Part One - Part Two - Part Three).
As always, let's take care of the basics first about this volcano. Shamshu Patera is located at 10.1° South Latitude, 63.0° West Longitude, or about 500 kilometers (310 miles) ESE of Hi'iaka Patera, a volcano of the week back in August. The volcano measures 115 kilometers (72 miles) north-to-south and 107 kilometers (67 miles) east-to-west. The height of the patera wall which marks the outer edge of the volcano is variable as the surrounding terrain is not constant, the result of debris flows coming off a mountain that abuts the northeastern margin of Shamshu Patera. Shadow measurements along the northwestern wall of Shamshu show that it is 500 meters (1,640 feet) tall, however the lack of a shadow along portions of its western wall suggest that it maybe less than 50 meters (160 feet) tall in some areas. Shamshu Patera was named at the IAU General Assembly in August 1997 after a pre-Islamic Arabian sun goddess.
a global view from June 1997.
A few obvious features stand out about Shamshu Patera. The volcano's floor is dominated by dark lava flows of varying albedos. The different levels of brightness of its flows suggest that different eruptions produced new flow lobes that covered a different potion of its floor. As the lava flows age, they cool and all more sulfur dioxide and sulfur to condense on their surfaces. So as they age, the lava flows slowly brighten. Despite how dark many of these lava flow lobes look, I can't see any evidence for surface changes at Shamshu during the Galileo mission, so either very little SO2 is deposited here, or they maybe persistently active. More on that in a bit. The shape of Shamshu's dark material, and the more central location in the patera, is more consistent with these resulting from lava flows rather than this volcano being a lava lake, like Pele or Loki. About half of the patera of covered in material that has the same brightness as the surrounding Ionian plains and has a brighter orange color. These areas likely haven't seen lava flows in recent times (> 100 years) or may have been coated in sulfur.
As far as current volcanic activity, Shamshu was observed by the Galileo Near-Infrared Mapping Spectrometer (NIMS) as an active hotspot on only one occasion, during orbit C10 in September 1997. The region was observed on several occasions before and since by Galileo and by New Horizons in 2007 and no additional activity was detected.
Next week, we will shift our focus from this fairly quiescent (at least in the present epoch) volcano to the more active Isum Patera on the opposite side of Io.
Bunte, M.; et al. (2010). "Geologic mapping of the Hi’iaka and Shamshu regions of Io". Icarus 207: 868–886.