User:Mathwhiz 29/cosmochemistry

Nice Work

-TA

Water and Carbon delivered by Comets

All the following section is taken from unless otherwise noted.

A significant fraction of Earth's volatiles may have been acquired during and around the Late Heavy Bombardment, around 4.5-3.8 bya. During this time, the inner solar system was bombarded by C-type asteroids and approximately 10¹¹ comets (explanation below), rich in complex organics as well as volatile molecules. We have begun looking at comets as a volatile delivery mechanism as the atmosphere of the early earth may have been neutral, and rich in CO₂ and NH₃ which are implied by "hot" accretion scenarios for Earth. In such an atmosphere, synthesis of prebiotic molecules would have been much more difficult. This is suggestive of cometary delivery of organic molecules and volatiles, but not any requirement.

The existence of complex terrestrial microorganisms by 4.5 bya, and evidence for biologically mediated carbon isotope fractionation in 3.8 bya Isua metasediments suggest chossing this period to assess cometary delivery of organics. This is 7x10⁸ years.

Impactor flux, derived from fitting the lunar crater record, and modified for the Earth's gravitational cross section, can be said to be between 10²⁰-10²² kg since 4.5 bya, with almost all the flux from during the Late Heavy Bombardment. Note that the poorly constrained fraction of mass flux that is cometary is assumed to be between 10-20%, and was set to be 10%. Currently, however, comets account for 10-30% of impact craters greater than 10km.

Comets are 18% carbon by mass, so this implies the earth has collected 2x10¹⁹-10²¹ kg of cometary carbon, when the total terrestrial surface carbon inventory of ~9x10¹⁹. Clearly, there is a discrepancy between the terrestrial carbon inventory predicted by the upper bound and that actually present on Earth. However, it is possible atmospheric erosion could reduce this discrepancy (C --> gaseous CO₂).

A similar discrepancy exists for water, which cannot be erased by atmospheric erosion. Spectroscopic observations of comet Halley, HH46, and W33A suggest that the abundance of water is about 40% water (in the ice, about 80%). Then, we would predict a water flux to the earth of 4x10¹⁹-3x10²² kg, when the water on earth today is about 1.3x10²¹ kg.

So it appears the volatile flux predicted by the upper bound is too large by a factor of ~100. Curiously enough, the mass flux is probably too small for the lower bound, as using lunar highland iridium and nickel abundances and scaling to the Earth yields M_comet (total, lower limit) = 5x10²¹ kg.

Let's use 10²² as a reasonable, middle ground of kg of comets that arrived, bringing water and organics. Then we have an average comet, around 10¹¹ kg, which is higher than normal but orders less than the largest, most rare and massive comets. I'm assuming a powerlaw distribution of objects, with most the mass in the normal, smaller comets. This is normal for space objects. Then we have a final number of about 10¹¹ comets. Cool.

D/H ratios

The abundance of deuterium in comet Hale–Bopp in the form of heavy water was found to be about twice that of Earth's oceans. If Hale–Bopp's deuterium abundance is typical of all comets, this implies that although cometary impacts are thought to be the source of a significant amount of the water on Earth, they cannot be the only source.

Deuterium was also detected in many other hydrogen compounds in the comet. The ratio of deuterium to normal hydrogen was found to vary from compound to compound, which astronomers believe suggests that cometary ices were formed in interstellar clouds, rather than in the solar nebula. Theoretical modelling of ice formation in interstellar clouds suggests that comet Hale–Bopp formed at temperatures of around 25–45 Kelvin.

That the Earth's water originated purely from comets is implausible, as a result of measurements of the isotope ratios of hydrogen in the three comets Halley, Hyakutake and Hale-Bopp by researchers like David Jewitt, as according to this research the ratio of deuterium to protium (D/H ratio) of the comets is approximately double that of oceanic water. What is however unclear is whether these comets are representative of those from the Kuiper Belt. According to A. Morbidelli the largest part of today's water comes from protoplanets formed in the outer asteroid belt that plunged towards the Earth, as indicated by the D/H proportions in carbon-rich chondrites. The water in carbon-rich chondrites point to a similar D/H ratio as oceanic water. Nevertheless, mechanisms have been proposed to suggest that the D/H-ratio of oceanic water may have increased significantly throughout Earth's history. Such a proposal is consistent with the possibility that a significant amount of the water on Earth was already present during the planet's early evolution.

Recent measurements of the chemical composition of Moon rocks suggest that Earth was born with its water already present. Investigating lunar samples carried to Earth by the Apollo 15 and 17 missions found a deuterium-to-hydrogen ratio that matched the isotopic ratio in carbonaceous chondrites. The ratio is also similar to that found in water on Earth. The findings suggest a common source of water for both objects. This supports a theory that Jupiter temporarily migrated into the inner Solar System, destabilizing the orbits of water-rich carbonaceous chondrites. As a result, some of the bodies could have fallen inwards and become part of the raw material for making Earth and its neighbors. The discovery of water vapor out-gassing from Ceres provides related information on water-ice content of the asteroid belt.