After images taken at the time could only really be processed over the last ten years, a new study has disclosed a nearly 1-square-mile, or 2.6-square-kilometre (sqkm), volcanic vent on Venus that changed in shape and grew over eight months in 1991.
Changes of this magnitude typically occur on Earth as a result of volcanic activity, either in the form of an explosion at the vent or the collapse and expansion of the vent walls due to magma movement beneath the vent.
Venus, therefore, appears to have volcanic activity, according to the research paper, which offers strong evidence to answer the lingering question about whether Earth's sister planet currently has eruptions and lava flows.
The research by Robert Herrick, research professor at the University of Alaska Fairbanks Geophysical Institute, US, is published in the journal Science.
According to the study, Herrick studied images taken in the early 1990s during the first two imaging cycles of NASA's Magellan space probe.
Until recently, comparing digital images to find new lava flows took too much time, the paper noted. As a result, few scientists have searched Magellan data for feature formation.
''It is really only in the last decade or so that the Magellan data has been available at full resolution, mosaicked and easily manipulable by an investigator with a typical personal workstation,'' Herrick said.
The new research focused on an area containing two of Venus' largest volcanoes, Ozza and Maat Mons.
''Ozza and Maat Mons are comparable in volume to Earth's largest volcanoes but have lower slopes and thus are more spread out,'' Herrick said.
Maat Mons contains an expanded vent that indicated volcanic activity, the study said.
Herrick compared a Magellan image from mid-February 1991 with a mid-October 1991 image and noticed a change to a vent on the north side of a domed shield volcano that is part of the Maat Mons volcano, he said.
The vent had grown from a circular formation of just under 1 square mile to an irregular shape of about 1.5 square miles, or 3.9 sqkm.
The later image indicated that the vent's walls became shorter, perhaps only a few hundred feet high and that the vent was nearly filled to its rim.
The researchers speculate that a lava lake formed in the vent during the eight months between the images, though whether the contents were liquid or cooled and solidified isn't known.
The researchers offered one caveat: a non-volcanic, earthquake-triggered collapse of the vent's walls might have caused the expansion.
They noted, however, that vent collapses of this scale on Earth's volcanoes have always been accompanied by nearby volcanic eruptions; magma withdraws from beneath the vent because it is going somewhere else, they said.
The surface of Venus is geologically young, especially compared to all the other rocky bodies except Earth and Jupiter's moon Io, Herrick said.
''However, the estimates of how often eruptions might occur on Venus have been speculative, ranging from several large eruptions per year to one such eruption every several or even tens of years,'' he said.
Herrick's research adds Venus to the small pool of volcanically active bodies in our solar system.
''We can now say that Venus is presently volcanically active in the sense that there are at least a few eruptions per year,'' he said.
''We can expect that the upcoming Venus missions will observe new volcanic flows that have occurred since the Magellan mission ended three decades ago, and we should see some activity occurring while the two upcoming orbital missions are collecting images,'' said Herrick.
With PTI inputs