book review

Solar System Astrophysics:
Planetary Atmospheres and the Outer Solar System, 2e
by E.F. Milone and W.J.F. Wilson

do the names Bebhionn, Fornjot, Hyrrokkin, and Jarnsaxa ring a bell? They are all moons of Saturn that have been discovered since 2004. Those moons are breeding like rats: Jupiter now has 67, including Hermippe, Orthosie, and Kale. Saturn has 62, and Uranus has 27, with names like Rosalind, Belinda, Sycorax, Margaret, and Puck; two more, still waiting to be named, may have just been discovered. Even (aka ♇), the formerly-planet known as Pluto, has five, and lowly Eris, which was never a planet, has a moon named Dysnomia, which was discovered in 2005.

Such is the knowledge you will gain from Solar System Astrophysics, vol.2, which is a continuation of vol.1 (Background Science and the Inner Solar System). Saturn is lying on its side with its pole pointing toward the Sun. Its rings have their own atmosphere, mostly dihydrogen and dioxygen (H₂ and O₂), as well as an ionosphere consisting mainly of O₂+, O+, and H+. The Earth may be destined to become a ringed planet. There's a lot of stuff about the atmosphere, planetary circulation effects, and ionosphere of Earth.

Astronomy excited the imaginations of early fiction writers. Edgar Allen Poe, for instance, wrote extensively about comets and predicted the Big Bang theory in his poem Eureka, which was harshly criticized as religious heresy. December 2016 Sky and Telescope has a good article on this.

The first mention of the moons of Mars was in Jonathan Swift's 1726 Gulliver's Travels. Swift described their discovery by the astronomers of the fictional flying island Laputa, and got their distances from Mars and their orbital periods almost exactly right. Although some like to credit Mary Shelley with inventing science fiction, Swift's science fiction predates Frankenstein by 97 years.

Astronomers sometimes propose to look for oxygen as a sign of life. But in fact O₂ is common in atmospheres of bodies in the outer solar system. Some of Jupiter's moons have atmospheres that are predominantly oxygen. They're all unique. One is dark red; another has oceans, rivers, and shorelines of liquid methane. Some have magnetic fields and a magnetosphere, indicating a solid metallic core. Saturn's moon Titan has a thick atmosphere of mostly nitrogen 1.4 times as dense as Earth. Lots of these moons have clouds.

Atmospheres aren't static; they change drastically over time. The atmosphere of Pluto, for example, condenses entirely during its winter season since it is 60% further from the Sun at aphelion than at perihelion. The solar wind gradually destroys the atmosphere of any body that lacks a magnetic field.

The asteroids aren't all in one big belt, either. There are at least four distinct asteroid belts between Mars and Jupiter, separated by Kirkwood gaps, the dynamics of which are similar to those that produce the many rings around Saturn.

Although an undergraduate science background is assumed, the authors go to great lengths to keep it interesting and reasonably easy for laymen to read. They debunk the quote falsely attributed to Thomas Jefferson, who supposedly said “It is easier to believe that two Yankee professors would lie than that stones would fall from heaven.” He never actually said that, as far as anyone knows, but the words ring true because early astronomers were reluctant to accept the concept of rocks falling out of the sky.

Perhaps the most famous meteorite is ALH-84001, an SNC-type achondrite originating from Mars. It contains carbonates, magnetite grains and polycyclic aromatic hydrocarbons, and some scientists once proposed that these might be evidence for Martian life. The authors also discuss a possible rare meteorite from Mercury.

And to anyone who still wonders whether a meteoroid becomes more dangerous when it fragments into smaller pieces, the answer once again is an emphatic No. The authors derive an equation on page 637 for the evaporation rate dm/dt that settles the matter once and for all:

where L is the heat of ablation in J/kg, Γ is the drag coefficient, R is the radius of the n fragments of mass m, ρ is density, and Λ is fraction of energy released as heat.

They explain it this way:

This means that a fragmented meteor decelerates more quickly and evaporates faster than an unfragmented meteor, resulting in an initial burst of light, followed by fading, higher in the atmosphere.

We can only hope somebody in Hollywood takes this equation into consideration before putting Bruce Willis in the awkward position of saying something scientifically incorrect.

Update: Of course it would be better if the asteroid could be persuaded to miss the Earth entirely, but then there'd be no movie.