How SOFIA Found Atomic Oxygen in the Atmosphere of Mars

SOFIA Detects Atomic Oxygen in the Martian Atmosphere

In May 2016 NASA announced that SOFIA, a telescope flown inside a modified Boeing 747, had detected atomic oxygen high in the atmosphere of Mars. It was the first measurement of this kind since the Viking and Mariner missions of the 1970s, and it found only about half as much oxygen as scientists had expected. The result was not a sign that Mars has breathable air. It was a new handle on how the thin Martian atmosphere loses energy and gas to space.

What exactly was found

The observation picked up a single spectral line, the signature that free oxygen atoms give off at a far-infrared wavelength of 63 micrometres. A team led by Ladislav Rezac of the Max Planck Institute for Solar System Research published the detection in the journal Astronomy & Astrophysics in 2015 under the title "First detection of the 63 μm atomic oxygen line in the thermosphere of Mars with GREAT/SOFIA." NASA publicised the finding the following year.

NASA's announcement placed the oxygen in the mesosphere, the upper layer of the Martian atmosphere. The research paper itself describes the signal as coming from the thermosphere, the even higher and thinner layer above it. Both are far above the dusty air near the surface that rovers sample.

Atomic oxygen is different from the oxygen we breathe. In the air on Earth, oxygen atoms come in pairs (O₂). A single oxygen atom is extremely reactive and readily bonds with almost every other element except the noble gases. On Mars, most of these lone atoms are produced when ultraviolet sunlight splits molecules of carbon dioxide, which makes up about 95 percent of the planet's atmosphere.

Why it took 40 years

The difficulty was not Mars but Earth. Water vapour in our own lower atmosphere absorbs far-infrared light, so ground-based telescopes cannot see the 63-micrometre line clearly. SOFIA project scientist Pamela Marcum put it simply: "Atomic oxygen in the Martian atmosphere is notoriously difficult to measure."

The answer was to fly above most of that water. SOFIA, the Stratospheric Observatory for Infrared Astronomy, was a Boeing 747SP with a hole cut in the rear of the fuselage and a telescope with a 2.5-metre mirror behind a sliding door. It observed from roughly 11 to 14 kilometres up (about 37,000 to 45,000 feet). The aircraft was a joint project of NASA and the German Aerospace Center, DLR, which paid about a fifth of the costs.

Flying high was only half the trick. Earth's air also contains oxygen, so the instrument had to tell Martian atoms from terrestrial ones. That job fell to GREAT, the German Receiver for Astronomy at Terahertz Frequencies, a spectrometer sensitive enough to separate the faint Martian signal from the much stronger local one.

Why one gas matters so much

Mars once had a thicker atmosphere and, almost certainly, liquid water on its surface. Today its surface pressure is less than one percent of Earth's at sea level. Working out where the rest went is one of the central questions of Mars science, and atomic oxygen is part of the answer.

High in the atmosphere, oxygen atoms collide with carbon dioxide molecules and help the gas radiate heat away to space. That cooling affects how warm and how puffed-up the upper atmosphere becomes, which in turn affects how easily gases leak away. Oxygen itself also escapes, particularly when the solar wind strikes the upper atmosphere, which is exactly what NASA's MAVEN orbiter, launched in 2013, was sent to study. A reliable measurement of atomic oxygen therefore helps researchers test the computer models they use to reconstruct the planet's past climate.

Half the expected amount: what it means

The SOFIA team found about half the atomic oxygen that their models predicted. That did not mean the models were broken. The amount of oxygen in the Martian upper atmosphere is expected to change with season, time of day, latitude and solar activity, and a single set of observations cannot capture all of that. The team said further flights would look for those variations. It is a good example of how a first measurement often raises new questions rather than settling old ones.

Could we breathe on Mars? Common misunderstandings

Headlines such as "NASA detects oxygen on Mars" led many readers to imagine a planet slowly becoming habitable. The reality is far less dramatic.

The oxygen was not breathable. Single oxygen atoms high in the sky are not the O₂ our lungs need, and molecular oxygen makes up only a tiny fraction of a percent of the Martian air.
It was not new oxygen. Viking and Mariner had detected oxygen decades earlier; SOFIA measured it again with a new method.
Terraforming remains speculation. A 2018 NASA-funded study concluded that Mars does not hold enough accessible carbon dioxide to warm the planet into a habitable state with today's technology.

The short video that accompanied this story shows one popular vision of a terraformed Mars. Treat it as an imaginative idea, not a plan.

From finding oxygen to making it

Five years later, a very different experiment showed how oxygen on Mars might actually be useful. MOXIE, an experimental instrument aboard NASA's Perseverance rover, made its first oxygen on 20 April 2021 in Jezero Crater by pulling carbon dioxide from the Martian air and splitting it with heat and electricity. By its 16th and final run on 7 August 2023 it had produced 122 grams of oxygen in total, at purities of 98 percent or better and at a peak rate of about 12 grams an hour, twice its original goal. Scaled up, the same technique could one day supply rocket propellant and breathing air for crews.

SOFIA itself did not last that long. After more than a decade of science, including the 2020 discovery of water molecules on the sunlit surface of the Moon, NASA and DLR ended the programme in 2022 following a review that criticised its cost relative to its scientific output. The aircraft flew its last science mission in September 2022 and is now displayed at the Pima Air & Space Museum near Tucson, Arizona.

The 2016 announcement is best remembered as a clever piece of observing: a jumbo jet carrying a telescope above the clouds to catch a whisper of oxygen from a world that never comes closer than about 55 million kilometres, and a reminder of how much we still have to learn about how Mars lost its air.

Sources: Sci.News, 10 May 2016: NASA's SOFIA detects atomic oxygen in upper atmosphere of Mars; Universe Today: Winged telescope detects Martian atomic oxygen; Wikipedia: Stratospheric Observatory for Infrared Astronomy; Wikipedia: Mars Oxygen ISRU Experiment (MOXIE). First shared on this site from ScienceAlert; video by Futurism.

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