page_title: Geology of Mercury
section_title: Impact basins and craters
hierarchical_section_title: Geology of Mercury / Surface features / Impact basins and craters
caption_attribution_description: English: Mariner 10 photomosaic of Caloris Basin on Mercury, with labels
context_page_description: The geology of Mercury is the least understood of all the terrestrial planets in the Solar System. This stems largely from Mercury's proximity to the Sun which makes reaching it with spacecraft technically challenging and Earth-based observations difficult.
  Mercury's surface is dominated by impact craters, basaltic rock and smooth plains, many of them a result of flood volcanism, similar in some respects to the lunar maria, and locally by pyroclastic deposits. Other notable features include vents which appear to be the source of magma-carved valleys, often-grouped irregular-shaped depressions termed "hollows" that are believed to be the result of collapsed magma chambers, scarps indicative of thrust faulting and mineral deposits inside craters at the poles. Long thought to be geologically inactive, new evidence suggests there may still be some level of activity.
  Mercury's density implies a solid iron-rich core that accounts for about 60% of its volume. Mercury's magnetic equator is shifted nearly 20% of the planet's radius towards the north, the largest ratio of all planets.
context_section_description: Craters on Mercury range in diameter from small bowl-shaped craters to multi-ringed impact basins hundreds of kilometers across. They appear in all states of degradation, from relatively fresh rayed-craters, to highly degraded crater remnants. Mercurian craters differ subtly from Lunar craters – the extent of their ejecta blankets is much smaller, which is a consequence of the 2.5 times stronger surface gravity on Mercury.
  The largest known crater is the enormous Caloris Basin, with a diameter of 1550 km, A basin of comparable size, tentatively named Skinakas Basin had been postulated from low resolution Earth-based observations of the non-Mariner-imaged hemisphere, but has not been observed in MESSENGER imagery of the corresponding terrain. The impact which created the Caloris Basin was so powerful that its effects are seen on a global scale. It caused lava eruptions and left a concentric ring over 2 km tall surrounding the impact crater. At the antipode of the Caloris Basin lies a large region of unusual, hilly and furrowed terrain, sometimes called "Weird Terrain". The favoured hypothesis for the origin of this geomorphologic unit is that shock waves generated during the impact traveled around the planet, and when they converged at the basin's antipode (180 degrees away) the high stresses were capable of fracturing the surface. A much less favoured idea was that this terrain formed as a result of the convergence of ejecta at this basin's antipode. Furthermore, the formation of the Caloris Basin appears to have produced a shallow depression concentric around the basin, which was later filled by the smooth plains (see below).
  Overall about 15 impact basins have been identified on the imaged part of Mercury. Other notable basins include the 400 km wide, multi-ring, Tolstoj Basin which has an ejecta blanket extending up to 500 km from its rim, and its floor has been filled by smooth plains materials. Beethoven Basin also has a similar-sized ejecta blanket and a 625 km diameter rim.
  As on the Moon, fresh craters on Mercury show prominent bright ray systems. These are made by ejected debris, which tend to be brighter while they remain relatively fresh because of a lesser amount of space weathering than the surrounding older terrain.
caption: Mercury's Caloris Basin is one of the largest impact features in the Solar System
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