Many sources of free oxygen
(appeared in May 2017)

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Space scientists and materials scientists working together refine a point in the quest for extra-terrestrial life, says S.Ananthanarayanan.

Ordinary oxygen, as free oxygen molecules, is abundant on the earth but is rare in other parts of the universe. This is because free oxygen is reactive and quickly combines with other elements, particularly to form water (H2O) or carbon dioxide (CO2). Oxygen is hence known to accumulate only if it is continuously generated, which happens on the earth by action of vegetable matter and photosynthesis

The discovery of sizeable molecular oxygen on the barren surface of the comet, 67P/Churyumov–Gerasimenko, just over 4 km long, hence raised questions that were difficult to answer. Konstantinos P. Giapis and Yunxi Yao, at the California Institute of Technology, while developing special materials for computer chips, have discovered that high speed water molecules striking mineral surfaces can give rise to molecular oxygen. They report the finding in the journal, Nature Communications and they say the same process appears to be going on in the comet, 67P.

Our planet, earth, too, did not have free oxygen for a very large part of its history. The first forms of life that evolved did not depend on oxygen. They did use the sun’s energy but they filled the atmosphere with methane. We still have some of these, in oxygen starved locations and which break down organic matter to create ‘marsh gas’. While there were also organisms, the cyanobacteria, which gave off oxygen, the oxygen promptly combined with hydrogen that was produced when methane decomposes. Methane, however, stays a gas at low temperatures and can get past the coldest part of the atmosphere and ascend to reach the extreme outer atmosphere. The light atoms of hydrogen given off by methane, at high altitudes, could then escape from the earth, which led to depletion of hydrogen.

When hydrogen levels fell, oxygen could accumulate. For millennia, however, oxygen got consumed in combining with free iron, to create oxides, which we see in the bands of iron ore in the earth’s crust. When all the iron got used up, oxygen began to accumulate. There was an explosion in vegetation and biodiversity and the existing life forms declined. Oxygen levels rose till they reached a balance of about 21% of the content of the atmosphere. The presence of oxygen also started using up hydrogen, which stopped the loss of hydrogen atoms out to space.

Quest for life

As finding oxygen in distant worlds is rare, the quest for life is focussed on finding other indicators, namely, carbon dioxide, methane, and ozone. Finding free, molecular oxygen, of course would be strong evidence of life processes. And along with chemical indicators of life, there is also the effort to find other signs, like smooth and undulating topography, as opposed to a zagged and rocky surface.

And yet, in the year 2014, a tiny, 4.1x4.3 km, barren, rocky comet, with barely any gravity, negligible atmosphere or anything earth-like, was found to contain molecular oxygen, in abundance from 1 to 10%, compared the comet’s content of water. As there was no question of the oxygen being from an organic source, it had to be proposed that it was of primordial origin, as thawing from a store frozen since when the solar system formed 4.6 billion years ago. This explanation, however is questioned, as it conflicts with how the formation of the solar system is understood and because the oxygen could not have kept from combining with other elements over such a long time.

Konstantinos P. Giapis, Professor of Chemical Engineering at Caltech has been working on the effects of charged atoms and molecules, or plasmas, on the surfaces of metals, at the nanoscale. When molecules are exposed to high energy radiation like ultra violet light, groups of atoms making up the molecule separate and float about as oppositely charged particles, in place of being together and neutral. The water molecule, of H2O, for instance, would split as H+ + OH-, or as 2H+ + O-. When such particles contact the surface of materials, they are attracted to the region very close to the atoms at the periphery of the material. The ions then get attached to the surface and can form a thin nano-layer. When this happens, there is a change in the distribution of atoms and charges that the attached ion presents to other ions of the surrounding plasma and reactions that are otherwise not possible can take place, or get accelerated.

Prof Giapis and his colleague, Yunxi Yao report that the collision of high speed H2O+ ions colliding with silicates and iron or nickel oxides found on the surface of the comet could lead to release of oxygen atoms from the water molecule as well as from the surface minerals. They say in their paper that they have verified in the laboratory that accelerated water ions can participate in such reactions when they strike oxidised surfaces at high speeds, to directly form molecular O2- ions.

When comets, in their elliptic orbits approach the sun, ice in the planet evaporates and forms a cloud or ‘coma’ around the comet. The water molecules are then ionised by ultra violet light from the sun and accelerated by the solar wind. These are the high-speed water ions that strike the comet surface, for molecular oxygen to be ejected. The ejected oxygen can then be replenished by oxygen from the dissociation of fresh water molecules or from the surface oxides, the paper says. This process would continuously populate the coma of the comet with oxygen, and in time, there would be a steady level of oxygen, with generation balanced by consumption in forming fresh water or oxides.

This insight, of the same processes studied in a ‘down at earth’ materials science lab being active in fragments of rock out in deep space, widens the range of sources of gases and substances found in the cosmos. Not only does the quest for life need to be circumspect while assigning the discovery of gases like oxygen to biological origin, even our understanding of the geology of distant worlds would need to be reviewed.

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The oxygen comet

The comet, 67P is a Jupiter family comet, from the Kuiper Belt, that was discovered by Soviet astronomers Churyumov and Gerasimenko in 1969. The European Space Agency launched the Rosetta mission to the comet in 2004 and Rosetta arrived at 67P ten years later, in 2014. The space craft then entered orbit around the comet, just over four km at its widest and longest, and sent down a landing craft, Philae, which conducted surface analysis of the comet.

Philae unfortunately landed in a shaded part of the comet and had to wait some months before its solar panels started working. But the data it sent based on battery power kept researchers busy without a break.

Water vapour on the comet was found to differ from water on the earth in the composition of hydrogen isotopes. This ruled out the 67P class of comets as a source of the water on the earth. And then, the abundance of molecular oxygen was perplexing!

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