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Extraterrestrial Life: Problem Set #2 Solutions 1) Discuss the evidence that suggests that the water on the Earth originated
 

Summary: Extraterrestrial Life: Problem Set #2 Solutions
1) Discuss the evidence that suggests that the water on the Earth originated
from impacts with bodies originally situated in the outer asteroid belt.
Why are comets not though to have dominated the delivery of the Earth's
water?
Two aspects are relevant to this question. First, we think that the
planetesimals forming in the protoplanetary disk at the location of the
Earth would have had rather little water content, because at the low
pressure of the disk water would have been in the vapor phase rather than
in the form of ice. This expectation is supported by the fact that meteorites
originating from the inner asteroid belt contain very little water, whereas
those from the outer belt are water rich.
However, comets as well as the outer belt asteroids contain water. We
believe that asteroids dominated the delivery of water to the Earth as a
result of measuring the ratio of deuterium to hydrogen in water on Earth,
and comparing that to the values found in meteorites and measured for
some comets. The D / H ratio on Earth is about 150 parts per million,
which is consistent with the range found for meteorites (and hence,
presumably, for the asteroids) but much smaller than the 300 ppm found
for comets.

  

Source: Armitage, Phil - Department of Astrophysical and Planetary Sciences, University of Colorado at Boulder

 

Collections: Physics