Sky & Telescope - July 2024 - 10

NEWS NOTES
BLACK HOLES
Strong Magnetic Fields
Swirl Near the Milky Way's
Black Hole
THE EVENT HORIZON TELESCOPE
(EHT) collaboration has released
another image of our galaxy's central
black hole, Sagittarius A* - this time
revealing the behavior of the magnetic
fields that thread the surrounding gas.
The results appear in two papers in the
April 1st Astrophysical Journal Letters.
The radio images that the EHT produces
are reconstructions of the black
hole's silhouette, a dark shadow framed
by the glow of light that has evaded
the event horizon. The radio emission
is synchrotron radiation, emitted by
electrons corkscrewing along magnetic
field lines.
This light is polarized, the waves
oscillating with a fixed orientation: As
electrons spiral around magnetic field
lines, the particles emit photons that
SOLAR SYSTEM
Webb Spies Evidence of Methane Volcanoes
ERIS AND MAKEMAKE are two of the
largest trans-Neptunian objects (TNOs),
denizens of the solar system beyond
Neptune. Most TNOs have extremely
dark surfaces, the result of being
weathered by solar and cosmic radiation
for billions of years. However, the
biggest TNOs - Eris and Makemake
among them - appear brighter thanks
to surface methane ice. Now, two teams
of scientists, led by Will Grundy (Lowell
Artist's concept
of Makemake
and its moon
Observatory) and Christopher Glein
(Southwest Research Institute), have
obtained James Webb Space Telescope
(JWST) observations of these worlds,
measuring and analyzing the isotopic
ratios of hydrogen in their ice. The publications
appear in the March 15th and
April Icarus, respectively.
Both objects formed with at least
some methane in their water and rocks,
and that methane would have had the
same proportion of deuterium (heavy
hydrogen) as other primordial objects,
such as comets. But JWST data showed
that Eris and Makemake's methane
has less deuterium than expected if the
methane were primordial. Instead, the
deuterium amount is similar to that of
water in other primordial objects, suggesting
that " hot " chemical reactions
with water generated methane, incorporating
the water's lighter hydrogen.
There are two main ways to make
methane (without invoking space cows).
10 JULY 2024 * SKY & TELESCOPE
pLines around the silhouette of the Milky
Way's supermassive black hole trace the light's
polarization, which in turn reveals the behavior
of magnetic fields close to the event horizon.
oscillate perpendicular to the field. The
amount and pattern of polarization
enables astronomers to map the magnetic
fields' structure and strength.
The polarization detected in Sgr A*'s
emission indicates that the magnetic
fields in its accretion flow are strong,
twisted, and orderly. Fields like these
can control and even choke the gas
flowing into a black hole, manipulating
the black hole's diet (S&T: Dec. 2022,
p. 12). They can also serve as a pipeline
for outflowing plasma jets.
EHT astronomers previously found
signs of strong, orderly magnetic fields
around the black hole in the galaxy
M87, too. Yet these two black holes are
very different: Sgr A* is a quiet, finicky
cat lapping from a trickle of gas; M87*
is a roaring lion that shoots out plasma
jets thousands of light-years long.
It could be that strong, orderly
magnetic environments are a universal
feature of the gas that skirts supermassive
black holes. Theoretically, such
fields should also power jets of plasma
that shoot out along the black hole's
poles. Observations have yet to find a
jet from Sgr A*, but data like these raise
hopes that one exists.
Based on comparing the polarization
data with simulations and calculations,
the researchers suggest that the black
hole may be spinning nearly as fast as
physically possible and tilts away from
us such that we're looking up at its
chin. But the team warns that these are
inferences, not measurements.
¢CAMILLE M. CARLISLE
The first is to heat up rocks to more
than 150°C (300°F), so that carbonrich
molecules break apart and loose
carbon atoms can take hydrogen from
water to make methane. Another way
to make methane is to run hot water
through rocks containing smaller carbon
molecules, such as carbon monoxide
and carbon dioxide.
Either scenario suggests Eris and
Makemake could have become hot
enough in the distant past to form
internal oceans. Depending on how hot
these worlds became, there could have
been some pretty spectacular methanedriven
volcanism, generated by the
buildup of gas within the deep ocean.
The JWST data suggest Eris and
Makemake probably experienced at least
some geology and, possibly, explosive
methane volcanism in their past. And
some of that activity might even still be
around, at least on the bigger Eris. The
researchers muse that " visits to Eris and
Makemake with spacecraft are intriguing
to ponder. " They sure are.
¢EMILY LAKDAWALLA
MAGNETIC FIELDS SWIRLING AROUND SGR A*: EHT COLLABORATION; ARTIST'S
CONCEPT OF MAKEMAKE: NASA / ESA / A. PARKER (SOUTHWEST RESEARCH INSTITUTE)
Sky & Telescope - July 2024

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