Venus, the closest planet to Earth in both size and proximity, remains a source of considerable mystery. Its reflective clouds prevent a clear view of the planet, and for centuries little was known about its surface and inner workings. But radar and gravity data acquired in the past few decades by spacecraft such as NASA's Magellan, which orbited Venus from 1990 to 1994, helped refine planetary scientists' understanding of Venus's past. The impact history recorded by the planet's craters indicated that Venus had been resurfaced by volcanic activity relatively recently, just a few hundred million years ago.

Now data from the European Space Agency's Venus Express spacecraft, launched in 2005, suggests that Venus has been geologically active even more recently than that. Suzanne Smrekar, a planetary scientist at the NASA Jet Propulsion Laboratory in Pasadena, Calif., and her colleagues reported in a study published online April 8 by Science that Venus Express infrared imagery reveal lava flows on the planet to be 2.5 million years old, and quite possibly much younger. Volcanism so recent in Venus's geologic history would indicate that the planet remains active today.

Smrekar and her co-authors examined so-called hot spots on Venus—raised volcanic landforms, somewhat similar to the Hawaiian Islands, that may indicate the presence of mantle plumes beneath. At three hot spots imaged by Venus Express, the team found an excess of thermal emissions, indicating a lack of surface weathering compared to the rest of the planet. In other words, the hot spots appear to host relatively fresh lava.

Because the weathering process on Venus is not well understood, the study's authors are unable to precisely determine the age of the lava flows, but their estimates, based on the volume of unweathered lava observed, set an upper bound of 2.5 million years. Because Venus Express infrared data are incomplete, however, covering only three of Venus's nine hot spots, Smrekar and her colleagues estimate that there is probably much more fresh lava to be found on the planet. So, based on that factor and others, the researchers conclude that Venus has likely been volcanically resurfacing just hundreds to tens of thousands of years in the past, and is likely still active today.

This is not the first time researchers have argued for recent or even extant volcanism on Venus—Smrekar herself published a similar study in Icarus, based on Magellan data, in 1994.

Infrared data from Venus Express overlaid on an exaggerated topographic map of a hot spot: ESA/NASA/JPL