A new insight into the extraterrestrial origins of our lakes, rivers and oceans
From: University of Maryland
March 15, 2023 -- A new
study brings scientists one step closer to answering the question of where
Earth's water came from.
Water makes up 71% of
Earth's surface, but no one knows how or when such massive quantities of water
arrived on Earth.
A new study published
in the journal Nature brings scientists one step closer to
answering that question. Led by University of Maryland Assistant Professor of
Geology Megan Newcombe, researchers analyzed melted meteorites that had been
floating around in space since the solar system's formation 4 1/2 billion years
ago. They found that these meteorites had extremely low water content -- in
fact, they were among the driest extraterrestrial materials ever measured.
These results, which
let researchers rule them out as the primary source of Earth's water, could
have important implications for the search for water -- and life -- on other
planets. It also helps researchers understand the unlikely conditions that
aligned to make Earth a habitable planet.
"We wanted to
understand how our planet managed to get water because it's not completely
obvious," Newcombe said. "Getting water and having surface oceans on
a planet that is small and relatively near the sun is a challenge."
The team of researchers
analyzed seven melted, or achondrite, meteorites that crashed into Earth
billions of years after splintering from at least five planetesimals -- objects
that collided to form the planets in our solar system. In a process known as
melting, many of these planetesimals were heated up by the decay of radioactive
elements in the early solar system's history, causing them to separate into
layers with a crust, mantle and core.
Because these
meteorites fell to Earth only recently, this experiment was the first time
anyone had ever measured their volatiles. UMD geology graduate student Liam
Peterson used an electron microprobe to measure their levels of magnesium,
iron, calcium and silicon, then joined Newcombe at the Carnegie Institution for
Science's Earth and Planets Laboratory to measure their water contents with a
secondary ion mass spectrometry instrument.
"The challenge of
analyzing water in extremely dry materials is that any terrestrial water on the
sample's surface or inside the measuring instrument can easily be detected,
tainting the results," said study co-author Conel Alexander, a scientist
at the Carnegie Institution for Science.
To reduce
contamination, researchers first baked their samples in a low-temperature
vacuum oven to remove any surface water. Before the samples could be analyzed
in the secondary ion mass spectrometer, the samples had to be dried out once
again.
"I had to leave
the samples under a turbo pump -- a really high-quality vacuum -- for more than
a month to draw down the terrestrial water enough," Newcombe said.
Some of their meteorite
samples came from the inner solar system, where Earth is located and where
conditions are generally assumed to have been warm and dry. Other rarer samples
came from the colder, icier outer reaches of our planetary system. While it was
generally thought that water came to Earth from the outer solar system, it has
yet to be determined what types of objects could have carried that water across
the solar system.
"We knew that
plenty of outer solar system objects were differentiated, but it was sort of
implicitly assumed that because they were from the outer solar system, they
must also contain a lot of water," said Sune Nielsen, a study co-author
and geologist at the Woods Hole Oceanographic Institution. "Our paper
shows this is definitely not the case. As soon as meteorites melt, there is no
remaining water."
After analyzing the
achondrite meteorite samples, researchers discovered that water comprised less
than two millionths of their mass. For comparison, the wettest meteorites -- a
group called carbonaceous chondrites -- contain up to about 20% of water by
weight, or 100,000 times more than the meteorite samples studied by Newcombe
and her co-authors.
This means that the
heating and melting of planetesimals leads to near-total water loss, regardless
of where these planetesimals originated in the solar system and how much water
they started out with. Newcombe and her co-authors discovered that, contrary to
popular belief, not all outer solar system objects are rich in water. This led
them to conclude that water was likely delivered to Earth via unmelted, or
chondritic, meteorites.
Newcombe said their
findings have applications beyond geology. Scientists of many disciplines --
and especially exoplanet researchers -- are interested in the origin of Earth's
water because of its deep connections with life.
"Water is
considered to be an ingredient for life to be able to flourish, so as we're
looking out into the universe and finding all of these exoplanets, we're
starting to work out which of those planetary systems could be potential hosts
for life," Newcombe said. "In order to be able to understand these
other solar systems, we want to understand our own."
https://www.sciencedaily.com/releases/2023/03/230315132425.htm#
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