Theory goes that oxygen was dislodged from carbon monoxide in the nebula of the early sun

The birth and evolution of the solar system, not to speak of life, often seem dependent on the chanciest of events. And, sometimes, deep mysteries are found in some of the most trivial of details. For example, scientists are baffled by the distribution of different varieties of oxygen in the solar system. Meteorites contain one variety of oxygen while all the planets and satellites contain another. So far scientists had thought that light from the early sun had played an important role in producing this anomaly. Now scientists at the University of California in the US disprove this theory, thus questioning our very understanding of how the solar system evolved.

The anomaly of oxygen isotopes was discovered in 1969, after analysing a meteorite that fell in Allende in Mexico. This meteorite mostly had the oxygen isotope O16 — which is the most abundant oxygen variety on earth — while rocks collected from the solar system contained more rarer heavier oxygen isotopes, namely O17 and O18 combined with other elements. This anomaly remained a mystery for a long time. Scientists had put forward three hypotheses to explain this mystery. One was external seeding: some other star system had thrust O16 on to the meteorites. The second theory was photochemistry: light from the sun somehow created O17 and O18. The third hypothesis is normal chemistry. “All these are theories,” says Subrata Chakraborty, scientist at the University of California, San Diego. “There is no experimental proof yet as how these isotopes were formed in the meteorites.”

The first theory, that of external seeding, was discarded some time ago. Chakraborty and his colleagues at the university and the Lawrence Berkeley National Laboratory designed some experiments to prove or disprove the second theory. The theory goes like this. The early sun — a proto sun, to be precise — had a thin nebula surrounding it. The most abundant substance in this nebula was carbon monoxide. When light from the sun passed through this nebula, it dislodged the oxygen atoms from the carbon monoxide. Each variety of oxygen absorbs a particular wavelength of light. It turns out, so goes the theory, that the most abundant variety of oxygen nearer to the sun shields those that are farther away from this particular wavelength.

This means that the rarer types of oxygen are not shielded — there is not enough of it in the nebula — even if it is far away from the light source.

Subrata Chakraborty in the University of California lab

Light knocks off this type of oxygen atoms from the carbon monoxide, freeing them to combine with other atoms. These combinations produce a substantial portion of the dust that formed the planets, and hence they have more of O17 and O18. But this was a hypothesis till recently.

To test this theory, the scientists used a facility named the Advanced Light Source. This generated a light in a tube that was filled with carbon monoxide. By creating precisely the proportion of gases thought to be present in the early solar system, the scientists could study what happened there when sunlight shone through the thin gaseous nebula. To their surprise, they did not see any shielding by the O16 atoms. This meant that the photochemistry hypothesis — relative shielding by O16 created dust rich in other oxygen varieties — was wrong. “Now there is only one explanation, that of chemistry,” says Chakraborty. Mark Theimens, another scientist in University of California, has shown that it was possible for chemical reactions to produce the anomaly in oxygen isotopes seen in meteorites.

What is the larger significance of these results? It shows that our understanding of the early solar system is far from correct. We do not know the precise chemistry and other processes that created the mixture of elements that we see in the planets and other objects in the solar system.

A clear understanding of such processes would help us understand how planetary systems form throughout the universe: special processes are rare in the universe. The University of California at San Diego, along with NASA, is studying samples sent by the Genesis spacecraft to understand the formation of stars and planets. This experiment would be a valuable input in analysing the data from the spacecraft.

To prove or understand how things really worked, it is sometimes useful to disprove many other theories.