Far from the Sun lurk icy rocks left over from the formation of our solar system 4.5 billion years ago. These objects are small and mysterious, but in 2019 we had our clearest view of them when NASA’s New Horizons spacecraft flew past Arrokoth, one of these Kuiper Belt objects.
That historic flyby showed the 35-kilometer-long (22-mile) Arrokoth to have two lobes and be shaped like a snowman. It has a big dent in its “head,” likely caused by an impact, and an otherwise relatively smooth surface. Weirdly, the object appeared to have a distinctly red hue, for reasons that couldn’t fully be explained. Now a group of researchers think they might have an unusual answer, with potential implications for the origin of life on Earth.
“We detected sugar molecules,” said Cornelia Meinert, an astrochemist at Côte d’Azur University in France and a coauthor on the study, published in the Proceedings of the National Academy of Sciences of the United States of America. “In those cold areas of the Kuiper Belt where Arrokoth is located, you can imagine we also have comets that contain similar chemistries. And those comets could have brought important sugars required for kick-starting life on the early Earth.”
When New Horizons flew past Arrokoth, it detected methanol ice, something not expected in abundance in the outer solar system. Meinert and her colleagues wondered whether this methanol could be responsible for Arrokoth’s coloration. In a laboratory on Earth, they blasted methanol ice with high-energy electrons to simulate the cosmic rays that would have bathed Arrokoth in the outer solar system.
They kept doing this “until we got to the same red-brownish colors as Arrokoth,” Meinert said. Some of the methanol ice had turned into organic material, specifically sugars such as ribose and glucose. “We believe at least 10% of the methanol [on Arrokoth] would be turned to sugars,” Meinert said. The experiment also produced polycyclic aromatic hydrocarbons (PAHs), which the team thought were the cause of the coloration. “The sugars [themselves] wouldn’t give the reddish color,” Meinert said.
“The question is, How much of those organics survived the formation of the solar system?”
The Kuiper Belt is thought to contain material left over from the nebula that surrounded the Sun in its infancy. Objects such as Arrokoth might therefore record the composition of that nebula.
Meinert and her team went further, suggesting that the presence of these sugars could partly explain the origin of early Earth’s organic material, with sugars like ribose and glucose crucial to the formation of life.
“The question is, How much of those organics survived the formation of the solar system?” Meinert said. Other studies have suggested that organic material was delivered to Earth on meteorites from the inner solar system or from the protoplanetary disk itself. “We propose a third source: that some of these molecules can also be formed in the Kuiper Belt,” she said, and be delivered to Earth on comets.
Special Delivery
Will Grundy, a planetary scientist at Lowell Observatory in Arizona and a coinvestigator for New Horizons, said he has a “split opinion” of the idea. “It’s an amazing view of what kind of chemistry gets created under these circumstances,” he said. “We think there’s methanol ice out there, and there’s got to be cosmic rays. So the process that they described in the paper has got to be happening.”
“It’s really not very hard to make red stuff.”
However, observations of other Kuiper Belt objects with telescopes suggest they are “similarly red” but lack an abundance of methanol, Grundy noted. “So that is maybe a vote against requiring methanol to make this red stuff.” Instead, it might be that objects such as Arrokoth were already red before the methanol process took place, perhaps gaining their hue earlier when energetic radiation broke bonds in carbon and hydrogen. “It’s really not very hard to make red stuff,” he said. Other distant objects such as Pluto also show red coloration.
Grundy also said that though delivery to Earth from the Kuiper Belt is plausible, it is not “necessarily the most efficient path” to get organic molecules to our planet. “It’s just so far away.” For objects that formed closer to the Sun, “it’s easier to perturb them to hit Earth,” he said.
We might learn more about the origin of Earth’s organic molecules by studying samples returned to Earth, such as those collected by Japan’s Hayabusa2 mission in 2020 and NASA’s OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer) in 2023, and by studying more Kuiper Belt objects. That can be done only remotely for now, however, because New Horizons lacks another reachable target as it makes its way out of the solar system, although scientists continue to look for one. “There’s a huge search effort going on right now,” Grundy said.
—Jonathan O’Callaghan (@astro_jonny), Science Writer
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