Numerous intersecting, linear features on Europa, possibly due to crustal cracking. Two distinct types of terrain, cratered and grooved, on Ganymede, suggesting that the entire ice-rich crust was once under tension. An ancient, heavily cratered crust on Callisto, with vestigial rings of enormous impact basins since erased by flow of the ice-laden crust. The elliptical shape of Amalthea (270 × 160 kilometers). A faint ring of material about Jupiter, with an outer edge of 128 000 kilometers from the center of the planet. Magnetosphere An electrical current of more than a million amperes flowing in the magnetic flux tube linking Jupiter and Io. Very strong ultraviolet emissions from ionized sulfur and oxygen in the Io plasma torus, indicating a hot (hundred thousand degree) plasma that evidently was not present at the time of Pioneer 10 encounter. Plasma electron densities exceeding 4500 per cubic centimeter in some regions of the Io plasma torus. A cold, corotating plasma inside 6 R_J with ions of sulfur, oxygen, and sulfur dioxide. High-energy trapped particles inside 6 R_J with significantly enhanced abundances of oxygen, sodium, and sulfur. Hot plasma near the magnetopause predominantly composed of protons, oxygen, and sulfur. Jovian radio emission at kilometer wavelengths, which may be generated by plasma oscillations in the Io plasma torus. Corotating plasma flows unexpectedly far from Jupiter in the dayside outer magnetosphere. Evidence suggesting a transition from closed magnetic field lines to a Jovian magnetotail at about 25 R_J from Jupiter. Whistler emission interpreted as lightning whistlers from the Jovian atmosphere. [1]Adapted from a paper by E. C. Stone and A. L. Lane in the Voyager 1 Thirty-Day Report. [2]Adapted from a summary prepared by E. C. Stone and A. L. Lane for the Voyager 1 Thirty-Day Report. [Illustration: The dramatic discovery of active volcanoes on Io was made by Linda Morabito and her colleagues from this navigation picture, taken March 8 at a range from Io of 4.5 million kilometers. On the bright edge, the immense plume of volcanic ash from Pele (P₁) rises nearly 300 kilometers above the surface. At the terminator, the border between day and night on Io, a second smaller cloud from the volcano Loki (P₂) catches the sunlight. These two eruptions—captured on this single discovery photograph—are much larger than the largest terrestrial volcanic eruption known. [P-21306 B/W]] [Illustration: Once the existence of giant volcanic eruptions on Io was recognized, a reexamination of the Voyager 1 encounter pictures revealed many more plumes. These two views of Prometheus (P₃) were found by Joseph Veverka and Robert Strom on March 12 when they reproduced earlier pictures.]