Sky & Telescope - March 2020 - 9

SOLAR SYSTEM

New Map Shows
Titan's Geological
History
WORKING WITH DATA FROM

NASA's Cassini mission, scientists
have produced the ﬁrst global map of
Titan's geological features. While it's
not the ﬁrst to portray these features,
this map shows the evolution of geology
on Saturn's largest moon.
Rosaly Lopes (Caltech) and colleagues combined more than 120 ﬂybys'
worth of data to create the map, as
described November 18th in Nature
Astronomy. In addition to high-resolution radar mapping that covered almost
half of the moon's surface, Cassini
also characterized the landscape using
lower-resolution radar and near-infrared images and spectra.
The team identiﬁes six main geological features and, based on mutual over-

Kraken Mare

Shangri-La
Xanadu
Dune
Huygens
Landing Site

p This map shows Titan on a 1:20,000,000
scale, with six main geological features marked
in color. NASA's Dragonfly mission, targeting a
2034 landing on the faraway moon, will investigate the Shangri-La Dune (S&T: Oct. 2019,
p. 8), near the Huygens landing site.

lap, estimates their relative ages from
newest to oldest: dunes, lakes, plains,
labyrinths, craters, and hummocky
regions. Plains and dunes dominate the
surface, covering two-thirds and oneﬁfth of the surface, respectively.
The youngest features, dunes and
lakes, don't overlap each other, so it's
not yet possible to tell which feature is
newer. Dunes are common at Titan's

Lakes

Hummocky

Crater

Labyrinth

Dunes

Plains

arid equatorial latitudes,
while lakes dominate the
more humid poles.
Some features are suggestive
of long-term evolution. For example,
global climate change might explain
why the north pole hosts most of the
wet lakes, while the south pole has
more dried-out lakebeds and labyrinths.
The latter likely formed when methane
rain dissolved elevated terrain, creating
steep channels, like karsts on Earth.
Spectra of the two oldest features -
craters and hummocky regions - reveal
the presence of water ice, hinting at
even longer-term changes. Excavating
impacts and the irregular mountainous
terrain appear to expose an ancient icy
crust that underlies the hydrocarboncoated surface.
■ MONICA YOUNG

GALAXIES

Three Black Holes May Merge in Nearby Galaxy

TITA N G EOLOGY M A P: N ASA / JPL- CA LTECH / ASU; G A L A X Y: N ASA / ESA / HUBBLE
HERITAG E (STSCI / AUR A) / HUBBLE COLL A BOR ATION / A . E VA NS (UNIV. OF VIRGINIA ,
CH A RLOT TESVILLE / NR AO / STON Y BROOK UNIV.); INSE T: P. WEILBACHER (AIP)

NEW OBSERVATIONS OF a galactic

crash site show that there are not just
two but three black holes on the verge
of union in the butterﬂy-shaped galaxy
NGC 6240. Two of them are the tightest
supermassive pair known.
Astronomers have long known that
there were two black holes in the galaxy.
Images taken by the Chandra space telescope in 2001 had revealed X-rays from
hot gas swirling into the twin maws.
Using the Multi Unit Spectroscopic
Explorer (MUSE) on the European
Southern Observatory's Very Large Telescope in Chile, Wolfram Kollatschny
(University of Göttingen, Germany)
and colleagues examined light emitted
by ionized elements, such as hydrogen, nitrogen, and sulfur, across the
galaxy. Spotting two peaks of emission
within one of the radio knots, they
realized that one of the black holes is
actually two, a close pair separated by
only 650 light-years. The discovery will
appear in Astronomy & Astrophysics.

By measuring the speeds of stars
whirling around each black hole, Kollatschny's team estimates that the
northern black hole has 360 million
Suns' worth of mass; the southern duo
has 710 million and 90 million solar
masses, respectively. The whole trio is
contained within a volume less than
3,000 light-years across.
"Such a concentration of three
supermassive black holes has so far
never been discovered in the universe,"
says team member Peter Weilbacher
(Leibniz Institute for Astrophysics Potsdam, Germany).
With three black holes in its core,
NGC 6240 is probably the remnant of a
three-galaxy merger. Theoretical calculations have shown that such multiple
mergers should be rare in the presentday universe, so this multiple-merger
system represents an opportunity to
study something that usually only
occurs in the distant universe.
■ MONICA YOUNG

N
S1

S2
1,000
light-years

p The relatively nearby galaxy NGC 6240
harbors three supermassive black holes at its
core: The northern black hole (N) is active and
was known before. New observations show
that the southern component actually consists
of two black holes (S1 and S2). The green color
indicates gas ionized by radiation surrounding the black holes, and the red lines show the
contours of the galaxy's starlight.
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Sky & Telescope - March 2020

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