INFINITY & BEYOND
Comet’s Mysterious Oxygen Stump Scientists
Oxygen has never been observed coming from a comet. But the comet known at 67P just changed that.
Mysterious oxygen has been found within comet 67P/Churyumov—Gerasimenko, a revelation that is forcing scientists to re-examine some things we thought we know about the formation of the solar system.
Since 2014, 67P/Churyumov—Gerasimenko has been orbited by the Rosetta space probe, launched and operated by the European Space Agency (ESA). Since then the comet has been physically sampling the chemicals emitted from the comet that form the surrounding cloud, called the “coma.”
In the past scientists have determined the chemicals of comet comas through spectroscopy, a method that quantifies chemical composition based on the amounts and varieties of light coming off something. Before this study, the coma of most comets was known to be over 95 percent water, carbon dioxide, and carbon monoxide.
While molecular oxygen (O2) has been observed to exist on other bodies in the solar system—such as moons of Jupiter and Saturn—it has never been observed to come from a comet.
The gases in a comet’s coma aren’t held there gravitationally like they are here on Earth. Rather, they are continually emitted by the comet as it is heated by the sun. Because the ice composition of the comet itself varies over its surface, so too do the gases emitted from it. Strangely, the amount of oxygen found was found to change in tandem with the amount of water; oxygen’s concentration always remaining close to 3.8 percent of water’s concentration. This suggests that these two chemicals were incorporated into the comet together as it formed. Herein lies the mystery.
While 67P/Churyumov—Gerasimenko’s orbit extends from just inside Mars’s orbit to just beyond Jupiter’s orbit (it is currently just outside Mars’s orbit), it is believed to originate from a region in the outskirts of the solar system called the Kupier Belt. Many comets are understood to have formed and reside within this region for a time, though the occasional one gets kicked into the planetary region by interactions with Jupiter’s gravity.
Back when this comet, and likewise the sun and planets, were forming from grains of dust in a nebula—sometimes called a “molecular cloud”—scientists have been able to predict how these grains of dust themselves formed. According to the paper, “models of gas grain chemistry in molecular clouds predict O2/H2O ratios at least an order of magnitude lower,” meaning the amount of oxygen measured was about 10 times what was predicted. If oxygen wasn’t present in the grains back when the solar system was a nebula, it must have showed up later. The paper proposes two hypotheses of how this might have happened.
The first hypothesis is that as the sun formed, it created conditions where grains could cool off rapidly, changing the water ice to “amorphous water ice.” This would allow any oxygen sitting on the surface of the grains (a phenomenon known to occur in the comet-forming regions of a nebula turning into a solar system) to become incorporated into the grains themselves.
The second hypothesis is that the oxygen came from the water ice itself. Water is made up of oxygen and hydrogen. Under certain circumstances, radiation from the sun or elsewhere in the universe can cause water to split into hydrogen and oxygen. Because the hydrogen molecules are so much smaller, they can diffuse out of the grains to leave the oxygen molecules behind.
This is just one more mystery that scientists are trying to solve about the origins of the solar system.