Sky & Telescope - July 2020 - 8

NEWS NOTES

March 28

March 30

Hubble images show Comet Borisov fragmenting in late March.

SOLAR SYSTEM

Is Interstellar Comet
2I/Borisov Breaking Up?
THE INTERSTELLAR COMET desig-

nated 2I/Borisov, discovered on its way
into the solar system in 2019 (S&T: Jan.
2020, p. 10), began shedding chunks in
late March - more than three months
after its closest approach to the Sun.
Two analyses of Hubble Space Telescope images taken between March
23rd and 30th show fragments drifting
from the comet.
Analysis of the images shows a
small piece of the comet breaking off
and ﬂoating slowly away, David Jewitt
(University of California, Los Angeles)
and colleagues found. The fragment was

bright and only 180 km (112 mi) from
the comet's core at the time Hubble
took the images.
What's more, after processing the
same Hubble images to increase the
contrast between the core and the
coma around it, Bryce Bolin (Caltech)
and colleagues spotted a second piece
farther out, ﬂoating away at more than
0.5 m/s. The two results were posted to
The Astronomer's Telegram on April 2nd
and April 3rd, respectively.
While the fragment that Jewitt's team
observed is about as
bright as the comet's
core, he thinks this is
u A more involved analysis
shows a second fragment
farther out from the core.

SOLAR SYSTEM

Why Have Parts of Mercury's Landscape Collapsed?
CHAOTIC TERRAIN , an unexplained

type of landscape on Mercury, might be
the legacy of vast amounts of volatile
materials that were once stored deep
underground.
For decades, planetary scientists have
thought this weird terrain was collateral damage from the impact that made
the Caloris Basin on the exact opposite
side of the planet. The impact could
have set off massive seismic ripples that
raced around the world, meeting on the
opposing side to jumble the landscape.
8

J U LY 2 0 2 0 * S K Y & T E L E S C O P E

But in a new study, Alexis Rodriguez
(Planetary Science Institute) and colleagues suggest that the terrain achieved
its chaotic look because of wholesale
collapse. Easily vaporized elements
stored underground escaped gradually;
once enough of these materials had
gone, the landscape deﬂated. If so, then
volatiles existed relatively recently on
Sun-baked Mercury. The ﬁndings, based
on observations made by NASA's Messenger spacecraft, appear in the March
16th Scientiﬁc Reports.

■ JEFF HECHT

* Read additional
details at https://is.gd/
borisovbreakup.

The researchers present crater
analyses to support their claim. Crater
counts provide a proxy of age, enabling
the researchers to estimate the chaotic
terrain's age at 1.8 billion years old - at
least 2 billion years younger than the
Caloris Basin. Also, ejecta rays from
recent craters disappear in some areas
of chaotic terrain, which aligns with
the idea that the ground-collapse events
occurred in geologically recent times.
Furthermore, the team found terrain with similar texture in regions
other than the one directly opposite the
Caloris Basin. These smaller areas don't
appear to be related to large impacts.

CORE FR AG MEN T: D. JE WIT T E T A L. / THE ASTR ONOMER'S
TELEGR AM 2020; SECOND FR AG MEN T: B. BOLIN E T A L. / THE
ASTR ONOMER'S TELEGR AM 2020

March 23

due more to its icy surface than its size.
Its mass, he estimates, is likely less than
1% of the whole comet, which would
make the split more like a side mirror dropping off a car than the whole
car falling apart. The piece that Bolin
reports is likewise small, no more than
100 meters across based on its visibility.
While the data provide evidence that
the comet is undergoing a fragmentation event, Bolin cautions that the
event hasn't yet destroyed the nucleus.
"When comets catastrophically disrupt,
their brightness drops very, very quickly
. . . we're not seeing that with Borisov,"
he explains.
The timing is strange, because comets are most likely to fragment when
nearest the Sun; however, Comet Borisov has been close enough for sunlight
to vaporize ices from its surface for the
past six months. That said, fragmentation was still possible in late March,
when it was still 2.6 a.u. from the Sun.
Bolin says the comet appears to have
been little altered when it arrived at
our solar system, despite eons traveling
through interstellar
space. But it may have
a narrow escape on the
way out.
https://www.is.gd/borisovbreakup https://www.is.gd/borisovbreakup
Sky & Telescope - July 2020

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