Home > Clement's Game, Hal Clement > SciFi Round Eleven: No, You Can’t Have A Hollow Planet. Not Even A Little One.

SciFi Round Eleven: No, You Can’t Have A Hollow Planet. Not Even A Little One.

This time, we’re going to echo back to the first post here.  I’ll be talking about another of Hal Clement’s books.  This one is called Still River.  As you’ll see, the world-building here is sloppy as compared to Clement’s usual standards.  But I still have a soft spot for the book, because it is a science fiction novel about planetary science graduate students.

Cover art for the hardcover printing of Still River (1987).  The thing in the center is a fusion-powered robot.  The person hanging from it upside down is a human graduate student.  The person in the background is an alien graduate student who just happens to have the same number of arms and legs.

Cover art for the hardcover printing of Still River (1987). The thing in the center is a fusion-powered robot. The person hanging from it upside down is a human graduate student. The person in the background is an alien graduate student who just happens to have the same number of arms and legs.

Field Camp

The protagonists of the book are a bunch of students studying for the degree of “Respected Opinion” at a many-species institution of higher learning based on a number of planets around stars in the Carina Nebula.  One wonders why a galaxy-spanning federation with relatively casual interstellar travel would put their university within 10 parsecs of a pending supernova, but at least the view is cool.


The federation built their university near this thing. It may have already gone pop. I think I’d go for a correspondence course…

Although only one of the students (Molly) is human, the others are only referred to by the names given to them by Molly’s universal translator – just like “Respected Opinion” is an alias.  The limitations of the translator are carefully explored, as are some of the quirks of the aliens’ psychologies.  No two of them are identical, and while they are all Starfish Aliens, they aren’t incomprehensible.  They all want to learn about various parts of planetary science and have written up proposals for field projects at one of the institute’s standard locations for field camps.

This small planet is called Enigma 88, and has been used for field exercises for many thousands of years (apparently, the federation has reached a state of technological near-stasis).  This is where things get strange.  Rather than having a standard set of things to explore while on the planet, with an expert guiding the students through and making sure they don’t miss something important, the students are not given access to the reports of the previous expeditions and are tossed down all by themselves to figure out why Enigma still has an atmosphere despite being so small and being bombarded by lots of high-energy photons from Eta Car.  These aliens aren’t following anything like human training methods: the unnecessary lack of background information nearly leads to the deaths of half of the students.  Doesn’t seem like the best strategy if the goal is to efficiently train planetary scientists.  The thing about aliens is they’re alien?

The Pun In The Title

The students hypothesize that Enigma’s atmosphere is being sustained by outgassing from its interior, like how methane is replenished in Titan’s atmosphere.  They approach and land on Engima; track the pattern of air flow and find outflows from a series of cave mouths near one pole; and start spelunking with armored environment suits, support robots powered by miniature fusion reactors, and kilometers of monofilament line.  They are looking for ice or minerals holding lots of chemically-bound volatiles.  In the caves they find a water-rich environment, supporting life.  The life is just contamination: thousands of years of field camps dumping their waste has given a biosphere made of a mis-mash of imported microbes and assorted larger forms that were transported as spores.  They find underground rivers, flowing downward to lower caves very slowly in the low gravity.  As they go further down, the temperature rises and the different components of the fluid in the down-going rivers evaporate into the outgoing air streams, so what was once dominated by water is now dominated by such lovely things as hydrogen peroxide.  The things are slowly moving hundred-kilometer-long distillation systems.  Eventually, the students descend to a point where the evaporation rate is high enough that the river goes no further: the gas venting from still further down carries away the material that’s coming down as fast as it comes in.  And the flow rate goes to zero.

And so we have an incredibly geeky pun.  Given that Clement wrote “Mission of Gravity”, “Star Light”, “Close to Critical”, and “The Nitrogen Fix”, we should not be surprised.

The Impossibility

But it wasn’t enough for Clement to have his fun distillation system be driven by outgassing from hot areas a few hundred kilometers inside Enigma.  Instead, he had there be a planet-wide interior circulation system.  The air flow out of the vents near one pole was balanced not by local down-welling, but by a series of vents at the opposite pole.  The cave system went all the way through the planet, including a single lumpy several-hundred-kilometer-wide void right in the center.  Enigma was supposedly a hollow planet.  We can expect nonsense like that from George Lucas, but Clement should have known better.

Very obviously, it is a lower energy-state for dense material to be at the center of a planet and lower density material on the outside.  The air inside the middle of Enigma is connected to the outside by the cave system.  It is being pushed inwards only by the pressure from the air along the line of the vents.  That pressure is hundreds to a thousand times less than the pressure from the rocks that are pushing the air in the central void outwards, since it is lower-energy for the rocks to occupy that space.  The only way to maintain the central void would be if the rock shell itself could resist the pressure from its own weight.  But as I mentioned in another earlier post, rocks aren’t that strong.  The pressure on the rocks on the inside of Enigma’s shell would be much lower than that in the center of the Earth, but it would still be measured in gigapascals, way above the strength of a large mass of any geologic material.  Enigma can’t exist; it would immediately and violently collapse in on itself.

There’s another part of the impossibility: not only did the students not know the place was hollow, apparently their teachers and all of the previous testing teams didn’t know either.  When they get back with their reports and get out of hospital, the students are rewarded with their degrees and the news that Enigma is now being promoted to active research area rather than field camp.  That makes no sense.  Even without landing on the place and doing seismology, it would be obvious in Enigma’s moments of inertia that it was hollow (and also in the higher-order terms in the gravity field since the voids aren’t spherically symmetric).  The big void space would also show up on very long-wave electromagnetic work, measuring the planet’s interaction with the surrounding interplanetary medium.  That’s how we know about the liquid layers inside Io, Europa, Ganymede, and Titan.

I can only conclude that Clement let his liking for chemistry overwhelm his knowledge of astronomy for this one.


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