Iceberg one piece5/29/2023 ![]() “We’ll see how the scientific community responds, but it definitely feels to me like this has to be the right answer,” he said. ![]() Seligman said it’s the most natural explanation: A water-rich comet is exactly what you would expect something ejected from an extrasolar planetary system to be. “So you could essentially start with something that looks like a water-rich comet and have this scenario make sense.” “The main takeaway is that Oumuamua is consistent with being a standard interstellar comet that just experienced heavy processing,” Bergner said. It was also consistent with missing dust: Oumuamua would not be sublimating dusty surface ice like comets with reflective tails, just releasing trapped interior gas. ![]() Given Oumuamua’s size – at roughly 400 feet wide and shaped like a pancake, an order of magnitude smaller than most comets – the researchers concluded such hydrogen outgassing was sufficient to power nongravitational acceleration. This radiolytically produced hydrogen is released when the ice is warmed and the amorphous ice becomes crystalline, rearranging its structure. It was shown in labs to trap pockets of molecular hydrogen when energetic radiation (akin to cosmic rays) hit the ice. Then Bergner, a former colleague at the University of Chicago, pointed to experiments involving amorphous ice, a disordered type of ice (as opposed to crystalline) that forms at very cold temperatures. In a paper published March 22 in Nature, Darryl Seligman, a postdoctoral researcher in the Department of Astronomy in the College of Arts and Sciences, and a National Science Foundation postdoctoral fellow, and first author Jennifer Bergner, an assistant professor of chemistry at the University of California, Berkeley, argue that hydrogen gas released by warming of amorphous water ice likely drove Oumuamua’s nongravitational acceleration, or acceleration beyond the sun’s influence. That led to Seligman’s own outside-the-box idea about hydrogen icebergs, one he now considers “out there,” but a step in the right direction. A breakthrough came when Seligman’s calculations showed that Oumuamua didn’t receive enough sunlight to power typical cometary outgassing by water or carbon dioxide, suggesting it might instead be composed of what are called hyper-volatiles – perhaps carbon monoxide or an exotic ice such as solid hydrogen or solid nitrogen. Outgassing had been Seligman’s focus since Oumuamua was discovered while he was a doctoral student at Yale University, but every explanation ran into a theoretical or observational problem. ![]() In a paper published March 22 in Nature, Seligman and first author Jennifer Bergner, an assistant professor of chemistry at the University of California, Berkeley, argue that hydrogen gas released by warming of amorphous water ice likely drove Oumuamua’s nongravitational acceleration, or acceleration beyond the sun’s influence. Insights from Oumuamua and dark comets – one of which a spacecraft will visit in less than a decade – could advance our understanding of planet formation in this solar system and others. “It turns out that we needed something that was as intensely scrutinized as Oumuamua, because it was interstellar, to realize that there’s this new population of dark comets within the solar system itself,” Seligman said. Now Seligman and colleagues believe they’ve unlocked another piece of the puzzle. In recently published research, Seligman helps put forward the most plausible explanation yet for what Oumuamua likely is – an icy planetary building block, broadly similar to comets – and argues that similar small bodies may be found in our midst. ![]() “It was a complete mystery from the very beginning.” “In every way we expected these interstellar comets to behave, Oumuamua just acted exactly the opposite,” said Darryl Seligman, a postdoctoral researcher in the Department of Astronomy in the College of Arts and Sciences, and a National Science Foundation postdoctoral fellow. Detected in 2017 and observable for only a few weeks, the first known interstellar object to pass through the solar system confounded astronomers, sparking speculation it could be a giant snowflake, hydrogen iceberg or piece of a Pluto-like planet – even an alien probe, an idea promoted in a best-selling book.Įach theory attempted to resolve an apparent contradiction: How could the interstellar interloper named Oumuamua (Hawaiian for “first messenger from afar,” and pronounced oh-MOO-uh-MOO-uh) be accelerating like a comet, but lack a comet’s bright, dusty tail? ![]()
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