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.

If the mental is physical, that is, if our experiences are physical features of the world, then in particular, our experience of temporal directionality (i.e., our experience of the direction of time, call it “the psychological arrow of time,” is a physical feature of the world, possibly of our brains. What kind of physical feature of the brain can the psychological arrow be? To explore this question, we first explain (briefly) what physicalism about the mind is. We argue very briefly that all forms of so-called non-reductive physicalism are dualism in disguise, and we propose instead a full-blown reductive physicalist theory of all the special sciences (called “Flat Physicalism”), including psychology. Flat Physicalism is a generalization of a reductive foundation of statistical mechanics, in which the notions of probability and entropy are deduced from mechanics (rather than being postulated), and therefore it is an especially convenient framework for studying the psychological arrow of time. There are three possible accounts of the psychological arrow, which we explore in the framework of Flat Physicalism. The first approach accepts that temporal directionality is a feature of the world, which is reflected in our experience. Unfortunately, all the existing attempts along these lines fail, in particular, the attempts based on postulating low entropy in the past. One reason for their failure is their employment of “typicality” arguments which are either a priori or circular. This first account also fails to explain how the direction of entropy increase is “sensed” in order to be reflected in our brains. We conclude that contemporary physics does not support this first option, and if one insists on there being a temporal directionality in the world “out there” (as it were), then one needs to change the fundamental principles of physics in a rather radical way. The second approach denies that the world is time-directed, but accepts that temporal directionality appears in our experience. In this case we need to reduce the experience of the psychological arrow of time to a nontemporal degree of freedom in the brain, and this has the radical implication that the psychological arrow is the fundamental arrow of time, and it explains what appears to be the arrow of time “out there,” rather than the other way around. The third approach denies not only that the world is time-directed, but also that we experience temporal directionality. This option is a version of “denialism” in the science and philosophy of mind, which is the view that first-person reports concerning the mental realm are not always reliable. Here the task is to explain the first-person reports in a different way (we do not address this third option in detail here and mention it only to complete the picture). The case of the psychological arrow of time turns out to be extremely important in contemporary science in two respects. If the first option is the case, then this exploration brings to the surface a huge lacuna in contemporary physics and the need for a radical change in it. If the second or third option is the case, then the arrow of time offers a unique case for studying the physics of the mind.

Our fundamental theories, that is, the quantum theory and general relativity, are invariant under time reversal. Only when we treat systems from the point of view of thermodynamics, that is, averaging between many subsystem components, an arrow of time emerges. The relation between thermodynamic and the quantum theory has been fertile, deeply explored and still a source of new investigations. The relation between the quantum theory and gravity, while it has not yet brought an established theory of quantum gravity, has certainly sparked in-depth analysis and tentative new theories. On the other hand, the connection between gravity and thermodynamics is less investigated and more puzzling. I review a selection of results in covariant thermodynamics, such as the construction of a covariant notion of thermal equilibrium by considering tripartite systems. I discuss how such construction requires a relational take on thermodynamics, similar to what happens in the quantum theory and in gravity.

This chapter offers a meta-level analysis in the sociology and history of physics in the context of the “Arrow of Time” or the so-called Two Times problem. In effect, it argues that the two topics are intertwined, and it is only by coming to grips with the sociological aspects, involving adherence to certain metaphysical, epistemological, and methodological assumptions. Our argument is that the so-called Arrow of Time Problem or Two Times Problem (TTP) is essentially a myth. It is an article of faith that is contradicted by actual theoretical practice, in which the applicable physical theory does possess an Arrow of Time and must do so in order to account for the propagation of real energy and other conserved currents. Belief in the TTP is upheld only through entrenched adherence to a set of primarily metaphysical beliefs of a predominant Received View of physics that themselves are contradicted by the empirical facts, by current theory, and by inconsistencies among the beliefs themselves.

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.