Caloris Planitia is a

plain In geography, a plain is a flat expanse of land that generally does not change much in elevation, and is primarily treeless. Plains occur as lowlands along valleys or at the base of mountains, as coastal plains, and as plateaus or uplands ...

within a large impact basin on Mercury, informally named Caloris, about in

diameter In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints lie on the circle. It can also be defined as the longest chord of the circle. Both definitions are also valid f ...

. It is one of the largest impact basins in the Solar System. "Calor" is

Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through ...

for "

heat In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is ...

" and the basin is so-named because the Sun is almost directly overhead every second time Mercury passes

perihelion An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any elli ...

. The crater, discovered in 1974, is surrounded by the Caloris Montes, a ring of

mountain A mountain is an elevated portion of the Earth's crust, generally with steep sides that show significant exposed bedrock. Although definitions vary, a mountain may differ from a plateau in having a limited summit area, and is usually higher ...

s approximately tall.

Appearance

MESSENGER first photo of unseen side of mercury cropped to Caloris

Caloris was discovered on images taken by the ''

Mariner 10 ''Mariner 10'' was an American robotic space probe launched by NASA on 3 November 1973, to fly by the planets Mercury and Venus. It was the first spacecraft to perform flybys of multiple planets. ''Mariner 10'' was launched approximatel ...

'' probe in 1974. Its name was suggested by Brian O'Leary, astronaut and member of the ''Mariner 10'' imagery team. It was situated on the terminator—the line dividing the daytime and nighttime hemispheres—at the time the probe passed by, and so half of the crater could not be imaged. Later, on January 15, 2008, one of the first photos of the planet taken by the ''

MESSENGER ''MESSENGER'' was a NASA robotic space probe that orbited the planet Mercury between 2011 and 2015, studying Mercury's chemical composition, geology, and magnetic field. The name is a backronym for "Mercury Surface, Space Environment, Geochem ...

'' probe revealed the crater in its entirety. The basin was initially estimated to be about in diameter, though this was increased to based on subsequent images taken by ''MESSENGER''. It is ringed by mountains up to high. Inside the crater walls, the floor of the crater is filled by lava plains, similar to the maria of the

Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...

. These plains are superposed by explosive vents associated with pyroclastic material. Outside the walls, material ejected in the impact which created the basin extends for , and concentric rings surround the crater. In the center of the basin is a region containing numerous radial troughs that appear to be

extensional fault An extensional fault is a fault caused by stretching of the Earth's crust. Stretching reduces the thickness and horizontally extends portions of the crust and/or lithosphere. In most cases such a fault is also a normal fault, but may create ...

s, with a crater,

Apollodorus Apollodorus (Greek: Ἀπολλόδωρος ''Apollodoros'') was a popular name in ancient Greece. It is the masculine gender of a noun compounded from Apollo, the deity, and doron, "gift"; that is, "Gift of Apollo." It may refer to: :''Note: A f ...

, located near the center of the pattern. The exact cause of this pattern of troughs is not currently known. The feature is named

Pantheon Fossae The Pantheon Fossae are a radial set of troughs in a region in the middle of Caloris Basin on Mercury. They appear to be a set of graben formed by extensional faults, with a 40 km crater located near the center of the pattern. The exact ori ...

Formation

The impacting body is estimated to have been at least 100 km (62 miles) in diameter. Bodies in the inner Solar System experienced a heavy bombardment of large rocky bodies in the first billion years or so of the Solar System. The impact that created Caloris must have occurred after most of the heavy bombardment had finished, because fewer

impact crater An impact crater is a circular depression in the surface of a solid astronomical object formed by the hypervelocity impact of a smaller object. In contrast to volcanic craters, which result from explosion or internal collapse, impact crater ...

s are seen on its floor than exist on comparably-sized regions outside the crater. Similar impact basins on the Moon such as the

Mare Imbrium Mare Imbrium (Latin ''imbrium'', the "Sea of Showers" or "Sea of Rains", "Sea of Tears") is a vast lava plain within the Imbrium Basin on the Moon and is one of the larger craters in the Solar System. The Imbrium Basin formed from the colli ...

and

Mare Orientale Mare Orientale (Latin ''orientāle'', the "eastern sea") is a lunar mare. It is located on the western border of the near side and far side of the Moon, and is difficult to see from an Earthbound perspective. Images from spacecraft have reveal ...

are believed to have formed at about the same time, possibly indicating that there was a 'spike' of large impacts towards the end of the heavy bombardment phase of the early Solar System. Based on ''MESSENGER''s photographs, Caloris' age has been determined to be between 3.8 and 3.9 billion years. A gravitational high, also known as a mascon, is centered on Caloris Planitia. Most large impact basins on the moon, such as

and

Mare Crisium Mare Crisium (Latin ''crisium'', the "Sea of Crises") is a lunar mare located in the Moon's Crisium basin, just northeast of Mare Tranquillitatis. The basin is of the Pre-Imbrian period, 4.55 to 3.85 billion years ago. Characteristics Mare Cri ...

, are also the site of mascons.

Antipodal chaotic terrain and global effects

The giant impact believed to have formed Caloris may have had global consequences for the planet. At the exact antipode of the basin is a large area of hilly, grooved terrain, with few small impact craters that are known as chaotic terrain (also "weird terrain"). It is thought by some to have been created as

seismic wave A seismic wave is a wave of acoustic energy that travels through the Earth. It can result from an earthquake, volcanic eruption, magma movement, a large landslide, and a large man-made explosion that produces low-frequency acoustic ener ...

s from the impact converged on the opposite side of the planet. Alternatively, it has been suggested that this terrain formed as a result of the convergence of ejecta at this basin's antipode. This hypothetical impact is also believed to have triggered

volcanic activity Volcanism, vulcanism or volcanicity is the phenomenon of eruption of molten rock (magma) onto the surface of the Earth or a solid-surface planet or moon, where lava, pyroclastics, and volcanic gases erupt through a break in the surface called a ...

on Mercury, resulting in the formation of smooth plains. Surrounding Caloris is a series of geologic formations thought to have been produced by the basin's ejecta, collectively called the Caloris Group.

Emissions of gas

Mercury has a very tenuous and transient atmosphere, containing small amounts of

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...

and

helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic ta ...

captured from the

solar wind The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between . The composition of the sol ...

, as well as heavier elements such as

sodium Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...

and

potassium Potassium is the chemical element with the symbol K (from Neo-Latin '' kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmos ...

. These are thought to originate within the planet, being "out-gassed" from beneath its crust. The Caloris basin has been found to be a significant source of sodium and potassium, indicating that the fractures created by the impact facilitate the release of gases from within the planet. The unusual terrain is also a source of these gases.

Gallery

File:Caloris Basin topography JMARS Colorized Shade 2km.jpg, Topographic map of Caloris basin File:Caloris basin labeled.png, Mosaic of eastern Caloris basin photographed by ''

'' in 1974–75. File:Spider crater on planet mercury.jpg,

in the center of Caloris basin, with

crater at center of the fossae. File:PIA19421-Mercury-Craters-MunchSanderPoe-20150416.jpg, Enhanced color image of craters Munch,

Sander A sander is a power tool used to smooth surfaces by abrasion with sandpaper. Sanders have a means to attach the sandpaper and a mechanism to move it rapidly contained within a housing with means to hand-hold it or fix it to a workbench. Woodw ...

, and

Poe Edgar Allan Poe (; Edgar Poe; January 19, 1809 – October 7, 1849) was an American writer, poet, editor, and literary critic. Poe is best known for his poetry and short stories, particularly his tales of mystery and the macabre. He is widel ...

, amid plains of Caloris basin File:PIA19450-PlanetMercury-CalorisBasin-20150501.jpg, Perspective view of Caloris – high (red); low (blue).

References