Nolan frowned. Those mud-colored patches were water. Undoubtedly. A narrow-band light filter proved it. But the areas which were neither sea nor cloud mass? There were three levels of brightness to be seen on the disk outside the polar areas. One was sea-bottom. One was cloud. The other....
Nolan fretted a little. There was something wrong. The solid ground surface of the planet was too light in color. It was such items that a person with a knack for it would notice sooner than a man without the knack. Vegetation should be more nearly midway between sea-bottom and cloud mass in color.
Nolan fitted in the chlorophyll filter. On the planet of a sol-type sun, vegetation had to use chlorophyll or else. Through this filter the clouds would show, of course. They were white and reflected all colors of light. But no color that chlorophyll didn't reflect could pass through the filter.
The cloud masses showed clearly. Nothing else appeared. The filters would have shown vegetation. It didn't. It said there wasn't any.
Nolan stepped up the magnification. He saw other things. He didn't like them. He got some maximum-magnification pictures and interpreted them with increasing grimness.
He went to make his report just as the system constants began to reach the skipper. The local sun's mass was 1.3 sols. The solar rotation period was thirty-four days. There were sunspots of perfectly familiar kinds. The Lauriac Laws about the size and distribution of planets in a sol-type system were borne out. One was small, and its sunward side was probably at a low red heat. This was like Mercury. Planet Two, like its analogue Venus in the home system, would be resolutely unoccupiable by man. Planet Four—analogous to Mars—was smaller than Three and had a very thin atmosphere. There were gas-giants in orbits six and seven. Then a novelty Lauriac's laws predicted things about fifth planets, too, but they'd never been verified because fifth planets were unstable. They blew up. Only fragments—asteroids—had so far been noted where fifth planets of sol-type suns ought to be. But there was a fifth planet here, rolling magnificently through emptiness. It matched the Lauriac predictions. It had an atmosphere, which should contain oxygen. It was the first sol-system fifth planet ever observed.
There was a babble in the skipper's office as the discoverers of the fifth planet told him about it.
Nolan said curtly, "I've something more urgent to report. Planet Three ought to be like Earth. It was. It isn't, any longer. It's dead!"
Nobody paid attention. There was a fifth planet! It was unparalleled! All the theories about the absence of fifth planets could now be checked!
"I'm telling you," said Nolan sharply, "that the third planet's dead! It was alive, and something happened to it! It has seas and clouds and ice-caps, and they're water! But its land surface is pure desert! Where life can exist, it does. Always! Life did exist here. Now it doesn't." He turned to the skipper, "Maybe bug-eyed monsters killed it, skipper. It looks to me like murder!"