MSGID: 1:317/3 641bd5f7   
   PID: hpt/lnx 1.9.0-cur 2019-01-08   
   TID: hpt/lnx 1.9.0-cur 2019-01-08   
    Surprisingly simple explanation for the alien comet 'Oumuamua's weird   
   orbit    
    2017 comet's unusual acceleration explained by hydrogen outgassing from   
   ice    
      
     Date:   
         March 22, 2023   
     Source:   
         University of California - Berkeley   
     Summary:   
         When the first interstellar comet ever seen in our solar system   
         was discovered in 2017, one characteristic -- an unexplained   
         acceleration away from the sun -- sparked wild speculation,   
         including that it was an alien spacecraft. An astrochemist   
         found a simpler explanation and tested it with an astronomer:   
         in interstellar space, cosmic rays converted water to hydrogen in   
         the comet's outer layers. Nearing the sun, outgassed hydrogen gave   
         the tiny comet a kick.   
      
      
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   FULL STORY   
   ==========================================================================   
   In 2017, a mysterious comet dubbed 'Oumuamua fired the imaginations of   
   scientists and the public alike. It was the first known visitor from   
   outside our solar system, it had no bright coma or dust tail, like most   
   comets, and a peculiar shape -- something between a cigar and a pancake --   
   and its small size more befitted an asteroid than a comet.   
      
      
   ==========================================================================   
   But the fact that it was accelerating away from the sun in a way that   
   astronomers could not explain perplexed scientists, leading some to   
   suggest that it was an alien spaceship.   
      
   Now, a University of California, Berkeley, astrochemist and a Cornell   
   University astronomer argue that the comet's mysterious deviations from   
   a hyperbolic path around the sun can be explained by a simple physical   
   mechanism likely common among many icy comets: outgassing of hydrogen   
   as the comet warmed up in the sunlight.   
      
   What made 'Oumuamua different from every other well-studied comet in   
   our solar system was its size: It was so small that its gravitational   
   deflection around the sun was slightly altered by the tiny push created   
   when hydrogen gas spurted out of the ice.   
      
   Most comets are essentially dirty snowballs that periodically approach the   
   sun from the outer reaches of our solar system. When warmed by sunlight,   
   a comet ejects water and other molecules, producing a bright halo or   
   coma around it and often tails of gas and dust. The ejected gases act   
   like the thrusters on a spacecraft to give the comet a tiny kick that   
   alters its trajectory slightly from the elliptical orbits typical of   
   other solar system objects, such as asteroids and planets.   
      
   When discovered, 'Oumuamua had no coma or tail and was too small and   
   too far from the sun to capture enough energy to eject much water, which   
   led astronomers to speculate wildly about its composition and what was   
   pushing it outward. Was it a hydrogen iceberg outgassing H2? A large,   
   fluffy snowflake pushed by light pressure from the sun? A light sail   
   created by an alien civilization? A spaceship under its own power?   
   Jennifer Bergner, a UC Berkeley assistant professor of chemistry who   
   studies the chemical reactions that occur on icy rocks in the cold vacuum   
   of space, thought there might be a simpler explanation. She broached   
   the subject with a colleague, Darryl Seligman, now an National Science   
   Foundation postdoctoral fellow at Cornell University, and they decided   
   to work together to test it.   
      
   "A comet traveling through the interstellar medium basically is getting   
   cooked by cosmic radiation, forming hydrogen as a result. Our thought   
   was: If this was happening, could you actually trap it in the body,   
   so that when it entered the solar system and it was warmed up, it would   
   outgas that hydrogen?" Bergner said. "Could that quantitatively produce   
   the force that you need to explain the non-gravitational acceleration?"   
   Surprisingly, she found that experimental research published in the 1970s,   
   '80s and '90s demonstrated that when ice is hit by high-energy particles   
   akin to cosmic rays, molecular hydrogen (H2) is abundantly produced and   
   trapped within the ice. In fact, cosmic rays can penetrate tens of meters   
   into ice, converting a quarter or more of the water to hydrogen gas.   
      
   "For a comet several kilometers across, the outgassing would be   
   from a really thin shell relative to the bulk of the object, so both   
   compositionally and in terms of any acceleration, you wouldn't necessarily   
   expect that to be a detectable effect," she said. "But because 'Oumuamua   
   was so small, we think that it actually produced sufficient force to power   
   this acceleration."  The comet, which was slightly reddish, is thought   
   to have been roughly 115 by 111 by 19 meters in size. While the relative   
   dimensions were fairly certain, however, astronomers couldn't be sure   
   of the actual size because it was too small and distant for telescopes   
   to resolve. The size had to be estimated from the comet's brightness and   
   how the brightness changed as the comet tumbled. To date, all the comets   
   observed in our solar system -- the short-period comets originating in the   
   Kuiper belt and the long-period comets from the more distant Oort cloud   
   have ranged from around 1 kilometer to hundreds of kilometers across.   
      
   "What's beautiful about Jenny's idea is that it's exactly what should   
   happen to interstellar comets," Seligman said. "We had all these stupid   
   ideas, like hydrogen icebergs and other crazy things, and it's just the   
   most generic explanation."  Bergner and Seligman will publish their   
   conclusions this week in the journal Nature. Both were postdoctoral   
   fellows at the University of Chicago when they began collaborating on   
   the paper.   
      
   Messenger from afar Comets are icy rocks left over from the formation of   
   the solar system 4.5 billion years ago, so they can tell astronomers about   
   the conditions that existed when our solar system formed. Interstellar   
   comets can also give hints to the conditions around other stars surrounded   
   by planet-forming disks.   
      
   "Comets preserve a snapshot of what the solar system looked like when   
   it was in the stage of evolution that protoplanetary disks are now,"   
   Bergner said.   
      
   "Studying them is a way to look back at what our solar system used to   
   look like in the early formation stage."  Faraway planetary systems   
   also seem to have comets, and many are likely to be ejected because   
   of gravitational interactions with other objects in the system, which   
   astronomers know happened over the history of our solar system. Some of   
   these rogue comets should occasionally enter our solar system, providing   
   an opportunity to learn about planet formation in other systems.   
      
   "The comets and asteroids in the solar system have arguably taught us more   
   about planet formation than what we've learned from the actual planets   
   in the solar system," Seligman said. "I think that the interstellar   
   comets could arguably tell us more about extrasolar planets than the   
   extrasolar planets we are trying to get measurements of today."  In the   
   past, astronomers published numerous papers about what we can learn from   
   the failure to observe any interstellar comets in our solar system.   
      
   Then, 'Oumuamua came along.   
      
   On Oct. 19, 2017, on the island of Maui, astronomers using the Pan-STARRS1   
   telescope, which is operated by the Institute for Astronomy at the   
   University of Hawaii in Manoa, first noticed what they thought was either   
   a comet or an asteroid. Once they realized that its tilted orbit and high   
   speed -- 87 kilometers per second -- implied that it came from outside   
   our solar system, they gave it the name 1I/'Oumuamua (oh MOO-uh MOO-uh),   
   which is Hawai'ian for "a messenger from afar arriving first." It was   
   the first interstellar object aside from dust grains ever seen in our   
   solar system. A second, 2I/Borisov, was discovered in 2019, though it   
   looked and behaved more like a typical comet.   
      
   As more and more telescopes focused on 'Oumuamua, the astronomers were   
   able to chart its orbit and determine that it had already looped around   
   the sun and was headed out of the solar system.   
      
   Because 'Oumuamua's brightness changed periodically by a factor of 12   
   and varied asymmetrically, it was assumed to be highly elongated and   
   tumbling end over end. Astronomers also noticed a slight acceleration   
   away from the sun larger than seen for asteroids and more characteristic   
   of comets. When comets approach the sun, the water and gases ejected   
   from the surface create a glowing, gaseous coma and release dust in the   
   process. Typically, dust left in the comet's wake becomes visible as   
   one tail, while vapor and dust pushed by light pressure from solar rays   
   produces a second tail pointing away from the sun, plus a little inertial   
   push outward. Other compounds, such as entrapped organic materials and   
   carbon monoxide, also can be released.   
      
   Why was it accelerating?  But astronomers could detect no coma, outgassed   
   molecules or dust around 'Oumuamua. In addition, calculations showed that   
   the solar energy hitting the comet would be insufficient to sublimate   
   water or organic compounds from its surface to give it the observed   
   non-gravitational kick. Only hypervolatile gases such as H2, N2 or carbon   
   monoxide (CO) could provide enough acceleration to match observations,   
   given the incoming solar energy.   
      
   "We had never seen a comet in the solar system that didn't have a   
   dust coma.   
      
   So, the non-gravitational acceleration really was weird," Seligman said.   
      
   This led to much speculation about what volatile molecules could be in   
   the comet to cause the acceleration. Seligman himself published a paper   
   arguing that if the comet was composed of solid hydrogen -- a hydrogen   
   iceberg -- it would outgas enough hydrogen in the heat of the sun to   
   explain the strange acceleration. Under the right conditions, a comet   
   composed of solid nitrogen or solid carbon monoxide would also outgas   
   with enough force to affect the comet's orbit.   
      
   But astronomers had to stretch to explain what conditions could lead   
   to the formation of solid bodies of hydrogen or nitrogen, which have   
   never been observed before. And how could a solid H2 body survive for   
   perhaps 100 million years in interstellar space?  Bergner thought that   
   outgassing of hydrogen entrapped in ice might be sufficient to accelerate   
   'Oumuamua. As both an experimentalist and a theoretician, she studies   
   the interaction of very cold ice -- chilled to 5 or 10 degrees Kelvin,   
   the temperature of the interstellar medium (ISM) -- with the kinds of   
   energetic particles and radiation found in the ISM.   
      
   In searching through past publications, she found many experiments   
   demonstrating that high-energy electrons, protons and heavier atoms could   
   convert water ice into molecular hydrogen, and that the fluffy, snowball   
   structure of a comet could entrap the gas in bubbles within the ice.   
      
   Experiments showed that when warmed, as by the heat of the sun, the   
   ice anneals -- changes from an amorphous to a crystal structure -- and   
   forces the bubbles out, releasing the hydrogen gas. Ice at the surface   
   of a comet, Bergner and Seligman calculated, could emit enough gas,   
   either in a collimated beam or fan- shaped spray, to affect the orbit   
   of a small comet like 'Oumuamua.   
      
   "The main takeaway is that 'Oumuamua is consistent with being a   
   standard interstellar comet that just experienced heavy processing,"   
   Bergner said. "The models we ran are consistent with what we see in the   
   solar system from comets and asteroids. So, you could essentially start   
   with something that looks like a comet and have this scenario work."   
   The idea also explains the lack of a dust coma.   
      
   "Even if there was dust in the ice matrix, you're not sublimating the ice,   
   you're just rearranging the ice and then letting H2 get released. So,   
   the dust isn't even going to come out," Seligman said.   
      
   'Dark' comets Seligman said that their conclusion about the source of   
   'Oumuamua's acceleration should close the book on the comet. Since 2017,   
   he, Bergner and their colleagues have identified six other small comets   
   with no observable coma, but with small non-gravitational accelerations,   
   suggesting that such "dark" comets are common. While H2 is not likely   
   responsible for the accelerations of dark comets, Bergner noted, together   
   with 'Oumuamua they reveal that there is much to be learned about the   
   nature of small bodies in the solar system.   
      
   One of these dark comets, 1998 KY26, is the next target for Japan's   
   Hayabusa2 mission, which recently collected samples from the asteroid   
   Ryugu. The 1998 KY26 was thought to be an asteroid until it was identified   
   as a dark comet in December.   
      
   "Jenny's definitely right about the entrapped hydrogen. Nobody had   
   thought of that before," he said. "Between discovering other dark comets   
   in the solar system and Jenny's awesome idea, I think it's got to be   
   correct. Water is the most abundant component of comets in the solar   
   system and likely in extrasolar systems, as well. And if you put a water   
   rich comet in the Oort cloud or eject it into the interstellar medium, you   
   should get amorphous ice with pockets of H2."  Because H2 should form in   
   any ice-rich body exposed to energetic radiation, the researchers suspect   
   that the same mechanism would be at work in sun-approaching comets from   
   the Oort cloud at the outer reaches of the solar system, where comets   
   are irradiated by cosmic rays, much like an interstellar comet would be.   
      
   Future observations of hydrogen outgassing from long-period comets could   
   be used to test the scenario of H2 formation and entrapment.   
      
   Many more interstellar and dark comets should be discovered by the Rubin   
   Observatory Legacy Survey of Space and Time (LSST), allowing astronomers   
   to determine if hydrogen outgassing is common in comets. Seligman has   
   calculated that the survey, which will be conducted at the Vera C. Rubin   
   Observatory in Chile and is set to become operational in early 2025,   
   should detect between one and three interstellar comets like 'Oumuamua   
   every year, and likely many more that have a telltale coma, like Borisov.   
      
   Bergner was supported by a NASA Hubble Fellowship grant. Seligman   
   was supported by the National Science Foundation (AST-17152) and NASA   
   (80NSSC19K0444, NNX17AL71A).   
      
       * RELATED_TOPICS   
             o Space_&_Time   
                   # Asteroids,_Comets_and_Meteors # Sun # Astronomy   
                   # Solar_Flare # Solar_System # Astrophysics #   
                   Extrasolar_Planets # Northern_Lights   
       * RELATED_TERMS   
             o Comet o Comet_Shoemaker-Levy_9 o Comet_Hale-Bopp o   
             Interstellar_medium o Sun o Solar_flare o Jupiter o Outer_space   
      
   ==========================================================================   
   Story Source: Materials provided by   
   University_of_California_-_Berkeley. Original written by Robert   
   Sanders. Note: Content may be edited for style and length.   
      
      
   ==========================================================================   
   Related Multimedia:   
       * An_artist's_depiction_of_the_interstellar_comet_`Oumuamua   
   ==========================================================================   
   Journal Reference:   
      1. Jennifer B. Bergner, Darryl Z. Seligman. Acceleration of   
      1I/`Oumuamua   
         from radiolytically produced H2 in H2O ice. Nature, 2023; 615   
         (7953): 610 DOI: 10.1038/s41586-022-05687-w   
   ==========================================================================   
      
   Link to news story:   
   https://www.sciencedaily.com/releases/2023/03/230322140338.htm   
      
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