Caves in space

Abstract

The definition of a cave has centered on its accessibility to humans, an understandable, but arbitrary, approach. Caves without humanly accessible entrances are still caves. As we reach out to the moons and planets of our solar system, and eventually the stars, we need to evaluate caves as possible reservoirs of extraterrestrial life and as potential human habitats. If humanly passable caves are a subset of all caves, then caves can exist from the tiny, subatomic dimension up to and including the entire universe. They can form and exist for only milliseconds or for billions of years. What's more, they can form in almost any solid material, and perhaps even in liquids, and contain within them vacuum, gases, liquids or solids. Caves can be initially classified as natural and artificial. Natural caves form either by constructive processes that build a boundary, which contains the void, or by destructional processes that remove or shift material to create a void, where none existed before. Caves are defined by answering five questions: 1. How did the void form? 2. How big is it? 3. How long has it lasted? 4. What does it contain? 5. How does it connect to exterior space? As humans move from a geocentric view of natural features to a more universal view, we need to see caves across a broader continuum of possibilities.

Planetary Caves: A Solar System View of Processes and Products

“…“A cave is a space rather than an object and consequently its definition involves the specification of its boundaries” (Curl, 1964, p. 1). On Earth, caves are either constructional, meaning a void has boundaries added to it, or they are destructional, in which a cave is created by excavating the void within an object (Mylroie, 2019). Examples of constructional caves include: talus caves , where the accumulation of blocks of rock subdivides open space into compartments; tufa caves form when the precipitation of CaCO 3 at a cliff or steep slope encloses open space; and reef caves , where organic precipitation of CaCO 3 encloses an open space by creating boundaries during coral reef formation.…”

“…The driving question that arises is whether we can use terrestrial speleology to define and understand cave formation processes on other planetary bodies. While Mylroie (2019) initially asserted that using Earth as a frame of reference was fraught with problems, that contention may not apply here. Earth has an active atmosphere and hydrosphere, a large moon, internal heat, and active plate tectonics that completes partial planetary resurfacing while preserving ancient terrains.…”

“…It is not only the size issue, but also a technological one. Mylroie (2019) indicated that a void filled with water was considered a "cave" as it could be explored by scuba divers or remotely operated vehicles, but a lava tube filled with molten lava would by this definition not be a cave as it cannot be directly examined by humans with current technology.…”

Fundamental Science and Engineering Questions in Planetary Cave Exploration

“…(#44; 75.6%) Most of the remaining unanswered geologic questions related to planetary caves involve formation mechanisms, distribution and occurrence, morphologies, and the potential for internal deposits across different planetary and geologic settings. These questions should be framed within the scope of cave size and duration of existence, which can be highly variable (Mylroie, 2019). From our survey, 46 geology questions were identified; of these, six were considered fundamental.…”

Lunar Pit Morphology: Implications for Exploration

“…Second, and the focus of this study, is the potential for accessing lunar caves (in this work, we use "cave" to refer to any sublunarean void space that is physically connected to a pit, regardless of scale or origin; see Mylroie (2019)). Lunar caves could preserve delicate structures for billions of years, and pits or caves could provide shelter for modern-day explorers, making them attractive destinations from both scientific and engineering perspectives.…”