Comet Shoemaker-Levy 9
Overview
 
Shoemaker–Levy redirects here. For other Shoemaker–Levy comets see List of periodic comets.

Comet Shoemaker–Levy 9 (formally designated D/1993 F2) was a comet
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...

 that broke apart and collided with Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

 in July 1994, providing the first direct observation of an extraterrestrial collision of solar system
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

 objects.
Encyclopedia
Shoemaker–Levy redirects here. For other Shoemaker–Levy comets see List of periodic comets.

Comet Shoemaker–Levy 9 (formally designated D/1993 F2) was a comet
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...

 that broke apart and collided with Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

 in July 1994, providing the first direct observation of an extraterrestrial collision of solar system
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

 objects. This generated a large amount of coverage in the popular media, and the comet was closely observed by astronomers
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...

 worldwide. The collision provided new information about Jupiter and highlighted its role in reducing space debris
Space debris
Space debris, also known as orbital debris, space junk, and space waste, is the collection of objects in orbit around Earth that were created by humans but no longer serve any useful purpose. These objects consist of everything from spent rocket stages and defunct satellites to erosion, explosion...

 in the inner solar system.

The comet was discovered by astronomers Carolyn
Carolyn S. Shoemaker
Carolyn Jean Spellmann Shoemaker is an American astronomer and is a co-discoverer of Comet Shoemaker–Levy 9. She holds the record for most comets discovered by an individual.- Personal life :...

 and Eugene M. Shoemaker
Eugene Merle Shoemaker
Eugene Merle Shoemaker , American geologist, was one of the founders of the fields of planetary science....

 and David Levy
David H. Levy
David H. Levy is a Canadian astronomer and science writer most famous for his co-discovery in 1993 of Comet Shoemaker–Levy 9, which collided with the planet Jupiter in 1994.-Biography:...

. Shoemaker–Levy 9, at the time captured by and orbiting Jupiter, was located on the night of March 24, 1993, in a photograph taken with the 40 centimetre Schmidt telescope
Schmidt camera
A Schmidt camera, also referred to as the Schmidt telescope, is a catadioptric astrophotographic telescope designed to provide wide fields of view with limited aberrations. Other similar designs are the Wright Camera and Lurie-Houghton telescope....

 at the Palomar Observatory
Palomar Observatory
Palomar Observatory is a privately owned observatory located in San Diego County, California, southeast of Pasadena's Mount Wilson Observatory, in the Palomar Mountain Range. At approximately elevation, it is owned and operated by the California Institute of Technology...

 in California
California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...

. It was the first comet observed to be orbiting a planet, and had probably been captured by the planet around 20 – 30 years earlier.

Calculations showed that its unusual fragmented form was due to a previous closer approach to Jupiter in July 1992. At that time, the orbit of Shoemaker–Levy 9 passed within Jupiter's Roche limit
Roche limit
The Roche limit , sometimes referred to as the Roche radius, is the distance within which a celestial body, held together only by its own gravity, will disintegrate due to a second celestial body's tidal forces exceeding the first body's gravitational self-attraction...

, and Jupiter's tidal force
Tidal force
The tidal force is a secondary effect of the force of gravity and is responsible for the tides. It arises because the gravitational force per unit mass exerted on one body by a second body is not constant across its diameter, the side nearest to the second being more attracted by it than the side...

s had acted to pull the comet apart. The comet was later observed as a series of fragments ranging up to 2 km (1.2 mi) in diameter. These fragments collided with Jupiter's southern hemisphere between July 16 and July 22, 1994, at a speed of approximately 60 km/s. The prominent scars from the impacts were more easily visible than the Great Red Spot and persisted for many months.

Discovery

While conducting a program of observations designed to uncover near-Earth object
Near-Earth object
A near-Earth object is a Solar System object whose orbit brings it into close proximity with the Earth. All NEOs have a perihelion distance less than 1.3 AU. They include a few thousand near-Earth asteroids , near-Earth comets, a number of solar-orbiting spacecraft, and meteoroids large enough to...

s, the Shoemakers and Levy discovered Comet Shoemaker–Levy 9 on the night of March 24, 1993 in a photograph taken with the 0.4 m (1.3 ft) Schmidt telescope
Schmidt camera
A Schmidt camera, also referred to as the Schmidt telescope, is a catadioptric astrophotographic telescope designed to provide wide fields of view with limited aberrations. Other similar designs are the Wright Camera and Lurie-Houghton telescope....

 at the Palomar Observatory
Palomar Observatory
Palomar Observatory is a privately owned observatory located in San Diego County, California, southeast of Pasadena's Mount Wilson Observatory, in the Palomar Mountain Range. At approximately elevation, it is owned and operated by the California Institute of Technology...

 in California
California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...

. The comet was thus a serendipitous
Serendipity
Serendipity means a "happy accident" or "pleasant surprise"; specifically, the accident of finding something good or useful without looking for it. The word has been voted as one of the ten English words hardest to translate in June 2004 by a British translation company. However, due to its...

 discovery, but one that quickly overshadowed the results from their main observing program.

Comet Shoemaker–Levy 9 was the ninth periodic comet (a comet whose orbital period is 200 years or less) discovered by the Shoemakers and Levy, hence its name. It was their eleventh comet discovery overall including their discovery of two non-periodic comets, which use a different nomenclature. The discovery was announced in IAU Circular
IAU Circular
The International Astronomical Union Circulars are notices that give information about astronomical phenomena. IAUCs are issued by the IAU's Central Bureau for Astronomical Telegrams at irregular intervals for the discovery and follow-up information regarding such objects as planetary...

 5725 on March 27, 1993.

The discovery image gave the first hint that comet Shoemaker–Levy 9 was an unusual comet, as it appeared to show multiple nuclei in an elongated region about 50 arcseconds long and 10 arcseconds wide. Brian Marsden of the Central Bureau for Astronomical Telegrams noted that the comet lay only about 4 degrees
Degree (angle)
A degree , usually denoted by ° , is a measurement of plane angle, representing 1⁄360 of a full rotation; one degree is equivalent to π/180 radians...

 from Jupiter as seen from Earth, and that while this could of course be a line of sight effect, its apparent motion in the sky suggested that it was physically close to the giant planet. Because of this, he suggested that the Shoemakers and David Levy had discovered the fragments of a comet that had been disrupted by Jupiter's gravity.

Jupiter-orbiting comet

Orbital studies of the new comet soon revealed that it was orbiting Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

 rather than the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

, unlike all other comets known at the time. Its orbit around Jupiter was very loosely bound, with a period of about 2 years and an apojove (furthest distance from Jupiter) of 0.33 AU. Its orbit around the planet was highly eccentric
Orbital eccentricity
The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...

 (e = 0.9986).

Tracing back the comet's orbital motion revealed that it had been orbiting Jupiter for some time. It seems most likely that it was captured from a solar orbit in the early 1970s, although the capture may have occurred as early as the mid-1960s. Several other observers found images of the comet in precovery
Precovery
Precovery is a term used in astronomy that describes the process of finding the image of an object in old archived images or photographic plates, for the purpose of calculating a more accurate orbit...

 images obtained before March 24, including Kin Endate
Kin Endate
is a Japanese amateur astronomer who has discovered 600 asteroids , placing him among the most prolific discoverers of minor planets. His discoveries include the Trojan asteroid 1990 VU1 and the Mars-crossing asteroid 6500 Kodaira...

 from a photograph exposed on March 15, S. Otomo
Satoru Otomo
is a Japanese astronomer. According to the Minor Planet Center, he is responsible for the discovery of 149 asteroids between 1991 and 1997, 15 of which were co-discoveries with Osamu Muramatsu. As of 1997, he was tied for 48th place with Tamara Mikhailovna Smirnova in the MPC's asteroid discovery...

 on March 17, and a team led by Eleanor Helin from images on March 19. No precovery images dating back to earlier than March 1993 have been found. Before the comet was captured by Jupiter, it was probably a short-period comet with an aphelion just inside Jupiter's orbit, and a perihelion interior to the asteroid belt
Asteroid belt
The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets...

.

The volume of space within which an object can be said to orbit Jupiter is defined by Jupiter's Hill sphere
Hill sphere
An astronomical body's Hill sphere is the region in which it dominates the attraction of satellites. To be retained by a planet, a moon must have an orbit that lies within the planet's Hill sphere. That moon would, in turn, have a Hill sphere of its own...

 (also called the Roche sphere). When the comet passed Jupiter in the late 1960s or early 1970s, it happened to be near its aphelion, and found itself slightly within Jupiter's Hill sphere. Jupiter's gravity nudged the comet towards it. Because the comet's motion with respect to Jupiter was very small, it fell almost straight toward Jupiter, which is why it ended up on a Jupiter-centric orbit of very high eccentricity that is to say, the ellipse was nearly flattened out.

The comet had apparently passed extremely close to Jupiter on July 7, 1992, just over 40000 km (24,854.9 mi) above the planet's cloud tops a smaller distance than Jupiter's radius of 70000 km (43,496.1 mi), and well within the orbit of Jupiter's innermost moon Metis
Metis (moon)
Metis , also known as ', is the innermost moon of Jupiter. It was discovered in 1979 in images taken by Voyager 1, and was named in 1983 after the first wife of Zeus, Metis...

 and the planet's Roche limit
Roche limit
The Roche limit , sometimes referred to as the Roche radius, is the distance within which a celestial body, held together only by its own gravity, will disintegrate due to a second celestial body's tidal forces exceeding the first body's gravitational self-attraction...

, inside which tidal force
Tidal force
The tidal force is a secondary effect of the force of gravity and is responsible for the tides. It arises because the gravitational force per unit mass exerted on one body by a second body is not constant across its diameter, the side nearest to the second being more attracted by it than the side...

s are strong enough to disrupt a body held together only by gravity. Although the comet had approached Jupiter closely before, the July 7 encounter seemed to be by far the closest, and the fragmentation of the comet is thought to have occurred at this time. Each fragment of the comet was denoted by a letter of the alphabet
Alphabet
An alphabet is a standard set of letters—basic written symbols or graphemes—each of which represents a phoneme in a spoken language, either as it exists now or as it was in the past. There are other systems, such as logographies, in which each character represents a word, morpheme, or semantic...

, from "fragment A" through to "fragment W", a practice already established from previously observed broken-up comets.

More exciting for planetary astronomers was that the best orbital solutions suggested that the comet would pass within 45000 km (27,961.8 mi) of the center of Jupiter, a distance smaller than the planet's radius, meaning that there was an extremely high probability that SL9 would collide with Jupiter in July 1994. Studies suggested that the train of nuclei would plow into Jupiter's atmosphere over a period of about five days.

Predictions for the collision

The discovery that the comet was likely to collide with Jupiter caused great excitement within the astronomical community and beyond, as astronomers had never before seen two significant solar system bodies collide. Intense studies of the comet were undertaken, and as its orbit became more accurately established, the possibility of a collision became a certainty. The collision would provide a unique opportunity for scientists to look inside Jupiter's atmosphere, as the collisions were expected to cause eruptions of material from the layers normally hidden beneath the clouds.

Astronomers estimated that the visible fragments of SL9 ranged in size from a few hundred metres to at most a couple of kilometres across, suggesting that the original comet may have had a nucleus up to 5 km (3.1 mi) across somewhat larger than Comet Hyakutake
Comet Hyakutake
Comet Hyakutake is a comet, discovered on January 31, 1996, which passed very close to Earth in March of that year. It was dubbed The Great Comet of 1996; its passage near the Earth was one of the closest cometary approaches of the previous 200 years. Hyakutake appeared very bright in the night...

, which became very bright when it passed close to the Earth in 1996. One of the great debates in advance of the impact was whether the effects of the impact of such small bodies would be noticeable from Earth, apart from a flash as they disintegrated like giant meteor
METEOR
METEOR is a metric for the evaluation of machine translation output. The metric is based on the harmonic mean of unigram precision and recall, with recall weighted higher than precision...

s.
Other suggested effects of the impacts were seismic waves travelling across the planet, an increase in stratospheric
Stratosphere
The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler...

 haze on the planet due to dust from the impacts, and an increase in the mass of the Jovian ring system
Rings of Jupiter
The planet Jupiter has a system of rings, known as the rings of Jupiter or the Jovian ring system. It was the third ring system to be discovered in the Solar System, after those of Saturn and Uranus. It was first observed in 1979 by the Voyager 1 space probe and thoroughly investigated in the 1990s...

. However, given that observing such a collision was completely unprecedented, astronomers were cautious with their predictions of what the event might reveal.

Impacts

Anticipation grew as the predicted date for the collisions approached, and astronomers trained terrestrial telescopes on Jupiter. Several space observatories did the same, including the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...

, the ROSAT
ROSAT
ROSAT was a German Aerospace Center-led satellite X-ray telescope, with instruments built by Germany, the UK and the US...

 X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...

 observing satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....

, and significantly the Galileo spacecraft
Galileo spacecraft
Galileo was an unmanned spacecraft sent by NASA to study the planet Jupiter and its moons. Named after the astronomer and Renaissance pioneer Galileo Galilei, it was launched on October 18, 1989 by the Space Shuttle Atlantis on the STS-34 mission...

, then on its way to a rendezvous with Jupiter scheduled for 1995. While the impacts took place on the side of Jupiter hidden from Earth, Galileo, then at a distance of 1.6 AU from the planet, was able to see the impacts as they occurred. Jupiter's rapid rotation brought the impact sites into view for terrestrial observers a few minutes after the collisions.

Two other satellites made observations at the time of the impact: the Ulysses spacecraft, primarily designed for solar
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

 observations, was pointed towards Jupiter from its location 2.6 AU away, and the distant Voyager 2
Voyager 2
The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space...

 probe, some 44 AU from Jupiter and on its way out of the solar system following its encounter with Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...

 in 1989, was programmed to look for radio emission in the 1–390 kHz range.

The first impact occurred at 20:13 UTC
Coordinated Universal Time
Coordinated Universal Time is the primary time standard by which the world regulates clocks and time. It is one of several closely related successors to Greenwich Mean Time. Computer servers, online services and other entities that rely on having a universally accepted time use UTC for that purpose...

 on July 16, 1994, when fragment A of the nucleus slammed into Jupiter's southern hemisphere at a speed of about 60 km/s. Instruments on Galileo detected a fireball which reached a peak temperature of about 24,000 K
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...

, compared to the typical Jovian cloudtop temperature of about 130 K, before expanding and cooling rapidly to about 1500 K after 40 s. The plume from the fireball quickly reached a height of over 3,000 km. A few minutes after the impact fireball was detected, Galileo measured renewed heating, probably due to ejected material falling back onto the planet. Earth-based observers detected the fireball rising over the limb of the planet shortly after the initial impact.

Astronomers had expected to see the fireballs from the impacts, but did not have any idea in advance how visible the atmospheric effects of the impacts would be from Earth. Observers soon saw a huge dark spot after the first impact. The spot was visible even in very small telescopes, and was about 6000 km (3,728.2 mi) (one Earth radius) across. This and subsequent dark spots were thought to have been caused by debris from the impacts, and were markedly asymmetric, forming crescent shapes in front of the direction of impact.

Over the next 6 days, 21 distinct impacts were observed, with the largest coming on July 18 at 07:33 UTC when fragment G struck Jupiter. This impact created a giant dark spot over 12,000 km across, and was estimated to have released an energy equivalent to 6,000,000 megatons of TNT
TNT equivalent
TNT equivalent is a method of quantifying the energy released in explosions. The ton of TNT is a unit of energy equal to 4.184 gigajoules, which is approximately the amount of energy released in the detonation of one ton of TNT...

 (600 times the world's nuclear arsenal). Two impacts 12 hours apart on July 19 created impact marks of similar size to that caused by fragment G, and impacts continued until July 22, when fragment W struck the planet.

Chemical studies

Observers hoped that the impacts would give them a first glimpse of Jupiter beneath the cloud tops, as lower material was exposed by the comet fragments punching through the upper atmosphere. Spectroscopic
Astronomical spectroscopy
Astronomical spectroscopy is the technique of spectroscopy used in astronomy. The object of study is the spectrum of electromagnetic radiation, including visible light, which radiates from stars and other celestial objects...

 studies revealed absorption lines in the Jovian spectrum due to diatomic sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...

 (S2) and carbon disulfide
Carbon disulfide
Carbon disulfide is a colorless volatile liquid with the formula CS2. The compound is used frequently as a building block in organic chemistry as well as an industrial and chemical non-polar solvent...

 (CS2), the first detection of either in Jupiter, and only the second detection of S2 in any astronomical object
Astronomical object
Astronomical objects or celestial objects are naturally occurring physical entities, associations or structures that current science has demonstrated to exist in the observable universe. The term astronomical object is sometimes used interchangeably with astronomical body...

. Other molecules detected included ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...

 (NH3) and hydrogen sulfide
Hydrogen sulfide
Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...

 (H2S). The amount of sulfur implied by the quantities of these compounds was much greater than the amount that would be expected in a small cometary nucleus, showing that material from within Jupiter was being revealed. Oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

-bearing molecules such as sulfur dioxide
Sulfur dioxide
Sulfur dioxide is the chemical compound with the formula . It is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel...

 were not detected, to the surprise of astronomers.

As well as these molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...

s, emission from heavy atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...

s such as iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

, magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...

 and silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...

 was detected, with abundances consistent with what would be found in a cometary nucleus. While substantial water was detected spectroscopically, it was not as much as predicted beforehand, meaning that either the water layer thought to exist below the clouds was thinner than predicted, or that the cometary fragments did not penetrate deeply enough. The relatively low levels of water were later confirmed by Galileo's atmospheric probe, which explored Jupiter's atmosphere directly.

Seismic waves

As predicted beforehand, the collisions generated enormous seismic wave
Seismic wave
Seismic waves are waves of energy that travel through the earth, and are a result of an earthquake, explosion, or a volcano that imparts low-frequency acoustic energy. Many other natural and anthropogenic sources create low amplitude waves commonly referred to as ambient vibrations. Seismic waves...

s which swept across the planet at speeds of 450 km/s and were observed for over two hours after the largest impacts. The waves were thought to be travelling within a stable layer acting as a waveguide
Waveguide
A waveguide is a structure which guides waves, such as electromagnetic waves or sound waves. There are different types of waveguides for each type of wave...

, and some scientists believed the stable layer must lie within the hypothesised tropospheric
Troposphere
The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of the atmosphere's mass and 99% of its water vapor and aerosols....

 water cloud. However, other evidence seemed to indicate that the cometary fragments had not reached the water layer, and the waves were instead propagating within the stratosphere
Stratosphere
The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler...

.

Other observations

Radio observations revealed a sharp increase in continuum
Black body
A black body is an idealized physical body that absorbs all incident electromagnetic radiation. Because of this perfect absorptivity at all wavelengths, a black body is also the best possible emitter of thermal radiation, which it radiates incandescently in a characteristic, continuous spectrum...

 emission at a wavelength of 21 cm after the largest impacts, which peaked at 120% of the normal emission from the planet. This was thought to be due to synchrotron radiation
Synchrotron radiation
The electromagnetic radiation emitted when charged particles are accelerated radially is called synchrotron radiation. It is produced in synchrotrons using bending magnets, undulators and/or wigglers...

, caused by the injection of relativistic
Theory of relativity
The theory of relativity, or simply relativity, encompasses two theories of Albert Einstein: special relativity and general relativity. However, the word relativity is sometimes used in reference to Galilean invariance....

 electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...

s electrons with velocities near the speed of light into the Jovian magnetosphere
Magnetosphere
A magnetosphere is formed when a stream of charged particles, such as the solar wind, interacts with and is deflected by the intrinsic magnetic field of a planet or similar body. Earth is surrounded by a magnetosphere, as are the other planets with intrinsic magnetic fields: Mercury, Jupiter,...

 by the impacts.

About an hour after fragment K entered Jupiter, observers recorded auroral emission near the impact region, as well as at the antipode
Antipodal point
In mathematics, the antipodal point of a point on the surface of a sphere is the point which is diametrically opposite to it — so situated that a line drawn from the one to the other passes through the centre of the sphere and forms a true diameter....

 of the impact site with respect to Jupiter's strong magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

. The cause of these emissions was difficult to establish due to a lack of knowledge of Jupiter's internal magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

 and of the geometry of the impact sites. One possible explanation was that upwardly accelerating shock wave
Shock wave
A shock wave is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium or in some cases in the absence of a material medium, through a field such as the electromagnetic field...

s from the impact accelerated charged particles enough to cause auroral emission, a phenomenon more typically associated with fast-moving solar wind
Solar wind
The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...

 particles striking a planetary atmosphere near a magnetic pole.

Some astronomers had suggested that the impacts might have a noticeable effect on the Io torus, a torus
Torus
In geometry, a torus is a surface of revolution generated by revolving a circle in three dimensional space about an axis coplanar with the circle...

 of high-energy particles connecting Jupiter with the highly volcanic
Volcano
2. Bedrock3. Conduit 4. Base5. Sill6. Dike7. Layers of ash emitted by the volcano8. Flank| 9. Layers of lava emitted by the volcano10. Throat11. Parasitic cone12. Lava flow13. Vent14. Crater15...

 moon Io
Io (moon)
Io ) is the innermost of the four Galilean moons of the planet Jupiter and, with a diameter of , the fourth-largest moon in the Solar System. It was named after the mythological character of Io, a priestess of Hera who became one of the lovers of Zeus....

. High resolution spectroscopic studies found that variations in the ion density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...

, rotational velocity, and temperatures at the time of impact and afterwards were within the normal limits.

Post-impact analysis

One of the surprises of the impacts was the small amount of water revealed compared to prior predictions. Before the impact, models of Jupiter's atmosphere had indicated that the break-up of the largest fragments would occur at atmospheric pressures of anywhere from 30 kilopascals to a few tens of megapascals (from 0.3 to a few hundred bar
Bar (unit)
The bar is a unit of pressure equal to 100 kilopascals, and roughly equal to the atmospheric pressure on Earth at sea level. Other units derived from the bar are the megabar , kilobar , decibar , centibar , and millibar...

), with some predictions that the comet would penetrate a layer of water and create a bluish shroud over that region of Jupiter.

Astronomers did not observe large amounts of water following the collisions, and later impact studies found that fragmentation and destruction of the cometary fragments in an 'airburst' probably occurred at much higher altitudes than previously expected, with even the largest fragments being destroyed when the pressure reached 250 kPa (36 psi), well above the expected depth of the water layer. The smaller fragments were probably destroyed before they even reached the cloud layer.

Longer-term effects

The visible scars from the impacts could be seen on Jupiter for many months. They were extremely prominent, and observers described them as more easily visible even than the Great Red Spot. A search of historical observations revealed that the spots were probably the most prominent transient features ever seen on the planet, and that while the Great Red Spot is notable for its striking color, no spots of the size and darkness of those caused by the SL9 impacts have ever been recorded before.

Spectroscopic observers found that ammonia and carbon sulfide persisted in the atmosphere for at least fourteen months after the collisions, with a considerable amount of ammonia being present in the stratosphere as opposed to its normal location in the troposphere.

Counterintuitively, the atmospheric temperature dropped to normal levels much more quickly at the larger impact sites than at the smaller sites: at the larger impact sites, temperatures were elevated over a region 15000 to 20000 km (9,320.6 to 12,427.5 mi) wide, but dropped back to normal levels within a week of the impact. At smaller sites, temperatures 10 K
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...

 higher than the surroundings persisted for almost two weeks. Global stratospheric temperatures rose immediately after the impacts, then fell to below pre-impact temperatures 2–3 weeks afterwards, before rising slowly to normal temperatures.

Frequency of impacts

SL9 is not unique in having orbited Jupiter for a time; five comets, (including 82P/Gehrels
82P/Gehrels
82P/Gehrels is a comet that was discovered on October 27, 1975 by Tom Gehrels at the Palomar Mountain Observatory in California. It has an estimated diameter of 1.46 km. This object has been identified as a quasi-Hilda comet, which means it is in a 3:2 mean-motion resonance with the planet Jupiter....

, 147P/Kushida–Muramatsu, and 111P/Helin–Roman–Crockett) are known to have been temporarily captured by the planet.
Cometary orbits around Jupiter are unstable, as they will be highly elliptical
Ellipse
In geometry, an ellipse is a plane curve that results from the intersection of a cone by a plane in a way that produces a closed curve. Circles are special cases of ellipses, obtained when the cutting plane is orthogonal to the cone's axis...

 and likely to be strongly perturbed
Perturbation (astronomy)
Perturbation is a term used in astronomy in connection with descriptions of the complex motion of a massive body which is subject to appreciable gravitational effects from more than one other massive body....

 by the Sun's gravity at apojove (the furthest point on the orbit from the planet).

By far the most massive planet in the solar system
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...

, Jupiter can capture objects relatively frequently, but the size of SL9 makes it a rarity: one post-impact study estimated that comets 0.3 km in diameter impact the planet once in approximately 500 years and those 1.6 km (0.994196378639691 mi) in diameter do so just once in every 6000 years.

There is very strong evidence that comets have previously been fragmented and collided with Jupiter and its satellites. During the Voyager missions to the planet, planetary scientists identified 13 crater chain
Crater chain
A crater chain is a line of craters along the surface of an astronomical body. The descriptor term for crater chains is catena , as specified by the International Astronomical Union's rules on planetary nomenclature....

s on Callisto
Callisto (moon)
Callisto named after the Greek mythological figure of Callisto) is a moon of the planet Jupiter. It was discovered in 1610 by Galileo Galilei. It is the third-largest moon in the Solar System and the second largest in the Jovian system, after Ganymede. Callisto has about 99% the diameter of the...

 and three on Ganymede
Ganymede (moon)
Ganymede is a satellite of Jupiter and the largest moon in the Solar System. It is the seventh moon and third Galilean satellite outward from Jupiter. Completing an orbit in roughly seven days, Ganymede participates in a 1:2:4 orbital resonance with the moons Europa and Io, respectively...

, the origin of which was initially a mystery. Crater chains seen on the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

 often radiate from large craters, and are thought to be caused by secondary impacts of the original ejecta, but the chains on the Jovian moons did not lead back to a larger crater. The impact of SL9 strongly implied that the chains were due to trains of disrupted cometary fragments crashing into the satellites.

Impact of July 19, 2009

On July 19, 2009, a new black spot about the size of the Pacific Ocean appeared in Jupiter's southern hemisphere. Thermal infrared measurements showed the impact site was warm and spectroscopic analysis detected the production of excess hot ammonia and silica-rich dust in the upper regions of Jupiter's atmosphere. Scientists have concluded that another impact event had occurred, but this time a more compact and strong object, probably a small undiscovered asteroid, was the cause.

Jupiter as a "cosmic vacuum cleaner"

The impact of SL9 highlighted Jupiter's role as a kind of "cosmic vacuum cleaner" (or in deference to the ancients' planetary correspondences to the major organs in the human body, a kind of "cosmic liver") for the inner solar system. The planet's strong gravitational influence leads to many small comets and asteroid
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...

s colliding with the planet, and the rate of cometary impacts on Jupiter is thought to be between two thousand and eight thousand times higher than the rate on Earth. If Jupiter were not present, the probability of asteroid impacts with the Solar System's inner planets would be greater.

The extinction of the dinosaurs at the end of the Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

 period is generally believed to have been caused by the impact event which created the Chicxulub crater
Chicxulub Crater
The Chicxulub crater is an ancient impact crater buried underneath the Yucatán Peninsula in Mexico. Its center is located near the town of Chicxulub, after which the crater is named...

, demonstrating that impacts are a serious threat to life on Earth. Astronomers have speculated that without Jupiter to mop up potential impactors, extinction events might have been more frequent on Earth, and complex life might not have been able to develop. This is part of the argument used in the Rare Earth hypothesis
Rare Earth hypothesis
In planetary astronomy and astrobiology, the Rare Earth hypothesis argues that the emergence of complex multicellular life on Earth required an improbable combination of astrophysical and geological events and circumstances...

.

Recently, however, it has been shown that the presence of a smaller planet at Jupiter's position in the solar system may in fact increase the impact rate of comets on the Earth significantly. A planet of Jupiter's mass still seems to provide increased protection against asteroids, but the total effect on all orbital bodies within the solar system is unclear.

See also

  • Atmosphere of Jupiter
    Atmosphere of Jupiter
    The atmosphere of Jupiter is the largest planetary atmosphere in the Solar System. It is mostly made of molecular hydrogen and helium in roughly solar proportions; other chemical compounds are present only in small amounts and include methane, ammonia, hydrogen sulfide and water. Although water is...

  • Juno
    Juno (spacecraft)
    Juno is a NASA New Frontiers mission to the planet Jupiter. Juno was launched from Cape Canaveral Air Force Station on August 5, 2011. The spacecraft is to be placed in a polar orbit to study the planet's composition, gravity field, magnetic field, and polar magnetosphere...

     probe to Jupiter
  • 73P/Schwassmann–Wachmann, a comet that broke into a chain upon encountering Earth's gravity

External links

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