I started getting curious about this question during my Flight Test Engineering class. My professor was talking about a part of the International Standard Atmosphere that is a similar pressure to that of Mars's surface. So I was wondering how a supersonic aircraft would perform in such a colder, emptier atmosphere. On one hand, because Mars's atmosphere is less dense than Earth's, there would be less pressure drag to slow it down, and the lower gravity could reduce the amount of lift the aircraft needs to fly. Hence, an aircraft would be able to fly faster and more efficiently than it could on Earth. But on the other hand, a lower temperature and a lower density would mean that the speed of sound is lower on Mars than it is on Earth. Thus supersonic effects, like wave drag and shockwaves, would start to form at lower speeds than they do on Earth; which could possibly negate the benefits of flying in a lower pressure environment.
So, assuming it is a rocket propelled spaceplane to neglect the effects on the propulsion system, how well would a supersonic aircraft perform when it is flying in the Martian atmosphere compared to Earth's? And how would it be possible to optimize such an aircraft for Martian supersonic flight?
Edit: I also posted this question onto r/AerospaceEngineering earlier in case anyone is interested in some of the answers there too.