Strange non-gravitational acceleration of comet 'Oumuamua explained

New Delhi, Mar 24 (PTI) Hydrogen gas releasing, as the ice present on the comet 'Oumuamua's surface changed its form from amorphous to crystal, could be the reason behind the strange acceleration of the comet while going away from the Sun, according to scientists.

The argument has been put forth by a University of California (UC), Berkeley, astrochemist and a Cornell University astronomer, both from the US, and the conclusions appear in the journal Nature.

The behaviour of 'Oumuamua, the comet that first made its appearance in 2017, perplexed scientists and public alike.

The first known visitor from outside our solar system and having a peculiar shape, the comet 'Oumuamua was unusual in that it had no bright coma - a luminous cloud of particles - or a dust tail and it was small in size akin to an asteroid.

The behaviour in question was the comet firing up and accelerating away from the Sun in a way that astronomers could not explain. Some even suggested that it was an alien spaceship.

Further, because the comet was so small, its gravitational deflection around the sun was slightly altered by the unusual acceleration.

Comets, which are icy rocks left over when the solar system formed 4.5 billion years ago, can tell astronomers about the conditions that existed when our solar system was formed.

Interstellar comets, present between or among stars, also can carry information about 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," Jennifer Bergner, a UC Berkeley assistant professor of chemistry.

Faraway planetary systems too appear to have comets. Because of the comets' gravitational interactions with other objects in those systems, it is likely that some of them are ejected and enter our solar system (rogue comets).

This helps astronomers learn about planet formation in other systems, the study 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," said Darryl Seligman, Cornell University.

Usually, in comets, as they approach the Sun, a glowing, gaseous coma is formed due to the water and gases being ejected out from their surface. Also, dust is released in the process, giving rise to the comet's "tail".

Further, a second tail pointing away from the Sun is produced by the vapour and dust being pushed out by light pressure from solar rays. A little inertial push outwards too is produced.

Other compounds, such as entrapped organic materials and carbon monoxide, could also be released.

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.

However, unlike other comets, astronomers did not detect a coma, or outgassed molecules or dust around 'Oumuamua.

"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.

Calculations showed that the solar energy hitting the comet would be insufficient to vaporise water or organic compounds from its surface to give it the observed non-gravitational kick, the study said.

So, what caused the acceleration? Bergner studies the interaction of very cold ice, such as that chilled down to the temperature prevalent in the interstellar medium (ISM) of 5 or 10 Kelvin, with the kinds of energetic particles and radiation found in the medium.

"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?" said Bergner.

From past experiments, Bergner found that high-energy electrons, protons and heavier atoms are capable of converting water ice into molecular hydrogen, which would be trapped in bubbles within the ice by the fluffy, snowball structure of the comet.

Under the heat of the Sun, the study said, the ice anneals, changing its form from an amorphous one to a crystallised one, thereby, forcing the bubbles out and releasing the hydrogen gas.

Bergner and Seligman also calculated in the study that the ice at the surface of a comet could emit enough gas to impact 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," said Bergner.

The study said that the explanation also provided for the lack of a dust coma.

"Even if there was dust in the ice matrix, you're not vaporising 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. PTI KRS KRS KRS