Trans-Neptunian object Arrokoth: Flattening of a snowman
Only in the course of several million years did Arrokoth, also known by
its nickname Ultima Thule, acquire its bizarre, pancake-flat shape

Date:
October 5, 2020
Source:
Max Planck Institute for Solar System Research
Summary:
The trans-Neptunian object Arrokoth, also known as Ultima Thule,
which NASA's space probe New Horizons passed on New Year's Day
2019, may have changed its shape significantly in the first 100
million years since its formation. Researchers now suggest that
the current shape of Arrokoth could be of evolutionary origin due
to volatile outgassing.



FULL STORY ==========================================================================
The many millions of bodies populating the Kuiper Belt beyond Neptune's
orbit are yet to reveal many of their secrets. In the 1980s, the space
probes Pioneer 1 and 2 as well as Voyager 1 and 2 crossed this region
-- but without cameras on board. NASA's spacecraft New Horizons sent
the first images from the outermost edge of the solar system to Earth:
in the summer of 2015 of dwarf planet Pluto and three and a half years
later of the trans-Neptunian object Arrokoth, about 30 kilometers in
size. Not yet officially named, the body was nicknamed Ultima Thule at
the time, in reference to the northernmost land point on Earth. After
all, the trans-Neptunian object is the body furthest away from the Sun
that has ever been visited and imaged by a human-made probe.


========================================================================== Especially Arrokoth's strange shape caused a sensation in the days after
the fly-by. The body is a contact binary, believed to be a result of low velocity merging of two separate bodies that formed close together. It
is composed of two connected lobes, of which the smaller one is slightly flattened, the larger one strongly so, creating the impression of a
squashed snowman. In their current publication, the researchers from
China, Germany, and the USA investigate how this shape came to be. A
pronounced bi-lobed shape is also known from some comets. However, there
is no other known body that is as flat as Arrokoth. Did Arrokoth already
look like this when it was created? Or did its shape develop gradually?
"We like to think of the Kuiper Belt as a region where time has more
or less stood still since the birth of the Solar System," explains
Dr. Ladislav Rezac from MPS, one of the two first authors of the current publication. More than four billion kilometers away from the Sun, the
bodies of the Kuiper Belt have remained frozen and unchanged, so is the
common belief. New Horizon's images of Arrokoth challenge this idea
by its apparently smooth surface without signs of frequent cratering
events and by its peculiar, flattened shape. Scientists assume that
the Solar System was formed 4.6 billion years ago from a disk of dust:
the particles from this nebula agglomerated into ever larger clumps;
these clumps collided and merged into even larger bodies. "There is as
yet no explanation as to how a body as flat as Arrokoth could emerge
from this process," says Rezac.

Another possibility would be that Arrokoth had a more ordinary shape
to begin with. It may have started as a merger between a spherical and
an oblate body at the time of its creation and only gradually become
flattened. Earlier studies suggest that during the formation of the
Solar System, the region where Arrrokoth is located could have been a
distinct environment in the cold, dust- shaded mid-plane of the outer
nebula. The low temperatures enabled volatiles such as carbon monoxide
and methane to freeze onto dust grains and compose planetesimals. When
the nebular dust cleared after Arrokoth's formation, solar illumination
would have raised its temperature and hence rapidly driven off the
condensed volatiles. Arrokoth's strange shape would then be a natural
outcome due to a favorable combination of its large obliquity, small eccentricity and mass-loss rate variation with solar flux, resulting in
nearly symmetric erosion between north and south hemispheres.

"For a body to change its shape as extremely as Arrokoth, its rotational
axis needs to be oriented in a special way," Rezac explains. Unlike
Earth's rotational axis, Arrokoth's is almost parallel to the orbital
plane. During its 298 year orbit around the Sun, one polar region of
Arrokoth faces the Sun continuously for nearly half the time while the
other faces away. Regions at equator and lower latitudes are dominated
by diurnal variations year round.

"This causes the poles to heat up the most, so that frozen gases
escape from there most efficiently resulting in a strong mass loss,"
says Dr. Yuhui Zhao from the Purple Mountain Observatory of the Chinese
Academy of Sciences. The flattening process most likely occurred early
in the evolution history of the body and proceeded rather quickly on a timescale of about one to 100 million years during the presence of super volatile ices in the near subsurface layers.

In addition, the scientists self-consistently demonstrated that the
induced torques would play a negligible role in the planetesimal's spin
state change during the mass loss phase.

"How many of such 'flattened snowman' bodies are in the Kuiper Belt
depends primarily on the probability of a body having a spin-axis
inclination similar to Arrokoth's and on the amount of super-volatile
ices present near its subsurface," Rezac says. There are reasons to
believe that even objects like Arrokoth had considerable amounts of super-volatiles that have escaped during its early evolution. For
instance, Pluto, due to its size and stronger gravity retains carbon
monoxide, nitrogen and methane gasses even today. In the case of smaller bodies, these volatiles would long have escaped into space.


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


========================================================================== Journal Reference:
1. Y. Zhao, L. Rezac, Y. Skorov, S. C. Hu, N. H. Samarasinha, J.-Y. Li.

Sublimation as an effective mechanism for flattened
lobes of (486958) Arrokoth. Nature Astronomy, 2020; DOI:
10.1038/s41550-020-01218-7 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/10/201005170843.htm

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