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Tuesday, March 31, 2015

Reading List: Living Among Giants

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.”

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