Resurgence

Those measurements could only be done on the surface, and that in turn implied their journey to Iceworld would be delayed by at least two extra days. Before the group left the Pride of Orion, Hans had suspected something like this might be necessary. With a warm world you could take high-resolution images from orbit. But if a world froze over and you wanted to know what it had been like before the freeze, you had to make measurements down on the ground. Also, Hans himself needed to visit the surface, no matter what the orbital measurements showed. You could never get a gut feel for a world from orbit.

It was useless to try to explain this to the others. He stared at the frosted ball of the planet, enhanced in the ship’s display to gleam faintly with reflected starlight. With luck, maybe Darya and the rest would conclude for themselves the need for a trip all the way down.

* * *

The trouble was, the Savior’s instruments were almost too good. They represented the best technology available to the Fourth Alliance, and from an orbital altitude of no more than two hundred kilometers the imaging sensors and radar altimeters left little to the imagination.

“Descend to the surface, to learn what?” Ben Blesh was watching on the display a revealing picture. It showed a succession of hills and valleys, all coated with a layer of blinding white. “It’s obvious what happened down there. The whole globe shows peaks and rifts and flat ocean surfaces, which the synthetic aperture radar confirms. There was no worldwide glaciation—no time for that. It’s clear that when the temperature dropped, all the water vapor and carbon dioxide precipitated out. You’d get one fall of water-snow and solid carbon dioxide. After that nothing would change. The air that remains still has some oxygen as well as nitrogen and argon, so things happened fast. There’s no doubt about the sequence of events. What can we possibly gain by going down to the surface?”

He was asking a question that had occurred to Hans long ago. Before they achieved parking orbit, he had fired off a question to the Pride of Orion. Suppose that the internal energy source of a main sequence star were somehow turned off in a short time span (weeks or months). How long would it take to cool down by normal radiative cooling? I’m looking for an order of magnitude result: are we talking years, centuries, millennia, or millions of years?

The first reply from Julian Graves was disappointing. I have consulted E.C. Tally, who is making his own calculations supplemented by the ship’s astrophysics library. Because the answer to your question depends on several other unknowns, in particular the star’s stage of progression along the main sequence, and the amount of gravitational potential energy contributed by the star’s own shrinkage during cooling, Tally is reluctant to provide a firm answer. He is, however, willing to provide a range of possibilities.

Hans could imagine. The embodied computer would hum and mutter and hedge his bets until you were ready to scream. Luckily it was Julian Graves who had to sit and listen, rather than Hans himself. Did that mean E.C. Tally was still aboard the Pride of Orion? He should have been on his way days ago.

Graves’s second answer was a bit better. At an absolute minimum, with limiting values of all variables, E.C. Tally indicates that radiative cooling would require twenty thousand years. A more likely value, including the gravitational energy provided by stellar shrinkage, would be between eight and eighteen million years. Any shorter value than twenty thousand years would prove that some external agent was employed to expedite cooling. One observational indicator is the ice fracture patterns, if any, on the planet’s surface. They will tell you if the cooling was rapid or slow. Here are the characteristic appearances associated with particular rates of cooling.

Hans examined the images sent from E.C. Tally, and then those of the surface of the world that they were approaching. Ben Blesh’s comment was correct. There were visible cracks and fissures, but all were coated with a surface of white. The fusion process in the central star had not simply ceased, to be followed by a slow decline in stellar temperature. The sun had gone out, and its surface had cooled from around seven thousand degrees to a few hundred degrees in a very short period—a few decades, or even a few hours.