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Hubble Probes Earth’s Atmosphere during Total Lunar Eclipse

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Taking advantage of a total lunar eclipse in January 2019, astronomers using the NASA/ESA Hubble Space Telescope have measured the amount of ozone in the atmosphere of our planet. This method serves as a proxy for how they will observe Earth-like exoplanets transiting in front of their host stars in search of extraterrestrial life.

An artist’s impression of Hubble observing the total lunar eclipse. Image credit: NASA/ ESA / Hubble / M. Kornmesser.

Hubble did not look at Earth directly. Instead, the astronomers used the Moon as a mirror to reflect sunlight, which had passed through Earth’s atmosphere, and then reflected back towards Hubble.

Using a space telescope for eclipse observations reproduces the conditions under which future telescopes would measure atmospheres of transiting exoplanets. These atmospheres may contain chemicals of interest to astrobiology, the study of and search for life.

Though numerous ground-based observations of this kind have been done previously, this is the first time a total lunar eclipse was captured at ultraviolet wavelengths and from a space telescope.

Hubble detected the strong spectral fingerprint of ozone, which absorbs some of the sunlight.

On Earth, photosynthesis over billions of years is responsible for our planet’s high oxygen levels and thick ozone layer. That’s one reason why scientists think ozone or oxygen could be a sign of life on another planet, and refer to them as biosignatures.

“Finding ozone is significant because it is a photochemical byproduct of molecular oxygen, which is itself a byproduct of life,” said Dr. Allison Youngblood, a researcher in the Laboratory for Atmospheric and Space Physics.

This image highlights the general region where astronomers used Hubble to measure the amount of ozone in Earth’s atmosphere. Image credit: NASA / ESA / Hubble / M. Kornmesser.

This image highlights the general region where astronomers used Hubble to measure the amount of ozone in Earth’s atmosphere. Image credit: NASA / ESA / Hubble / M. Kornmesser.

Although ozone in Earth’s atmosphere had been detected in previous ground-based observations during lunar eclipses, the new study represents the strongest detection of the molecule to date because ozone — as measured from space with no interference from other chemicals in the Earth’s atmosphere — absorbs ultraviolet light so strongly.

Hubble recorded ozone absorbing some of the Sun’s ultraviolet radiation that passed through the edge of Earth’s atmosphere during a lunar eclipse that occurred on January 20 to 21, 2019.

Several other ground-based telescopes also made spectroscopic observations at other wavelengths during the eclipse, searching for more of Earth’s atmospheric ingredients, such as oxygen and methane.

“One of NASA’s major goals is to identify planets that could support life,” Dr. 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.”

The findings were published today in the Astronomical Journal.


Allison Youngblood et al. 2020. The Hubble Space Telescope’s Near-UV and Optical Transmission Spectrum of Earth as an Exoplanet. AJ 160, 100; doi: 10.3847/1538-3881/aba0b4

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