Venus

Venus has been very prominent in the early evening these last couple of months. It was the subject of a feature in the last series by Brian Cox on the Solar System. I was interested in the apparent "snow line" effect on the mountains of Venus. It's a kind of metallic frost. It has to be something that melts and evaporates at 460 Celsius at the lowest levels on Venus but solidifies at cooler temperatures with altitude. Brian Cox names lead sulfide as a possible candidate. Looking that up, I see a melting point of over 1000 Celsius. This must be at Earth's standard atmospheric pressure and will be different on Venus. This article names bismuth sulfide and tellurium as other possible candidates.

Retrograde motion

Mars has been in retrograde motion recently as Earth has been overtaking it. Earth has a higher angular velocity. Mars is still going forward but looks like it is going backwards relative to a fixed reference point. OK, there are no fixed reference points in space but the background of stars are so far away that it doesn't make much difference. Here Mrs B was overtaking a lorry coming up to a bridge. Above, the cab lines up with a bridge in the distance. 

In the second view there is daylight between the cab and the bridge and below the cab is nowhere near lining up with the bridge. You could achieve the same effect if we were still and the lorry went backwards. But all the while it was moving forward at 50mph. This is what we mean by retrograde motion.