Photographs taken today of Titan, the largest of Saturn's 15 moons and the largest moon in the solar system, suggest that it has an atmosphere like the one that gave birth to life on earth more than 3 billion years ago.
"We think it's much too cold for life on Titan but what we're seeing so far is quite similar to what took place on earth and what still might be taking place on Venus," said Dr. Garry Hunt as he studied pictures from the Voyager spacecraft moving close to Saturn. "We think Titan is a cold and minature version of the planets close to the sun, especially Venus."
Twice the size of the earth's moon and the brightest moon in the solar system, Titan has long been looked on as one of the most fascinating bodies in the heavens. It has a rich methane atmosphere that reflects sunlight like a planet, not like an airless moon. It may be the only moon in the solar system large enough to have a gravitational field with the strength to keep an atmosphere.
One of the main goals of the Voyager mission to Saturn is to observe Titan, which was only 2,500 miles away from the spacecraft at 12:41 a.m. Washington time on Wednesday. The 1,800-lb. spacecraft flew under the south pole of Titan, then followed a curving path to bring it near the south pole of Saturn and beyond the planet during the late evening hours on Wednesday.
Approaching Titan at 38,000 miles an hour, Voyager sent back photographs today of the mysterious moon that showed a glowing gold-streaked globe whose atmosphere was covered by a thick haze reaching out as far as 60 miles above its atmosphere. The northern hemisphere of Titan was darker than its southern half and a dark patch that descended from the moon's north pole made it look like Titan was divided into three parts.
Scientists believe the dark patch at the pole is a hood of ice rich in methane, ethane and acetylene that grows so heavy sometimes that pieces break off and fall into the lower atmosphere. The dark colors in the polar hood and in the region beneath it were due to seasonal changes in the northern hemisphere, which on Titan is just coming out of winter being in it for almost 20 years.
Another reason colors in Titan's northern hemisphere are so dark right now is that its upper atmosphere is being bombarded by cosmic particles raining on the moon out of the magnetosphere that surrounds so much of Saturn. The magnetosphere, which extends more than 746,000 miles from the surface of Saturn, traps particles that have streamed out to Saturn from the sun. It forms radiation belts around the planet powerful enough to damage delicate electronics abroad a passing spacecraft.
The protons and electrons that make up the trapped particles are also believed to heat up the upper reaches of the atmosphere of Titan as it passes through the radiation belts. Enough of this heating has been observed from earth for scientists to believe that the atmosphere of Titan is a lot warmer than a moon more than a billion miles from the sun should be.
Dr. Hunt, a scientist at the University of London, said that even though Titan revolves on its axis only once every 16 days, the atmosphere heating should be enough to generate windstorms on the moon. He said the weather patterns observed on Titan might be quite similar to those observed on Venus, where clouds in the upper atmosphere whirl around the planet at more than 200 miles an hour.
The haze that surrounded Titan today was so thick that Scientists despaired of seeing through it to the surface, which is believed to be made up of liquid methane mixed in with lakes of semi-frozen acetylene and ethane.
"We're not going to see through this northern haze," Hunt said. "The only chance we'll have to see the surface is, when we pass under the south pole some of the clouds might be moving so fast they'll open up a hole in the haze for a little while."
Scientists speculate that the heating of the atmosphere gives Titan a surface temperature of about 150 degrees above absolute zero, which is at least 60 degrees warmer than it should be at its distance from the sun. While this is far below zero Fahrenheit, it is not too much colder than it gets at the North Pole on earth in winter.
Titan's atmosphere of methane and ammonia is believed to be similar to the primitive atmosphere of the earth, which changed to what it is today 3 billion years ago through the continuous action of the sun, violent electrical storms and erupting valcanoes on its surface.
As it neared Saturn, Voyager made another discover: it found that two of the more than 100 rings circling the planet are not uniform and are not concentric around Saturn. One ring abruptly goes from 25 miles wide to 60 miles wide. Another shows an almost identical disturbance.
"The mystery of the rings keeps getting deeper and deeper to where we now feel we're in a bottomless pit," said Dr. Bradford Smith of the University of Arizona. "We have no idea what makes these two rings so eccentric and so different from the others."