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Planetary regoliths form through impact fragmentation and micrometeorite bombardment, creating soil-like layers of broken rock. Weathering processes (thermal cycling, chemical alteration, ice sublimation) depend on atmosphere, surface temperature, and water availability; rates and styles differ dramatically between planets.
From your study of impact cratering mechanics, you know that collisions shatter target rock and eject debris across the surrounding terrain. Now scale that process up to billions of years of continuous bombardment — from giant impacts early in solar system history down to a steady rain of micrometeoroids today — and you get regolith: a blanket of fragmented, pulverized material covering a planetary surface. On the Moon, this layer ranges from a few meters to over 15 meters deep, accumulated over 4 billion years of impact gardening. Every square centimeter of the lunar surface has been churned, shattered, and re-shattered countless times.
But regolith formation is only the beginning. Once fragmented material sits on a surface, it is subject to space weathering — a suite of processes that alter its physical and chemical properties without any atmosphere or water involved. On airless bodies like the Moon and Mercury, solar wind ions (mostly hydrogen and helium nuclei) implant into grain surfaces, while micrometeorite impacts create tiny melt splashes that coat grains with nanoscale iron particles. These nanophase iron coatings progressively darken and redden the surface, which is why fresh lunar craters appear bright against the older, darkened terrain. The effect is so systematic that space weathering maturity has become a relative age-dating tool: the darker and redder the surface, the longer it has been exposed.
On bodies with atmospheres, entirely different weathering regimes take over. Mars has both mechanical and chemical weathering. Extreme diurnal temperature swings (from -80°C at night to +20°C by day) drive thermal fracturing, cracking rocks along grain boundaries as minerals expand and contract at different rates. Mars also has chemical weathering from acidic dust-water interactions in its past and ongoing oxidation of iron-bearing minerals by atmospheric peroxides, producing the planet's characteristic rust-red color. Venus, with its 460°C surface temperature and dense CO₂ atmosphere laced with sulfuric acid, weathers rock through high-temperature chemical reactions that would be impossible on any other terrestrial planet. On Titan, methane rain erodes ice bedrock much as water rain erodes silicate rock on Earth, creating eerily familiar river valleys and rounded pebbles — but made of water ice shaped by liquid hydrocarbons.
The critical insight is that weathering style is a direct fingerprint of surface environment. By identifying which weathering processes have acted on a surface — space weathering versus chemical alteration versus freeze-thaw cycling — planetary scientists can reconstruct atmospheric history, water availability, and temperature regimes even on worlds we have never visited with landers. Regolith is not just broken rock; it is a diary of every environmental condition the surface has experienced.