Ghost craters on the planet

Mercury Mercury commonly refers to: * Mercury (planet), the nearest planet to the Sun * Mercury (element), a metallic chemical element with the symbol Hg * Mercury (mythology), a Roman god Mercury or The Mercury may also refer to: Companies * Merc ...

have

tectonic Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building, the growth and behavior of the strong, old cores of continents k ...

features such as

graben In geology, a graben () is a depressed block of the crust of a planet or moon, bordered by parallel normal faults. Etymology ''Graben'' is a loan word from German, meaning 'ditch' or 'trench'. The word was first used in the geologic contex ...

and

wrinkle ridge A wrinkle ridge is a type of feature commonly found on lunar maria, or basalt plains. These features are low, sinuous ridges formed on the mare surface that can extend for up to several hundred kilometers. Wrinkle ridges are tectonic features cr ...

s. These features were formed by extensional and contractional forces originating in tectonic processes such as

uplift Uplift may refer to: Science * Geologic uplift, a geological process ** Tectonic uplift, a geological process * Stellar uplift, the theoretical prospect of moving a stellar mass * Uplift mountains * Llano Uplift * Nemaha Uplift Business * Uplif ...

and global contraction. The combination of graben and wrinkle ridges inside ghost craters found on Mercury has not been observed on any of the other

terrestrial planet A terrestrial planet, telluric planet, or rocky planet, is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets accepted by the IAU are the inner planets closest to the Sun: Mercury, Ve ...

Discovery

The tectonic features on Mercury were not studied in precise detail until the ''MESSENGER'' spacecraft was sent to Mercury; by 2009 the ''MESSENGER'' spacecraft had mapped 98% of the planet's surface. By 2013, Messenger mapped 100% of the planet's surface. Mercury Dual Imaging System cameras and Mercury Laser Altimeter equipment on the messenger spacecraft allowed for researchers to obtain high-resolution images of the planet's surface for analyzing Mercury's tectonics.

Ghost craters

Ghost crater A palimpsest , in planetary astronomy, is an ancient crater that has been degraded over time. They may also be referred to as "ghost craters", "degraded craters", "buried craters", or "pathological craters". Palimpsests have been identified on ...

s are

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 craters ...

s that have been filled with

volcanic A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often found where tectonic plates a ...

deposits. Ghost craters can be observed on the surface 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 ...

Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury (planet), Mercury. In the English language, Mars is named for the Mars (mythology), Roman god of war. Mars is a terr ...

, Mercury, and may possibly exist on

Venus Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never fa ...

. On Mercury, ghost craters are most commonly found in the northern hemisphere of the planet in the smooth plains. There are three types of ghost craters on Mercury. Type one ghost craters contains a wrinkle ridge that forms a ring while type two ghost craters contain a wrinkle ridge ring and graben in the interior. Type three ghost crater's wrinkle ridge ring either does not exist or can not be observed. Instead, it has a ring of graben along its border.

Tectonic features

Graben

A graben is a tectonic feature associated with normal faulting. The footwall is raised while the hanging wall is lowered, creating a depression. The graben are thought to have formed from extensional stresses due to uplift and contraction of the volcanic deposits that filled the crater. Uplift is possible by isostatic rebound in response to the formation of the crater or from the volcanic material being deposited. The volcanic material will contract as it cools, and this cooling generates extensional stresses. The orientation of a graben is determined based on the stress at the time of its formation. Radial graben form when basin-circumferential stresses are the most extensional. When

radial stress Radial stress is stress towards or away from the central axis of a component. Pressure vessels The walls of pressure vessels generally undergo triaxial loading. For cylindrical pressure vessels, the normal loads on a wall element are: * the lon ...

es are the most extensional, circumferential graben form. If radial stress and circumferential stress are equal, then polygonal patterns form.

Wrinkle ridges

Wrinkle ridges are tectonic feature where sections of the crust are uplifted. Wrinkle ridges are associated with compressional stresses. On Mercury, the planet's interior is cooling and contracting. This contraction creates planet-wide compression. Wrinkle ridges on Mercury are thought to come from material loaded on top of the volcanic plains, in addition to the cooling. When plains have volcanic material deposited on top, the

lithosphere A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust (geology), crust and the portion of the upper mantle (geology), mantle that behaves elastically on time sca ...

can flex and create more compressional stresses.

Temporal relationships

Scientists used cross-cutting relationships in order to determine the sequence of events that formed ghost craters. Most likely, an impact struck Mercury. Then volcanic material flooded over the surface and filled the inside of the crater. Next, the graben or wrinkle ridges formed. When the graben are all included within the wrinkle ridges, then they are interpreted to have formed after the wrinkle ridges. If a graben cuts across a ridge and appears to be modified, then it is interpreted to be older, and to have formed before the ridges.

Importance

The presence of both graben and wrinkle ridges within ghost craters is a feature unique to Mercury. This combination is because of the tectonic processes that formed the graben and wrinkle ridges. The graben and wrinkle ridges came from rapidly accumulating lava flows that cooled within impact craters while the planet was experiencing global contraction from interior cooling. Other planetary bodies such as the Moon and Mars do not have both tectonic features within their ghost craters, possibly due to the accumulation rates of volcanic material on the Moon and Mars being slower than the rates on Mercury.

References

{{Mercury (planet) Impact craters on Mercury Geology of Mercury