Both photos NASA<br >The thick atmosphere of Venus obscures its surface (left)， as photographed by the Pioneer Venus orbiter. At right， the<br >radar altimeter of the orbiter sees through the cloud cover to allow mapping of the planet s terrain.<br > today it is volcanically dead. ~ Spherical grains<br > have been discovered that are similar to granules<br > from meteorites in the oldest Earth rocks. These<br > grains may indicate that large impacts played a<br > more important role in the history of the solar<br > system than had previously been thought.<br > Scientists at the Jet Propulsion Laboratory<br > (JPL) suggest that， based on Viking orbiter<br > pictures， Mars may have had ancient lakes or<br >ASTRONOMY AND SPACE seas covering up to t5 percent of the planet. ~<br >SCIENCE The Sun， as described by the "solar constant"<br > observed by various means， appears to be fading<br >The year 1986 began in an exc ，ting fashion. In by 0.02 percent per year. This phenomenon<br >early January， the robot spacecraft Voyager 2 seems linked to a solar cycle， such as the<br >flew past the planet Uranus and returned to 11-year sunspot or 22-year magnetic cycles. If<br >waating scientists on Earth remarkable photos and the trend continues over decades， our climate<br >information about the mysterious Uranian clouds， could be affected.<br >moons， and rings. A new theory suggests that the Moon may<br > But as scientists anticipated their fullest and have been formed from the solar nebula when a<br >most rewarding year of space exploration since a roughly Mars-sized object smashed into the<br >human first walked the dusty soil of the Moon in proto-Earth. Debris ejected as a result of the<br >1969， a string of disasters rocked the American collision formed a prelunar disk from which the<br >space program and changed the complexion of Moon later formed. More work is needed before<br >space exploration and commercialization forever， the hypothesis can be confirmed. ~ Comet<br > Halley， well studied by an international team of<br >THE SOLAR SYSTEM satellites， was found to have a two-lobed<br >Voyager 2 passed Uranus in January 1986， velvet-black nucleus subject to bright outbursts.<br >providing discoveries of at least 10 new moons， (See page 8.) ~ Comet Wilson， discovered<br >many rings， a surprising magnetic field， August 5， may be the first known visitor from the<br >unexpected atmospheric winds， a new emission Oort Cloud. The Oort Cloud is a theoretical ring of<br >in the atmosphere called electroglow， and the comets that circles the solar system outside the<br >