Australian_Geographic_2015_07-08.

NASA/JPL-CALTECH/ASI/USGS

T

HINKING out-
side the box is
normal for most
space scientists, but
when the box includes
Saturn’s biggest moon,
Titan, ideas have to be
even more audacious
than usual. What sets Titan apart is
not mere size: it is the only world,
other than Earth, known to have large
stable bodies of liquid on its surface.
And Titan’s lakes and seas drive a
weather cycle of evaporation and rain-
fall, exactly as the oceans on Earth do.
That, however, is where the similarity
ends. With Titan’s average ambient
temperature of –180°C, the moisture in
its atmosphere is not water vapour, but a
melange of hydrocarbons that are best
thought of as liquid natural gas – ethane,
methane and other compounds. These
liquids pool in basins in Titan’s bedrock,
which is actually frozen water.
Titan’s seas and lakes dominate its
northern polar region, but there are a few
in the south too. And they are large –
comparable with North America’s Great
Lakes in the case of the three biggest,
which are designated as ‘maria’, or seas.

Some 30 smaller lakes, ranging from a

few kilometres in length to a couple of

hundred have also been identified.

The discovery and exploration of

these lakes by remote sensing from the

orbiting Cassini spacecraft has raised

questions. Are they fed by methane and

ethane springs from a ‘water table’?

Could cyclones form over the larger seas?

And do they have hydrocarbon ‘icebergs’

• as radar observations suggest?
  The best way to answer these ques-
  tions, says Ralph D. Lorenz, a scientist
  at Johns Hopkins University in the
  USA, is to send a submarine to Titan.
  It would be a robotic submarine similar
  to the unmanned underwater vehicles
  already in use for military, scientific and
  commercial purposes. And it could be
  carried there by a mini space shuttle
  able to glide through Titan’s thick
  atmosphere – perhaps similar to the
  US military’s secret X-37B space shuttle.
  Exploring the sea floors of Titan to
  reveal its chemical and climatic history
  could be the next big step in our
  understanding of the Solar System.

Are galaxies rotating? Astron-

omy has only been around for

a few hundred years; surely the

rotation measured would be so

small as to be negligible in such

a short time period?

David Frankland, Perth

You’re quite right that the visible

rotation of galaxies is infinitesi-

mally small on human timescales.

Instead, we measure their rotation

with the Doppler effect, which

shifts the wavelength of light

emitted by moving sources.

Measuring the rainbow spectra of

galaxies lets us determine their

rotational velocities very accurately.

If you have a space question for Fred,

email it to [email protected]

Fred answers

your questions

x1

NAKED EYE Venus and

Jupiter are visible, low

in the western evening sky. The

‘Goddess of Love’ is the brighter

of the planets. Their meeting with

the thin crescent on 19 July will

be impressive. In August they are

seen only early in the month.

BINOCULARS Look high

in the evening sky to

Scorpius the Scorpion. Where the

tail makes a right-angle turn is an

impressive triple star formed by

Zeta (1 and 2), Sco and HR 6266.

Its star field is rich, including the

open star cluster NGC 6231.

SMALL TELESCOPE

NGC 6397, in the

southern constellation of Ara,

is considered to be the fourth-

brightest globular star cluster in

the sky. It has a brilliant, compact

core with the stars more loosely

scattered towards the edge.

Glenn Dawes is a co-author

of Astronomy Australia 2015

(Quasar Publishing).

Glenn Dawes

looking up

x10

x10 0

Exploring the seas of Titan

Getting to the bottom of Titan’s unique geography might

require an underwater mission, says Fred Watson.

SPACE

FRED WATSON is astronomer-in-charge

of the Australian Astronomical Observatory.

Surface detail.

Cassini has captured

images of the lakes

and seas around

Titan’s north pole.

22 Australian Geographic 圀漀爀氀搀䴀愀最猀⸀渀攀琀圀漀爀氀搀䴀愀最猀⸀渀攀琀

WorldMags.net