JOURNEY TO THE CENTER OF THE EARTH

Jules Verne wrote about a human journey to the center of the earth. However, his and other science fiction on the subject was based on false ideas about what the earth was like. This one is based on current (1999) science.

Well, not by people, only instruments, and maybe not to the center but only to the core. Consider a 400 meter long rod of tungsten. Tungsten is enough heavier than ordinary rock that the pressure at the bottom of the rod is enough greater than the ambient pressure plus the compressive strength of ordinary rock. Therefore, the rod will sink, maybe close to the center of the earth. The object is to instrument the rod to report back to geophysicists on the surface about the composition and other measurable properties of the layers it passes. A number of questions arise.

1. Is 400 meters, a rough calculation, the right length? Will it penetrate better if it is pointed?
2. What diameter must it have so that it won't be held up by friction on its sides? What about buckling?
3. How fast will it sink? Presumably it will sink slower as it gets deeper and the gravitational attraction of the earth below it lessens. The attraction is proportional to the distance from the center, so it won't slow down very fast.
4. Presumably it won't melt, because the melting point of tungsten is higher than that of the layers through which it passes.
5. It will be best if its instruments can function at ambient temperature and pressure, since it will be difficult to maintain an instrument compartment at surface temperature and pressure. Is it impossible?
6. What are the instruments? (1) temperature and pressure. (2) seismometer. (3) Backscatter from a radioactive source as is used in oil well logging. Multiple sources will be wanted. (4) Chemical analysis of the layers it passes.
7. What is the power source? Presumably a nuclear reactor powered heat engine, but it must have a T₁ greater than ambient, and a T₂ at ambient or somewhat higher, so it will be a different kind of nuclear reactor and a different kind of heat engine than is used on the surface.
8. How does information get back to the surface? (1) The penetrator trails a cable. This seems chancy. (2) Information is coded in neutrinos produced by an accelerator. This would require quite a large neutrino detector, but who says this project would be cheap. (3) Sound waves, possibly from explosions. If none of these worked, maybe seismic waves would bounce off it detectably, at least giving its current location.

I hope to provide some numbers in a future web page, but it would be better if they were provided by someone with better qualifications than mine.

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Send comments to jmc@cs.stanford.edu. I sometimes make changes suggested in them. - John McCarthy

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