Volatile (astrogeology)

• 1. Introduction

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  Volatiles are the chemical elements and compounds that vaporize with relative ease, and they sit at the heart of some of the most dramatic events in the solar system. A volcano erupting in a towering column of ash, a comet trailing a brilliant tail across the sky, the icy outer planets of our solar system spinning in the dark: all of these are, in a fundamental sense, stories about volatiles. The list of substances that qualify is long and surprising. Hydrogen, oxygen, water, ammonia, methane, carbon dioxide, sulfur dioxide, hydrogen sulfide, phosphine, the halogens, the noble gases: these are the restless members of the periodic table, the ones that refuse to stay put. Their opposites, the refractory substances, are the stable, heat-resistant compounds that hold their form under conditions that would send volatiles fleeing as vapor. What makes this distinction matter is that volatiles behave very differently depending on where they are. On Earth, they concentrate inside magma and drive volcanic eruptions. Across the solar system, they define the character of entire planets. The questions that follow are worth sitting with: why does the same molecule behave as a gas on one world and as an ice on another? And why, when magma rises toward the surface of the Earth, do those dissolved gases become so catastrophically dangerous?

• 2. Gases, Ices, and Giant Worlds

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  Planetary scientists draw a useful line through the volatile family based on melting points. Those with exceptionally low melting points, hydrogen and helium being the prime examples, are classed as gases. Those with melting points above roughly 100 K, which is about -173 degrees Celsius or -280 degrees Fahrenheit, fall into the category scientists call ices. The terminology can catch newcomers off guard, because in planetary science neither "gas" nor "ice" refers strictly to a physical state. A compound described as an ice may actually exist as a solid, a liquid, or a gas depending on local conditions. Jupiter and Saturn earn the label gas giants because their bulk compositions are dominated by hydrogen and helium. Uranus and Neptune, by contrast, are ice giants, dominated by heavier volatiles. Yet the interiors of all four worlds are composed mostly of a hot, dense fluid that grows denser as the center is approached. In the case of Neptune, temperatures inside may reach 5,100 degrees Celsius. Closer to the Sun, cometary behavior is largely governed by water ice sublimating as comets approach inside Jupiter's orbit. Farther out, supervolatiles such as carbon monoxide and carbon dioxide can drive cometary activity as far out as 25.8 astronomical units from the Sun, well beyond the main planets.

• 3. Volcanoes From the Inside Out

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  In igneous petrology, the word "volatile" narrows to a more specific target: the gaseous components dissolved within magma, chiefly water vapor and carbon dioxide. These two substances, largely invisible until an eruption, govern whether a volcano vents quietly or explodes with extraordinary force. The key variable is viscosity. A magma that is felsic, meaning it has a higher silica content, tends to be thick and viscous. When volatiles are trapped in such a magma, the pressure builds until the eruption becomes explosive. A mafic magma, with lower silica content and lower viscosity, allows volatiles to escape more gently, producing effusive eruptions that pour lava in flowing sheets and can give rise to lava fountains. Some explosive eruptions are triggered by the interaction between water and magma at the surface, a sudden release of energy when the two meet. Other eruptions, though, trace their violence to the gas already dissolved inside the magma itself. As magma climbs toward the surface, pressure drops and dissolved volatiles come out of solution, forming bubbles that circulate through the liquid. Those bubbles link up into networks, and the connected gas promotes the breakup of magma into small drops, sprays, or clots carried in an expanding cloud. A typical magma is 95-99 percent liquid rock by composition, but that small fraction of gas represents an enormous volume once it reaches atmospheric pressure.

• 4. Pressure, Depth, and Solubility

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  Three factors govern how volatiles disperse through magma: the pressure confining the melt, the composition of the magma, and its temperature. Pressure and composition are the dominant parameters. Scientists use empirical equations to model how much gas a given magma can hold at a given pressure. For water dissolved in a generic magma, the relationship is expressed as n equals 0.1078 times P, where n is the dissolved gas as a weight percentage and P is the pressure in megapascals. In rhyolite, a silica-rich volcanic rock, the equation becomes n equals 0.4111 times P, meaning rhyolite holds considerably more water than the baseline. For carbon dioxide in the same system, the coefficient drops to just 0.0023 times P, a much lower capacity. These equations assume a single volatile in isolation; in reality, magmas carry multiple volatiles at once, and the chemical interactions between them complicate every calculation. As magma ascends, it moves from undersaturated conditions, where it holds less water than the maximum possible, toward saturation, and then into supersaturation. Once supersaturated, the excess water can be ejected as bubbles or vapor. Carbon dioxide behaves differently from water because its solubility is far lower. It begins exsolving at greater depth, well before the magma reaches the chamber, and tends to leak upward through cracks into overlying calderas. By the time a full eruption begins, the magma has already shed most of its carbon dioxide, leaving water as the dominant volatile driving the event.

• 5. The Life of a Bubble

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  Bubble nucleation begins the moment a volatile becomes saturated in the magma. At that threshold, volatile molecules spontaneously aggregate in a process called homogeneous nucleation. Surface tension works against this process, trying to shrink each bubble back into the liquid. The irregular surfaces provided by solid crystals stored inside the magma chamber offer a physical foothold that makes nucleation far easier. Those crystals become prime nucleation sites, reducing the energy barrier that volatiles must overcome to form stable bubbles. When nucleation is delayed, the magma becomes progressively more supersaturated, and the relationship between supersaturation pressure and bubble radius follows the equation delta P equals 2 sigma divided by r, with the supersaturation pressure reaching values around 100 megapascals and sigma representing surface tension. In a delayed-nucleation scenario, the bubbles that eventually form are closely packed because the distance between existing bubble sites is small. The faster magma rises, the further the system departs from equilibrium, and the more violently the eventual nucleation occurs. A competition runs constantly inside rising magma: new volatile molecules can either join existing bubbles, enlarging them, or seed entirely new ones. The spacing between molecules determines which outcome dominates, and that spacing, in turn, determines the texture and behavior of the magma as it approaches the surface.