Volcanoes display the awesome power of Nature like few other
events. Earlier this year, ash from an Icelandic volcano disrupted
air travel throughout much of northern Europe. Yet this recent
eruption pales next to the fury of Jupiter's moon Io, the most
volcanic body in our solar system.
Now that astronomers are finding rocky worlds orbiting distant
stars, they're asking the next logical questions: Do any of those
worlds have volcanoes? And if so, could we detect them? Work by
theorists at the Harvard-Smithsonian Center for Astrophysics
suggests that the answer to the latter is a qualified "Yes."
"You would need something truly earthshaking, an eruption that
dumped a lot of gases into the atmosphere," said Smithsonian
astronomer Lisa Kaltenegger. "Using the James Webb Space Telescope,
we could spot an eruption 10 to 100 times the size of Pinatubo for
the closest stars," she added.
Astronomers are decades away from being able to image the
surface of an alien world, or exoplanet. However, in a few cases
they have been able to detect exoplanet atmospheres for gas giants
known as "hot Jupiters." An eruption sends out fumes and various
gases, so volcanic activity on a rocky exoplanet might leave a
telltale atmospheric signature.
To examine which volcanic gases might be detectable, Kaltenegger
and her Harvard colleagues, Wade Henning and Dimitar Sasselov,
developed a model for eruptions on an Earth-like exoplanet based on
the present-day Earth. They found that sulfur dioxide from a very
large, explosive eruption is potentially measurable because a lot
is produced and it is slow to wash out of the air.
"Our first sniffs of volcanoes from an alien Earth might be
pretty rank!" Kaltenegger said. "Seeing a volcanic eruption on an
exoplanet will show us similarities or differences among rocky
worlds."
The 1991 eruption of Mount Pinatubo in the Philippines spewed
about 17 million tons of sulfur dioxide into the stratosphere - a
layer of air 6 to 30 miles above Earth's surface. The largest
volcanic eruption in recorded history, the 1815 Tambora event, was
about 10 times more powerful.
Such gigantic eruptions are infrequent, so astronomers would
have to monitor many Earth-sized planets for years to catch one in
the act. However, if alien worlds are more volcanically active than
Earth, success might be more likely.
"A Tambora-sized eruption doesn't happen often here, but could
be more common on a younger planet, or a strongly tidally active
planet - analogous to Io," said Henning. "Once you detected one
eruption, you could keep watch for further ones, to learn if
frequent eruptions are common on other planets."
To look for volcanic sulfur dioxide, astronomers would rely on a
technique known as the secondary eclipse, which requires the
exoplanet to cross behind its star as seen from Earth. By
collecting light from the star and planet, then subtracting the
light from the star (while the planet is hidden), astronomers are
left with the signal from the planet alone. They can search that
signal for signs of particular chemical molecules.
Due to its proximity, a hypothetical Earth or super-Earth
orbiting Alpha Centauri would offer a best-case scenario for a
sun-like star. A super-Earth orbiting a smaller host star close to
our own Sun would show the biggest signal. But any Earth-like
planet less than 30 light-years away could show faint signs of
volcanism when studied with the James Webb Space Telescope.
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