Scientists first used the moon as a mirror to study the Earth’s atmosphere

Taking advantage of the total lunar eclipse, astronomers using NASA’s Hubble Space Telescope discovered ozone in the Earth’s atmosphere. This method simulates how astronomers and astrobiological researchers will seek evidence of life beyond Earth by observing potential “biosignatures” on exoplanets (planets around other stars). The study is published by the Astronomical Journal.

Hubble did not look directly at Earth. Instead, astronomers used the moon as a mirror to reflect sunlight that passed through Earth’s atmosphere and then bounced back towards the Hubble. Using a space telescope to observe eclipses mimics the conditions under which future telescopes would measure exoplanet atmospheres. These atmospheres may contain chemicals of interest to astrobiology and the study and search for life.

Although there have been numerous ground-based observations of this kind, this is the first time that a total lunar eclipse has been recorded in the ultraviolet wavelength range and using a space telescope. Hubble has found a strong spectral imprint of ozone that absorbs some of the sunlight. Ozone is important for life because it acts as a “protective shield” in the Earth’s atmosphere.



On Earth, photosynthesis has been responsible for billions of years of high oxygen levels on our planet and a thick ozone layer. This is one of the reasons why scientists believe that ozone or oxygen can be signs of life on another planet, and they call them biosignatures.

Detecting ozone is important because it is a photochemical byproduct of molecular oxygen, which itself is a byproduct of life, the scientists emphasize.

Although ozone in the Earth’s atmosphere has been detected in previous ground-based observations during lunar eclipses, the Hubble study represents the most important molecular detection to date. Ozone, measured from space without interfering with other chemicals in the Earth’s atmosphere, absorbs ultraviolet light particularly strongly.

Hubble recorded ozone absorbing some of the sun’s ultraviolet radiation that passed over the edge of the Earth’s atmosphere during a lunar eclipse that occurred from 20 to 21 January 2019. Several other ground-based telescopes have also performed spectroscopic observations at other wavelengths during the eclipse, looking for more of the Earth’s atmospheric components such as oxygen and methane.

One of NASA’s primary goals is to identify planets on which life could exist. But how would we recognize an inhabited or uninhabited planet if we saw it? What would they look like with the methods astronomers have at their disposal for characterizing exoplanet atmospheres? This is why it is so important to develop models of the Earth’s spectrum as a template for categorizing atmospheres on extrasolar planets.

Allison Youngblood of the Laboratory for Atmospheric and Space Physics in Boulder, Colorado, Lead Observing Investigator for Hubble

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