1.   Methane condenses to form a cloud layer above the ammonia clouds, leaving a gas that is almost entirely hydrogen and helium.

2.   The Magellan radar-mapping mission was designed to penetrate the dense cloud layer and return detailed radar images of the surface geology.

3.   While heavy overcast blocks much of the radiation, a light cloud layer does little to reduce it.

4.   The three cloud layers are believed to consist of ammonia ice, ammonium hydrosulfide and a mixture of ice and water.

5.   The infrared sensors would pick up the fiery plume of a ballistic missile as it climbs through the cloud layer, air force officials said.