When one makes an analogy, one Maps features of the Source onto the Target. In the case of the Earth-Mars analogy, the Source is the Earth and the Target is Mars. By Mapping features known about the earth onto mars it is possible to make predictions about the types of features that will be found on Mars, such as water and life. Thus, analogy is a very powerful mental tool to discover new things. As well as distinguishing between the Source and the Target researchers in analogical reasoning often distinguish between Superficial and Structural feature.
Superficial features are things such as Mars being round or having a red hue.
Thus, making an analogy between Mars and a red Stop sign would be an analogy based upon the feature red, which is a superficial attribute. In the case of Mars, stuructural relations might be the Seasonal movement of dark streaks across the surface of the planet and the presence of dust devils. The major virtue of analogy is that it allows a person to go beyond the superficial. Analogy in Science . We have investigated the use of analogy by scientists.