Every news headline I saw over the last few weeks that referred to this article wondered aloud if there are active volcanoes. Even the ESA website poses the news as a question. So, does Venus have active volcanism? First, let's talk about the paper and what it reports.
Emmanuel Marcq and colleagues used ultraviolet spectrometer data collected from 2007 to 2012 using the SPICAV instrument aboard the Venus Express spacecraft to examine the density of sulphur dioxide above the clouds of Venus. They found that SO2 column densities increased prior to 2007, and then decreased by a factor of 5 over the next five years.
This finding is quite similar to observations made by the Pioneer Venus Orbiter in the 1970s and 1980s, which revealed a ten-fold decrease in SO2 column density. At the time, Larry Esposito (1984) of the Laboratory for Atmospheric and Space Physics in Boulder interpreted this decline to have occurred following an episode of volcanogenic upwelling from the lower atmosphere (it is important to note that SO2 is abundant and ubiquitous in the lower atmosphere of Venus).
Marcq, et al. conclude that the SO2 variability observed from the 1970s to the present is the result of long-timescale fluctuations in upward transport from the troposphere to the mesosphere.
What they do not know is whether this is the result of 1) episodic increased buoyancy from volcanic plumes, or 2) intrinsic dynamic variability in the upward component of the global circulation.
Back to our original question: Does Venus possess active volcanoes? This study cannot answer that question. The authors seem to want it to be so, but say in their conclusion: "By Occam's razor, we are inclined to think that this variability originates from intrinsic dynamical variability in the ascending sub-solar branch of the global circulation at cloud-top level on a decennial timescale rather than from an external forcing such as extra buoyancy caused by volcanic eruptions, but we cannot dismiss a volcanic forcing through our study alone."
It's pretty clear that most people want active volcanoes on Venus, but the jury is still out, I'm afraid.
Esposito, L. W. (1984). Sulfur dioxide: Episodic injection shows evidence for active venus volcanism. Science (New York, N.Y.), 223(4640), 1072-1074. doi: 10.1126/science.223.4640.1072
Marcq, E., Bertaux, J., Montmessin, F., & Belyaev, D. (2012). Variations of sulphur dioxide at the cloud top of Venus’s dynamic atmosphere Nature Geoscience DOI: 10.1038/NGEO1650