1I/‘Oumuamua, a strange object of extrasolar origin discovered on October 19, 2017 by the Pan-STARRS 1 telescope, was small, about half as long as a city block and only as thick as a three story building, but it was very shiny; its shininess is about the same as the surfaces of Pluto and Triton, which are covered in exotic ices. In new research, a duo of astrophysicists at Arizona State University looked at several different ices and the push they would give ‘Oumuamua as they evaporated and found that the best ice is nitrogen, which would explain many of the things the researchers know about the interstellar object. They also suggest ‘Oumuamua was probably thrown out of a young planetary system about half a billion years ago.
An artist’s impression of ‘Oumuamua as a piece of nearly pure nitrogen ice. Image credit: William Hartmann / Arizona State University.
“In many ways ‘Oumuamua resembled a comet, but it was peculiar enough in several ways that mystery surrounded its nature, and speculation ran rampant about what it was,” said Professor Steven Desch, a researcher in the School of Earth and Space Exploration at Arizona State University.
From observations of ‘Oumuamua, Professor Desch and his colleague, Dr. Alan Jackson, determined several characteristics of the interstellar object that differed from what would be expected from a comet.
In terms of speed, the object entered the Solar System at a velocity a bit lower than would be expected, indicating that it had not been traveling in interstellar space for more than a billion years or so.
In terms of size, its pancake shape was also more flattened than any other known solar system object.
The scientists also observed that while ‘Oumuamua acquired a slight push away from the Sun, the push was stronger than could be accounted for.
Finally, the object lacked a detectable escaping gas, which is usually depicted visibly by a comet’s tail.
In all, ‘Oumuamua was very much like a comet, but unlike any comet that had ever been observed in the Solar System.
The authors hypothesized that the object was made of different exotic ices and they calculated how quickly these ices would sublimate as ‘Oumuamua passed by the Sun.
From there, they calculated the rocket effect, the object’s mass and shape, and the reflectivity of the ices.
“That was an exciting moment for us. We realized that a chunk of ice would be much more reflective than people were assuming, which meant it could be smaller,” Professor Desch said.
“The same rocket effect would then give ‘Oumuamua a bigger push, bigger than comets usually experience.”
Illustration of a plausible history for ‘Oumuamua: origin in its parent system around 400 million years ago; erosion by cosmic rays during its journey to the Solar System; and passage through the Solar System, including its closest approach to the Sun on September 9, 2017, and its discovery in October 2017. At each point along its history, this illustration shows the predicted size of ‘Oumuamua, and the ratio between its longest and shortest dimensions. Image credit: S. Selkirk / Arizona State University.
The team found one ice in particular — solid nitrogen — that provided an exact match to all the object’s features simultaneously.
And since solid nitrogen ice can be seen on the surface of Pluto, it is possible that a cometlike object could be made of the same material.
“We knew we had hit on the right idea when we completed the calculation for what albedo would make the motion of ‘Oumuamua match the observations,” Dr. Jackson said.
“That value came out as being the same as we observe on the surface of Pluto or Triton, bodies covered in nitrogen ice.”
The researchers then calculated the rate at which chunks of solid nitrogen ice would have been knocked off the surfaces of Pluto and similar bodies early in our Solar System’s history.
And they calculated the probability that chunks of solid nitrogen ice from other solar systems would reach ours.
“It was likely knocked off the surface by an impact about half a billion years ago and thrown out of its parent system,” Dr. Jackson said.
“Being made of frozen nitrogen also explains the unusual shape of ‘Oumuamua.”
“As the outer layers of nitrogen ice evaporated, the shape of the body would have become progressively more flattened, just like a bar of soap does as the outer layers get rubbed off through use.”
The team’s two papers were published in the Journal of Geophysical Research: Planets.
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Alan P. Jackson & Steven J. Desch. 1I/‘Oumuamua as an N2 ice fragment of an exo-Pluto surface: I. Size and Compositional Constraints. Journal of Geophysical Research: Planets, published online March 16, 2021; doi: 10.1029/2020JE006706
Alan P. Jackson & Steven J. Desch. 1I/‘Oumuamua as an N2 ice fragment of an exo-Pluto surface II: Generation of N2 ice fragments and the origin of ‘Oumuamua. Journal of Geophysical Research: Planets, published online March 16, 2021; doi: 10.1029/2020JE006807
