Sky & Telescope - March 2021 - 28

S CERES This view from NASA's Dawn spacecraft shows the floor of
Ceres' Juling Crater. The crater floor shows evidence of the flow of ice
and rock, similar to rock glaciers in Earth's polar regions.

come from the outer solar system. McKinnon later calculated
that, as the giants danced their complicated steps, icy objects
from the outskirts could have been hurled inward. Of them,
only one would likely have been successfully implanted in the
belt, while the others were ejected or tossed into the Sun.
Raymond says that as few as 10 Ceres-size objects in the
outer solar system could explain Ceres' existence as the sole
survivor. According to his simulations, roughly 10% of the
planetesimals stirred up by Jupiter and Saturn under the Nice
model should have wound up in the asteroid belt. Ironically,
that makes things more difﬁcult, because it suggests that the
asteroid belt should have more carbon-rich asteroids than it
currently holds. Why it doesn't remains a mystery.
According to Nesvorný, one of the best arguments for
Ceres' implantation comes from meteorites. Rocky and
carbon-rich meteorites are " different beasts, " he says, that
appear to have formed in separate places. C-type, carbon-rich
meteorites best resemble Ceres and most likely come from
the outskirts of the solar system. The parent bodies for these
meteorites tend to orbit in the outer part of the asteroid belt,
farthest from the Sun.
But they aren't the only asteroids making up the outer
belt; stony S-type asteroids are also intermixed with their
carbonaceous siblings. For years, scientists have recognized
differences between C- and S-type meteorites. Now, new
research has quantiﬁed those differences, revealing a gap
in the isotopes found inside the meteorites themselves. The

Four asteroids contain 55%
of the main belt's total mass:
Ceres, Pallas, Vesta, and Hygiea

MARCH 2 021 * SK Y & TELESCOPE

variations between the asteroids' isotopes indicates that they
formed progressively farther out, with C-types placed ﬁ rmly
in the outer solar system.
" There has been this dichotomy between [meteorites] for
decades, " says meteorite scientist Conel Alexander (Carnegie
Institution for Science). " What's changed is now these new
isotope anomalies have been found in the system that hadn't
been looked at carefully before. It all comes down to their
isotopes. "
Combined, the simulations, meteorite isotopes, and
Dawn's observations make a strong case for Ceres having
formed in the outer solar system.
Vesta's origins are harder to pin down. The second most
massive object in the belt, Vesta is differentiated, having
separated into a crust, mantle, and core early in its lifetime.
The fact that it has all three layers makes it fairly unique in
the belt and might be key in solving an ongoing puzzle: the
mystery of the missing mantle.
When newborn objects grow large enough, the heaviest
elements in them sink to the center, while the lightest elements rise to the top. The result is iron-rich cores wrapped in
rocky mantles. Astronomers have found iron-rich asteroids
in the belt and collected iron-rich meteorites on Earth. But
the lighter elements that would make up mantle material are
surprisingly hard to spot in both.
One option, proposed by Bottke and his colleagues in
2006, is that differentiated asteroids formed closer to the
T VESTA Various craters and grooves along the equator stand out in this
image from the Dawn spacecraft of 4 Vesta. The grooves may be due to
a massive, glancing impact that made the interior crack and collapse.

CERES: N ASA / JPL- CA LTECH / UCL A / MPS / DLR / IDA / ASI / IN A F;
V ESTA: N ASA / JPL- CA LTECH / UCL A / MPS / DLR / IDA

Solar System Rubble

Sky & Telescope - March 2021

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