Cross-hatched Venus puzzles astronomers.

Cross-hatched Venus puzzles astronomers 

Magellan's radar views of cloud-enveloped Venus reveal some clearly unexpected new surface features. The most unusual one recorded since the craft began mapping the planet on Sept. 15 consists of a remarkably regular, cross-hatched pattern (left photo), formed when several parallel, linear features intersect a group of brighter ones at almost right angles. Researchers have never seen such terrain either on Venus or the other planets," says project scientist R. Stephen Saunders of the Jet Propulsion Laboratory in Pasadona, Calif.

The unusual pattern marks a low rise in the northern hemisphere between two plains called Sedna Planitia and Guinevere Planitia. Lines in the fainter of the two groups are regularly spaced about 1 kilometer apart, Saunders says, and are about as narrow as the radar can detect -- roughly 120 meters. The brighter of the cross-hatched lines are less regular, and in some places appear to begin where they intersect the faint lines. Magellan researchers do not know whether the lines represent fractures of some kind, but Saunders says that parts of the puzzling pattern look as though they may be associated with some past volcanic activity.

Magellan has also returned images revealing two craters. Clear differences between them suggest the craters had vastly different origins. Features in the smaller, 1-km-diameter crater (upper right photo) indicate explosive volcanism. For example, the radar-bright surface deposit that broadens away from the south side appears to have been created by erupted debris.

About 37 km across, the larger crater (pictured at lower right), located in Guinevere Planitia, presents the classic look of a meteorite impact. A lack of debris on the crater's south side suggests the meteorite was descending from the south at a shallow angle to the surface when it hit. The crater's central peak probably formed when the floor, compressed by the impact, rebounded.

Other images include what Saunders describes as "lots" of low domes only a few kilometers across and resembling what some geophysicists say are shield volcanoes (SN: 6/23/90, p. 392). There is also "a very curious field of little craters, which have little sinuous channels coming out of them," Saunders says.