Although Callisto was heavily covered with craters up to 200 kilometers in diameter, Dr. Soderblom commented on the absence of larger impact basins. Perhaps there was one in the huge bullseye feature; however, it was not a “standard” looking basin like those on the Moon. Callisto was “extremely smooth and free of any relief. The structure [the bullseye], if impact caused, shows no relief; the limb does not show any relief; maybe it’s possible that Callisto cannot support relief.” By the next day, the geologists on the Imaging Team were becoming more convinced that the Callisto “bullseye” was the frozen remnant of an enormous impact into Callisto’s surface. Since Callisto is composed in large part of water and has an icy crust, the team speculated that any raised features created by the impact would eventually “slide” back into the surface, “and the ripple marks from the shock wave caused by the impact were frozen” into the surface. [Illustration: Callisto was the last of the Galilean satellites to be studied by Voyager 1. In this photo, taken March 5 from a distance of 1.2 million kilometers, with a resolution of about 25 kilometers, the extensive cratering of the surface began to be apparent. Near the upper left edge is the large impact basin Valhalla; the numerous light spots are craters 100 kilometers or more in diameter. This is the same side of Callisto that was photographed at higher resolution during the Voyager 1 flyby of the satellite a day later. [P-21284C]] Lyle Broadfoot announced that the ultraviolet spectrometer had detected very strong auroral emission in Jupiter’s north and south polar regions. The aurora seemed to be caused primarily by the excitation of molecular hydrogen, although some atomic hydrogen was also detected. Auroral emissions from helium atoms were not detected. [Illustration: The largest ancient impact basin on Callisto is called Valhalla. The central light area is about 600 kilometers across. Surrounding it is a set of concentric low ridges, looking like frozen ripple marks, extending about 1500 kilometers from the center. This picture was taken by Voyager 1 on March 6 at a range of 350 000 kilometers. [P-21287C]] The IRIS infrared measurements required more computer processing than other Voyager data, and therefore they were not available until a day or two after the observations were made. However, Rudy Hanel already had two new results to report. First, the Great Red Spot was about 3° C cooler than its surroundings, and this cooling extended many kilometers above the clouds, into the thin upper atmosphere. Second, the thermal emission from Io was peculiar, with an unexpected shape to the spectrum. Tentatively, Hanel suggested there might be some hot spots on the surface of the satellite. Jupiter has been full of surprises, but the excitement was far from over. Major discoveries were yet to be made.