WHAT I REALLY WANT TO KNOW IS…
What causes Venus’s dark spots?
Venus is one of the closest planets to Earth. But much about our inner neighbour is still shrouded in mystery thanks to the permanent cloud cover that masks its surface.
We began to learn what the surface of Venus was like in the last century, thanks to radar observations from Earth and then visits by space probes. They told us it is an inhospitable place, with crushing air pressure, searing temperatures and a poisonous, sulphurous atmosphere.
A NASA spacecraft called Magellan orbited Venus from 1990 to 1994. It made extensive radar observations to map the planet’s landscape, plus it measured radio emissions given off by the surface heat. One thing that was noticed was that surface features became brighter as they got higher, and a number of theories have been put forward to explain why this might be so. A handful of dark spots were also noticed at the highest elevations, which left scientists baffled.
An error? Unlikely
We used modern techniques to analyse two areas in the Ovda Regio highlands region of Venus in higher spatial resolution to try to explain why they were bright. For the most part, our data showed the same brightening with elevation that previous scientists had found, up to about 4,500m. But we noticed that at the highest elevations, above 4,700m, it was suddenly dropping off. Instead of getting brighter as we expected, there was a sudden sharp drop to lower levels, like those seen in the lowlands, showing radio waves were not being reflected. And to our surprise, whereas the early studies showed just a few dark spots, we were seeing a couple of hundred of them. This told us that these were real features on Venus, and not just random errors in the data. Venus isn’t like Earth, where it is hot at the equator and much colder at the poles. The temperature is pretty much the same all around the planet, more than twice the maximum temperature in a domestic oven. But just like on Earth, if you were to climb a mountain it would begin to get cooler. So we think the dark spots are like our own mountaintops, where you reach a snowline and see snow above but not below. Instead of snow, we suggest that some kind of metallic compound produced by Venus’s extraordinary conditions is falling, like rain or snow, to form a strange kind of frost. Around four per cent of Venus’s landscape is covered in active volcanoes, and almost all its surface is made up of lava flows and magma that has erupted from within the planet. So it is a very basaltic, igneous world. What we are trying to find out is what sort of minerals could be present to be producing such unique chemical signatures. Some sort of insight into the composition of the lava that is erupting from those volcanoes would be very useful information to help our research.
So how do we find out what is there? Another space probe would be awesome. Europe’s recent visitor, Venus Express, has not been able to help us because it has been mainly studying the planet’s atmosphere.
The data from NASA’s Magellan probe had a spatial resolution of 75m: in other words, one pixel in an image represented an area 75m across. That’s great in terms of Venus research because it was hugely better than had been managed by previous, older spacecraft. But probes imaging the Moon have managed to get down to a resolution of 30cm.
It would be great to look at Venus in similar detail. That could help solve the riddle of the dark spots and tell us a lot more besides. We simply need to go back to Venus.