Scientists find that cometary dust affects interpretation of spacecraft measurements, reopening the case for comets like 67P as potential sources of water for early Earth.
Researchers have found that water on Comet 67P/Churyumov–Gerasimenko has
a similar molecular signature to the water in Earth’s oceans. Contradicting
some recent results, this finding reopens the case that Jupiter-family comets
like 67P could have helped deliver water to Earth.
Water was essential for life to form and flourish on Earth and it
remains central for Earth life today. While some water likely existed in the
gas and dust from which our planet materialized around 4.6 billion years ago,
much of the water would have vaporized because Earth formed close to the Sun’s
intense heat. How Earth ultimately became rich in liquid water has remained a
source of debate for scientists.
Research has shown that some of Earth’s water originated through vapor vented from volcanoes; that vapor condensed and rained down on the oceans. But scientists have found evidence that a substantial portion of our oceans came from the ice and minerals on asteroids, and possibly comets, that crashed into Earth. A wave of comet and asteroid collisions with the solar system’s inner planets 4 billion years ago would have made this possible.
This image, taken by ESA’s Rosetta navigation camera,
was taken from a about 53 miles from the center of Comet
67P/Churyumov-Gerasimenko on March 14, 2015. The image resolution is 24 feet
per pixel and is cropped and processed to bring out the details of the comet's
activity.
ESA/Rosetta/NAVCAM
While the case connecting asteroid
water to Earth’s is strong, the role of comets has puzzled scientists. Several
measurements of Jupiter-family comets — which contain primitive material
from the early solar system and are thought to have formed beyond the orbit of
Saturn — showed a strong link between their water and Earth’s. This link
was based on a key molecular signature scientists use to trace the origin of
water across the solar system.
This signature is the ratio of
deuterium (D) to regular hydrogen (H) in the water of any object, and it gives
scientists clues about where that object formed. Deuterium is a rare, heavier
type — or isotope — of hydrogen. When compared to Earth’s water, this
hydrogen ratio in comets and asteroids can reveal whether there’s a
connection.
Because water with deuterium is
more likely to form in cold environments, there’s a higher concentration of the
isotope on objects that formed far from the Sun, such as comets, than in
objects that formed closer to the Sun, like asteroids.
Measurements within the last couple
of decades of deuterium in the water vapor of several other Jupiter-family
comets showed similar levels to Earth’s water.
“It was really starting to look
like these comets played a major role in delivering water to Earth,” said Kathleen Mandt, planetary scientist at NASA’s Goddard Space Flight
Center in Greenbelt, Maryland. Mandt led the research, published in Science Advances on Nov. 13, that revises the abundance of
deuterium in 67P.
But in 2014, ESA’s (European Space
Agency) Rosetta mission to 67P challenged the idea that Jupiter-family comets
helped fill Earth’s water reservoir. Scientists who analyzed Rosetta’s water
measurements found the highest concentration of deuterium of any comet, and about three times more
deuterium than there is in Earth’s oceans, which have about 1 deuterium atom
for every 6,420 hydrogen atoms.
“It was a big surprise and it made
us rethink everything,” Mandt said.
Mandt’s team decided to use an
advanced statistical-computation technique to automate the laborious process of
isolating deuterium-rich water
in more than 16,000 Rosetta measurements. Rosetta made these measurements in
the “coma” of gas and dust surrounding 67P. Mandt’s team, which included
Rosetta scientists, was the first to analyze all of the European mission’s
water measurements spanning the entire mission.
The researchers wanted to understand what physical processes caused the variability in the hydrogen isotope ratios measured at comets. Lab studies and comet observations showed that cometary dust could affect the readings of the hydrogen ratio that scientists detect in comet vapor, which could change our understanding of where comet water comes from and how it compares to Earth’s water.
What are comets made of? It's one of the questions
ESA's Rosetta mission to comet 67P/Churyumov-Gerasimenko wanted to answer.
“So I was just curious if we could
find evidence for that happening at 67P,” Mandt said. “And this is just one of
those very rare cases where you propose a hypothesis and actually find it
happening.”
Indeed, Mandt’s team found a clear
connection between deuterium measurements in the coma of 67P and the amount of
dust around the Rosetta spacecraft, showing that the measurements taken near
the spacecraft in some parts of the coma may not be representative of the
composition of a comet’s body.
As a comet moves in its orbit
closer to the Sun, its surface warms up, causing gas to release from the
surface, including dust with bits of water ice on it. Water with deuterium
sticks to dust grains more readily than regular water does, research suggests.
When the ice on these dust grains is released into the coma, this effect could
make the comet appear to have more deuterium than it has.
Mandt and her team reported that by
the time dust gets to the outer part of the coma, at least 75 miles from the
comet body, it is dried out. With the deuterium-rich water gone, a spacecraft
can accurately measure the amount of deuterium coming from the comet body.
This finding, the paper authors
say, has big implications not only for understanding comets’ role in delivering
Earth’s water, but also for understanding comet observations that provide
insight into the formation of the early solar system.
“This means there is a great
opportunity to revisit our past observations and prepare for future ones so we
can better account for the dust effects,” Mandt said.
By Lonnie Shekhtman
NASA’s Goddard Space Flight Center, Greenbelt, Md.
Source: NASA-Led Team Links Comet Water to Earth’s Oceans - NASA Science
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