Whedon’s ‘Verse

September 19th, 2007

Joss Whedon’s “Firefly” reflects Joss’s disinterest in science in a subtle way. As a series it got so much right, like no noise in space, but it does have a few oddities from the point of view of Hard SF. Aside from that weird space-drive, I mean.

For example, the terraformed moons & planets of the ‘Verse are said to have had their atmospheres and gravity fixed by the terraforming process. Atmosphere is OK (in decades, mind), but gravity?

Here’s a speculative, but hopefully Hard SF take on fixing a planet/moon’s gravity. Question: without adding mass how do you increase a planet’s surface gravity?

Ans: Shrink the planet.

Consider: materials under compression increase in density. Intense gravitational and electromagnetic fields, perhaps even strong nuclear fields, cause materials to compress into denser forms. Some such are metastable, like diamond too, thus remain dense after the pressure subsides. Some fretful types still worry that particle accelerators might create bits of quark matter (strangelets) which can catalyse catastrophic collapse of the Earth to nuclear density. There’s several reasons why that’s unlikely, but what if there was a nuclear process that can collapse a planet’s metallic core and leave the silicate mantle?

The energetics are actually in favour of that occurring since shrinking a mass releases gravitational binding energy. If ‘Verse engineers found a way of shrinking a metallic core to 0.1% of its previous size then a planet would contract and its surface gravity would increase. At the core/mantle boundary the core’s gravity has increased 100-fold, thus enhancing compression of the silicates of the mantle.

How much would gravity increase? To double the surface gravity a spherical body would need to shrink to 70.7% of its previous size. Doesn’t sound like much, but it means the average density increases by sqrt(8)= 2.83. Escape velocity increases by just 19%, but that’s a second-order problem. Earth, so shrunk, would be just 9,010 km across.

One result I can’t parameterize is where the released gravitational energy would end up – some would become heat and probably melt much of the mantle, but that might be needed to create volcanism and revive a magnetic field. The rest would end up in the chemical bonds of the new high density phases of the compressed mantle.

Anyway there’s a new trick to add to fiction: compressed planets. I’m sure someone can imagine a way of limiting strong nuclear material, like quark matter, to just compacting a metallic core in the 500 years between Now and the ‘Verse.