Could the asteroid Ryugu be a remnant of an extinct comet? Scientists
now answer

Date:
March 22, 2022
Source:
Nagoya City University
Summary:
The Hayabusa2 mission has recently uncovered information on the
physical characteristics of the asteroid 'Ryugu,' which, according
to the conventional theory, forms from a collision between larger
asteroids.

Now, a study by scientists from Japan suggests that Ryugu is, in
fact, an extinct comet. With a simple physical model that fits
currently available observations, the study provides a better
understanding of comets, asteroids, and the evolution of our
solar system.



FULL STORY ========================================================================== Asteroids hold many clues about the formation and evolution of planets
and their satellites. Understanding their history can, therefore,
reveal much about our solar system. While observations made from
a distance using electromagnetic waves and telescopes are useful,
analyzing samples retrieved from asteroids can yield much more detail
about their characteristics and how they may have formed. An endeavor
in this direction was the Hayabusa mission, which, in 2010, returned to
Earth after 7 years with samples from the asteroid Itokawa.


==========================================================================
The successor to this mission, called Hayabusa2, was completed near
the end of 2020, bringing back material from Asteroid 162173 "Ryugu,"
along with a collection of images and data gathered remotely from
close proximity. While the material samples are still being analyzed,
the information obtained remotely has revealed three important features
about Ryugu. Firstly, Ryugu is a rubble- pile asteroid composed of small
pieces of rock and solid material clumped together by gravity rather than
a single, monolithic boulder. Secondly, Ryugu is shaped like a spinning
top, likely caused by deformation induced by quick rotation. Third,
Ryugu has a remarkably high organic matter content.

Of these, the third feature raises a question regarding the origin of
this asteroid. The current scientific consensus is that Ryugu originated
from the debris left by the collision of two larger asteroids. However,
this cannot be true if the asteroid is high in organic content (which will confirmed once the analyses of the returned samples are complete). What
could, then, be the true origin of Ryugu? In a recent effort to answer
this question, a research team led by Associate Professor Hitoshi Miura
of Nagoya City University, Japan, proposed an alternative explanation
backed up by a relatively simple physical model. As explained in their
paper published in The Astrophysical Journal Letters, the researchers
suggest that Ryugu, as well as similar rubble-pile asteroids, could,
in fact, be remnants of extinct comets. This study was carried out in collaboration with Professor Eizo Nakamura and Associate Professor Tak
Kunihiro from Okayama University, Japan.

Comets are small bodies that form on the outer, colder regions of
the solar system. They are mainly composed of water ice, with some
rocky components (debris) mixed in. If a comet enters the inner solar
system -- the space delimited by the asteroid belt "before" Jupiter --
heat from the solar radiation causes the ice to sublimate and escape,
leaving behind rocky debris that compacts due to gravity and forms a rubble-pile asteroid.

This process fits all the observed features of Ryugu, as Dr. Miura
explains, "Ice sublimation causes the nucleus of the comet to lose
mass and shrink, which increases its speed of rotation. As a result
of this spin-up, the cometary nucleus may acquire the rotational speed
required for the formation of a spinning-top shape. Additionally, the
icy components of comets are thought to contain organic matter generated
in the interstellar medium. These organic materials would be deposited
on the rocky debris left behind as the ice sublimates." To test their hypothesis, the research team conducted numerical simulations using a
simple physical model to calculate the time it would take for the ice to sublimate and the increase in rotational speed of the resulting asteroid
due to it. The results of their analysis suggested that Ryugu has likely
spent a few tens of thousands of years as an active comet before moving
into the inner asteroid belt, where the high temperatures vaporized its
ice and turned it into a rubble-pile asteroid.

Overall, this study indicates that spinning top-shaped, rubble-pile
objects with high organic content, such as Ryugu and Bennu (the target
of the OSIRIS- Rex mission) are comet-asteroid transition objects
(CATs). "CATs are small objects that were once active comets but have
become extinct and apparently indistinguishable from asteroids," explains
Dr. Miura. "Due to their similarities with both comets and asteroids,
CATs could provide new insights into our solar system." Hopefully,
detailed compositional analyses of the samples from both Ryugu and Bennu
will shed more light on these issues. Make sure to stay tuned!

========================================================================== Story Source: Materials provided by Nagoya_City_University. Note:
Content may be edited for style and length.


========================================================================== Related Multimedia:
* From_comet_to_asteroid:_The_journey_of_Ryugu_at_bottom_of_page ========================================================================== Journal Reference:
1. Hitoshi Miura, Eizo Nakamura, Tak Kunihiro. The Asteroid 162173
Ryugu: a
Cometary Origin. The Astrophysical Journal Letters, 2022; 925 (2):
L15 DOI: 10.3847/2041-8213/ac4bd5 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/03/220322111331.htm

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