Astronomy

22 ASTRONOMY • JANUARY 2018

Saturn’s largest moon, Titan, is
one of the solar system’s most
intriguing worlds. It hosts a
thick atmosphere denser than
Earth’s and harbors lakes and
rivers of a methane-ethane
mixture on its surface. The
saturnian satellite even has
a complex weather system
that circulates this liquid,
which evaporates into the
atmosphere, condenses into
clouds, and then rains back
to the surface. Could Titan
even harbor some form of life
that relies on hydrocarbons in
the way Earth organisms rely
on water? Perhaps. Scientists
continue to find more
complex chemistry at the
satellite (although, no aliens).
A July 28 paper in
Science Advances shows
Titan is astrobiologically
interesting. While looking
for the molecule acetonitrile
isotopolog in archived
observations taken with the
Atacama Large Millimeter/
submillimeter Array (ALMA),
researchers found a signal
from vinyl cyanide, “a more
exciting molecule,” says
Maureen Palmer of NASA’s
Goddard Space Flight Center
and lead author of the
discovery paper. Laboratory
experiments and computer
simulations suggest that this
particular molecule would be
a stable material from which
to form something called a

lipid bilayer membrane, a

structure that separates and

protects a cell’s innards from

its environment. All living

organisms on Earth have such

cell membranes.

Palmer and her colleagues

looked through several

months of ALMA observations

of Titan and found three

spectral lines associated

with vinyl cyanide. Each of

these lines is produced as

an energized vinyl cyanide

molecule settles to a lower

energy state, giving off a

photon. The brightness of

each spectral line relates to

the number of photons ALMA

received. “And the number of

photons is dependent on how

many molecules are changing

energy levels and releasing

these photons,” adds Palmer.

The researchers modeled

different scenarios to estimate

the overall abundance of vinyl

cyanide at Titan, but their

data weren’t sensitive enough

to create a map of where in

the atmosphere the molecule

is most abundant. They’ve

taken more observations

of Titan with ALMA and are

currently analyzing them.

This type of work —

hunting for biologically

important chemistry — can

begin to address how life

forms, at other locales and

here on Earth. Perhaps soon,

we’ll find out we’re not alone.

A perfectly positioned

standard candle

On September 5, 2016, the 48-inch telescope at Palomar

Observatory captured a stellar explosion, dubbed iPTF16geu.

Astronomers published the discovery April 21 in Science.

This isn’t just any type of explosion — it’s one that’s used as

a ruler to measure cosmic distances, called a type Ia supernova

(or type Ia SN). Each of these blasts has a nearly identical light

curve, which measures brightness over time. The fainter the

explosion, the farther it must lie from us. Astronomers can cal-

culate distances to high precision by comparing these blasts

because of their similarities. Scientists used this type of mea-

surement in the late 1990s to show the universe’s expansion is

speeding up, as some type Ia SN explosions are fainter than

expected, meaning they are farther than expected.

But iPTF16geu is more than just a type Ia supernova.

Astronomers found four images of that same blast at the site

of a galaxy lying about 2 billion light-years away. It turns out

the supernova lay directly behind that galaxy as viewed from

Earth. When objects line up like this, background light is bent

around the foreground object — in this case, the massive gal-

axy. (Think of putting a straw into a glass of water; the straw

looks like it bends.) As a result, the light from the blast took

four separate paths around the galaxy, leading to four images of

the same stellar explosion. And with four images, there’s more

science that can be done. That’s because the light in each image

traveled a slightly different path around the intervening galaxy.

Slightly different paths equate to different distances and differ-

ent amounts of time. “If you measure the arrival times of the

different images, that turns out to be a good way to measure the

expansion rate of the universe,” lead author Ariel Goobar said

in a press release.

Astronomers expect this is the first of many similar discov-

eries, as new surveys come online and software better under-

stands how to pick out multiply lensed explosions. Not only will

finding more lensed supernovae improve our ability to measure

the expansion rate of the universe, new gravitational lenses will

also allow astronomers to more accurately map the distribution

of normal and dark matter throughout the cosmos.

Titan’s complex chemistry

— and astrobiology?

A distant supernova, iPTF16geu lit up the sky not once, but four times,

thanks to the phenomenon of gravitational lensing. The Hubble Space

Telescope caught this image of the lensed supernova, which appears as

four bright spots surrounding a blue foreground galaxy. The supernova

itself occurred at a distance of 4.3 billion light-years. ESA/HUBBLE, NASA

Cassini’s high-

resolution camera

penetrated the

haze around Titan

and spotted some

of the moon’s

largest seas and

lakes of liquid

hydrocarbons.

These seas could

sustain life:

Researchers found

a signal from

vinyl cyanide,

a molecule that

could act as a

cell membrane.

NASA/JPL-CALTECH/SPACE

SCIENCE INSTITUTE