the collisions of planetesimals with
the Earth from the orbits, which entirely located inside Jupiter's
orbit. Although the fraction of these orbits is relatively small (<=
0.01), such bodies spend in Earth-crossing orbits much longer time
(perhaps, by two orders of magnitude) and collide with the Earth
from orbits of smaller eccentricities than Jupiter-crossing objects.
To sum up, the total amount of water delivered to the Earth during
the formation of the giant planets could be comparable to that in
the Earth oceans. The amount of water delivered to Venus is less by
a factor of 3, and that delivered to Mars is less by a factor of 2
for Jupiter-family comets and by a factor of 30 for near-Earth
objects. We acknowledge support of this work by NASA grant
NAG5-10776, the RFP ``Astronomy", RFBR, and INTAS (00-240).
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Owen, T. C. and Bar-Nun , A., Contributions of Icy Planetesimals to
the Earth's Early Atmosphere, Origins Life Evolution Biosphere, 2001,
Aug, vol 31, 435--458
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2001OLEB...31..435O&db_key=AST
Abstract
Laboratory experiments on the trapping of gases by ice forming at low
temperatures implicate comets as major carriers of the heavy noble
gases to the inner planets. These icy planetesimals may also have
brought the nitrogen compounds that ultimately produced atmospheric
N_2. However, if the sample of three comets analyzed so far is
typical, the Earth's oceans cannot have been produced by comets alone,
they require an additional source of water with low D/H. The highly
fractionated neon in the Earth's atmosphere may also indicate the
importance of non-icy carriers of volatiles. The most important
additional carrier is probably the rocky material comprising the bulk
of the mass of these planets. Venus may require a contribution from
icy planetesimals formed at the low temperatures characteristic of the
Kuiper Belt.
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Owen, T. C. and Bar-Nun , A., From the Interstellar Medium to
Planetary atmospheres via Comets, Collisional processes in the solar
system, ed. by: Mikhail Ya. Marov and Hans Rickman, Astrophysics and
Space Science Library, Volume 261, Dordrecht: Kluwer Academic
Publishers, 2001, p.249-264, 2001.
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2001cpss.book..249O&db_key=AST
Abstract
Laboratory experiments on the trapping of gases by ice forming at low
temperatures implicate comets as major carries of the heavy noble
gases to the inner planets. Recent work on deuterium in Comet
Hale-Bopp provides good evidence that comets contain some unmodified
interstellar material. However, if the sample of three comets analyzed
so far is typical, the Earth's oceans cannot have been produced by
comets alone. The highly fractionated neon in the Earth's atmosphere
also indicates the importance of non-icy carriers of volatiles, as do
the noble gas abundances in meteorites from Mars.
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Laufer, D. and Notesco, G. and Bar-Nun, A. and Owen, T.,
From the Interstellar Medium to Earth's Oceans via Comets-
An Isotopic Study of HDOH2O, Icarus, 1999, Aug, vol 140, 446--450.
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1999Icar..140..446L&db_key=AST
Abstract
The isotopic enrichment of HDO over H2O when water vapor freezes
into ice at 60-170 K was studied experimentally. No such enrichment
was detected (1.003-1.007 in the 95% confidence interval). Thus HDO
cannot be enriched when ice is formed by freezing of water vapor.
The very similar D/H ratio in the water of Comets Halley, Hyakutake,
and Hale-Bopp (~3 ? 10-4) is 10-20 times larger then the D/H ratio
in the solar nebula. Therefore the cometary water had to originate
in a giant molecular cloud, where the HDO is enriched by
ion-molecule reactions. We cannot determine whether the ice grains
which agglomerated into these comets were formed in a ~50 K warm
clump in the giant molecular cloud and settled intact to the solar
nebula or sublimated and refroze in the ~50 K Uranus-Neptune region.
The HDO/H2O ratio in Earth's oceans suggests that the water was
delivered by both comets and rocky material formed in Earth's region
of the solar nebula.
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Bogard, D. D. and Clayton , R. N. and Marti , K. and Owen , T.
and Turner , G., Martian Volatiles: Isotopic Composition, Origin,
and Evolution, Space Science Reviews, 2001, Apr, vol 96, p.425-458.
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2001SSRv...96..425B&db_key=AST
Abstract
Information about the composition of volatiles in the Martian
atmosphere and interior derives from Viking spacecraft and
ground-based measurements, and especially from measurements of
volatiles trapped in Martian meteorites, which contain several
distinct components. One volatile component, found in impact glass
in some shergottites, gives the most precise measurement to date of
the composition of Martian atmospheric Ar, Kr, and Xe, and also
contains significant amounts of atmospheric nitrogen showing
elevated ^15N/^14N. Compared to Viking analyses, the ^36Ar/^132Xe
and ^84Kr/^132Xe elemental ratios are larger in shergottites, the
^129Xe/^132Xe ratio is similar, and the ^40Ar/^36Ar and ^36Ar/^38Ar
ratios are smaller. The isotopic composition of atmospheric Kr is
very similar to solar Kr, whereas the isotopes of atmospheric Xe
have been strongly mass fractionated in favor of heavier isotopes.
The nakhlites and ALH84001 contain an atmospheric component
elementally fractionated relative to the recent atmospheric
component observed in shergottites. Several Martian meteorites also
contain one or more Martian interior components that do not show the
mass fractionation observed in atmospheric noble gases and nitrogen.
The D/H ratio in the atmosphere is strongly mass fractionated, but
meteorites contain a distinct Martian interior hydrogen component.
The isotopic composition of Martian atmospheric carbon and oxygen
have not been precisely measured, but these elements in meteorites
appear to show much less variation in isotopic composition,
presumably in part because of buffering of the atmospheric component
by larger condensed reservoirs. However, differences in the oxygen
isotopic composition between meteorite silicate minerals (on the one
hand) and water and carbonates indicate a lack of recycling of these
volatiles through the interior. Many models have been presented to
explain the observed isotopic fractionation in Martian atmospheric
N, H, and noble gases in terms of partial loss of the planetary
atmosphere, either very early in Martian history, or over extended
geological time. The number of variables in these models is large,
and we cannot be certain of their detailed applicability.
Evolutionary data based on the radiogenic isotopes (i.e.,
^40Ar/^36Ar, ^129Xe/^132Xe, and ^136Xe/^132Xe ratios) are
potentially important, but meteorite data do not yet permit their
use in detailed chronologies. The sources of Mars' original
volatiles are not well defined. Some Martian components require a
solar-like isotopic composition, whereas volatiles other than the
noble gases (C, N, and H_2O) may have been largely contributed by a
carbonaceous (or cometary) veneer late in planet formation. Also,
carbonaceous material may have been the source of moderate amounts
of water early in Martian history.
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Owen, T. C., Inner Planet Atmospheres: The Case for Cometary
Contributions, Meteoritics & Planetary Science, vol. 36, Supplement,
p.A156, 2001, Sep.
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2001M%26PSA..36..156O&db_key=AST
Abstract
Abundances and isotope ratios of nitrogen and noble gases together
with the values of D/H in water can be used to constrain models for
volatile delivery to the inner planets. Mars is particularly useful
for this study, owing to the thinness of its atmosphere and its lack
of vigorous techtonic activity that could mix crustal materials with
the interior. The mixing line defined by abundances of 36Ar, 84Kr, and
132Xe measured in Martian meteorites (shergottites) passes through the
point corresponding to the Earth's atmosphere. Martian and terrestrial
xenon isotopes exhibit virtually identical relative abundances. Both
of these relationships are completely outside the limits set by noble
gases in chondrites, suggesting a non-meteoritic external source for
these noble gases. D/H in SNC minerals varies from the Mars
atmospheric value, enriched by escape of H, to a low value that is
identical to the D/H in the water from the three comets measured to
date, about twice the value in Standard Mean Ocean Water (SMOW). A
similar relationship exists for 15N/14N on Mars: both atmospheric and
original components have been identified. Venus exhibits a different
endowment of noble gases from that of Earth and Mars, that may have
been provided by the same class of icy planetesimals that enriched the
atmosphere of Jupiter. The DS-1 and CONTOUR missions to comets, which
will provide in-situ data on abundances and isotope ratios of cometary
volatiles in the next two years, will test these hints of cometary
contributions.
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Morbidelli, A., and Chambers, J., and Lunine, J. I., and
Petit , J. M., and Robert , F., and Valsecchi , G. B., and Cyr , K. E.,
Source regions and time scales for the delivery of water to Earth,
Meteoritics and Planetary Science, 2000, Nov, vol 35, p.1309-1320,
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2000M%26PS...35.1309M&db_key=AST
Abstract
In the primordial Solar System the most plausible sources of the water
accreted by the Earth were in the outer asteroid belt, in the giant
planet regions and in the Kuiper belt. We investigate the implications
on the origin of Earth's water of dynamical models of primordial
evolution of solar system bodies and check them with respect to
chemical constraints. We find that it is plausible that the Earth
accreted water all along its formation, from the early phases when the
solar nebula was still present to the late stages of gas-free sweepup
of scattered planetesimals. Asteroids and the comets from the
Jupiter-Saturn region were the first water deliverers, when the Earth
was less than half its present mass. The bulk of the water presently
on Earth was carried by a few planetary embryos, originally formed in
the outer asteroid belt and accreted by the Earth at the final stage
of its formation. Finally, a late veneer, accounting for at most 10%
of the present water mass, occurred due to comets from the
Uranus-Neptune region and from the Kuiper belt. The net result of
accretion from these several reservoirs is that the water on Earth had
essentially the D/H ratio typical of the water condensed in the outer
asteroid belt. This is in agreement with the observation that the D/H
ratio in the oceans is very close to the mean value of the D/H ratio
of the water inclusions in carbonaceous chondrites.
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Petit, J.-M., and Morbidelli, A., and Chambers, J. E., and
Lunine , J. I., and Robert , F., and Valsecchi , G. B., and
Cyr , K. E., Asteroid belt Clearing and Delivery of Water to Earth,
AAS/Division for Planetary Sciences Meeting, 2000, Oct.
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2000DPS....32.5206P&db_key=AST
Abstract
We begin our numerical study of the formation of the terrestrial
planets assuming the presence of planetary embryos spread between ~0.5
and 4 AU at the time of formation of Jupiter or its core. Due to their
mutual gravitational interaction and with the growing Jupiter, the
orbits of the embryos begin to cross each other and they collide,
forming bigger bodies. A few planets are formed in a stable
configuration in the terrestrial planet region, while the asteroid
belt is usually cleared of embryos. Due to the combined gravitational
influence of Jupiter and of the embryos, most of the asteroids are
ejected from the system in a very short time ( ~1 My). Less than 1% of
the asteroids survive, mostly in the 2.8-3.3 region, and their
eccentricity and inclination distributions qualitatively resemble
those observed. The surviving particles have undergone changes in
semi-major axis of several tenth of an AU, which could explain the
radial mixing of asteroid taxonomic types. Some of the particles end
up on very inclined eccentric orbits in the inner Solar System, on
orbits with a longer decay time. These particles could be the source
of the Late Heavy Bombardment. In this scenario, the Earth would
continuously accrete water during its formation, from the earliest
phases when the solar nebula was still present, to the late stages of
gas-free sweepup of scattered planetesimals. Asteroids and the comets
from the Jupiter-Saturn region were the first water deliverers, when
the Earth was less than half its present mass. The bulk of the water
presently on Earth was carried by a few planetary embryos, originally
formed in the outer asteroid belt and accreted by the Earth at the
late stages of its formation. Finally , a late veneer, accounting for
at most 10% of the present water mass, occurred due to comets from the
Uranus-Neptune region and from the Kuiper belt.
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Taylor , S. R., The Leonard Award Address: On the Difficulties of
Making Earth-Like Planets,
Meteoritics and Planetary Science, 1999, may, vol 34, p.317--329.
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1999M%26PS...34..317T&db_key=AST
Abstract
Here I discuss the series of events that led to the formation and
evolution of our planet to examine why the Earth is unique in the
solar system. A multitude of factors are involved. These begin with
the initial size and angular momentum of the fragment that separated
from a molecular cloud. These are crucial in determining whether a
planetary system or a double star develops from the resulting nebula.
Another requirement is that there must be an adequate concentration of
heavy elements to provide the two percent 'rock' and 'ice' components
of the original nebula. An essential step in forming rocky planets in
the inner nebula is loss of gas and depletion of volatile elements due
to early solar activity, that is linked to the mass of the central
star. The lifetime of the gaseous nebula controls the formation of gas
giants. In our system, fine timing was needed to form the gas giant,
Jupiter before the gas in the nebula was depleted. Although Uranus and
Neptune eventually formed cores large enough to capture gas, they
missed out and ended as ice giants The early formation of Jupiter is
responsible for the existence of the asteroid belt (and our supply of
meteorites) and the small size of Mars while the gas giant now acts as
a gravitational shield for the terrestrial planets. The Earth and the
other inner planets accreted long after the giant planets in a
gas-free inner nebula from volatile-depleted planetesimals that were
probably already differentiated into metallic cores and silicate
mantles. The accumulation of the Earth from such planetesimals was
essentially a stochastic process, accounting for the differences among
the four rocky inner planets including the startling contrast between
those two apparent twins, Earth and Venus. Impact history and
accretion of a few more or less planetesimals were apparently crucial.
The origin of the Moon by a single massive impact with a body larger
than Mars accounts for the obliquity (and its stability) and spin of
the Earth in addition to explaining the angular momentum, orbital
characteristics and unique composition of the Moon. Plate tectonics,
unique among the terrestrial planets, led to the development of the
continental crust on the Earth, an essential platform for the
evolution of Homo sapiens. Random major impacts have punctuated the
geological record, accentuating the directionless course of evolution.
Thus a massive asteroidal impact terminated the Cretaceous Period,
resulted in the extinction of at least 70% of species living at that
time and led to the rise of mammals. This sequence of events that
resulted in the formation and evolution of our planet were thus unique
within our system. The individual nature of the eight planets is
repeated among the 60-odd satellites: no two seem identical. This
survey of our solar system raises the question whether the random
sequence of events that led to the formation of the Earth are likely
to be repeated in detail elsewhere. Preliminary evidence from the 'new
planets' is not reassuring. The discovery of other planetary systems
has removed the previous belief that they would consist of a central
star surrounded by an inner zone of rocky planets and an outer zone of
giant planets beyond a few AU. Jupiter-sized bodies in close orbits
around other stars probably formed in a similar manner to our giant
planets at several AU from their parent star and subsequently migrated
inwards becoming stranded in close but stable orbits as 'hot
Jupiters', when the nebula gas was depleted. Such events would prevent
the formation of terrestrial-type planets in such systems.
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Halliday, A. N. and Porcelli , D., In Search of Lost Planets - The
Paleocosmochemistry of the Inner Solar System, Earth and Planetary
Science Letters, 2001, Nov, 192, 545--559.
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2001E%26PSL.192..545H&db_key=AST
Abstract
The depletion of moderately volatile elements in planetesimals and
planets is generally considered to be a result of removal of hot
nebula gases. This theory can be tested with Sr isotopes. The
calculated initial 87Sr/86Sr of the angrite parent body (APB), eucrite
parent body (EPB), the Moon and the Earth are significantly higher
than the initial Sr isotopic composition of the solar system despite
the volatile-depleted nature of all of these objects. Calculated
time-scales required to accomplish these increases in 87Sr/86Sr with a
solar Rb/Sr in a nebula environment are >2 Myr for the APB, >3 Myr for
the EPB and >10 Myr for the Moon. These times are more than an order
of magnitude longer than that expected for cooling the nebula in the
terrestrial planet-forming region and correspond to the period during
which most of the mass already should have been accreted into sizeable
planetesimals and even planets. Therefore, incomplete condensation of
the nebula does not provide an adequate explanation for the depletion
in moderately volatile elements. The data are better explained by a
protracted history of depletion via more than one mechanism, including
processes completely divorced from the earliest cooling of the
circumstellar disk. The Sr model ages are maximum formation ages of
the APB and EPB and indicate that these are most probably secondary
objects. With independent estimates of their minimum age, a
time-integrated Rb/Sr can be calculated for the precursor materials
from which they formed. These are consistent with accretion of the APB
and EPB from objects that at one stage may have resembled carbonaceous
chondrite parent bodies in terms of volatile budgets. At some late
stage there were large losses of volatiles, the most likely mechanism
for which is very energetic collisions between planetesimals and
proto-planets that, in the case of the Asteroid Belt, have since been
lost. The same applies to the Moon, which presently has Rb/Sr=0.006
even though the material from which it formed had a time-integrated
Rb/Sr ratio of ~0.07, consistent with a precursor planet (Theia) that
was even less volatile element-depleted than the present Earth
(Rb/Sr=0.03). The time-integrated Rb/Sr of Theia is similar to the
present Rb/Sr of Mars (0.07). There is suggestive evidence of a
similar time-integrated value for the proto-Earth (~0.09). Therefore,
prior to the later stages of planet formation involving giant impacts
between large objects, the inner solar system may have had relatively
uniform concentrations of moderately volatile elements broadly similar
to those found in volatile-depleted chondrites. Correlations of the
present Rb/Sr ratios in planets and planetesimals with ratios of other
volatile elements to Sr can be used to infer the time-integrated
composition of precursor materials. The time-integrated inferred K/U
ratios of the proto-Earth, as well as Theia, were ~20000, so that
early radioactive heat production may have been ~40% greater than that
calculated by extrapolating back from the Earth's present K/U. Higher
C and S bulk concentrations may have led to concentrations in
proto-cores of 0.6-1.5% C and 4-10% S. These are significantly higher
than those anticipated from the degree of volatile depletion of the
present silicate Earth (~0.12% C, ~1.3% S). If the late history of
accretion did not involve large-scale re-equilibration of silicates
and metal, the present core may have inherited such high C and S
concentrations. In this case, S would be the dominant light element in
the present core.
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Related, up-coming symposium:
ESLAB36: "Earth-like Planets and Moons", ETSTEC, Noordwijk, The
Netherlands, 3-8 June 2002.
http://ssd.esa.int/resources/conferences/eslab36
-- ************************************************************************ Amara Graps, PhD | Max-Planck-Institut fuer Kernphysik Heidelberg Cosmic Dust Group | Saupfercheckweg 1 +49-6221-516-543 | 69117 Heidelberg, GERMANY Amara.Graps@mpi-hd.mpg.de * http://www.mpi-hd.mpg.de/dustgroup/~graps ************************************************************************ "We came whirling out of Nothingness scattering stars like dust." --Rumi
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