Recent Discoveries about Oxygen on Earth and in Outer Space
This 4th of July, I took a date to the Hollywood Bowl. On the way out of the stadium, I spotted Mars: bright, red, and high in the sky. I pointed it out to her, and we got to talking about planets and stars.
“Didn’t I hear that they just discovered oxygen on other planets?” she asked.
I was surprised to hear that. “I doubt it,” I said. “Because if they’ve found oxygen, then they’ve found life!”
“Really?” she second-guessed me. “There can’t be oxygen without life?”
I thought about it for a second. I thought I was sure, but suddenly I wasn’t. She had me stumped. It seems to be “common knowledge” that there can be no oxygen unless plants, algae, and other living things make it with photosynthesis – but why should that have to be? I came home to look further into this question. I learned quite a bit, including the comforting fact that this is not a trivial question. There also happens to be a bevy of interesting research news about oxygen in space and Earth. (For the record, I did not see any such news bulletins that oxygen has recently been discovered on an exoplanet, so I don’t know exactly what she had heard).
What Is Oxygen, Anyway?
Let’s clarify an important distinction right away. There are multiple forms of oxygen. Oxygen is an element, which means that its smallest unit is one atom. When oxygen is considered one atom at a time, it is called elemental or atomic oxygen, abbreviated O. However, this form is extremely rare, because other elements find it very attractive. Oxygen is one of the hottest babes on the periodic table. Just as you’d expect to find gorgeous women surrounded by men, friends, or admirers, atomic oxygen is just about always bound to other atoms to form molecules.
One common compound, at least here on Earth, is two oxygen atoms bound to each other. This substance is called diatomic oxygen, molecular oxygen, or oxygen gas (abbreviated O2). This is the gas that plants produce and we inhale. It’s the substance that rusts iron and feeds fire. It is a stable molecule, and it constitutes 20% of our atmosphere, but common knowledge is right – it’s virtually nonexistent in the rest of the universe.
What gives? Oxygen is the third most abundant element in the universe and our solar system. Almost half of Earth’s atoms are oxygen! So why can’t all those oxygen atoms just pair up and fill outer space with O2 ? Why was even Planet Earth devoid of oxygen gas for the first half of its history?
In outer space, hydrogen (H) is much more abundant than oxygen. Odds are, then, that when a lone oxygen atom is zipping through space, the first atom it will bind to will be H. OH, hydroxide, is also very attractive and will immediately bond to something yet again, very often another H to produce water. In fact, water is one of the most common molecules in space. It is most often found as very thin vapor or chunks of solid ice, almost never under the right conditions to be liquid.
Still, even if it’s outnumbered, we would expect some O2 to result from random collisions of O atoms. From what we can tell from surveys of outer space, it isn’t there at all.
Believe it or not (this is what surprised me) astronomers and chemists didn’t have a good answer to that question themselves until very recently. In fact, they didn’t even realize that celestial oxygen gas was so rare until they expressly looked for it within the last couple of decades. 1 It was only in 2015 that a team from Syracuse University and San Jose State University, led by Jiao He, found a key factor. It turns out that elemental oxygen ranks very highly in what we call “bonding energy”. 2 This means that O binds very tightly to other particles or “space dust”. Bonding energy is different from O’s sheer electrical attraction. Not only do other atoms “want” to bond to O, but when they do, it is a very tight hug. Once an oxygen atom clings to a speck of dust, it’s hard to dislodge it. On that speck of dust, it tends to get bound up in solids such as ice or silicate (sand). Carbon dioxide, CO2, also forms naturally in space, and early Earth had plenty in its atmosphere. As I discussed in TEOH Section 9.II, certain microbes called cyanobacteria evolved a pathway to “breathe in” CO2 to photosynthesize glucose and then “breathe out” O2 as a waste product. Cyanobacteria and their cousins, chloroplasts, which now live inside plant cells, are the sole source of oxygen gas in our atmosphere.
Oxygen Without Life
So, could oxygen possibly exist on planets without life? Yes, but only under particular peculiar circumstances. Theoretical astrophysicists have dreamed up at least two ways that other planets could have oxygen gas without life. Let’s call these scenarios Planet Vapo and Planet Oceania. Planet Vapo has water vapor in its atmosphere. Under the right conditions, sunlight can photolysize the molecules of this vapor, tearing them apart to form hydrogen and oxygen gases.In one such setting, Vapo orbits close to a special kind of sun called an M dwarf star and is exposed to extreme ultraviolet light. 3 In a more Earth-like scenario, Vapo is situated within the habitable zone of its sun. The habitable zone is the happy-medium distance where a planet can support liquid water. In this case, Vapo’s sun can be just about any kind of star, but Vapo must have a low-nitrogen atmosphere if photolysis is to occur. 4
The imaginary planet Oceania is also situated in a habitable zone. Oceania contains a large amount of a space mineral called titania (TiO2). Just add a dose of ordinary sunlight, and the titania catalyzes the dissolution of water into H2 and O2. 5 This pathway could theoretically yield a fairly high level of oxygen even with a small percentage of titania in the sea.
Note that Earth does not meet any of these requirements. Earth is high in nitrogen gas and has virtually no titania. We don’t orbit an M dwarf star. All of our oxygen was biologically synthesized.
Planets Vapo and Oceania might be unlikely hypotheticals, but certainly no less likely than our own planet laden with life. It is important to keep the Vapo and Oceania possibilities in mind. In case we do ever discover another planet surrounded by oxygen, we need to understand that it could be a false lead. We’d want to check that possibility before getting too excited, warning the public of a war of the worlds, and spending a quadrillion dollars to visit Oceania.
Life Without Oxygen
The converse of the Life = Oxygen assumption is not so simple either. It is demonstrably possible for a planet without oxygen to support simple life forms like bacteria. After all, that’s how Earth’s biology began. It is doubtful, though, that life can advance very far without inhaling. Alternative energy sources such as sulfur and iron are much less effective than oxygen, and they seriously constrain the size and complexity of organisms. 6 You might think that if we look at a planet without oxygen, it has no potential for supporting complex life. But if you thought that, you might have bypassed Earth just as it was on the verge of a breakthrough.
Billions of years ago, cyanobacteria began releasing oxygen gas into the ocean. As discussed in TEOH Section 9.II, this oxygen didn’t get very far at first, because there were substances such as iron in the ocean to absorb it. In fact, the availability of oxygen spurred the evolution of protists, which consumed the oxygen, just as we breathe it in today. Eventually, though, oxygen saturated the ocean and percolated into the atmosphere. This Great Oxygenation Event (GOE) happened about 2.4 BYA.
New research led by Matthew Koehler at the University of Washington shows that the GOE was a little more dramatic than we had thought. Koehler has shown that oxygenation was stop-and-go for hundreds of millions of years before the GOE. The scientists have detected long intervals when the atmosphere became oxygenated before the GOE – some almost 300 million years earlier. These oxygenations were transient; high-oxygen cycles were followed by crashes and low-oxygen cycles. This intuitively makes sense; it’s a classic case of population dynamics. When oxygen is just barely high enough to support aerobic respiration, the cells that breathe it in and replace it with CO2 will quickly deprive themselves of oxygen. Their populations will plummet until a sufficient store of oxygen is restored. It seems reasonable that they might have to go through this cycle a few times before oxygen reaches sustainable levels. These cycles mean that if we find a planet with no detectable oxygen, it could be teeming with microbial life but just having a “bad air day”.Interestingly, Koehler’s findings were corroborated by another (apparently independent) study published in the exact same month, July of 2018. 7 This Caltech team, headed by Mark Torres, also found evidence of oxygen in the atmosphere as long as 2.7 BYA. Oxygen is a potent gas; its release ushered in a whole new chemical regime on Earth. Koehler’s and Torres’ studies both looked at clues left behind by other elements that were impacted by O2. Whereas Koehler studied nitrogen and the exotic metal selenium in Australia, Torres studied sulfur signatures in some of Earth’s oldest exposed rock, in Canada and South Africa. It’s remarkable that these projects examined different elements on different continents, and both got a date of 2.7 billion years for the first significant concentrations of O2 in the atmosphere. Eventually, of course, Earth’s plant-like and animal-like life forms reached equilibrium. Today, the O2:CO2 ratio is about 500:1.
Oxygen has a long and complex history on Earth, and presumably the same would be true on other life-bearing worlds. We take it for granted, but it is one of the things that makes Earth truly exceptional. We know of no other place in the universe where creatures can shoot off fireworks in the atmosphere while they point upward, breathe deeply, and wonder about life on blue planets.
- E.A. Bergin et al., “Implications of Submillimeter Wave Astronomy Satellite Observations for Interstellar Chemistry and Star Formation”, The Astrophysical Journal Letters, vol. 539, no. 2 (8/16/2000), http://iopscience.iop.org/article/10.1086/312843 (accessed and saved 10/24/18). ↩
- Jiao He et al., “A New Determination of the Binding Energy of Atomic Oxygen on Dust Grain Surfaces: Experimental Results and Simulations”, The Astrophysical Journal, vol. 801, no. 2 (3/12/2015), http://iopscience.iop.org/article/10.1088/0004-637X/801/2/120/meta (accessed and saved 8/21/18). ↩
- Feng Tian et al., “High stellar FUV/NUV ratio and oxygen contents in the atmospheres of potentially habitable planets”, Earth and Planetary Science Letters vol. 385, pp. 22-27 (1/01/2014), https://www.sciencedirect.com/science/article/pii/S0012821X13005876?via%3Dihub (accessed and saved 11/13/18). ↩
- Robin Wordsworth and Raymond Pierrehumbert, “Abiotic Oxygen-Dominated Atmospheres on Terrestrial Habitable Zone Planets”, Astrophysical Journal Letters, 785:L20 pp. 1-4 (4/20/2014), http://iopscience.iop.org/article/10.1088/2041-8205/785/2/L20/meta#apjl493070s3 (accessed and saved 9/01/18). ↩
- Norio Narita et al., “Titania may produce abiotic oxygen atmospheres on habitable exoplanets”, Scientific Reports 5, Article no. 13977 (9/10/2015), https://www.nature.com/articles/srep13977 (accessed and saved 11/13/18). ↩
- David C. Catling et al., “Why O2 Is Required by Complex Life on Habitable Planets and the Concept of Planetary ‘Oxygenation Time’”, Astrobiology vol. 5 no. 3 (6/07/2005), http://iopscience.iop.org/article/10.1088/0004-637X/801/2/120/meta (accessed and saved 8/21/18). https://www.liebertpub.com/doi/10.1089/ast.2005.5.415 (abstract accessed 10/23/18). ↩
- Mark A. Torres et al., “Riverine evidence for isotopic mass balance in the Earth’s early sulfur cycle”, Nature Geoscience 11, 661-664 (7/23/2018), https://www.nature.com/articles/s41561-018-0184-7 (accessed and saved 11/15/18). ↩