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Comet 67P's Water: New Research Links Cometary Origins to Earth's Oceans

Recent research reveals that the water on Comet 67P/Churyumov–Gerasimenko shares a molecular composition similar to Earth's ocean water. This discovery challenges recent findings and reignites the hypothesis that Jupiter-family comets, such as 67P, may have played a role in delivering water to Earth.

The Origins of Earth's Water

Water has always been fundamental to the emergence and sustenance of life on Earth. Although some water may have been present in the gas and dust that formed Earth around 4.6 billions years ago, much of it likely vaporized due to the planet's proximity to the sun. The origins of Earth's abundant liquid water remain a topic of scientific investigation.

Volcanic Activity and Asteroid Impact as Sources of Water

Research indicates that volcanic eruptions released vapor that condensed into rain, filling Earth's early oceans. Yet, scientists have also uncovered evidence suggesting that much of our planet's water was delivered by asteroid and comet impact. A surge in such collisions with the inner planets around 4 billion years ago likely played a pivotal role in this process.

The Role of Comets in Water Delivery

Though the connection between asteroid water and Earth's is well-supported, the role of comets remains an enigma. Jupiter-family comets, containing primitive material from beyond Saturn's orbit, have shown molecular signatures linking their water to Earth's. These signatures serve as a crucial tool for tracing water's origin across the solar system.

Understanding the Deuterium-to-Hydrogen (D/H) Ratio

The Deuterium-to-Hydrogen (D/H) ratio in water serves as a molecular signature, offering scientists insights into an object's formation region. Deuterium, a heavier hydrogen isotope, provides a critical comparison point. By analyzing the D/H ratio in comets and asteroids relative to Earth's water, scientists can evaluate potential connections.

Deuterium Enrichment in Comets

Water enriched with deuterium predominantly forms in colder regions, leading to higher concentrations of the isotope in distant objects like comets compared to those formed nearer the sun, such as asteroids.

Recent Findings from the Rosetta Mission

Over the past two decades, measurements of deuterium in the water vapor of various Jupiter-family comets have revealed levels comparable to those in Earth's water.

Kathleen Mandt's New Study on Comet 67P

"These findings increasingly suggested that comets significantly contributed to Earth's water supply," remarked Kathleen Mandt, a planetary scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. Mandt spearheaded the study, published in Science Advances on November 13, which reevaluates deuterium levels in Comet 67P.

The Unexpected Discovery in 2014

In 2014, the European Space Agency's (ESA) Rosetta mission to Comet 67P questioned the hypothesis that Jupiter-family comets contributed significantly to Earth's water reservoir. Rosetta's water measurements revealed the highest deuterium concentration recorded in a comet, approximately three times higher than Earth's oceans, where the ratio is about one deuterium atom per 6,420 hydrogen atoms.

What constitutes the composition of comets? This was a central question ESA's Rosetta mission to Comet 67P/Churyumov-Gerasimenko sought to address. Credit: NASA.

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"It was an unexpected revelation that forced us to reevaluate everything we thought we knew," Mandt explained.

Reevaluating Deuterium Levels with Advanced Techniques

To streamline the complex task of identifying deuterium-rich water in over 16,000 Rosetta measurements, Mandt's team employed an advanced statistical-computation technique. These measurements, taken within the gas and dust 'coma' surrounding 67P, were analyzed comprehensively for the first time by Mandt's team, including Rosetta mission scientists.

Dust's Impact on Isotope Measurements

Researchers sought to ex plore the physical mechanisms responsible for variations in hydrogen isotope ratios in comets. Their lab experiments and observations indicated that dust in comets could impact the hydrogen ratio readings detected in cometary vapor, potentially reshaping our knowledge of the origins of cometary water and its similarity to Earth's water.

New Insights from Comet Dust Interaction

"I was intrigued to see if we could find proof of that occurring at 67P," said Mandt. "It's an uncommon situation where you hypothesize something and actually observe it happening."

New Discoveries About Deuterium in Comet Dust

Mandt's team discovered a distinct link between deuterium levels in the coma of 67P and the dust surrounding the Rosetta spacecraft. This finding indicates that certain measurements taken near the spacecraft in parts of the coma may not accurately reflect the comet's overall composition.

Dust Dehydration and Its Effect on Measurements

As a comet travels closer to the sun its orbit, its surface heats up, leading to the release of gases and dust containing water ice. Research indicates that deuterium-enriched water binds more readily to dust grains than regular water. This can result in an overestimation of deuterium levels when the ice from these grains is released into the coma.

Dehydration of Dust Beyond 75 Miles from the Comet

Mandt and her team discovered that by the time dust travels to the outer region of the coma, over 75 miles from the comet's core, it becomes dehydrated. Once the deuterium-rich water is lost, spacecraft can measure the deuterium directly from the comet's body with precision.

Implications for Understanding Earth's Water Origins

According to the paper's authors, this findings is crucial for understanding both the contribution of comets to Earth's water supply and the insights that comet observations provide into the early solar system's formation.

Revisiting Observations for Future Research

Mandt mentioned that this discovery provides an excellent opportunity to revisit earlier observations and make necessary preparations for future ones, enhancing the consideration of dust effects.

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