Sif Mons is an active

shield volcano A shield volcano is a type of volcano named for its low profile, resembling a shield lying on the ground. It is formed by the eruption of highly fluid (low viscosity) lava, which travels farther and forms thinner flows than the more viscous lava ...

located in Eistla Regio on

Venus Venus is the second planet from the Sun. It is often called Earth's "twin" or "sister" planet for having almost the same size and mass, and the closest orbit to Earth's. While both are rocky planets, Venus has an atmosphere much thicker ...

. It has a diameter of and a height of . 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 In Norse mythology, Sif is a golden-haired goddess associated with earth. Sif is attested in the ''Poetic Edda'', compiled in the 13th century from earlier traditional sources, and the ''Prose Edda'', written in the 13th century by Snorri Sturl ...

; the name Sif Mons was officially adopted by the

International Astronomical Union The International Astronomical Union (IAU; , UAI) is an international non-governmental organization (INGO) with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach, education, and developmen ...

(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 A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcanic eruption. An eruption that ejects large volumes of magma over a short period of time can cause significant detriment to the str ...

roughly 50 km in diameter. The caldera appears bright on radar imagery, with distinct

lava flow Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a Natural satellite, moon onto its surface. Lava may be erupted at a volcano or through a Fissure vent, fractu ...

s 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 A pyroclastic flow (also known as a pyroclastic density current or a pyroclastic cloud) is a fast-moving current of hot gas and volcanic matter (collectively known as tephra) that flows along the ground away from a volcano at average speeds of b ...

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 Volcanic cones are among the simplest volcanic landforms. They are built by ejecta from a volcanic vent, piling up around the vent in the shape of a cone with a central crater. Volcanic cones are of different types, depending upon the nature and s ...

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 The ''Magellan'' spacecraft was a robotic space probe launched by NASA on May 4, 1989. Its mission objectives were to map the surface of Venus by using synthetic-aperture radar and to measure the planetary gravitational field. The ''Magell ...

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 field A dune is a landform composed of wind- or water-driven sand. It typically takes the form of a mound, ridge, or hill. An area with dunes is called a dune system or a dune complex. A large dune complex is called a dune field, while broad, flat ...

s 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 Hawaii ( ; ) is an island U.S. state, state of the United States, in the Pacific Ocean about southwest of the U.S. mainland. One of the two Non-contiguous United States, non-contiguous U.S. states (along with Alaska), it is the only sta ...

during a 3-month eruption in 2018.

References

{{Venus Volcanoes of Venus Shield volcanoes Active volcanoes