Sif Mons

Sif Mons is an active shield volcano located in Eistla Regio on Venus. It has a diameter of 500 km and a height of 2.2 km. The volcano is one of the few identified active extraterrestrial volcanoes, having last erupted between 1990 and 1992. It is named after the Norse goddess Sif; the name Sif Mons was officially adopted by the International Astronomical Union (IAU) in 1982.

Geology and characteristics
Sif Mons is a broad shield volcano with an approximate radius of ~1,000 km and a measured peak elevation of ~2.2 km. Sif Mons's slopes are very shallow, with a maximum slope on its southwestern flank of just 0.2° and a minimum slope on its northern flank of less than 0.04°. Sif Mons's peak is occupied by an unusually shallow or infilled circular summit caldera roughly 50 km in diameter. The caldera appears bright on radar imagery, with distinct lava flows radiating down the eastern flank. Small radar-dark regions under 50 km in diameter lie near the summit; the dark appearance in radar imagery may be due to smooth lava flows or smooth material not covered by other flows. The dark regions have irregular edges, indicating that they may be pyroclastic flow deposits. As the radar-dark regions follow Sif Mons's topography, planetary scientists Bruce A. Campbell and Donald B. Campbell interpreted the features as smooth lava flows.

Additional subsidiary flows have been identified on Sif Mons's volcanic edifice, likely originating from independent eruptions from multiple vents. Other subsidiary features include an uplifted region ~1.7 km high to the southeast of Sif Mons and a volcanic cone 5–8 km in diameter to the north. The uplifted region is circular in profile, likely being a central vent for additional lava flows or a ring of small domes. The northern volcanic cone, meanwhile, is the source for a single major flow that extends down the flanks of Sif Mons for ~200 kilometers before continuing into southern Sedna Planitia.

Current volcanic activity
On 27 May 2024, a paper was published reporting evidence of ongoing volcanic activity at Sif Mons. Using synthetic-aperture radar (SAR) data collected during the Magellan spacecraft mission, the authors identified changes in radar backscatter on the flank of Sif Mons and across a western regions of Niobe Planitia. These changes, which took place between 1990 and 1992, are likely due to lava flows that erupted within the aforementioned timeframe. Alternative causes for the observed changes were ruled out; variations from different viewing angles were accounted for in the analysis, and the observed changes were inconsistent with dune fields observed elsewhere on Venus. The lava flows on Sif Mons occupy its western flank, flowing downslope and covering older flows. The lava flows are estimated to be roughly 30 km2 in area, comparable in size to flows erupted from the Kīlauea volcano in Hawaii during a 3-month eruption in 2018.