Could there be volcanoes erupting on Venus?
Scientists think the answer is yes based on the latest data from European Space Agency's Venus Express, which completed an eight-year mission of Earth's neighboring planet last year.
Using a near-infrared channel of the spacecraft's Venus Monitoring Camera (VMC) to map thermal emission from the surface, an international team spotted localized changes in surface brightness between images taken only a few days apart.
"We have now seen several events where a spot on the surface suddenly gets much hotter, and then cools down again," said Eugene Shalygin from the Max Planck Institute for Solar System Research in Germany, lead author of the paper reporting the results in Geophysical Research Letters.
"These four hotspots are located in what are known from radar imagery to be tectonic rift zones, but this is the first time we have detected that they are hot and changing in temperature from day to day. It is the most tantalizing evidence yet for active volcanism."
The latest work builds on several other findings that had offered clues to volcanism on Venus.
In a study published in 2010, scientists reported that the infrared radiation coming from three volcanic regions was different from that of the surrounding terrain. They interpreted this as coming from relatively fresh lava flows. Found to be less than 2.5 million years old, the study could not establish whether there is still active volcanism on the planet.
Then in 2012, scientists noted a sharp rise in the sulphur dioxide content of the upper atmosphere of Venus in 2006-2007, followed by a gradual fall over the following five years. This may have been caused by wind patterns, but the more intriguing possibility was that episodes of volcanic activity were injecting vast amounts of sulphur dioxide into the upper atmosphere.
The latest hotspots are located along the Ganiki Chasma rift zone close to the volcanoes Ozza Mons and Maat Mons. Rift zones - results of fracturing of the surface - are often associated with upwelling of magma below the crust. This process can bring hot material to the surface, where it may be released through cracks in the surface as a lava flow.
"These observations are close to the limits of the spacecraft's capabilities and it was extremely difficult to make these detections with Venus' thick clouds impairing the view," said co-author Wojciech Markiewicz, also from the Max Planck Institute for Solar System Research. "But the VMC was designed to make these systematic observations of the surface and luckily we clearly see these regions that change in temperature over time, and that are notably higher than the average surface temperature."
Because VMC's view is blurred by the clouds, the areas of increased emission appear spread out over large areas more than 62 miles across, although the hot regions on the surface below are probably smaller. One hotspot known as Object A, for example, had a temperature of 830 degrees C, much higher than the global average of 480 degrees C.
"It looks like we can finally include Venus in the small club of volcanically active solar system bodies," said Hakan Svedhem, ESA's Venus Express project scientist. "Our study shows that Venus, our nearest neighbor, is still active and changing in the present day - it is an important step in our quest to understand the different evolutionary histories of Earth and Venus."