The mystery of why Earth has so much water, allowing our “blue
marble” to support an astounding array of life, is clearer with new
research into comets. Comets are like snowballs of rock, dust, ice, and other
frozen chemicals that vaporize as they get closer to the Sun, producing the
tails seen in images. A new study reveals that the
water in many comets may share a common origin with Earth’s oceans, reinforcing
the idea that comets played a key role in bringing water to our planet billions
of years ago.
The Stratospheric Observatory for Infrared Astronomy, SOFIA, the world’s largest airborne observatory, observed
Comet Wirtanen as it made its closest approach to Earth inDecember
2018. Data collected from the high-flying observatory
found that this comet contains “ocean-like” water. Comparing
this with information about other comets, scientists suggest in a new study
that many more comets than previously thought could have delivered water to
Earth. The findings were published in Astronomy and
Astrophysics Letters.
“We have identified a vast
reservoir of Earth-like water in the outer reaches of the solar system,”
said Darek Lis, a scientist at NASA’s Jet Propulsion Laboratory, in Pasadena,
California, and lead author of the study. “Water was crucial for the
development of life as we know it. We not only want to understand how Earth’s
water was delivered, but also if this process could work in other planetary
systems.”
Dirty Snowballs
Planets
form from debris orbiting in a disk shape around a
star; small pieces of debris can stick together and grow larger over time.
Leftover debris remains in regions of our own solar system like the Kuiper Belt,
beyond Neptune, or theOort Cloud,
far past Pluto. Comets come from these areas, but we can only see them when
their orbits bring them closer to the Sun. The heat from the Sun causes some of
the dirty snow to vaporize, creating the fuzzy halo or “coma” of
water vapor, dust and ice grains seen in comet images.
Scientists predict that the water in Earth’s
oceans came from water-carrying bodies in the early solar system that collided
with our planet, similar to today’s ice-rich asteroids or comets. But
scientists do not know where in the formative disk these objects originated.
Water
Types
Water is also known by its chemical name H2O
because it’s made of two hydrogen atoms and one oxygen atom. But using special
instruments, scientists can detect two types: regular water, H2O,
and heavy water, HDO, which has an extra neutrally-charged particle called a
neutron inside one of the hydrogen atoms. Scientists compare the amount of
heavy to regular water in comets. If comets have the same ratio of these water
types as Earth’s oceans, it indicates that the water in both may share a common
origin.
But measuring this ratio is difficult. Ground
and space telescopes can study this level of detail in comets only when they
pass near Earth, and missions to visit comets, like Rosetta, are rare. Scientists have only been able to study this ratio in about
a dozen comets since the 1980s. Additionally, it is difficult
to study a comet’s water from the ground because water in Earth’s atmosphere blocks
its signatures.
New Observations
Observing at high altitudes above much of the
Earth’s atmospheric water allowed SOFIA to accurately measure the ratio of
regular to heavy water in Comet Wirtanen. The data showed that Comet Wirtanen’s
water ratio is the same as the Earth’s oceans.
When the team compared the new SOFIA data with previous
studies of comets, they found a surprising commonality. The ratio of regular to
heavy water was not linked to the origin of the comets – whether they were from
the Oort Cloud or the Kuiper Belt. Instead, it was related to how much water
was released from ice grains in the comet’s coma compared to directly from the
snowy surface. This could imply that all comets could have a heavy-to-regular
water ratio similar to Earth’s oceans, and that they could have delivered a
large fraction of water to Earth.
“This is the first
time we could relate the heavy-to-regular water ratio of all comets to a single
factor,” noted Dominique Bockelée-Morvan, scientist at the Paris
Observatory and the French National Center for Scientific Research and second
author of the paper. “We may need to rethink how we study comets because
water released from the ice grains appears to be a better indicator of the
overall water ratio than the water released from surface ice.”
More studies are needed to see if these findings
hold true for other comets. The next time a comet is forecast to fly close
enough for this type of study will be in November 2021.
SOFIA, the Stratospheric Observatory
for Infrared Astronomy, is a Boeing 747SP jetliner modified to carry a 106-inch
diameter telescope. It is a joint project of NASA and the German Aerospace
Center, DLR. NASA’s Ames Research Center in California’s Silicon Valley manages
the SOFIA program, science and mission operations in cooperation with the
Universities Space Research Association headquartered in Columbia, Maryland,
and the German SOFIA Institute (DSI) at the University of Stuttgart. The
aircraft is maintained and operated from NASA’s Armstrong Flight Research
Center Building 703, in Palmdale, California.
News Media Contact
Nicholas Veronico
SOFIA Science Center
Ames Research Center, Silicon Valley, California
650-604-4589 / 650-224-8726
nicholas.a.veronico@nasa.gov
Elizabeth Landau
NASA Headquarters, Washington
818-359-3241
elandau@jpl.nasa.gov
Calla Cofield
Jet Propulsion Laboratory, Pasadena, Calif.
626-808-2469
calla.e.cofield@jpl.nasa.gov
2019-097
Comet Provides New Clues to Origins of Earth"s Oceans - Jet Propulsion Laboratory
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