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.

The conceptual problems of quantum theory make a particularly strong appearance in contexts such as black hole physics, or the physics of the very early universe, where the theory must be used with nothing that could be reasonably given the “role of observer” or a “measuring device.” As such, those situations offer a rather fertile ground, where proposals for dealing with those problems could produce results that actually differ substantially from the ones obtained within the “standard type” of studies, where those questions are essentially ignored. We will explore the ways in which one of the proposals to address the so-called measurement problem affects various specific issues that arise within the above-mentioned fields. We will see that in our specific approach to the subject several well-known and concrete problems seem to simply disappear, and in particular, that it could offer a novel and unexpected account for the nature of the entropic arrow of time in cosmology.

According to the standard account of time reversal, namely the account found in physics books, a time-reversal transformation involves a temporal operator 𝑇 that, when acting on a sequence of states, inverts the order with which states happen, and suitably changes the properties of the entities in the state so as to make the theory time-reversal invariant. This ‘symmetry first’ approach imposes symmetries on the theory: the changes in the states are a consequence of requiring the theory to be time-reversal invariant. Some (Albert, Callender) find this view unjustified: we discover a theory has a given symmetry, on the basis of the theory’s ontology, not the other way around. So, they propose a ‘metaphysics first’ approach, sometimes dubbed ‘pancake account’ of time reversal: 𝑇 inverts the order of the states but does nothing else. Consequently, since there are no obvious independent reasons for the state to change as 𝑇 prescribes to preserve time-reversal symmetry, then the theory is not time-reversal invariant. In this chapter I wish to further motivate the pancake account of time reversal by arguing the standard account is far more problematical than has been suggested. Moreover, I defend the pancake account from recent objections raised by Roberts. Finally, since I value symmetries, I propose an alternative account, which aims at retaining the best of both approaches: the 𝑇 operator changes the order of the states, it leaves the state unaffected (like the pancake account), but also makes the theory time-reversal invariant (like the standard account).

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 a 2002 paper, I offered a novel way of thinking about the compatibility of free will with determinism, one that depended on appealing to the typical understanding of time of the philosopher of physics as simply one of the four dimensions of the Block Universe, albeit an especially interesting and important one. I argued that rejecting the everyday notion of “passage of time,” and of the explanatory privilege that we usually give to past → future determination as opposed to future → past determination, allowed one to articulate a novel way of defending free action in a Block world subject to deterministic laws. The problem is, most of the time these days I no longer believe in the Block and do believe in the passage of time! But I still believe that human action is (often) free, and that physics poses no genuine threat to our freedom. In this paper I will explore how the core idea behind “Freedom from the Inside Out” can be modified to be compatible with a metaphysical picture in which time passes, and explanation is not fully time-symmetric.

Working inside the control-theoretic framework for understanding thermodynamics, I develop a systematic way to characterize thermodynamic theories via their compatibility with various notions of coarse-graining, which can be thought of as parametrizing an agent’s degree of control of a system’s degrees of freedom, and explore the features of those theories. Phenomenological thermodynamics is reconstructed via the ‘equilibration’ coarse-graining where a system is coarse-grained to a canonical distribution; finer-grained forms of thermodynamics differ from phenomenological thermodynamics only in that some states of a system possess a free energy that can be extracted by reversibly transforming the system (as close as possible) to a canonical distribution. Exceeding the limits of phenomenological thermodynamics thus requires both finer-grained control of a system and finer-grained information about its state. I consider the status of the second law in this framework, and distinguish two versions: the principle that entropy does not decrease, and the Kelvin/Clausius statements about the impossibility of transforming heat to work, or moving heat from a cold body to a hotter body, in a cyclic process. The former should be understood as relative to a coarse-graining, and can be violated given finer control than that coarse-graining permits; the latter is absolute and binds any thermodynamic theory compatible with the laws of physics, even the entirely reversible limit where no coarse-graining is appealed to at all. I illustrate these points via a discussion of Maxwell’s demon.

There is a long-cherished hope, which has its origins in the work of Boltzmann, that all that we are going to need to do in order to account for all the of the differences there are between the past and the future is to add to the fundamental time-reversal-symmetric dynamical laws, and to the standard statistical-mechanical probability-measure over the space of possible fundamental physical states, a simple postulate – a so-called “past hypothesis” – about the initial microstate of the universe as a whole. And there are various widespread and perennial sorts of puzzlement about how a hope like that can even seriously be entertained – puzzlements (that is) about how it is that the macrocondition of the universe 15 billion years ago, all by itself, can even imaginably be up to the job of explaining so much about the feel, today and on Earth, of the passing of time. I want to try to alleviate those puzzlements here. I will begin with a number of very general observations – and then, by way of illustration, I will present a new and detailed analysis of how it is that a simple pendulum clock invariably arranges to turn its hands clockwise in the temporal direction that points away from the Big Bang.

We conduct a case study analysis of a proposal for the emergence of time based upon the approximate derivation of three grades of temporal structure within an explicit quantum cosmological model which obeys a Wheeler–DeWitt type equation without an extrinsic time parameter. Our main focus will be issues regarding the consistency of the approximations and derivations in question. Our conclusion is that the model provides a self-consistent account of the emergence of chronordinal, chronometric, and chronodirected structure. Residual concerns relate to explanatory rather than consistency considerations.

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.