What
is questioned is the role of comets in the initial creation of the
Earth. Instead it could be that a mass of condritic material within
the inner solar system envelope is presumed to be the mass
contributor. This has the advantage of eliminating the angular
momentum problem presented by comets.
It
appears also sufficient to deliver the necessary volatiles.
Thus
it represents the simplest solution and comet bombardment is way
rarer than we imagined. It in fact eliminates the serious question
of the non bombardment of everything else in the inner solar system
and why Earth is so lucky.
With
the new model, everything besides Venus is too small and as
previously posted, Venus is a later event that happened after the
inner solar system was scoured for condritic rock.
Asteroid Crashes
Likely Gave Earth Its Water
by SPACE.com Staff
Date: 12 July 2012
Time: 02:01 PM ET
Asteroids from the
inner solar system are the most likely source of the majority of
Earth's water, a new study suggests.
The results contradict
prevailing theories, which hold that most of our planet's water
originated in the outer solar system and was delivered by
comets or asteroids that coalesced beyond Jupiter's orbit, then
migrated inward.
"Our results
provide important new constraints for the origin of volatiles in the
inner solar system, including the Earth," lead author Conel
Alexander, of the Carnegie Institution of Washington, said in a
statement. "And they have important implications for the current
models of the formation and orbital evolution of the planets and
smaller objects in our solar system."
Alexander and his
colleagues analyzed samples from 86 carbonaceous chondrites. These
primitive meteorites are thought to be key sources of the early
Earth's volatile elements, such as hydrogen and nitrogen.
The team measured the
abundance of different hydrogen, nitrogen and carbon isotopes in the
chondrite samples. Isotopes are versions of an element that have
different numbers of neutrons in their atomic nuclei. For example,
the isotope deuterium — also known as "heavy hydrogen" —
contains one neutron, while "normal" hydrogen has none.
The amount of
deuterium in celestial bodies' water ice sheds light on where the
objects formed in the solar system's early days. In general,
bodies that took shape farther from the sun have relatively higher
concentrations of deuterium, researchers said.[interesting
assertion that I do not trust]
The 86 chondrite
samples' deuterium content — which the team gleaned from clays, the
remnants of water ice — suggest the meteorites' parent bodies
formed relatively close to the sun, perhaps in the mainasteroid belt
between Mars and Jupiter.
Comets, by contrast,
have much higher deuterium ratios. As a result, scientists think most
of them were born in the solar system's frigid outer reaches.
The isotopic
composition of the bulk Earth appears to be more consistent with
chondrites than with comets, researchers said. There are many
different types of chondrites, and no single group is a perfect
match. So our planet probably accreted its water and other volatiles
from a variety of chondrite parent asteroids, they added.
The study appears
today (July 12) online in the journal Science.
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