Despite five decades of analysis, many aspects of Mars crater morphology and evolution remain enigmatic, and it seems likely that new types of data will be needed to find the answers. As a final section in this chapter, we offer new approaches to solving these questions. Finding the answers will require a new orbital data set. Our recommendation is for a new data set that is comparable to many that have been collected for other planets in the Solar System and thus well within the capabilities of the National Air and Space Administration (NASA) and other international space agencies.

We take the younger examples, as illustrated in Chapter 4, and show some of the common ways that craters may be modified. Even craters that are classified as morphologically fresh may have experienced modification. This might take the form of chemical weathering of the floor or deposition of eolian or ice deposits within the crater cavity.

This chapter reviews impact craters throughout the Solar System, looking first at craters formed on Earth, where we have the best field knowledge. We then investigate craters formed on airless rocky bodies (the Moon and Mercury), where the cratering process is not affected by atmospheric effects. We follow this with a glimpse of craters on volatile-rich bodies that also lack an atmosphere, specifically Ganymede, 1 Ceres, and Charon. Here the target material is most likely water ice. Finally, we examine craters formed on bodies with thick atmospheres (Venus and Titan) to see what landforms may have been formed by the interaction of the projectile and the ejecta with the atmosphere.

Here we delve into greater detail of the morphology of individual craters. We review what the freshest, and hence the most likely youngest, craters look like.

We introduce the mode of formation of craters on planetary surfaces to set the stage for comparisons of crater morphology throughout the Solar System and on Mars specifically.

We review the principal types of Martian ejecta morphology. We then delve more deeply into the attributes of these ejecta deposits.

In this chapter, we explore more of the ejecta diversity. There is a much wider range of morphologies, particularly when smaller diameter craters or craters formed in the Northern Plains are considered.

We consider the types of information available to the planetary geomorphologist to investigate craters on Mars. This information primarily takes the form of images, as well as topographic and compositional data, collected from Mars orbit by a variety of spacecraft. We then review aspects of the chronology of Mars, from the earliest geologic epoch (the Noachian) until the most recent (the Amazonian), and how the rocks formed during these time periods are distributed across the planet. We discuss that what can be observed on Mars today is not the way in which the planet has appeared throughout its history.