- Carroll, Michael.
Living Among Giants.
Cham, Switzerland: Springer International, 2015.
ISBN 978-3-319-10673-1.
-
In school science classes, we were taught that the solar system, our
home in the galaxy, is a collection of planets circling a star,
along with assorted debris (asteroids, comets,
and interplanetary dust). Rarely did we see a representation of
either the planets or the solar system to scale, which
would allow us to grasp just how different various parts of the solar
system are from another. (For example, Jupiter is more massive than
all the other planets and their moons combined: a proud
Jovian would probably describe the solar system as the Sun, Jupiter,
and other detritus.)
Looking more closely at the solar system, with the aid of what has
been learned from spacecraft exploration in the last half century,
results in a different picture. The solar system is composed of
distinct neighbourhoods, each with its own characteristics. There
are four inner “terrestrial” or rocky planets: Mercury,
Venus, Earth, and Mars. These worlds huddle close to the Sun, bathing
in its lambent rays. The main asteroid belt consists of worlds like
Ceres, Vesta, and Pallas, all the way down to small rocks. Most orbit
between Mars and Jupiter, and the feeble gravity of these bodies and
their orbits makes it relatively easy to travel from one to another
if you're patient.
Outside the asteroid belt is the domain of the giants, which are the subject
of this book. There are two gas giants: Jupiter and Saturn, and two ice
giants: Uranus and Neptune. Distances here are huge compared to the
inner solar system, as are the worlds themselves. Sunlight is dim
(at Saturn, just 1% of its intensity at Earth, at Neptune 1/900 that
at Earth). The outer solar system is not just composed of the four giant
planets: those planets have a retinue of 170 known moons (and doubtless
many more yet to be discovered), which are a collection of worlds as diverse
as anywhere else in the domain of the Sun: there are sulfur-spewing
volcanos, subterranean oceans of salty water, geysers, lakes and rain of
hydrocarbons, and some of the most spectacular terrain and geology known.
Jupiter's moon Ganymede is larger than the planet Mercury, and appears to
have a core of molten iron, like the Earth.
Beyond the giants is the
Kuiper Belt,
with Pluto its best known denizen. This belt is home to a
multitude of icy worlds—statistical estimates are that
there may be as many as 700 undiscovered worlds as large or larger
than Pluto in the belt. Far more distant still, extending as far
as two light-years from the Sun, is the
Oort cloud,
about which we know essentially nothing except what we glean from
the occasional comet which, perturbed by a chance encounter or
passing star, plunges into the inner solar system. With our present
technology, objects in the Oort cloud are utterly impossible
to detect, but based upon extrapolation from comets we've observed,
it may contain trillions of objects larger than one kilometre.
When I was a child, the realm of the outer planets was shrouded
in mystery. While Jupiter, Saturn, and Uranus can be glimpsed by
the unaided eye
(Uranus,
just barely, under ideal conditions, if
you know where to look), and Neptune can be spotted with a modest
telescope, the myriad moons of these planets were just specks of
light through the greatest of Earth-based telescopes. It was not until
the era of space missions to these worlds, beginning with the
fly-by probes
Pioneer
and
Voyager,
then the orbiters
Galileo
and
Cassini,
that the wonders of these worlds were revealed.
This book, by science writer and space artist Michael Carroll, is a tourist's
and emigrant's guide to the outer solar system. Everything here is on an
extravagant scale, and not always one hospitable to frail humans. Jupiter's magnetic
field is 20,000 times stronger than that of Earth and traps radiation so
intense that astronauts exploring its innermost large moon Io would
succumb to a lethal dose of radiation in minutes. (One planetary
scientist remarked, “You need to have a good supply of
grad students when you go investigate Io.”) Several of the
moons of the outer planets appear to have oceans of liquid water
beneath their icy crust, kept liquid by tidal flexing as they orbit
their planet and interact with other moons. Some of these oceans
may contain more water than all of the Earth's oceans. Tidal flexing
may create volcanic plumes which inject heat and minerals into these
oceans. On Earth, volcanic vents on the ocean floor provide the
energy and nutrients for a rich ecosystem of life which exists
independent of the Sun's energy. On these moons—who knows?
Perhaps some day we shall explore these oceans in our submarines
and find out.
Saturn's moon
Titan is an
amazing world. It is larger than Mercury, and has an atmosphere 50%
denser than the Earth's, made up mostly of nitrogen. It has rainfall,
rivers, and lakes of methane and ethane, and at its mean temperature of
93.7°K, water ice is a rock as hard as granite. Unique among worlds
in the solar system, you could venture outside your space ship on Titan
without a space suit. You'd need to dress very warmly, to be sure,
and wear an oxygen mask, but you could explore the shores, lakes, and
dunes of Titan protected only against the cold. With the dense
atmosphere and gravity just 85% of that of the Earth's Moon, you
might be able to fly with suitable wings.
We have had just a glimpse of the moons of Uranus and Neptune as
Voyager 2
sped through their systems on its way to the outer darkness. Further
investigation will have to wait for orbiters to visit these planets, which
probably will not happen for nearly two decades. What
Voyager 2 saw was tantalising. On Uranus's moon Miranda,
there are cliffs 14 km high. With the tiny gravity, imagine the extreme
sports you could do there! Neptune's moon
Triton
appears to be a Kuiper Belt object captured into orbit around Neptune
and, despite its cryogenic temperature, appears to be geologically
active.
There is no evidence for life on any of these worlds. (Still, one
wonders about those fish in the dark oceans.) If barren,
“all these worlds are ours”, and in the fullness of time
we shall explore, settle, and exploit them to our own ends. The outer
solar system is just so much bigger and more grandiose than the
inner. It's as if we've inhabited a small island for all of our
history and, after making a treacherous ocean voyage, discovered an
enormous empty continent just waiting for us. Perhaps in a few
centuries residents of these remote worlds will look back toward the
Sun, trying to spot that pale blue dot so close to it where their
ancestors lived, and remark to their children, “Once, that's all
there was.”
March 2015 