Surprise Discovery: Water Found on a Nearby Comet

New findings from the James Webb Space Telescope indicate the presence of water on a comet situated within the asteroid belt, which is located between the orbits of Mars and Jupiter. This discovery strengthens the hypothesis that such comets may have been the primary source of water on Earth and elsewhere in our solar system. In September 2022, scientists successfully captured images of the comet 238P/Read, named after astronomer Michael T. Read, who discovered it in 2005. Using the advanced capabilities of the space telescope’s spectrometers, the researchers analyzed the composition of the gas cloud surrounding the comet and trailing behind it. In an article published in Nature journal, the researchers report that this cloud indeed contains traces of water, likely remnants of the water that formed at the outskirts of our young solar system.

Despite its location within the asteroid belt, Comet Read exhibits the typical characteristics of ice-based comets - a coma and tail that appear around it -  as it approaches the sun during its orbit.  As these comets draw nearer to the sun, some of the ice sublimates, releasing gases and dust particles that were trapped inside, which form the coma and tail. For many years, the conventional explanation was that comets were formed at the outskirts of the solar system, for example in the Kuiper Belt or Oort Cloud, where the low temperatures due to the distance from the sun, help preserve their icy composition. Only a small portion of this ice sublimates when their orbit brings them closer to the sun. However, recent estimates suggest that comets composed predominantly of frozen water could also exist closer to the sun, although solid evidence of this remained elusive until the current discovery made with the James Webb Space Telescope. "In the past, we've seen objects in the main belt with all the characteristics of comets, but only with this precise spectral data from [the] Webb [telescope] can we say yes, it's definitely water ice that is creating that effect," explained the head of the research team, astronomer and principal research scientist Michael Kelley, from the University of Maryland. "With Webb's observations of Comet Read, we can now demonstrate that water ice from the early solar system can be preserved in the asteroid belt."

The abundance of water on Earth enables the existence and sustainment of life as we know it, and interestingly, water is also present in numerous locations across our solar system. Mars, for example, was likely a water-rich planet in its distant past, and even today, substantial ice deposits are found at its poles, alongside evidence of subsurface water reservoirs. Furthermore, there is compelling evidence pointing to the existence of oceans of liquid water beneath the frozen surfaces of Jupiter and Saturn’s large moons. Even smaller planetary bodies, such as the dwarf planet Ceres, show evidence of water. Many comets and asteroids also contain frozen water. The prevailing theory suggests that collisions of such celestial bodies with Earth during its early formation served as the source of most of the planet’s water. However, alternative studies propose that water was synthesized here on Earth from oxygen and hydrogen that had originated from elsewhere, potentially from those very same comets and asteroids.

“Our water-soaked world, teeming with life and unique in the universe as far as we know, is something of a mystery — we’re not sure how all this water got here,” said Stefanie Milam, one of the coauthors of the article and a member of the James Webb Space Telescope research team.“Understanding the history of water distribution in the solar system will help us to understand other planetary systems, and if they could be on their way to hosting an Earth-like planet.”

Frozen water in a comet much situated much closer than previously discovered. An artist's rendition of the comet 238P/Read | Source: NASA, ESA

Continuing to Investigate

Alongside the intriguing findings of discovering water in a celestial body relatively close to the sun, these discoveries also present a puzzle. The gases that the comet releases do not contain carbon dioxide, a circumstance that is quite unusual, as carbon dioxide typically constitutes about ten percent of the gas volume in similar comets. The researchers suggest two possible explanations for this enigma. One hypothesis is that this comet originally contained carbon dioxide, but over its billions of years in the asteroid belt, it has already sublimated, given that the evaporation temperature of carbon dioxide is lower than that of water. An alternative explanation is that the comet might have been formed in a region of the solar system that was relatively warm, where any carbon dioxide had already sublimated away. Thus, the comet would not have contained carbon dioxide to begin with.

Water without carbon dioxide. The emission spectrum of comet 238P/Read as measured by James Webb Space Telescope (bottom line) compared with past measurements from another comet (top line) | Source: (NASA, ESA, CSA, and J. Olmsted (STScI)

The next phase of the research will entail using the telescope to investigate other such comets in the asteroid belt, comparing the data, and expanding our knowledge about these celestial bodies. These objects  are exceptionally challenging to observe, due to their faintness, relatively small size - only hundreds of meters in diameter -  and their significant distance from the sun, which is twice ours, and even greater. Only a potent telescope such as the James Webb Space Telescope currently has the capacity to unveil their secrets, which is precisely what the researchers intend to continue doing. “Now that Webb has confirmed there is water preserved as close as the asteroid belt, it would be fascinating to follow up on this discovery with a sample collection mission, and learn what else the main belt comets can tell us,” Milam added.