52 Australian Geographic
THIS PAGE: WIKIMEDIA / ITTIZ; OPPOSITE: ALEX RIESAir Force’s ‘secret’ X-37B mini-shuttle (AG 128), could
bring the sub close to the surface before deploying the
craft on a parachute. The prospect of a torpedo-shaped
submarine floating down onto the waves of an alien sea
is fantastic beyond belief. But there is a good chance this
will become reality before the middle of the century.I
F THERE IS ONE thing we have learnt from the robotic
exploration of the Solar System, it is that water and
ice have played a fundamental role in its history. The
Earth’s oceans, for example, are thought to have arrived,
at least partly, during impacts by icy comets from the
Solar System’s fringes. They, in turn, are composed of
the debris of the giant gas cloud that gave birth to our
star and its planets, assembled by gravity. Even the bone-
dry landscape of Mars shows clear evidence of having
been shaped by water. Most planetary scientists agree
Mars’s northern hemisphere has every sign of having
once harboured an ocean; its low elevation and paucity
of impact craters suggesting a regenerated surface con-
trasting strongly with the rugged terrain to its south.
That is not to say Mars is now devoid of water. Much
of it is still there, locked up as ice in the polar caps, or
beneath the surface soil as permafrost at lower latitudes.
Ground-penetrating radar aboard orbiting spacecraft
has revealed glaciers overlain by a thin layer of soil, even
at temperate latitudes. And the overall quantity of ice onMars is far from limited. Data from the ESA’s Mars
Express orbiter have revealed that if just the southern
polar cap melted, it would produce enough water to flood
the entire planet to an average depth of 11m.
It is believed that wet conditions on Mars lasted well
into the planet’s so-called Hesperian era, which occurred
between 3.7 and 2.9 billion years ago. This is the period
during which we know life was beginning on Earth. The
oldest undisputed fossilised terrestrial bacteria date from
3 billion years ago, with speculative evidence of micro-
organisms existing another half a billion years earlier.
Perhaps further robotic exploration of Mars will find
evidence of past, or even present, biological activity. That
is exactly what ESA’s two-stage ExoMars mission, sched-
uled for launch in 2016 and 2018, will be looking for. It
will include a rover capable of drilling 2m into the soil
of Mars, where microbes could be producing the myste-
rious methane emissions that have been detected in the
planet’s atmosphere.T
HERE IS STILL much to learn about the Solar
System’s oceans. The dramatic New Horizons
fly-by of Pluto in July (AG 126) raised questions
about the dwarf planet’s internal heat source – questions
that might also have a bearing on our understanding of
the warmth of Saturn’s moon Enceladus. Being an isolated
world, Pluto is devoid of any tidal heating. Yet data from
New Horizons astonished scientists by revealing a surface
that has been geologically renewed relatively recently.
That suggests an unknown heat source – such as a
radioactive core, or perhaps even the heat given up by a
subsurface ocean as it slowly freezes. An ocean under the
ice of Pluto would be a discovery indeed, and the still-
incoming New Horizons data will be intensely scrutinised
for any evidence of this.
Beyond the Solar System is our wider Milky Way
galaxy, in which we now know planetary systems are
commonplace. Of the 2000 or so ‘exoplanets’ currently
catalogued, only a handful are Earth-like, and none are
proved to have oceans – although several, including a
planet orbiting a red dwarf star, Gliese 581, are within
the ‘habitable zone’ where the temperature is right for
liquid water. Our capability to discover such worlds is
currently limited by technology that is still in its infancy.
But within the next decade or so, it is likely there will be
evidence of liquid surfaces on some exoplanets, and a
better understanding of the occurrence of ice-moons
throughout the galaxy. If our Solar System is anything to
go by, they could number in the hundreds of billions.
Proof of oceans on exoplanets will not come from their
exploration in the near future because the distances are
simply too great. A recently discovered Earth-like planet
in the habitable zone of its parent star – known as Kepler
452b – is relatively close at 1400 light-years away, but
even the fastest spacecraft ever launched would take 30
million years to reach it. So for now – until we’ve found
a way to travel faster than light – we’ll need to rely on the
next generation of large telescopes to look for spectral
signatures of water in light reflected by these worlds.
With such a wealth of proposals aimed at investigat-
ing the Solar System’s seas and oceans, the future for this
kind of research is bright. Meanwhile, Cassini remains
operational and will return more data about the strange
ice-moons of Saturn before its mission ends in 2017.
There’s much to be excited about in our exploration
of waterworlds. And that is certainly quite something to
reflect on, when you next find yourself floating over the
limpid, life-filled waters of our own blue planet. AGFuture robotic exploration of
Mars could find evidence of past
or present biological activity.
Red water. Mars as it
may have looked with
oceans, 3 billion years
ago. If its south polar
cap melted today,
water would cover its
surface to 11m in depth.MARS