抄録
The distribution of clay minerals in the universe is not well understood. However, recent observations of molecular clouds and star formation regions show evidence for the presence of silicates and water molecules, suggesting that clay minerals could be formed under certain conditions. The presence of clay minerals in such a star formation region is directly found in some meteorites. Carbonaceous chondrites, that are primitive materials formed in the solar nebula about 4.56 billion years ago, contain abundant aqueous alteration products such as saponite and serpentine. These samples are important to understand the conditions of aqueous alteration in the very early stage of the solar system history.
However, the presence of clay minerals in the present solar system seems pessimistic because liquid water is absent except for the earth. The planet Mars will be the only promising place where abundant clay minerals could be present as far as we currently know. The evidence for the presence of clay minerals on Mars is also found in meteorites. Some meteorites are widely believed to have originated from Mars due to many pieces of evidence (e.g., matching gas compositions between trapped gas in meteorites and martian atmosphere as analyzed by the Viking lander). Although all known martian meteorites are igneous rocks, some of them contain aqueous alteration products texturally and compositionally similar to terrestrial “iddingsite”. Martian iddingsites are found in a particular martian meteorite class called “nakhlite”. Nakhlite is a clinopyroxenite with minor abundance of olivine and mesostasis. The fractures and rims of olivine and mesostasis are altered into brown color similar to iddingsite. Martian iddingsite mainly consist of smectite and ferrihydrite with some minor components. Geochronological and mineralogical studies show that martian meteorites with clay minerals crystallized 1.3 billion to 180 million years ago and the crystallization of magma occurred near the surface (-30 m from the surface). Thus, the presence of aqueous alteration products in martian meteorites suggests that liquid water was present near the surface of Mars until recently although the abundance of water was small and the duration was short. Nevertheless, aqueous alteration products in martian meteorites are important samples to understand the history of volatile evolution on Mars and have recently provided interesting results.
The presence of clay minerals on Mars is also supported by recent observations of Mars by spacecrafts. 2001 Mars Odyssey found evidence for the possible wide distribution of water-bearing minerals in low latitude regions as well as abundant water ice in polar caps. Mars Exploration Rovers recently landed on two different places on Mars just started various analyses to look for evidence for past liquid water. Some early results are consistent with the presence of clay minerals. The results of thermodynamic calculation assuming martian environment also suggest that clay mineral are stable on Mars. Thus, claysphere is clearly present beyond earth and clay science is important to understand geological history of Mars.
