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 or 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. It is 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.
Duricrust is often studied during missions to Mars because it may help prove that 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.