What subsurface feature observed by the MESSENGER mission suggests ancient liquid reservoirs?
Chaotic terrain characterized by jumbled blocks and depressions
Data collected by the MESSENGER mission indicated the presence of chaotic terrain features on Mercury, described as areas composed of jumbled blocks and depressions. The leading scientific hypothesis explaining this topography suggests it resulted from the collapse of the planet's surface layer. This collapse is thought to have occurred over material beneath the crust that was either receding or solidifying after being liquid—specifically, ancient, salty liquid water or brine. The implication is that Mercury once possessed substantial liquid reservoirs deep below its surface, which, if true, provides evidence that conditions once existed that might have sustained microbial life.
