The coldest place in the Solar System is not the most distant planet, but localized pockets much closer to home. While distance from the Sun causes a drop in a body’s average temperature, the absolute record for coldness is held within specific, perpetually shaded topographical features on airless bodies in the inner Solar System. The conditions that create this extreme cold require a unique combination of geometry and physics that allows heat to escape into space without being replenished by solar energy.
The Mechanism: How Permanent Shadowing Creates Extreme Cold
The formation of the Solar System’s coldest regions relies on radiative cooling, the continuous emission of thermal energy into the vacuum of space. On airless bodies like the Moon and Mercury, this process is highly efficient because there is no atmosphere to trap escaping heat. Any surface not exposed to sunlight is free to radiate its internal heat outward.
The crucial ingredient for achieving record low temperatures is the body’s small axial tilt, or the angle of its rotation axis relative to its orbit. Both the Moon and Mercury have a small axial tilt, meaning their polar regions are angled almost perpendicularly to the Sun’s incoming rays. This low angle results in certain deep craters near the poles having floors that never receive direct sunlight.
These perpetually dark zones are referred to as Permanently Shadowed Regions (PSRs). Once heat radiates away from these PSRs, no new heat can enter, turning the crater floor into a “cold trap.” Volatile molecules, such as water vapor, that wander into these traps quickly freeze solid due to the extremely low temperatures and lack of atmospheric pressure. This constant heat loss with zero replenishment drives the temperature down to near-absolute zero levels.
The Absolute Record Holders: Permanently Shadowed Regions
The coldest measured spots in the Solar System are found within the Permanently Shadowed Regions (PSRs) of the Moon. Data from the Lunar Reconnaissance Orbiter (LRO) confirmed the existence of these thermal sinks. Specific areas within craters near the Moon’s south pole have registered temperatures as low as approximately 25 Kelvin.
To put this into perspective, 25 Kelvin is equivalent to about -248 degrees Celsius or -415 degrees Fahrenheit. These temperatures are substantially colder than the average surface temperature of the dwarf planet Pluto. The extreme cold is a direct result of these crater floors being shielded from solar illumination for potentially billions of years.
While Mercury is much closer to the Sun, its own PSRs also harbor remarkably low temperatures, though typically slightly warmer than the Moon’s record holders. The significance of these findings extends beyond temperature records; the intense cold preserves volatile compounds. Water ice, which would otherwise sublimate in the Sun’s heat, is trapped in these areas, making them valuable targets for future resource utilization and scientific study.
Distant Worlds: Bulk Temperatures of the Outer Solar System
The vast distances of the outer Solar System result in extremely low average temperatures for the gas and ice giants, but they do not hold the record for the absolute lowest localized temperature. Neptune, the farthest major planet, maintains an average atmospheric temperature of about -200 degrees Celsius. Even Pluto, a dwarf planet, has an average surface temperature of around 40 Kelvin, or approximately -233 degrees Celsius.
These low bulk temperatures are predominantly a function of the minimal solar energy they receive. This cold is distributed across the entire body, unlike the deep-freeze pockets in PSRs. The lowest atmospheric temperature measured on a planet is found on Uranus, where the minimum temperature of its atmosphere can drop to 49 Kelvin.
The key distinction lies between a low average temperature and a localized extreme. The absence of an atmosphere on inner Solar System bodies allows permanent shadowing and radiative cooling to achieve far more profound temperature drops in isolated spots. This results in the Moon’s shadowed craters being measurably colder than the surface of Pluto, despite the enormous difference in distance from the Sun.