“But Neptune’s mean surface gravity is the same as Earth’s because of its low density,” Rob replied. “Like Saturn, another big puffball, Neptune has a small rocky core surrounded by huge layers of ice and atmosphere. Both Neptune and Earth have a greenish cast, and each has a satellite of about the same size and at about the same distance away.” “There’s one big difference, though, Rob,” Duff said. “Neptune’s a zillion times colder.” “It’s still not as cold as we’re going to find Pluto,” Rob reminded him, “near absolute zero!” The redhead made a wry face. “Why did you have to say that? It’s so warm and comfortable in here!” As the _Rigel_ drove onward, thousands of miles a minute, day upon day, Rob grew impatient to reach Pluto. He was thinking of Jim Hawley and the _Capella_ crew undergoing unknown hardship and peril. The radio circuit with the ill-fated space ship had been left open in case she was able to get a message through. But none had come during all this time, and Rob was beginning to doubt that he would ever see his fun-loving friend again. The day finally came when the _Rigel_ hovered over the little planet, which was not quite as large a world as Mars. Rob and Duff, with some of the other crewmen in the pilots’ compartment, stared down upon trackless wastes of incredible frozen beauty. Ever since the ship had dropped low enough to reveal the dazzling surface features of the solar system’s most distant planet, no one had spoken. The bizarre landscape seemed to have awed everyone into a state of silent fascination. Suddenly Duff broke the quiet. “Look, what a pretty blue lake!” Rob saw the small body of water partly surrounded by a canyon of towering ice cliffs. In the twilight glow of stars and the weak sun, the lake and peaks sparkled with a clarity that reminded Rob of great jewels. “It’s a lake rightly enough,” Spacemaster O’Leary said. “You can see the ripples, but that’s no water.” He checked the thermocouple. “It’s 348 degrees below zero Fahrenheit down there! That’s a lake of _liquid oxygen_. I’ve seen them on the dark side of Mercury.” Rob gasped in astonishment. He had visited most of the planets, but there was nothing to compare with a wonder such as this. Lieutenant Stone then spoke. “Those ice cliffs don’t look to be frozen water. Do you think they might be chunks of dry ice, sir?” “That’s my opinion,” the spacemaster replied, “—solid carbon dioxide. Notice those other crystal peaks off to the right. They are probably ammonia. I’ve seen them on Mercury, too.”