Can ice exist in a vacuum?

The question of ice persistence in the near-total absence of atmosphere is not straightforward; it requires careful consideration of temperature, pressure, and the direction of heat flow. In a vacuum, the environment aggressively promotes the transition of solid directly to gas—a process called sublimation—because the pressure is typically far below the critical threshold where liquid water can stably exist. ^[1][6] For water, this stability boundary is the triple point, a specific combination of temperature ($\sim 0.01^\circ \text{C}$) and pressure ($\sim 0.6 \text{ kPa}$) where solid, liquid, and gas phases can coexist in equilibrium. ^[1][6] Any pressure significantly lower than this point drives ice to sublime directly into vapor. ^[1]

# Pressure Dynamics

At standard terrestrial pressure, around $101 \text{ kPa}$, ice requires a temperature above $0^\circ \text{C}$ to melt, or a heat source to sublime, though sublimation in a typical home freezer (often around $-17.77^\circ \text{C}$) is negligible because the pressure is so much higher than the triple point pressure. ^[1] It is the pressure that dictates whether a solid turns into a liquid or jumps straight to vapor. In the hard vacuum of space, or on surfaces like the Moon that lack a significant atmosphere, the pressure is extremely low, meaning the triple point is bypassed entirely for any transition involving liquid water, forcing ice to sublime. ^[1]

# Vacuum Freezing

Paradoxically, ice can form even when the surrounding ambient temperature is quite warm, provided the pressure is rapidly lowered. This phenomenon, often observed in vacuum chamber experiments or discussions about HVAC systems, is called vacuum freezing. ^[6] When liquid water is exposed to a strong vacuum, its boiling point plummets. The necessary energy for this rapid boiling (the latent heat of vaporization) is drawn directly from the water molecules themselves. ^[6] Since this energy is removed from the system via the escaping vapor, the remaining liquid cools drastically. If the cooling is fast enough, the temperature drops below freezing before all the water has turned to vapor, resulting in the formation of ice. ^[6] In a closed system, this process continues until the water freezes or the remaining vapor pressure equals the equilibrium vapor pressure of the resulting ice, which corresponds to the triple point conditions. ^[6] This contrasts sharply with a standard stovetop boil where an external heat source provides the energy for the phase change, maintaining the water temperature near $100^\circ \text{C}$. ^[6]

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# Cosmic Ice Survival

If a vacuum causes boiling and eventual freezing of liquid water, how do celestial bodies like comets, which are fundamentally composed of ice, survive for billions of years far from the Sun? The answer lies in the extremely low energy input in deep space. For an ice cube near Earth's orbit, the heat transfer is dictated by solar radiation, which is intense (about $1 \text{ kW}$ per square meter). ^[4] This energy flux is high enough to drive rapid sublimation, potentially causing a $1 \text{ m}^2$ surface area of ice at $0^\circ \text{C}$ to lose over $5 \text{ MJ}$ per second due to the required latent heat, which dwarfs the incoming solar energy at $1 \text{ AU}$. ^[4]

However, comets spend most of their time in regions like the Kuiper Belt, perhaps $50 \text{ AU}$ from the Sun. The solar radiation intensity drops off by the square of the distance, meaning the heat input is only $1/2500$th of what Earth receives. ^[4] At these frigid temperatures, the rate of sublimation drops exponentially, allowing the ice to persist for vast stretches of time. ^[4] The ice cube near the space shuttle would sublime significantly faster than a comet far out in the solar system, perhaps taking $2500$ times longer in the Kuiper Belt than near Earth, assuming similar starting conditions. ^[4]

# Sublimation Science

Scientific investigation, such as a detailed experimental and theoretical study funded by $\text{NASA}$, has moved beyond simple kinetic theory to precisely quantify ice behavior in a simulated vacuum environment. ^[4] This research involved suspending individual ice particles in a vacuum chamber better than $10^{-5} \text{ torr}$ (where heat transfer by gas conduction is negligible) and measuring mass loss while simultaneously controlling and measuring surface temperature. ^[4]

A key finding was the evaporation coefficient ($\alpha_e$), which corrects the theoretical sublimation rate (based on simple kinetic gas theory) to match the experimentally observed rate. ^[4] Near the melting point ($0^\circ \text{C}$), the actual sublimation rate was found to be substantially lower—up to two orders of magnitude less—than the theoretical prediction. ^[4] The functional dependence of this coefficient on temperature was not random; it followed a general exponential trend, suggesting a similarity to a Fermi distribution function. ^[4] This implies that the rate of molecules ready to evaporate, influenced by surface diffusion and supply, dictates the actual observed loss, deviating from equilibrium expectations as temperature rises. ^[4]

This deviation has profound implications: if the experimental data is used, rather than the simpler theoretical kinetic model, the estimated distance from the Sun required for ice to melt (and thus potentially become liquid) on a comet increases from about $0.03 \text{ AU}$ to approximately $0.2 \text{ AU}$. ^[4] The complex interplay between incoming radiation, particle size, and surface characteristics determines the lifespan of cosmic ice grains. ^[4]

An interesting observation arising from the vacuum experiments involved the freezing of suspended water droplets. When degassed water froze under vacuum, the surface temperature would stabilize exactly at $0^\circ \text{C}$ for a measurable duration, even as the sublimation rate was actively declining after an initial peak. ^[4] This behavior, which seems contradictory—constant temperature but decreasing mass loss—was attributed to a complex, multi-stage freezing process involving dendritic ice growth on the surface followed by the slower crystallization of the interior, which affects the release of latent heat over time. ^[4]

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# Sunlight and Distance

The environment surrounding the ice particle dictates its fate. In the $\text{NASA}$ study, when simulating sunlight with a xenon arc lamp, the sublimation rate was found to be size-dependent; smaller particles showed a decreased rate, though the effect of surface roughness due to preparation (air-saturated vs. boiled water) was considered a more significant factor in the structure than the overall mass loss rate. ^[4]

If you were to drop an ice cube outside the International Space Station (where the environment is roughly $1 \text{ AU}$ from the Sun), the side facing the Sun would absorb energy at roughly $1 \text{ kW}/\text{m}^2$, potentially keeping its surface near $300 \text{ K}$ (though it would cool quickly through evaporation and radiation). Conversely, the side in shadow would radiate heat into space and cool extremely rapidly. ^[4] The net result is that the ice cube would not simply sublime at a steady rate; it would experience extreme thermal gradients, with the shadowed portion cooling to temperatures where sublimation is incredibly slow (e.g., $170 \text{ K}$ where vapor pressure is $130$ million times less than atmospheric pressure). ^[4] The directionality of the solar heating, which is often ignored in simple models, is a critical factor in determining whether the ice survives for hours or days before complete vaporization or melting. ^[4]

# Impurities and Rate Changes

The composition of the ice block is another critical element influencing how quickly it can vanish in a vacuum. Ice found in nature, such as on comets, is never pure $\text{H}_2\text{O}$. ^[4] The presence of impurities alters how much energy is absorbed from the radiation field. Experiments using cores mixed with materials showed that:

• Charcoal Powder: Being strongly absorbing across the spectrum, higher concentrations led to a significantly higher sublimation rate. ^[4]
• Aluminum Microspheres: These were absorbing in the ultraviolet and scattering in the visible spectrum. They also increased the sublimation rate with concentration. ^[4]
• Aluminum Oxide ($\text{Al}_2\text{O}_3$): Characterized by strong scattering in the visible spectrum but low absorption there, adding this impurity had almost no effect on the sublimation rate compared to pure ice, demonstrating that scattering is less influential than direct absorption by an embedded impurity. ^[4]

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# Liquid Water Existence

The experimental data confirming that the actual sublimation rate remains low even close to the melting point ($\sim 0^\circ \text{C}$) in a vacuum suggests that ice can withstand heating for longer periods than predicted by older kinetic models alone. ^[4] This has an intriguing consequence for solar system objects: since a comet's ice requires closer proximity to the Sun to match the theoretical rate of sublimation, using the experimental (slower) rate means that liquid water could potentially exist on comet surfaces at distances up to $0.2 \text{ AU}$ away from the Sun, increasing the time and distance over which these icy bodies might exhibit liquid phases during their orbits. ^[4] Therefore, ice can exist in a vacuum, provided the environment is cold enough (like deep space far from a star) or the transition process is managed by energy removal (like vacuum freezing of ambient water).