Readers ponder mysterious origins of oxygen on comets and Earth

Molecule mystery

The Rosetta spacecraft discovered oxygen leaking from comet 67P, Christopher Crockett reported in “Oxygen in comet surprises scientists” (SN: 11/28/15, p. 6). The molecules’ presence supports a long-held belief that comets are pristine relics of the solar system’s birth.

Reader R. Powell questioned that assumption. The story notes “that the pristine oxygen is released from the [comet’s] water-ice when the sun’s heat sublimes the ice,” he wrote in an e-mail. “How is it known that the oxygen was not created by [ultraviolet] light breaking down the water into oxygen and hydrogen?” If the latter is true, Powell said, the oxygen may not be primordial.

The researchers spent a lot of time on this very question, Crockett says. Water can generate oxygen in a couple of ways: either through exposure to UV light or bombardment with electrons and ions. Comet 67P spent billions of years in the Kuiper belt beyond Neptune, where a rain of charged particles could have created oxygen. But such rain only penetrates a few meters below a comet’s surface, the researchers say, and a comet loses several meters of its surface after every close encounter with the sun. So any oxygen that 67P built up while hanging out in the Kuiper belt is long gone.

Solar wind can also break apart water molecules, but it can’t keep up with the rate at which a comet sheds its skin. If wind was the primary source of 67P’s oxygen, the ratio between oxygen and water on the comet’s surface would decrease as the comet approached the sun. But the ratio has stayed relatively constant, Crockett says. That relative stability over a period of several months suggests that the oxygen is coming from a deep reservoir within the comet, which would have been buried 4.6 billion years ago.

Boring billions

About 1.8 billion years ago, Earth’s planetary processes seemingly stalled during a low-oxygen period, Thomas Sumner wrote in “Earth’s (not so boring) boring billion” (SN: 11/14/15, p. 18). But new chemical clues hint that the planet’s midlife was anything but monotonous.

Reader Steve Schlosser wanted to know what caused oxygen levels to take such a roller coaster ride after the stagnant boring billion. “If the burst of oxygen levels to well above today’s levels occurred after this period, what caused the levels to drop to those we know today?” he asked in an e-mail.

Oxygen levels are largely controlled by photosynthesis but they are also influenced by decomposition, Sumner says. Generally, the more plants and photosynthesizing microbes, the more oxygen there is. A few hundred million years ago, for example, Earth was a lot warmer than today with an abundance of oxygen-producing plants. When these plants died, much of that organic matter ended up buried in swamps, stalling decay that would have actually drawn down oxygen. So oxygen levels rose until the climate cooled. But, Sumner notes, these more recent ups and downs in oxygen concentration are pretty minor compared with the large-scale changes earlier in Earth’s history. For comparison, during the boring billion oxygen levels were less than a hundredth of what they are now.

As for the greater mystery of oxygen stagnation during the boring billion, no one actually knows why the levels stayed low and stable for so long, Sumner says. Scientists have kicked around a few ideas, but none have gained wide acceptance yet. The new excitement and focus on that era will hopefully lead to an answer, he says.

Correction

Reader Ann Harmer enlightened us about a recent cover image, which we described as a “plasticized” brain (SN: 11/14/15, p. 1). “The process by which the beautiful vascular specimen of the human brain was produced is plastination,” she wrote in an e-mail. “Therefore, the brain was plastinated, not plasticized (a common error). The process was invented by Dr. Gunther von Hagens and patented in 1978.”