A lunar cold spot is a region of anomalously low nighttime surface temperature surrounding young impact craters on the Moon's surface.^[1] Hypervelocity impacts displace and rarefy material, and ejecta falls to form a “fluffy” surface layer that is less densely packed than surrounding lunar regolith.^[2] The resulting region of low surface density has a lower thermal inertia than other material and cools faster after sunset.^[2]

These ray-like spots extend 10 to 100 times the radius of the impact crater and are 2–10 Kelvin^[3] cooler than the surrounding lunar surface.^[1] Depending on size, cold spots fade to background levels after roughly 0.5–1 million years^{[n 1]} as successive impacts and seismic waves recompact the material.^[4] They cover over 1% of the lunar surface.^[2]

The phenomenon was first observed by Apollo 17, which carried an infrared radiometer.^[1] Cold spots were systematically characterized and mapped by Joshua Bandfield and other planetary scientists at the Space Science Institute in 2011 using data from the Diviner Lunar Radiometer Experiment (DLRE) on NASA's Lunar Reconnaissance Orbiter.^[2] Over 2,200 cold spots have been identified on the lunar surface between the latitudes 50° N and 50° S, with source craters ranging from 43 meters to 2.3 kilometers in diameter.^[1]

Cold spots have been found only on the Moon but are predicted to be present on Mercury and other airless bodies.^[5]

The command and service module on Apollo 17 was equipped with an infrared scanning radiometer. During its lunar orbit in December 1972, the instrument detected low temperature anomalies near certain craters. Observations were reported by planetary scientists Wendell W. Mendell and Frank J. Low in 1974 but attracted little further investigation for several decades.^[1]

With the launch of NASA's Lunar Reconnaissance Orbiter in June 2009, a systematic global mapping campaign was carried out using the Diviner Lunar Radiometer Experiment. The instrument measured emitted thermal radiation and reflected sunlight across the lunar surface and could derive surface kinetic temperatures to a precision of 1–2 K.^[6] Using seven spectral channels spanning the visible spectrum and infrared wavelengths, Diviner collected over 500 billion radiometric measurements and mapped surface temperatures across the entire surface of the Moon.^[6] The instrument built composite coverage by taking measurements at multiple local times over successive orbits, producing a map with a spatial resolution of 180 by 300 meters per pixel.^[6]

Analysis of the dataset by Joshua L. Bandfield and other scientists at the Space Science Institute (SSI) in Boulder, Colorado identified cold spots as a formally distinct class of surface feature in 2011.^[2] Cold spots appear in Diviner nighttime temperature maps as ray-like or irregular patches of suppressed temperature radiating outward from a central young crater. They are absent from daytime maps, radar backscatter imagery, and visible or near-infrared reflectance data.^[2] Detection is dependent on night-time thermal infrared mapping. Over 2,200 spots were characterized in greater detail by 2014.^[1]

Cold spots possess thermal inertia values of 40–50 J m⁻² K⁻¹ s⁻¹/² at kilometer scales, 10 to 30 percent lower than the global average for lunar regolith fines.^[7] In the largest cold spots, values can fall below 20 J m⁻² K⁻¹ s⁻¹/², among the lowest recorded anywhere on the lunar surface.^[7] The largest temperature contrast occurs just after sunset, when the surrounding surface has the greatest amount of trapped heat, and gradually diminishes through the night.^[2] The cold spot layer is estimated to be at least 5 centimeters thick to explain the persistent thermal signature across the entire lunar night.^[7] The total volume of decompacted material in a cold spot is typically more than ten times the volume of the source crater itself.^[7] This suggests that the impact modifies the surrounding regolith to distances and depths far exceeding what direct ejecta deposition could account for.^[2]

All new craters greater than 40 meters in diameter possess cold spots, suggesting that the phenomenon is universal to recent impacts above this size threshold.^[8] Larger impacts produce more pronounced cold spots extending across greater distances.^[8] For example, the Einthoven cold spot, an informal name for the 1.1 kilometer impact crater located on the far side of the Moon south of the older and much larger crater Einthoven crater, is one of the largest ever catalogued, covering approximately 2,070 square kilometers of terrain and extends up to 50 kilometers from the crater.^[9]

Cold spots are not uniformly distributed, with double the density near the Moon's apex (0° N, 90° W), the leading side in its orbital motion with a higher degree of impact cratering.^[1] The largest cold spots, with source crater diameters a kilometer or wider, are concentrated on the antapex in the trailing hemisphere.^[1]

Analysis of Apollo 16 astronaut footprint depths revealed that they were slightly deeper in the cold spot surrounding South Ray crater, providing physical evidence for lower density regolith around cold spots.^[10] This agrees with thermal modelling, which requires a lower average density to explain the reduced nighttime temperatures.^[10]

The mechanism by which an impact creates cold spots at large distances remains an active area of research. Since ejecta deposition alone cannot account for the amount of low-density material or its vast distance from the crater,^[2] three main hypotheses have been proposed for how impacts decompactify surrounding regolith:

• Granular flow and secondary impacts: A laterally propagating granular flow driven by the impact under vacuum redistributes surface material close to the crater.^[2][11] Further out, a cascade of secondary ballistic impacts from the primary crater could progressively churn and loosen the regolith across the wider cold spot area.^[12] Eclipse and twilight thermal data are consistent with small surface rocks deposited by this granular flow process.^[8]

• Expanding vapor cloud: A rapidly expanding cloud of material vaporized by impact travels outward across the surface, entraining and disaggregating regolith grains without net deposition.^[2] Thermophysical modelling of Diviner data found this grain-lifting and turbulent mixing model to be the best match to the observed temperature patterns.^[7]

• Seismic wave transmission: Impact energy propagates outward through meter-scale boulders in the megaregolith immediately below the fine regolith layer, driving a surface solitary wave that dilates the overlying soil across large distances.^[4][13] Impact-generated seismic waves passing through the regolith have also been proposed as a contributing mechanism.^[14]

Cold spots are transient features. Comparison of cold spot crater size and frequency distributions with established crater chronology models constrains their retention time to no more than 0.5 to 1 million years.^[1] Smaller cold spots may fade within a few hundred thousand years.^[1] All cold spots decay toward background thermal properties at roughly the same rate, governed by the rate at which meteorite bombardment and micrometeorite impacts recompact and homogenize the decompacted regolith layer.^[1] Bombardment overturns regolith at a depth exceeding 5 centimeters over a few hundred thousand years.^[1]

Because cold spots are associated exclusively with the youngest impact craters, they provide a practical tool for identifying craters formed within the last million years, which in turn constrains models for impact flux rate on the Moon.^[1]

Cold spots also reveal the rate and depth at which the lunar surface is continuously turned over and mixed by meteorite bombardment.^[15] Their rapid degradation implies that the uppermost regolith is actively processed on geologically short timescales.^[1] Moreover, the modelled formation ages of the largest cold spot craters coincide with the 0–1.4 million-year cosmic ray exposure ages of known lunar meteorites on Earth, making cold spot craters strong candidates for their source sites.^[16][1]

Cold spots have exclusively been found on the Moon. However, the regolith decompaction process that generates lunar cold spots is expected to occur on any airless, silicate-rich body with a fine-grained surface layer.^[4] Thermal modelling predicts that cold spots 6 K cooler than ambient should exist on Mercury.^[5] In 2018, BepiColombo was launched by the European Space Agency and JAXA carrying a thermal radiometer that will search for cold spots on Mercury.^[5]