Origin Of Water On Earth

Hydrogen peroxide found in space

Hydrogen peroxide has been detected for the first time in interstellar space, offering hints about the origin of water, a similar molecule.

The chemical signature of hydrogen peroxide molecules, which consist of two oxygen atoms bookended by two hydrogen atoms, was detected in dense clouds of cosmic gas and dust in a distant part of our own galaxy, the Milky Way, reported the study published Wednesday in Astronomy & Astrophysics.

The light signals were captured by the European Southern Observatory's Atacama Pathfinder Experiment telescope (APEX) in the Chilean Andes.

But the chemical best known on Earth as an antiseptic, rocket fuel and bleach isn't likely to give interstellar tourists blond highlights in their hair, even if they somehow manage to make the 400 light-year journey from Earth.

"The amount of hydrogen peroxide in the cloud is just one molecule for every 10 billion hydrogen molecules, so the detection required very careful observations," said Per Bergman, lead author of the study, in a statement. Bergman is an astronomer at Onsala Space Observatory in Sweden.

The discovery is exciting for astronomers because water can be produced when hydrogen peroxide reacts with hydrogen under the right conditions. A lot of the water on Earth is believed to have originally formed in space, but scientists aren't sure how.

"We don't understand yet how some of the most important molecules here on Earth are made in space," said Berengere Parise, an astrochemist at the Max-Planck institute for Radio Astronomy in Germany who co-authored the paper. "But our discovery of hydrogen peroxide with APEX seems to be showing us that cosmic dust is the missing ingredient in the process."

The finding is consistent with previous studies that showed hydrogen peroxide doesn't form efficiently from hydrogen and oxygen gas, but proposed it could be formed on grains such as dust.

The clouds where the molecules were found are a region of the galaxy near the star Rho Ophiuchi. The area is a "stellar nursery" where new stars are formed. The clouds are made mostly of hydrogen, but contain traces of other molecules.

Origin of water in the Earth-Moon system «

Features a couple of interesting articles on the origin of water in the Earth-Moon system. By way of an introduction, Robert (2011) reviews the known ratios of deuterium (heavy hydrogen; one proton, one neutron) to hydrogen (one proton) of various planetary bodies in the solar system: the proto-Sun, Earth, and Moon, along with carbonaceous chondrite meteorites and comets (Fig. 1).

Fig. 1. Deuterium/hydrogen ratios of the proto-Sun (peach), Earth (blue), Moon (red), carbonaceous chondrites (black), and comets (green). The D/H ratio is multiplied by 10^6 reflecting parts-per-million quanities of deuterium with respect to hydrogen. Adapted from Robert (2011).

As you can see, there are a couple of interesting isotopic associations. The Earth overlaps strongly with carbonaceous chondrites, while the Moon spans a range of D/H values, potentially indicating a significant affinity with comets.

Why might this disparity between the Earth and Moon exist? The prevailing hypothesis for the formation of the Moon is the impact of Theia  with Earth, so within this framework, it stands to reason that water on Earth appeared after the formation of the Moon via significant contributions of water from carbonate chondrites.

By comparison, Greenwood et al. (2011) discovered that abundant lunar water exists bound up within the hydrous mineral apatite, which represents a mafic phase within the mare basalts and anorthositic highlands. The interesting thing about the Moon, however, is that a number of sources are identified including solar protons, the lunar mantle, and comets (D/H ratios increasing respectively, with the solar fraction as the lightest). These different sources potentially explain the wide range of D/H ratios observed in lunar rocks.

This appears to be a tidy hypothesis, but as Robson (2011) hints, how do you explain the prominent influence of comets on lunar water, and its apparent absence in terrestrial water? The Earth and Moon are next-door neighbours in the context of the solar system, and Greenwood et al. (2006) which indicates that the relatively high D/H ratio on Venus arises from the equivalent loss of a terrestrial ocean; something which most certainly did not occur on Earth.

References

Greenwood, J.P., Itoh, S., Sakamoto, N., Warren, P., Taylor, L., and Yurimoto, H.