mirror, mirror

Hubble telescope uses the Moon as a mirror to study Earth

Looking at our own planet can help us find life on other worlds.

On January 20-21, 2019, planet Earth came between the Sun and the Moon, draping the rocky body with its shadow in a total lunar eclipse.

The Moon may have appeared dim to us here on Earth, but it served as a giant lunar mirror from space.

NASA's Hubble Space Telescope used the Moon to reflect sunlight in order to observe Earth's atmosphere, detecting the fingerprint of the planet's ozone. By looking at Earth's ozone, scientists will be able to detect the conditions capable of hosting life on other worlds beyond our own.

Hubble's observations were detailed in a study published this week in The Astronomical Journal.

This diagram illustrates the total lunar eclipse, showing the sunlight reflected off of the Moon.

M. Kornmesser (ESA/Hubble), NASA, and ESA

Hubble was launched in 1990, and the space telescope has been roaming low Earth orbit ever since.

In order to observe Earth's atmosphere, the telescope did not look directly at the planet but rather used the Moon to reflect sunlight that had passed through Earth's atmosphere during the total lunar eclipse as the planet became wedged between the Moon and the Sun.

Astronomers have used this method before, but this is the first time a total lunar eclipse was captured in ultraviolet wavelengths, which is somewhere between visible light and X-rays, from a space telescope.

By doing so, the space telescope was able to detect the spectral fingerprint of ozone. Ozone is a gas made up of three oxygen atoms in Earth's upper atmosphere, and serves as a protective shield from the Sun's ultraviolet radiation.

Ozone forms naturally when oxygen in Earth's atmosphere is exposed to strong concentrations of ultraviolet light, acting as a blanket around our planet.

"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," Giada Arney, a scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, and co-author of the new study, said in a statement. "These necessary ingredients should be common on habitable planets."

As scientists look for signs of life on other planets, ozone serves as a strong indication of habitability in other worlds.

"Finding ozone is significant because it is a photochemical byproduct of molecular oxygen, which is itself a byproduct of life," Allison Youngblood, a researcher at the Laboratory for Atmospheric and Space Physics in Boulder, Colorado, and lead author of the new study, said in a statement.

However, finding oxygen alone is not an indication that life abounds on another planet, there are plenty of other factors to consider before astronomers can confirm an exoplanet's habitability.

As exoplanets transit in front of their host stars, telescopes detect the signatures of chemicals in the planets' atmospheres as they filter out certain colors in the star's light. However, smaller planets similar to Earth have a thinner atmosphere, which makes this type of detection much more difficult.

Therefore, larger telescopes will be needed in order to look at smaller exoplanets.

The recent observations by Hubble of Earth is part of ongoing experiments to perfect these types of observations since our planet is the only small, rocky world that we know of that hosts life. Earth serves as the perfect, and only, analogue, for finding life on other planets.

Abstract: We observed the 2019 January total lunar eclipse with the Hubble Space Telescope's STIS spectrograph to obtain the first near-UV (1700–3200 Å) observation of Earth as a transiting exoplanet. The observatories and instruments that will be able to perform transmission spectroscopy of exo-Earths are beginning to be planned, and characterizing the transmission spectrum of Earth is vital to ensuring that key spectral features (e.g., ozone, or O3) are appropriately captured in mission concept studies. 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. We describe the observations and methods used to extract a transmission spectrum from Hubble lunar eclipse spectra, and identify spectral features of O3 and Rayleigh scattering in the 3000–5500 Å region in Earth's transmission spectrum by comparing to Earth models that include refraction effects in the terrestrial atmosphere during a lunar eclipse. Our near-UV spectra are featureless, a consequence of missing the narrow time span during the eclipse when near-UV sunlight is not completely attenuated through Earth's atmosphere due to extremely strong O3 absorption and when sunlight is transmitted to the lunar surface at altitudes where it passes through the O3 layer rather than above it.
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