Comets may have delivered water to Earth

Scientists have reopened the debate about comets as potential sources of Earth’s water.

A recent study challenges earlier findings and sheds light on the influence of cometary dust on spacecraft measurements, particularly focusing on Comet 67P/Churyumov-Gerasimenko.

The research may reshape our understanding of water’s origins on Earth and the early solar system.

Mystery of Earth’s water origins

Water is vital for life, yet its origin on Earth remains a topic of debate. Earth’s proximity to the Sun during its formation likely caused much of its initial water to vaporize.

Over time, water condensed from volcanic vapor and rained down, contributing to oceans. Additionally, asteroids and possibly comets delivered ice and minerals during a period of intense collisions around 4 billion years ago.

While the asteroid hypothesis has gained strong support, the role of comets remains unclear.

Earth’s water and extraterrestrial sources

Scientists use the deuterium-to-hydrogen (D/H) ratio to trace water’s origin. Deuterium, a heavier isotope of hydrogen, forms more readily in colder environments.

Objects like comets, formed far from the Sun, typically have higher D/H ratios than objects formed closer, like asteroids. By comparing these ratios, researchers can identify potential links between Earth’s water and extraterrestrial sources.

Jupiter-family comets, including 67P, contain primitive material from the early solar system. Initial measurements of their D/H ratios suggested a match with Earth’s water, bolstering the case for comets as water carriers.

“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.

Twist in the comet-water connection

The European Space Agency’s Rosetta mission, launched in 2004, studied 67P closely. However, its findings in 2014 contradicted earlier studies.

Rosetta’s measurements revealed a much higher D/H ratio in 67P’s water vapor – three times that of Earth’s oceans. “It was a big surprise and it made us rethink everything,” Mandt explained.

This discrepancy led Mandt’s team to reanalyze over 16,000 Rosetta measurements. Using advanced statistical techniques, they identified a significant factor affecting D/H readings: cometary dust.

Comet dust distorts water origins

Cometary dust influences the accuracy of D/H ratio measurements. As a comet approaches the Sun, its surface warms, releasing gas and dust. Water with deuterium adheres to dust particles more readily than regular water.

When these dust grains release their ice into the coma (the gas and dust cloud surrounding the comet), they skew D/H measurements. Mandt’s team found that measurements taken near Rosetta often reflected dust contamination rather than the comet’s true composition.

“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.”

The analysis showed that by the time dust particles reach the outer coma – about 75 miles from the comet’s surface – the deuterium-rich water is gone. At this distance, spacecraft can accurately measure the comet’s true D/H ratio.

Implications for comet studies

This research suggests that earlier D/H measurements of 67P and similar comets may need revisiting.

The dust effect likely influenced past observations, affecting conclusions about the roles of comets in Earth’s water delivery. By accounting for these dust effects, scientists can refine their understanding of comet composition and their contributions to 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 noted.

Potential of Jupiter-family comets

With these findings, Jupiter-family comets like 67P could once again be considered potential sources of Earth’s water.

Their D/H ratios, when measured accurately, might align more closely with Earth’s oceans than previously thought. This research not only reshapes our view of Earth’s water origins but also highlights the need for improved techniques in future comet studies.

Understanding the role of comets in delivering water to Earth remains a complex puzzle. By addressing the influence of cometary dust on spacecraft measurements, scientists have taken a crucial step forward.

As researchers revisit past data and plan future missions, comets may prove to be key players in the story of Earth’s water and the formation of the solar system.

The study is published in the journal Science Advances.

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