Duricrust
Duricrust is a hard layer on or near the surface of soil. Duricrusts can range in thickness from a few millimeters or centimeters to several meters.
It is a general term for a zone of chemical precipitation and hardening formed at or near the surface of sedimentary bodies through pedogenic and non-pedogenic processes. It is typically formed by the accumulation of soluble minerals deposited by mineral-bearing waters that move upward, downward, or laterally by capillary action, commonly assisted in arid settings by evaporation. There are different types of duricrusts, each distinguished by a dominant mineralogy. For example, ferricrete is dominated by sesquioxides of iron; alcrete is dominated by sesquioxides of aluminum; silcrete by silica; calcrete by calcium carbonate, and gypcrete by gypsum.
Duricrusts need to be formed in absolute accumulation, therefore they must have a source, transfer and precipitation.
Duricrust is often studied during missions to Mars because it may help prove the planet once had more water. Duricrust was found on Mars at the Viking 2 landing site, and a similar structure, nicknamed "Snow Queen", was found under the Phoenix landing site. Phoenix's duricrust was later confirmed to be water-based.
