Radar anomaly

The high regions are covered by a material that reflects radar waves unusually strongly. Is it some kind of snow? Or is it a material that weathers differently where it is “cool”?

The plate shows the eastern margin of Maxwell Montes at the “snow line”, covered by radar-bright material. It also shows an impact crater whose lower-lying floor lacks the “snow cover”. Of course there is no (water) snow on Venus. The surface is at 460°C, and even the cooler parts of the 10 km high Maxwell Montes reach 360°C. This is some other material. It clearly covers only the higher ground.

Even the earliest radar observations from Earth showed that some areas reflect the radar signal much more strongly than the rest of the surface. These were the first features to be named: Alpha, Beta and Maxwell. In these “anomalous” areas the radar reflection is much stronger than elsewhere, and the radar emission is much weaker. They are also the highest areas on Venus. They are dark in the near infrared, so they are cooler.

At the equator the anomaly begins at 2.5 km altitude; closer to the poles, at 5 km. On the highest ground, near 10 km, it weakens again. These areas also have a very rough surface, and this alone could affect how they look.

Early theories suggested that the material might be pyrite, formed as basalt weathers. Later, lead was also considered. Yet one high place, the volcano Maat Mons, shows no such anomaly. Perhaps its surface materials are different. The style of volcanism and the temperature (that is, the altitude) may also play a part. Brossier et al. (2020) explain this “anomaly of the anomaly” by age. At 8 km, Maat Mons is very high, but it may be younger than other areas at similar altitude. Its volcanic materials may not yet have weathered into the material that causes the anomaly elsewhere.

The most widely accepted theory is that materials that conduct in an electric field cause the anomaly. Ferroelectric materials are suggested for the equatorial Ovda Regio. One example is chlorapatite (apatite-(CaCl)), a mineral that forms from basalt; it also occurs in the Balaton Uplands in Hungary. For Maxwell Montes, Treiman et al. (2016) think that semiconducting materials cover the surface.

Several studies have also asked which materials could condense on Venus at exactly this temperature, around 350°C, and have electrical properties that match the radar data. Brackett et al. (1995) proposed a strongly dielectric volatile – a substance that turns into gas at Venus temperatures. It would come from volcanic outgassing. It would then condense from the atmosphere on the cool highlands as a kind of “heavy-metal snow”.