"The thing is, Bliss, I'm curious, too. Look how Golan changed as soon as he found out there were no craters on the world we're heading for. I haven't the slightest idea what difference that makes. Do you?"
"Not a bit. Still he knows much more planetology than we do. We can only assume he knows what he's doing."
"I wish I knew."
"Well, ask him."
Pelorat grimaced. "I'm always afraid I'll annoy him. I'm sure he thinks I ought to know these things without being told."
Bliss said, "That's silly, Pel. He has no hesitation in asking you about any aspect of the Galaxy's legends and myths which he thinks might be useful. You're always willing to answer and explain, so why shouldn't he be? You go ask him. If it annoys him, then he'll have a chance to practice sociability, and that will be good for him."
"Will you come with me?"
"No, of course not. I want to stay with Fallom and continue to try to get the concept of the Universe into her head. You can always explain it to me afterward-once he explains it to you."
64.
PELORAT entered the pilot-room diffidently. He was delighted to note that Trevize was whistling to himself and was clearly in a good mood.
"Golan," he said, as brightly as he could.
Trevize looked up. "Janov! You're always tiptoeing in as though you think it's against the law to disturb me. Close the door and sit down. Sit down! Look at that thing."
He pointed to the planet on the viewscreen, and said, "I haven't found more than two or three craters, each quite small."
"Does that make a difference, Golan? Really?"
"A difference? Certainly. How can you ask?"
Pelorat gestured helplessly. "It's all a mystery to me. I was a history major at college. I took sociology and psychology in addition to history, also languages and literature, mostly ancient, and specialized in mythology in graduate school. I never came near planetology, or any of the physical sciences."
"That's no crime, Janov. I'd rather you know what you know. Your facility in ancient languages and in mythology has been of enormous use to us. You know that. And when it comes to a matter of planetology, I'll take care of that."
He went on, "You see, Janov, planets form through the smashing together of smaller objects. The last few objects to collide leave crater marks. Potentially, that is. If the planet is large enough to be a gas giant, it is essentially liquid under a gaseous atmosphere and the final collisions are just splashes and leave no marks.
"Smaller planets which are solid, whether icy or rocky, do show crater marks, and these remain indefinitely unless an agency for removal exists. There are three types of removals.
"First, a world may have an icy surface overlying a liquid ocean. In that case, any colliding object breaks through the ice and splashes water. Behind it the ice refreezes and heals the puncture, so to speak. Such a planet, or satellite, would have to be cold, and would not be what we would consider a habitable world.
"Second, if a planet is intensely active, volcanically, then a perpetual lava flow or ash fallout is forever filling in and obscuring any craters that form. However, such a planet or satellite is not likely to be habitable either.
"That brings us to habitable worlds as a third case. Such worlds may have polar ice caps, but most of the ocean must be freely liquid. They may have active volcanoes, but these must be sparsely distributed. Such worlds can neither heal craters, nor fill them in. There are, however, erosion effects. Wind and flowing water will erode craters, and if there is life, the actions of living things are strongly erosive as well. See?"
Pelorat considered that, then said, "But, Golan, I don't understand you at all. This planet we're approaching-"
"We'll be landing tomorrow," said Trevize cheerfully.
"This planet we're approaching doesn't have an ocean."
"Only some thin polar ice caps."
"Or much of an atmosphere."
"Only a hundredth the density of the atmosphere on Terminus."
"Or life."
"Nothing I can detect."
"Then what could have eroded away the craters?"
"An ocean, an atmosphere, and life," said Trevize. "Look, if this planet had been airless and waterless from the start, any craters that had been formed would still exist and the whole surface would be cratered. The absence of craters proves it can't have been airless and waterless from the start, and may even have had a sizable atmosphere and ocean in the near past. Besides, there are huge basins, visible on this world, that must have held seas, and oceans once, to say nothing of the marks of rivers that are now dry. So you see there was erosion and that erosion has ceased so short a time ago, that new cratering has not yet had time to accumulate."
Pelorat looked doubtful. "I may not be a planetologist, but it seems to me that if a planet is large enough to hang on to a dense atmosphere for perhaps billions of years, it isn't going to suddenly lose it, is it?"
"I shouldn't think so," said Trevize. "But this world undoubtedly held life before its atmosphere vanished, probably human life. My guess is that it was a terraformed world as almost all the human-inhabited worlds of the Galaxy are. The trouble is that we don't really know what its condition was before human life arrived, or what was done to it in order to make it comfortable for human beings, or under what conditions, actually, life vanished. There may have been a catastrophe that sucked off the atmosphere and that brought about the end of human life. Or there may have been some strange imbalance on this planet that human beings controlled as long as they were here and that went into a vicious cycle of atmospheric reduction once they were gone. Maybe we'll find the answer when we land, or maybe we won't. It doesn't matter."