A new analysis of our solar system’s interstellar interloper, 3I/ATLAS, reveals that it’s spewing huge amounts of water — and astronomers can’t immediately explain why.
The object, which is widely believed to be comet, showed strong ultraviolet emissions that are unmistakable telltales of hydroxyl gas (OH), a byproduct of water, when astronomers imaged it with NASA’s Neil Gehrels Swift space telescope before it disappeared behind the Sun. The emissions could only be spotted from space because the ultraviolet light would get absorbed in the atmosphere.
Their findings, detailed in a new study published in The Astrophysical Journal Letters, argue that the presence of all this OH indicates the comet is ejecting water vapor at a torrential rate of about 88 pounds per second — around the same rate as a fire hose running at full blast, according to a press release about the findings.
The most extraordinary thing is that this was spotted happening pretty far from the Sun, at a heliocentric distance of about three astronomical units (AU) away, or three times the distance between the Earth and our star. Typically, comets stray much closer to the Sun before the water ice in their core, called a nucleus, begins to sublimate, or instantly transform from a solid to a gas. Something else must be driving all the water dumping from 3I/ATLAS — which also implies, tantalizingly, that the comet must harbor considerable stores of water for this process to keep going.
“When we detect water — or even its faint ultraviolet echo, OH, — from an interstellar comet, we’re reading a note from another planetary system,” coauthor Dennis Bodewits, a professor of physics at Auburn University, said in the release. “It tells us that the ingredients for life’s chemistry are not unique to our own.”
It’s another example of the fascinating strangeness of interstellar objects like 3I/ATLAS. Think of it as a sample of somewhere very far away, perhaps tens of millions of light years, careening straight past our doorstep. That it’s in many ways bizarre compared to local comets hints at just how unique these unimaginable alien realms must be, and how we have so much more to understand of how star systems form and how their structures may evolve.
Typically, a comet’s coma, a huge halo of gas and dust that give comets their glowing appearance, begin to form as the object nears the Sun — or another star, presumably — and heats up. The heat either sublimates or vaporizes the material in its nucleus, which is many times smaller than the tail that catches our eyes from the ground, stretching behind the comet.
3I/ATLAS’s coma has already surprised us in many ways. Its chemistry is strange compared to our own comets, and it appears to have an astonishingly high ratio of carbon dioxide to water.
What’s causing the outpouring of water vapor is still unclear. The astronomers speculate that sunlight might be heating up the ice grains released from the nucleus, which then get vaporized into the surrounding coma.
Astronomers believe that 3I/ATLAS came from the center of the Milky Way, where it was likely booted out of its original star system by a gravitational disturbance like the close flyby of another star, braving interstellar space before eventually cruising through our solar neighborhood. Based on these inferences, astronomers estimated that the comet must be billions of years old, perhaps three billion years older than the Sun itself. It’s not only a snapshot of a different part of the galaxy, but a different era of the cosmos altogether.
Right now, 3I/ATLAS is flying behind the Sun, so we can’t observe it from Earth. But scientists have been able to catch a glimpse of it using spacecraft stationed near Mars, and it’ll soon swing back into full view in late November.
“Every interstellar comet so far has been a surprise,” said lead author Zexi Xing, a postdoctoral researcher at Auburn University, said in a statement about the work referencing the two previously discovered interstellar objects. “‘Oumuamua was dry, Borisov was rich in carbon monoxide, and now ATLAS is giving up water at a distance where we didn’t expect it.”
“Each one,” Xing added, “is rewriting what we thought we knew about how planets and comets form around stars.”
More on space: Astronomer: 30+ Percent Probability Interstellar Object Is Alien Craft Disguised as Comet