A ray system comprises radial streaks of fine ''

ejecta Ejecta (from the Latin: "things thrown out", singular ejectum) are particles ejected from an area. In volcanology, in particular, the term refers to particles including pyroclastic materials (tephra) that came out of a volcanic explosion and magma ...

'' thrown out during the formation of an impact crater, looking somewhat like many thin spokes coming from the hub of a wheel. The rays may extend for lengths up to several times the

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 ...

of their originating crater, and are often accompanied by small secondary craters formed by larger chunks of ejecta. Ray systems have been identified on 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 ...

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's sur ...

(

Kamil Crater The Kamil Crater is a wide and deep (original depth, a part covered by sand at present) meteorite impact crater in the East Uweinat Desert in southwestern New Valley Governorate, Egypt, Only north of the border with the Sudan and above ...

), Mercury, and some moons of the outer planets. Originally it was thought that they existed only on

planet A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a you ...

s or moons lacking an

atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A ...

, but more recently they have been identified on

Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin at ...

in infrared images taken from orbit by '' 2001 Mars Odyssey''s thermal imager. Rays appear at visible, and in some cases infrared wavelengths, when ejecta are made of material with different reflectivity (i.e.,

albedo Albedo (; ) is the measure of the diffuse reflection of solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body that refle ...

) or thermal properties from the surface on which they are deposited. Typically, visible rays have a higher albedo than the surrounding surface. More rarely an impact will excavate low albedo material, for example

basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...

ic-

lava Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or ...

deposits on the lunar maria. Thermal rays, as seen on Mars, are especially apparent at night when slopes and shadows do not influence the infrared energy emitted by the Martian surface. The layering of rays across other surface features can be useful as an indicator of the relative age of the impact crater, because over time various processes obliterate the rays. On non-atmosphered bodies such as the Moon, space weathering from exposure to

cosmic ray Cosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our own ...

s and micrometeorites causes a steady reduction of the differential between the ejecta's albedo and that of the underlying material. Micrometeorites in particular produce a glassy melt in the regolith that lowers the

. Rays can also become covered by

flows (such as those of Lichtenberg on the moon), or by other impact craters or ejecta. Kuiper crater EW1065899876G

Lunar rays

The physical nature of lunar rays has historically been a subject of speculation. Early hypotheses suggested that they were deposits of salt from evaporated water. Later they were thought to be deposits of volcanic ash or streaks of dust. After the impact origin of craters became accepted, Eugene Shoemaker suggested during the 1960s that the rays were the result of fragmented ejecta material. Recent studies suggest that the relative brightness of a lunar ray system is not always a reliable indicator of the age of a ray system. Instead the albedo also depends on the portion of

iron oxide Iron oxides are chemical compounds composed of iron and oxygen. Several iron oxides are recognized. All are black magnetic solids. Often they are non-stoichiometric. Oxyhydroxides are a related class of compounds, perhaps the best known of wh ...

(FeO). Low portions of FeO result in brighter materials, so such a ray system can retain its lighter appearance for longer periods. Thus the material composition needs to be factored into the albedo analysis to determine age. Among the lunar craters on the near side with pronounced ray systems are Aristarchus,

Copernicus Nicolaus Copernicus (; pl, Mikołaj Kopernik; gml, Niklas Koppernigk, german: Nikolaus Kopernikus; 19 February 1473 – 24 May 1543) was a Renaissance polymath, active as a mathematician, astronomer, and Catholic canon, who formulat ...

Kepler Johannes Kepler (; ; 27 December 1571 – 15 November 1630) was a German astronomer, mathematician, astrologer, natural philosopher and writer on music. He is a key figure in the 17th-century Scientific Revolution, best known for his laws o ...

Proclus Proclus Lycius (; 8 February 412 – 17 April 485), called Proclus the Successor ( grc-gre, Πρόκλος ὁ Διάδοχος, ''Próklos ho Diádokhos''), was a Greek Neoplatonist philosopher, one of the last major classical philosophe ...

Dionysius The name Dionysius (; el, Διονύσιος ''Dionysios'', "of Dionysus"; la, Dionysius) was common in classical and post-classical times. Etymologically it is a nominalized adjective formed with a -ios suffix from the stem Dionys- of the name ...

, Glushko, and Tycho. Smaller examples include

Censorinus Censorinus was a Roman grammarian and miscellaneous writer from the 3rd century AD. Biography He was the author of a lost work ''De Accentibus'' and of an extant treatise ''De Die Natali'', written in 238, and dedicated to his patron Quintus C ...

, Stella, and Linné. Similar ray systems also occur on the

far side ''The Far Side'' is a single-panel comic created by Gary Larson and syndicated by Chronicle Features and then Universal Press Syndicate, which ran from December 31, 1979, to January 1, 1995 (when Larson retired as a cartoonist). Its surrealist ...

of the Moon, such as the rays radiating from the craters

Giordano Bruno Giordano Bruno (; ; la, Iordanus Brunus Nolanus; born Filippo Bruno, January or February 1548 – 17 February 1600) was an Italian philosopher, mathematician, poet, cosmological theorist, and Hermetic occultist. He is known for his cosmolog ...

, Necho,

Ohm Ohm (symbol Ω) is a unit of electrical resistance named after Georg Ohm. Ohm or OHM may also refer to: People * Georg Ohm (1789–1854), German physicist and namesake of the term ''ohm'' * Germán Ohm (born 1936), Mexican boxer * Jörg Ohm (bor ...

, Jackson,

King King is the title given to a male monarch in a variety of contexts. The female equivalent is queen regnant, queen, which title is also given to the queen consort, consort of a king. *In the context of prehistory, antiquity and contempora ...

, and the small but prominent Pierazzo. Most lateral transport of primary ejecta from impact craters is limited to a distance of a few crater radii, but some larger impacts, such as the impacts that made the

and Tycho craters, launched primary ejecta halfway around the moon. North Ray and South Ray craters, each with a clear ray system, were observed from the ground by the astronauts of

Apollo 16 Apollo 16 (April 1627, 1972) was the tenth human spaceflight, crewed mission in the United States Apollo program, Apollo space program, administered by NASA, and the fifth and penultimate to Moon landing, land on the Moon. It was the second o ...

in 1972. Image:AS15-94-12836.jpg, Asymmetrical ray system about the lunar crater

(

Apollo 15 Apollo 15 (July 26August 7, 1971) was the ninth crewed mission in the United States' Apollo program and the fourth to land on the Moon. It was the first J mission, with a longer stay on the Moon and a greater focus on science than ear ...

image) Image:Pierazzo crater Clementine mosaic.jpg, Pierazzo crater (Mosaic of Clementine images) File:Giordano Bruno crater rays AS11-44-6665HR.jpg, The rays of

extend for hundreds of kilometers from the small crater (

Apollo 11 Apollo 11 (July 16–24, 1969) was the American spaceflight that first landed humans on the Moon. Commander Neil Armstrong and lunar module pilot Buzz Aldrin landed the Apollo Lunar Module ''Eagle'' on July 20, 1969, at 20:17 UTC, ...

image) File:South Ray crater AS16-P-4618 ASU.jpg, South Ray (

image)

References

Citations

Sources

* Martel, L.M.V. (Sept., 2004)
Lunar Crater Rays Point to a New Lunar Time Scale
', Planetary Science Research Discoveries. http://www.psrd.hawaii.edu/Sept04/LunarRays.html. (Accessed 9/15/2005) * Burnham, R. (March, 2005)
Chipping pieces off Mars: Martian craters with rays may be the long-sought sources for the Mars meteorites found on Earth
', Astronomy Magazine Online. http://www.astronomy.com/asy/default.aspx?c=a&id=2980 (Accessed 8/07/2006) {{The Moon, state=collapsed Impact craters

Lunar rays

See also

References

Citations

Sources