Wednesday, October 31, 2012

Where hydrogen is consumed

kw: observations, solar physics

Immediately after reviewing the prior book this morning, I began reading 2132 by Kim Stanley Robinson. It is big, more than 560 pages, so it will take a while, even at my normal breakneck reading pace. Right on page three, I saw this lovely bit of prose, describing the view from Mercury:
"[…look] at the sun's photosphere, and even magnify your view of it, until the burning tops of the convection cells are revealed in their squiggling thousands, each a thunderhead of fire burning furiously, all together torching five million tons of hydrogen a second—at which rate the star will burn another four billion years."
I really like Robinson's writing, and this is a nice image, but not entirely correct. The solar spicules do look like flames, but the "torching" is going on almost 700,000 km deeper, where the temperature is some 15 million K and the pressure is unimaginable. The convection cells simply bring the heat up from below to a level from which it can radiate into space.

Mercury intercepts about 440 trillionths of this energy on average, and at perihelion its subsolar point can reach 700 K (about 430°C or 800°F). Earth actually intercepts slightly more solar energy, on average, but spreads it out over a body having nearly 7 times the surface area. If Earth were airless like the moon, its subsolar point would reach 390 K (about 115°C or 240°F).

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