This recent lunar eclipse provided astronomers a chance to test an observational technique on a familiar target — our own homeworld. The precise alignment of the Sun, Earth, and Moon during a total eclipse is similar to the arrangement of exoplanets as they transit — pass “in front of” — their star as seen from the Earth.
The Earth is far too bright to be directly imaged by Hubble. Therefore, astronomers looked at light, reflected off the Moon, which had passed through the atmosphere of Earth.
Prior to the eclipse, the Moon shone brightly. Engineers directed Hubble to direct its sights on one small region of the lunar surface, carefully tracking its motion relative to the orbiting observatory.
Observations revealed the presence of ozone (O3) in the atmosphere of our homeworld. Although hardly a shocking revelation in itself, the technique used in this measurement may be applied to worlds orbiting other stars.
“O3 is photochemically produced from O2, a product of the dominant metabolism on Earth today, and it will be sought in future observations as critical evidence for life on exoplanets. Ground-based observations of lunar eclipses have provided the Earth’s transmission spectrum at optical and near-IR wavelengths, but the strongest O3 signatures are in the near-UV,” researchers described in The Astronomical Journal.
Ozone signatures had previously been detected during eclipses using ground-based telescopes. However, ultraviolet light is largely blocked by the atmosphere of our world, making this the strongest detection of the atmospheric molecule ever recorded.
Photosynthesis drives the production of ozone in the atmosphere of ancient Earth, building up high levels of oxygen and a significant ozone layer on our world. By 600 million years ago, atmospheric ozone had built up to the point where the surface of the Earth became shielded from the deleterious effect of ultraviolet light from the Sun, allowing lifeforms to crawl from the ancient oceans onto the barren land for the first time.
“Finding ozone in the spectrum of an exo-Earth would be significant because it is a photochemical byproduct of molecular oxygen, which is a byproduct of life,” Allison Youngblood of the Laboratory for Atmospheric and Space Physics, lead researcher on the Hubble observations, stated.
In addition to ozone, researchers also tested the technique, looking for signs of oxygen, methane, carbon monoxide, and water.