Saturn Rukh by Robert L. Forward

through clenched teeth, “Only a half minute more.”
     
    The roaring finally stopped, and they went from three gees to free fall.
     
    “Trajectory looks good,” said Chastity, reading her pilot screen. “We took off nineteen klecs and are on track for Titan. We should get there in three days.”
     
    “If you wouldn’t mind, Chastity,” said Sandra, “I’d appreciate it if you would rotate us so our telescope port is pointing north so I can get some shots of the underside of the rings. We’ve already passed under most of the D ring, and the C ring is coming up.”
     
    “Okay,” said Chastity, rotating Sexdent until the telescope had a good view of the rings above them.
     
    “I see some spokes coming up on the B ring,” said Dan, who had unlatched the biviewer and was spotting for Sandra again. “From the underside, the spokes show up brighter than the rings rather than darker.”
     
    “Forward scattering,” said Sandra as she started collecting images. “The spokes are generated by small charged dust particles kicked loose from the bigger rocks by electrical currents. The dust particles scatter light better than the big chunks.”
     
    ~ * ~
     
    As soon as they cleared the backside of Saturn and could see Earth again, Jeeves set up a direct comm link to Earth through the high data rate laser communicator dishes. There were three dishes, each thirty centimeters in diameter, spaced 120 degrees around the ship so that one of them always had Earth in sight. The minute the receivers were turned on, they found a message streaming through space to them. It was from the Mission Control Center, congratulating them on their successful perigee burn, which had happened over an hour ago. Bemused, Rod responded. “Engines worked fine. We’re on our way to Titan.”
     
    ~ * ~
     
    Three days later, they met Titan out at its orbit.
     
    “Still just a featureless brown ball of smog,” said Dan dejectedly as he scanned Titan through the biviewer.
     
    “You’re just looking in the wrong frequency band,” said Sandra. “Both microwaves and near infrared can penetrate the smog clouds. Why don’t you put down that biviewer and help me. You take the near-infrared telescope, while I set up the radar imager.”
     
    A few hours later they had moderately good images of the visually hidden surface of Saturn’s largest moon—bigger than the planet Mercury and second only to Jupiter’s moon Ganymede in size.
     
    “Here it is—Titan undressed,” said Sandra, as she gave the rest of the crew a tour of the orange smog moon on her holoviewport. “Since Titan is tidally locked to Saturn, it has six poles—the usual north and south spin poles, the inner pole that always faces Saturn, the outer pole that always faces away from Saturn, the leading pole that faces the direction of Titan’s motion in its orbit, and the trailing pole on the opposite side of that.”
     
    She switched the holoviewport above the engineering console to show the infrared image of one side of Titan. It had a large dark splotch in the center that covered nearly the whole hemisphere.
     
    “This is the trailing pole image,” she said. “The dark regions are deposits of hydrocarbons all the way from simple compounds like methane—C-H-four—to complex carbon-chain hydrocarbons like oil and tar. Right here is a small ocean—mostly methane—but probably containing ethane and other stuff dissolved in it.”
     
    “How do you know it’s methane?” asked Chastity.
     
    “Because Titan’s atmosphere still has lots of methane in it. It’s ninety-five percent nitrogen and argon, and five percent methane. Since methane is easily broken up by sunlight to carbon and hydrogen, and hydrogen is easily lost to space, there must be a reservoir of methane on Titan’s surface to keep the atmosphere replenished—in this case the reservoir is a small ocean. Besides, the surface pressure and temperature of one-and-a-half bars at ninety-four kelvin are