Astronomers using NASA’s Hubble Space Telescope have discovered Earth-specific sunscreen ozone in our atmosphere by taking advantage of a total moon eclipse. By looking for potential “biosignatures” on exoplanets, these technique models how astronomers and astrobiology researchers will look for proof of life elsewhere in the universe (planets around other stars).
Hubble did not directly observe Earth. Instead, they used the Moon as a mirror to send sunlight back in Hubble’s direction after it had traveled through the atmosphere of Earth.
The circumstances in which future telescopes would measure the atmospheres of transiting exoplanets are replicated by using a space telescope for eclipse observations. Astrobiology, the study of life and the hunt for it, may be interested in molecules found in these atmospheres.
This is the first total lunar eclipse to be recorded at ultraviolet wavelengths and from a space telescope, despite the fact that several ground-based observations of this kind have been made in the past.
Ozone, which filters some sunlight, has a distinct spectral imprint that Hubble has discovered. Because it is the source of the shielding that covers the Earth’s atmosphere, ozone is essential to life.
On Earth, the high oxygen content and thick ozone layer are results of photosynthesis that has occurred over billions of years. Ozone and oxygen are referred to as biosignatures by scientists because they may be indicators of life on distant planets for this reason.
“Finding ozone is significant because it is a photochemical byproduct of molecular oxygen, which is itself a byproduct of life,” explained Allison Youngblood of the Laboratory for Atmospheric and Space Physics in Boulder, Colorado, lead researcher of Hubble’s observations.
Since ozone absorbs ultraviolet light so intensely when measured from space and is not influenced by other chemicals in the Earth’s atmosphere, Hubble’s study represents the strongest detection of the molecule to date, even though ozone in Earth’s atmosphere had previously been discovered in ground-based observations made during lunar eclipses.
During a lunar eclipse that took place on January 20 to 21, 2019, Hubble captured ozone absorbing part of the Sun’s UV energy as it travelled past the edge of Earth’s atmosphere.
During the eclipse, a number of additional ground-based telescopes conducted spectroscopic investigations at various wavelengths to look for additional components of Earth’s atmosphere, such as oxygen and methane.
We think Earth had low concentrations of ozone before the mid-Proterozoic geological period (between roughly 2.0 billion to 0.7 billion years ago) when photosynthesis contributed to the build up of oxygen and ozone in the atmosphere to the levels we see today. But because the ultraviolet-light signature of ozone features is very strong, you would have a hope of detecting small amounts of ozone. The ultraviolet may therefore be the best wavelength for detecting photosynthetic life on low-oxygen exoplanets.
Giada Arney
“One of NASA’s major goals is to identify planets that could support life,” Youngblood said. “But how would we know a habitable or an uninhabited planet if we saw one? What would they look like with the techniques that astronomers have at their disposal for characterizing the atmospheres of exoplanets? That’s why it’s important to develop models of Earth’s spectrum as a template for categorizing atmospheres on extrasolar planets.”
Her paper is available online in The Astronomical Journal.
Sniffing Out Planetary Atmospheres
If an extrasolar planet transits a crossing of the parent star’s surface a phenomenon known as a transit, the atmospheres of some extrasolar planets can be studied. Starlight passes through the atmosphere of the backlit exoplanet during a transit.
(If viewed close up, the planet’s silhouette would look like it had a thin, glowing “halo” around it caused by the illuminated atmosphere, just as Earth does when seen from space.)
Certain colors of starlight are filtered out by atmospheric chemicals, leaving their own mark. Hubble-using astronomers invented this method of investigating exoplanets. This is especially surprising considering that Hubble was not primarily intended for such investigations and that extrasolar planets had not yet been identified at the time of the 1990 launch of the space observatory.
Hubble has so far been used by astronomers to study the atmospheres of gas giant planets and super-Earths (planets with several times the mass of Earth) when they pass in front of their sun. However, terrestrial planets that are comparable to Earth in size are significantly smaller bodies with thinner atmospheres than an apple’s skin.
It will be far more difficult to extract these signs from exoplanets the size of Earth. Because of this, scientists will require space telescopes that are significantly larger than Hubble in order to gather the weak starlight that transits through the atmospheres of these tiny planets.
To develop a significant signal, these telescopes will need to observe planets for a longer period of time many hundreds of hours. Astronomers chose to carry out tests on Earth, the closest and only known populated terrestrial planet, in order to get ready for these larger telescopes.
During a total lunar eclipse, our planet is perfectly aligned with the Sun and Moon, simulating the geometry of a terrestrial planet passing a star. The Moon is extremely brilliant, yet because its surface is speckled with bright and dark patches, it is not a perfect reflector, making observations difficult.
The Moon is also so close to Earth that, despite the Moon’s motion in relation to the space observatory, Hubble had to attempt and maintain a steady watch on a single area. Youngblood’s team had to adjust their study to take the Moon’s movement into account.
Where There’s Ozone, There’s Life?
Ozone in the atmosphere of a terrestrial extrasolar planet does not prove the presence of life on its surface.
“You would need other spectral signatures in addition to ozone to conclude that there was life on the planet, and these signatures cannot necessarily be seen in ultraviolet light,” Youngblood said.
When oxygen in the Earth’s atmosphere is exposed to intense amounts of ultraviolet light, ozone spontaneously develops. The Earth is shielded from harmful ultraviolet rays by an ozone layer.
“Photosynthesis might be the most productive metabolism that can evolve on any planet, because it is fueled by energy from starlight and uses cosmically abundant elements like water and carbon dioxide,” said Giada Arney of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, a co-author of the science paper. “These necessary ingredients should be common on habitable planets.”
Similar to how Earth’s seasons affect plant development, seasonal variations in the ozone signature may likewise point to biological synthesis of oxygen. But when nitrogen and oxygen are exposed to sunshine, ozone can also be created in the absence of life.
Astronomers must look for combinations of biosignatures in order to boost confidence that a particular biosignature was produced by life. Each of the several biosignatures is more readily identified at wavelengths specific to those signatures, hence a multiwavelength campaign is required.
“Astronomers will also have to take the developmental stage of the planet into account when looking at younger stars with young planets. If you wanted to detect oxygen or ozone from a planet similar to the early Earth, when there was less oxygen in our atmosphere, the spectral features in optical and infrared light aren’t strong enough,” Arney explained.
“We think Earth had low concentrations of ozone before the mid-Proterozoic geological period (between roughly 2.0 billion to 0.7 billion years ago) when photosynthesis contributed to the build up of oxygen and ozone in the atmosphere to the levels we see today. But because the ultraviolet-light signature of ozone features is very strong, you would have a hope of detecting small amounts of ozone. The ultraviolet may therefore be the best wavelength for detecting photosynthetic life on low-oxygen exoplanets.”
Similar observations in infrared light could be made by NASA’s James Webb Space Telescope, which is scheduled to launch in 2018. This telescope has the potential to find methane and oxygen in the atmospheres of exoplanets. Webb is currently scheduled to launch in 2021.
