Sky & Telescope - May 2019 - 11

SOLAR SYSTEM

Saturn Hasn't Always
Had Rings
ANALYSES OF DATA from the ﬁnal
days of NASA's Cassini spacecraft suggest Saturn's rings are a late addition
and may be related to the formation
(and destruction) of its moons.
As Cassini's gas gauge sank toward
empty, mission planners rerouted
the orbiter to take 22 daredevil dives
between the planet and its rings. Every
time it passed through, the spacecraft
felt the gravitational tug of both the
planet and the rings.
Luciano Iess (Sapienza University of
Rome) and colleagues teased out the
rings' contribution from the planet's,
reporting January 17th in Science that
the rings have a mass of 1.5 × 1019 kg.
That's equivalent to roughly 40% of
Saturn's mid-size moon Mimas.
Cassini data have also indicated that
some 10 times more space dust is raining down on the rings than expected,
meaning they can't be really old and
still look as bright as they do. The low
mass measurement, combined with
information on space dust as well as
simulations of the rings' evolution, suggests that the rings are young, roughly
10 million to 100 million years old.

These results jibe with work appearing in Icarus by James O'Donaghue
(NASA Goddard) and colleagues, who
re-analyzed ground-based data taken in
2011 at the Keck II Telescope on Mauna
Kea, Hawai'i. They found that icy ring
particles, in addition to raining down
on the equator via gravity, also ﬂow in
along magnetic ﬁeld lines to reshape the
chemistry of the planet's outer atmosphere. Combining this analysis with
Cassini data, O'Donaghue's team also
pegs the rings' lifetime at less than 100
million years.
The rings' age range echoes that previously suggested for several of Saturn's
mid-size icy moons (S&T: Aug. 2016,
p. 14). Some planetary scientists think
these satellites formed from the rubble
of a previous generation of moons. How
that event and the current ring system
might be connected remains unclear,
however: It's hard to understand why
the detritus that makes up the rings
would have migrated so close to the
planet instead of clumping together
into additional moons farther out, Iess's
team notes in the paper. One alternate
idea is that Saturn caught a passing
comet or icy asteroid and tore it apart,
then festooned itself in the wreckage.

IN BRIEF

■ CAMILLE M. CARLISLE & MONICA
YOUNG

The glow of a star torn up by a supermassive black hole - a tidal disruption
event discovered in 2014 and designated
ASASSN-14li - has enabled researchers
to measure how fast the black hole whirls
around. Reporting January 9th at the winter meeting of the American Astronomical
Society and in the February 1st Science,
Dheeraj Pasham (MIT) and colleagues have
used an innovative method to add to the
list of a couple dozen supermassive black
holes with measured spins. Using archival
observations from NASA's Chandra and
Swift space telescopes and the European Space Agency's XMM-Newton, the
researchers found a bright, periodic X-ray
pulse that repeated every 131 seconds.
The signal lasted a surprisingly long time
- at least 450 days. To endure for more
than a year, the pulse is likely related to
material orbiting the black hole. For the gas
to come as close as the pulse's frequency
implies, the black hole itself has to be
spinning rapidly, dragging the gas tightly
around itself. The spin is at least 70% of
the black hole's theoretical maximum.
■ CAMILLE M. CARLISLE

N ASA / JPL- CA LTECH / SPACE SCIENCE INSTIT U TE

Saturn's rings appear to
dwarf its moon Mimas,
but in fact the rings
contain a fraction of
Mimas's mass.

A Bevy of Fast Radio
Bursts Announced
A new radio array, the Canadian Hydrogen
Intensity Mapping Experiment (CHIME),
has spotted 13 new fast radio bursts
(FRBs) - including the second one known
to repeat. These mysterious flashes probably occur thousands of times a day, but
they're so fleeting that astronomers miss
most of them. The new FRBs were detected over a period of about three weeks
last summer, when CHIME was running
at only a fraction of its full capacity. The
highlight of the bounty is a single burst that
flared time and again. Appearing January
9th in Nature and announced at the winter
meeting of the American Astronomical Society, the repeating burst was first detected
on August 14th, and then CHIME saw it
pop up five additional times. The only other
known repeating FRB was first detected in
2012 (S&T: June 2016, p. 12) and has reappeared hundreds of times since. The two
repeaters share striking similarities in their
radio signals that could help astronomers
understand their origins.
■ SHANNON HALL

Read more about the repeating FRB at
https://is.gd/CHIMEFRB.

Annihilated Star Clocks
Black Hole's Spin

s k y a n d t e l e s c o p e . c o m * M AY 2 0 1 9

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https://www.is.gd/CHIMEFRB http://www.skyandtelescope.com
Sky & Telescope - May 2019

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