A cold trap is a concept in planetary sciences that describes an area cold enough to freeze (trap) volatiles. Cold-traps can exist on the surfaces of airless bodies or in the upper layers of an adiabatic atmosphere. On airless bodies, the ices trapped inside cold-traps can potentially remain there for geologic time periods, allowing us a glimpse into the primordial solar system. In adiabatic atmospheres, cold-traps prevent volatiles (such as water) from escaping the atmosphere into space.
Cold-traps on airless planetary bodies
The obliquity (axial tilt) of some airless planetary bodies in the Solar System such as Mercury, the Moon and Ceres is very close to zero. Harold Urey first noted that depressions or craters located near the poles of these bodies will cast persistent shadows that can survive for geologic time periods (millions–billions of years). The absence of an atmosphere prevents mixing by convection, rendering these shadows extremely cold. If molecules of volatiles such as water ice travel into these permanent shadows, they will become trapped for geologic time periods.
Studying cold-traps on airless bodies
As these shadows receive no insolation, most of the heat they receive is scattered and emitted radiation from the surrounding topography. Usually, horizontal heat conduction from adjacent warmer areas can be neglected due to the high porosity and therefore low thermal conductivity of the uppermost layers of airless bodies. Consequently, the temperatures of these permanent shadows can be modeled using ray casting or ray tracing algorithms coupled with 1D vertical heat conduction models. In some cases, such as bowl-shaped craters, it is possible to obtain an expression for the equilibrium temperature of these shadows.
Additionally, the temperatures (and therefore the stability) of cold-traps can be remotely sensed by an orbiter. The temperatures of lunar cold-traps have been extensively studied by the Lunar Reconnaissance Orbiter Diviner radiometer. On Mercury, evidence for ice deposits inside cold-traps has been obtained through radar, reflectance and visible imagery. On Ceres, cold-traps have been detected by the Dawn spacecraft.
Atmospheric cold-traps
In atmospheric science, a cold-trap is a layer of the atmosphere that is substantially colder than both the deeper and higher layers. For example, for Earth's troposphere, the temperature of the air drops with increasing height reaching a low point (at about 20 kilometers height). This region is called a cold-trap, because it traps ascending gases with high boiling points, forcing them to drop back into Earth.
For biological life-forms on Earth, the most important gas to be kept in that way is water vapor. Without the presence of a cold-trap in the atmosphere, the water content would gradually escape into space, making life impossible. The cold trap retains one-tenth of a percent of the water in the atmosphere in the form of a vapor at high altitudes. Earth's cold-trap is also a layer above which ultraviolet intensity is strong, since higher up the amount of water vapor is negligible. Oxygen screens out ultraviolet intensity.