Water has long been considered a key ingredient for the development of life on Earth. For decades, scientists have debated how Earth became rich in water despite its proximity to the Sun, where the intense heat would have caused much of the water to vaporize during the planet’s formation around 4.6 billion years ago. While a variety of explanations have been proposed, recent research, particularly concerning comets, has added an exciting twist to the ongoing discussion about the origins of Earth’s water.
The idea that comets might have played a crucial role in delivering water to Earth gained traction in recent years following studies of the molecular composition of water found on certain comets. One notable example is Comet 67P/Churyumov–Gerasimenko, a Jupiter-family comet that has been the subject of extensive study by the European Space Agency’s (ESA) Rosetta mission. In a study published in Science Advances on November 13, 2023, researchers have reported new findings that suggest the water on Comet 67P bears a molecular signature closely resembling that of Earth’s oceans. This discovery reignites the debate about whether comets, particularly Jupiter-family comets like 67P, could have been significant contributors to the Earth’s water supply.
The challenge of understanding how Earth acquired its water stems from the fact that much of the planet’s early environment was inhospitable to the preservation of water. Earth formed from a cloud of gas and dust surrounding the early Sun, but the temperature close to the Sun would have been too high for liquid water to remain stable. Some of Earth’s water likely originated from volcanic outgassing, where water vapor released from the planet’s interior condensed to form clouds and rain, contributing to the formation of oceans. However, this volcanic source alone could not account for the vast amounts of water on the planet today, and an alternative or additional source had to be identified.
Scientists have long theorized that water could have been delivered to Earth by asteroids and comets that collided with the planet during the early bombardment period, around 4 billion years ago. This period, known as the Late Heavy Bombardment, saw a large number of comets and asteroids impacting the inner planets, potentially delivering water ice and organic compounds to Earth’s surface. While evidence supporting asteroid contributions to Earth’s water is strong, the role of comets, and specifically Jupiter-family comets, has been more difficult to assess.
The key to understanding the relationship between cometary water and Earth’s water lies in the measurement of deuterium, a heavier isotope of hydrogen. The ratio of deuterium to regular hydrogen (D/H) in a sample of water can provide vital clues about where the water originated. Deuterium is more prevalent in the water of objects that formed farther from the Sun, as cooler temperatures in the outer solar system allow deuterium to condense more readily onto water molecules. Thus, comets, which are thought to have formed in the cold regions beyond Saturn’s orbit, should exhibit a higher deuterium-to-hydrogen ratio compared to water from objects that formed closer to the Sun, like asteroids.
In the past couple of decades, scientists have measured the deuterium levels in water vapor emitted from several Jupiter-family comets, revealing a striking similarity to the D/H ratio in Earth’s oceans. This observation seemed to suggest that comets might have played a major role in delivering water to Earth. Kathleen Mandt, a planetary scientist at NASA’s Goddard Space Flight Center, and her team, were among the researchers who examined this potential link, particularly focusing on Comet 67P. Their earlier findings, which indicated a strong match between the deuterium levels in 67P’s water and Earth’s, reinforced the idea that Jupiter-family comets could have contributed significantly to the planet’s water supply.
However, a major surprise came in 2014 with the arrival of ESA’s Rosetta spacecraft at Comet 67P. The spacecraft’s measurements revealed that the comet’s water had an unexpectedly high concentration of deuterium—three times higher than the levels found in Earth’s oceans. This discovery initially seemed to contradict the hypothesis that comets could have delivered water to Earth, prompting scientists to reconsider their assumptions.
Mandt and her colleagues were undeterred by this discrepancy. They hypothesized that the high deuterium content might be due to the influence of dust in the comet’s coma— the cloud of gas and dust that surrounds the comet’s nucleus. As the comet approached the Sun, it warmed, causing water ice from its surface to sublimate and be released into the coma. Some of this sublimated water could be associated with dust particles, and research suggested that deuterium sticks to dust grains more readily than regular hydrogen. This means that measurements taken from the coma, especially those close to the comet’s surface, might have been skewed by the dust and may not have accurately reflected the composition of the comet’s core water.
To test this hypothesis, Mandt’s team used an advanced statistical-computation technique to analyze over 16,000 individual measurements of deuterium taken by Rosetta during its mission. The team examined the data across different regions of the comet’s coma, paying particular attention to the role of dust in the readings. They discovered a significant correlation between the deuterium measurements and the amount of dust in the region of the coma where the measurements were taken. This finding suggested that dust, particularly from the outer parts of the coma, could be responsible for artificially inflating the deuterium ratio in earlier measurements.
Mandt and her team concluded that when the spacecraft measured the water vapor farther from the comet, at least 75 miles from the nucleus, the dust effects had largely dissipated, providing a more accurate measure of the deuterium levels in the comet’s water. By this distance, the water vapor was less contaminated by dust, and the deuterium levels were found to be much lower, much closer to the levels observed in Earth’s oceans.
This result is a game-changer for our understanding of cometary water and its potential role in Earth’s water history. It suggests that prior observations of Comet 67P, which showed unusually high deuterium levels, may not have been representative of the comet’s bulk composition. Instead, the dust in the coma played a significant role in skewing the measurements, and by accounting for this factor, scientists may be able to revisit earlier data and gain a clearer picture of the comet’s true water signature.
The implications of this discovery go beyond just Earth’s water supply. Understanding the water composition of comets like 67P offers important insights into the early solar system. Comets are some of the oldest, most primitive objects in the solar system, containing materials that date back to the time of its formation. By studying cometary water, scientists can gain clues about the conditions in the outer solar system during the formation of the planets and the role that these icy bodies played in delivering not only water but also organic compounds that could have been essential for life.
Mandt’s findings highlight the importance of revisiting past observations and refining the techniques used to study comets. As technology advances and future missions continue to explore these icy bodies, it is likely that new, more accurate measurements will continue to reshape our understanding of comets and their contributions to the Earth’s water reservoir.
Source: NASA