Was Its Weird Trajectory Caused by Hydrogen Outgassing? Dr. Avi Loeb Says, ‘No’.
‘Oumuamua’, the mysterious cigar-shaped interstellar 800-ft.-by-100-ft.-by-100-ft. object, which late in 2017, sped though the solar system, raised many questions about its nature and origins, but scientists are still unable to tell us what it was.
Now, a University of California, Berkeley, astrochemist and a Cornell University astronomer is arguing 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, say the new study’s authors, 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, they explain, 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.”
Dr. Avi Loeb Responds
Chief Harvard astronomer, Dr. Avi Loeb, author of the book Extraterrestrial, and leading proponent of the notion that Oumuamua might be evidence of an extra-solar civilization, is, however, standing by his case that the object in question was NO comet. According the editors of EarthSky, the online magazine, Dr. Loeb responded on March 24 to the new study: “In a new paper that I submitted this morning for publication in collaboration with Thiem Hoang, we show that the paper published today in Nature by Jennifer Bergner and Darryl Seligman miscalculated the surface temperature of Oumuamua. “Bergner and Seligman suggested that the peculiar acceleration of Oumuamua can be explained if it was made of water ice which was partly dissociated into hydrogen by cosmic-rays in interstellar space. However, their surface temperature calculation near the sun ignored the crucial cooling effect of evaporating hydrogen.
“By adding the cooling from hydrogen evaporation, our new paper shows that the surface temperature of the iceberg is reduced by an order of magnitude.”
Dr. Loeb continued, with a numerical explanation:
“The correct calculation of the surface temperature is straightforward. It balances heating by sunlight with radiative cooling from the surface and the additional losses from the energy invested in dislodging hydrogen atoms from the lattice. The latter component was omitted in the thermal model presented by Bergner and Seligman [in the ‘Methods’ section to their new study], leading to an overestimate of the surface temperature by a factor of 9.
“As a result of the decrease in surface temperature, the thermal speed of outgassing hydrogen is reduced by a factor of 3 …”
And he explained why flawed numbers presented in the comet study would lead to a flawed conclusion:
“The original model required that about a third of the hydrogen atoms be separated from water by cosmic-rays, and hence the new result requires all the hydrogen to be separated from water. This makes the model untenable because a full-hydrogen surface resembles the hydrogen iceberg model proposed in a previous 2020 paper by Darryl Seligman.
“Following this original proposal, I wrote a paper with Thiem Hoang, showing that heating by interstellar starlight would quickly destroy pure hydrogen layers, not allowing them to reach the solar system as Oumuamua did.
“Moreover, the lower surface temperature further limits the thermal annealing of water ice, a key process that is appealed to by Bergner and Seligman as a mechanism for releasing molecular hydrogen.” (https://earthsky.org/space/oumuamua-a-comet-avi-loeb-responds/)
AR #133
Oumuamua Mystery Still Unsolved
