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A planetary surface is where the solid (or liquid) material of the outer crust on certain types of astronomical objects comes into contact with atmosphere or outer space; planetary surfaces can be found on terrestrial planets, dwarf planets, natural satellites and planetesimals.〔(【引用サイトリンク】 Planetary Surface materials )〕 The study of planetary surfaces is referred to as surface geology but also a focus of a number of fields including planetary cartography, topography, geomorphology, atmospheric sciences and astronomy. Land (or ground) is the term given to a planetary surface that is not covered by liquid, the term "landing" is used to describe the collision of an object with a planetary surface and is usually at a velocity in which the object can remain intact and remain attached. In differentiated bodies, the surface is where the crust meets the planetary boundary layer. Anything below this is regarded as being sub-surface or sub-marine. The inner atmospheres of some astronomical bodies such as stars and gas giants may have a contiguous liquid phase but are generally not regarded as a surface. Planetary surfaces and surface life are of particular interest to humans as it is the species' primary habitat, on which they evolved to breathe air and to move over land. As such, it is a major focus of space exploration and space colonization. The Earth and Moon are the only such surfaces to have been directly explored by humans. Due to the difficulty involved in exploration, relatively limited "on the ground" exploration has been undertaken. As conclusions are difficult to make from observation at distance through flyby or orbit, the exact composition and properties of many planetary surfaces remain unconfirmed. Space probes including lander spacecraft have encountered the surfaces of planetary mass objects including Mars, Venus, Titan as well as other astronomical bodies including 433 Eros, 25143 Itokawa, Tempel 1 and 67P/Churyumov–Gerasimenko. Mars is currently the only other planet to have had its surface explored by a mobile surface probe (rover). ==Distribution and conditions== Planetary surfaces are found throughout the Solar System, from the inner terrestrial planets, to the asteroid belt, the natural satellites of the gas giant planets and beyond to the Trans-Neptunian objects. Surface conditions, temperatures and terrain vary significantly due to a number of factors including Albedo often generated by the surfaces itself. Measures of surface conditions include surface area, surface gravity, surface temperature and surface pressure. Surface stability may be affected by erosion through Aeolian processes, hydrology, subduction, volcanism, sediment or seismic activity. Some surfaces are dynamic while others remain unchanged for millions of years. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「planetary surface」の詳細全文を読む スポンサード リンク
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