1I / ‘Oumuamua, a strange object of extrasolar origin discovered on October 19, 2017 by the Pan-STARRS 1 telescope, was small, about half the length of a city block and only the thickness of a three-story building, but it was very shiny; its luster is about the same as the surfaces of Pluto and Triton, which are covered with exotic ice. In new research, a duo of Arizona State University astrophysicists looked at different types of ice 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 that ‘Oumuamua was likely thrown out of a young planetary system about half a billion years ago.
An artistic impression of ‘Oumuamua as a piece of almost pure nitrogen ice. Image credit: William Hartmann / Arizona State University.
“In many ways’ Oumuamua was like a comet, but it was curious enough in several ways that mystery surrounded its nature, and speculation was rampant as to what it was,” said Professor Steven Desch, a researcher at the School of Earth and Space. Exploration at Arizona State University.
Based on observations from ‘Oumuamua, Professor Desch and his colleague, Dr. Alan Jackson, determined several features of the interstellar object that differed from what would be expected of a comet.
In terms of speed, the object entered the solar system at a speed slightly slower than expected, indicating that it has not traveled in interstellar space for over a billion years or so.
In terms of size, the pancake shape was also more flattened than any other known object in the solar system.
The scientists also noted that although ‘Oumuamua received a slight push from the sun, the push was stronger than could be explained.
Finally, the object lacked a detectable escaping gas, which is usually visible through the tail of a comet.
All in all, ‘Oumuamua was very much like a comet, but unlike any comet ever seen in the solar system.
The authors hypothesized that the object was made of several exotic types of ice and calculated how quickly these ice creams would sublimate if ‘Oumuamua passed the sun.
From there they calculated the rocket effect, the mass and shape of the object and the reflectivity of the ice creams.
“That was an exciting moment for us. We realized that a patch of ice would be much more reflective than people thought, which meant it could be smaller, ”said Professor Desch.
“The same rocket effect would give ‘Oumuamua a bigger push, bigger than comets usually experience.”
Illustration of a plausible history for ‘Oumuamua: origin in the mother system about 400 million years ago; cosmic ray erosion during its journey to the solar system; and transit through the Solar System, including the closest approach to the Sun on September 9, 2017, and its discovery in October 2017. At any point in 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 with all of the object’s characteristics at the same time.
And since solid nitrogen ice can be seen on Pluto’s surface, it is possible that a comet-like object was made of the same material.
“We knew we had found the right idea when we completed the calculation for which albedo would match ‘Oumuamua’s movement with the observations,” said Dr. Jackson.
“That value turned out to be the same as we observe on the surface of Pluto or Triton, bodies covered with nitrogen ice.”
The researchers then calculated the rate at which chunks of solid nitrogen ice would have struck off Pluto’s surfaces 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 probably hit by a surface impact about half a billion years ago and thrown out of its mother system,” said Dr. Jackson.
“Made from 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 more and more flattened, much like a bar of soap does when the outer layers are rubbed away with use.”
The team’s two articles are published in the Journal of Geophysical Research: Planets
Alan P. Jackson and Steven J. Desch. 1I / ‘Oumuamua as an N2 ice fragment of an exo-Pluto surface: I. Size and compositional constraints. Journal of Geophysical Research: Planetspublished online March 16, 2021; doi: 10.1029 / 2020JE006706
Alan P. Jackson and 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: Planetspublished online March 16, 2021; doi: 10.1029 / 2020JE006807