IS IT THE ENGINE?” someone asked I when this view (above) of Mimas was first shown. The “engine,” a crater 130 ki¬lometers across, remains from a collision, probably with another moon, that nearly blew Mimas apart perhaps four billion years ago. Grooves on the satellite’s oppo¬site side may have been caused by the tre¬mendous stresses of impact on a body whose gravity is only five thousandths that of Earth.

The impact created steep crater walls, but a natural process called isostatic rebound formed a central peak (painting, left) that rises six kilometers from the crater floor. The effect would not be greater if Delaware were rolled into a ball and Mount Everest dropped on it.lunaiapetus

suspected methane. Methane is only a minor constituent, as water vapor is on Earth.
It is announced today that Titan’s atmo¬sphere is at least as dense as Earth’s. It in¬cludes hydrogen cyanide. That news would not stop presses. But to biologists it is signifi¬cant: Hydrogen cyanide is a critical building block for the more complex molecules of life.
The temperature at cloud tops is far too cold for life. But some scientists speculate that Titan’s thick clouds could trap enough heat down below to make life imaginable.

There are not that many atmospheres in the solar system. Titan has one because it is massive enough to hold onto its gases gravi¬tationally. Also, its temperatures are so cold that gas molecules do not have the energy to escape its grasp, as happened on the large moons of Jupiter when they formed.

snaps of satellite images

“At Titan we may have a snapshot—a fro¬zen record of the composition of Earth’s early atmosphere,” says Hanel.

Earth and Titan are different today pri¬marily because Titan’s low temperature keeps water frozen. On much warmer Earth there were oceans where life evolved. The oxygen released by living things utterly changed our planet’s character.

Titan might have known warmer days as well. Gaseous ammonia in its early atmo¬sphere may have trapped enough heat to permit liquid ammonia or even water to run across the moon’s surface. Life could have begun and then frozen out.