Hubble uses Earth as proxy for identifying oxygen on potentially
habitable exoplanets

Date:
August 6, 2020
Source:
NASA/Goddard Space Flight Center
Summary:
Taking advantage of a total lunar eclipse, Hubble used the Moon
as a mirror to study sunlight that had passed through Earth's
atmosphere. As a result, Hubble detected Earth's own brand of
sunscreen - ozone - in our atmosphere. The technique simulates
how scientists will search for evidence of life on planets around
other stars.



FULL STORY ========================================================================== Taking advantage of a total lunar eclipse, astronomers using NASA's Hubble Space Telescope have detected Earth's own brand of sunscreen -- ozone --
in our atmosphere. This method simulates how astronomers and astrobiology researchers will search for evidence of life beyond Earth by observing potential "biosignatures" on exoplanets (planets around other stars).


========================================================================== 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. Ozone is important to life because it is the source of the
protective shield in Earth's atmosphere.

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," explained
Allison Youngblood of the Laboratory for Atmospheric and Space Physics
in Boulder, Colorado, lead researcher of Hubble's observations.

Although ozone in Earth's atmosphere had been detected in previous
ground-based observations during lunar eclipses, Hubble's 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," 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 The atmospheres of some extrasolar
planets can be probed if the alien world passes across the face of its
parent star, an event called a transit. During a transit, starlight
filters through the backlit exoplanet's atmosphere. (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.) Chemicals in the atmosphere leave their telltale
signature by filtering out certain colors of starlight. Astronomers
using Hubble pioneered this technique for probing exoplanets. This is particularly remarkable because extrasolar planets had not yet been
discovered when Hubble was launched in 1990 and the space observatory
was not initially designed for such experiments.



==========================================================================
So far, astronomers have used Hubble to observe the atmospheres of gas
giant planets and super-Earths (planets several times Earth's mass)
that transit their stars. But terrestrial planets about the size of
Earth are much smaller objects and their atmospheres are thinner, like
the skin on an apple.

Therefore, teasing out these signatures from Earth-sized exoplanets will
be much harder.

That's why researchers will need space telescopes much larger than
Hubble to collect the feeble starlight passing through these small
planets' atmospheres during a transit. These telescopes will need to
observe planets for a longer period, many dozens of hours, to build up
a strong signal.

To prepare for these bigger telescopes, astronomers decided to conduct experiments on a much closer and only known inhabited terrestrial planet: Earth. Our planet's perfect alignment with the Sun and Moon during a
total lunar eclipse mimics the geometry of a terrestrial planet transiting
its star.

But the observations were also challenging because the Moon is very
bright, and its surface is not a perfect reflector because it is mottled
with bright and dark areas. The Moon is also so close to Earth that
Hubble had to try and keep a steady eye on one select region, despite
the Moon's motion relative to the space observatory. So, Youngblood's
team had to account for the Moon's drift in their analysis.

Where There's Ozone, There's Life? Finding ozone in the skies of a
terrestrial extrasolar planet does not guarantee that life exists on
the 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.

On Earth, ozone is formed naturally when oxygen in the Earth's atmosphere
is exposed to strong concentrations of ultraviolet light. Ozone forms
a blanket around Earth, protecting it from harsh ultraviolet rays.

"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." Seasonal variability in the ozone
signature also could indicate seasonal biological production of oxygen,
just as it does with the growth seasons of plants on Earth.

But ozone can also be produced without the presence of life when nitrogen
and oxygen are exposed to sunlight. To increase confidence that a given biosignature is truly produced by life, astronomers must search for combinations of biosignatures. A multiwavelength campaign is needed
because each of the many biosignatures are more easily detected at
wavelengths specific to those signatures.

"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." NASA has a forthcoming observatory called the
James Webb Space Telescope that could make similar kinds of measurements
in infrared light, with the potential to detect methane and oxygen in
exoplanet atmospheres. Webb is currently scheduled to launch in 2021.

Video: https://www.youtube.com/watch?v=OHbiPO8bAts&feature=emb_logo

========================================================================== Story Source: Materials provided by
NASA/Goddard_Space_Flight_Center. Note: Content may be edited for style
and length.


========================================================================== Journal Reference:
1. Allison Youngblood, Giada N. Arney, Antonio Garci'a Mun~oz, John T.

Stocke, Kevin France, and Aki Roberge. The Hubble Space
Telescope's Near- UV and Optical Transmission Spectrum of
Earth as an Exoplanet. The Astronomical Journal, 2020 DOI:
10.3847/1538-3881/aba0b4 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200806122835.htm

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