Venus is a rocky world that is similar in size to Earth, but it’s also a world that remains mysterious because it’s permanently shrouded in cloud. In the 1990s, NASA’s Magellan mission used radar to pierce the clouds and map the planet’s surface, revealing a landscape covered with volcanoes.
Crater counts suggest none of the surface is more than about 750 million years old, but we don’t know over what timescale it was laid down. Radar tells us the topography and elevation of the surface, revealing that there are higher mountains which are cone-shaped, like a typical volcano.
It also allows us to differentiate terrain types and shows features that are consistent with the presence of volcanoes, such as lava flows. But what it does not tell us is whether or not those volcanoes are still active.
ESA’s Venus Express satellite has been orbiting the planet since 2006. Most of its instruments focus on the atmosphere, but some can see right down to the ground in infrared wavelengths and we are seeing the hot glow of the surface itself. One thing we are doing is simply looking for regions of high temperature, as you might find from a lava lake or a still-hot lava flow. A problem here is that the glow is scattered by the clouds, so we get a hazy picture instead of a pin-sharp one. We haven’t seen any lava lakes yet, but we’re still looking.
The other thing we can do is look to see just how black the surface is in the infrared. Fresh lava flows are expected to be much darker than old ones, a result of the older flows being weathered over time. We have already discovered some unusually black terrain next to a volcano, which volcanologists tell us is likely to be a fresh lava flow less than two million years old. That’s geologically recent, but it’s still not the present day.
Apart from mapping minerals we can also look in detail at the atmosphere, which is where Venus Express excels. Compared to the surface of the planet, the atmosphere is more dynamic and changes more quickly, so we should be able to see alterations taking place on a shorter timescale.
Venus’s atmosphere contains a lot of sulphur dioxide. On Earth, the only natural source of this gas is volcanic activity. Given that sulphur dioxide is removed from Venus’s atmosphere through reactions with the surface and with atmospheric water, any that we detect must have come from a volcano that was active within the past few million years. If we look at changes on a shorter timescale, we can see whether there were any injections of sulphur dioxide recently. Venus Express observed a sharp rise in the amount of sulphur dioxide early on in its mission, followed by a tenfold decrease – an enormous drop. Similar behaviour was observed in the 1980s. This paints a picture of massive injections of sulphur dioxide into the upper atmosphere roughly once a decade or so, with smaller injections more frequently. It is very tempting to suggest that these are the direct signature of volcanic eruptions – but they could also be due to weather patterns we don’t understand yet.
In the short term, we can continue examining Venus Express data to look for lava flows and lakes, and try to make better sense of the sulphur dioxide signatures in the atmosphere to work out whether they are due to volcanic eruptions or weather patterns. In the longer term, we could send an orbiter with a new generation of radar technology to measure millimeter-scale changes in surface height. This would help identify active volcanism.
Ultimately, we’d like to send a rover to Venus to search for the suspected volcanoes – but it would need to be able to withstand the 450”C surface heat and enormous atmospheric pressure.