At least some of the water in Earth’s oceans arrived here with Jupiter family comets. This conclusion was reached by scientists after analyzing samples delivered from comet 67P/Churiumov–Herasymenko. Previously, this statement was questioned.
Mystery of Earth’s water enrichment
Researchers have discovered that the water on comet 67P/Churiumov–Herasymenko has a molecular signature similar to the water in Earth’s oceans. Contrary to some recent results, this finding reaffirms that Jupiter family comets, such as 67P, could have helped bring water to Earth.
Water was essential for life to form and thrive on Earth, and it remains key to life today. Although some water probably existed in the gas and dust from which our planet materialized about 4.6 billion years ago, a greater proportion of the water would have evaporated as the Earth formed in close proximity to intense solar radiation. The way the Earth eventually became enriched with liquid water remains a source of debate for scientists.
Research showed that some of Earth’s water was formed from vapor coming out of volcanoes; this vapor condensed and rained down on the oceans. But scientists found evidence that much of our oceans were formed from ice and minerals on asteroids and possibly comets that crashed into Earth. A wave of comet and asteroid collisions with the inner planets of the Solar System 4 billion years ago would have made this possible.
Similarities between cometary and terrestrial water
While the link between asteroid and terrestrial water is very 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 believed to have formed beyond Saturn’s orbit, have shown a strong connection between their water and Earth’s. This connection is based on a key molecular signature that scientists use to trace the origin of water in the Solar System.
This signature is the ratio of deuterium (D) to ordinary 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. Compared to Earth’s water, this ratio of hydrogen in comets and asteroids can show if there is a connection between them.
Measurement of “deuterium water” in comets
“It was really starting to look like these comets played a major role in delivering water to Earth,” said Kathleen Mandt, a planet scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. Mandt led a study published in Science Advances on November 13 revisiting the deuterium content of 67P.
But in 2014, the European Space Agency’s Rosetta mission to 67P called into question the idea that comets in Jupiter’s family of comets helped fill Earth’s bodies of water. Scientists who analyzed Rosetta’s water measurements found the highest concentration of deuterium among all comets, and about three times the amount of deuterium found in Earth’s oceans, which have about 1 deuterium atom for every 6,420 hydrogen atoms.
Mandt’s team decided to use advanced statistical-computing technology to automate the labor-intensive process of isolating deuterium-rich water in the Rosetta probe’s more than 16,000 measurements. It made these measurements in the “coma” of gas and dust surrounding 67P. Mandt’s team was the first to analyze all of the European mission water measurements covering the entire mission period.
The researchers wanted to understand what physical processes caused variability in the ratio of hydrogen isotopes measured on comets. Laboratory studies and observations of comets have shown that comet dust can affect the hydrogen ratio readings that scientists find in a comet pair, which could change our understanding of where comet water comes from and how it compares to Earth’s water.
Mandt’s team found a clear link between measurements of deuterium in coma 67P and the amount of dust around the Rosetta spacecraft, suggesting that measurements taken near the spacecraft in some parts of the coma may not reflect the comet’s body composition.
Accuracy of deuterium measurements on comets
As the comet orbits closer to the Sun, its surface heats up, causing it to release gas from the surface, including dust with bits of water ice on it. Studies show that water with deuterium sticks to dust particles more easily than plain water. When the ice on these dust particles enters the comet, this effect can cause the comet to appear to contain more deuterium than it actually does.
Mandt and her team reported that by the time the dust reaches the outer part of the comet, at least 75 miles away from the comet’s body, it has dried up. When the deuterium-rich water disappears, the spacecraft can accurately measure the amount of deuterium coming from the comet’s body.
This conclusion, according to the authors of the paper, is important not only for understanding the role of comets in supplying water to Earth, but also for understanding observations of comets that provide insight into the formation of the early Solar System.
Provided by phys.org
