Earth's Moon
Two particularly interesting missions conducted in the 1990’s by NASA were Clementine and The Lunar Prospector. These two missions are controversial and interesting because they both reveal separate results on the topic of lunar ice. Lunar ice is very important because it would show that water exists on the Moon and would prove the Moon could sustain life.
The Strategic Defense Initiative Organization and NASA conducted the Clementine mission in 1994. The mission objective was to test spacecraft components under extended exposure to normal conditions experience in outer space and to make scientific observations of the Moon. Clementine was launched on January 25, 1994 from Vandenberg Air Force Base aboard a Titan IIG rocket. After two Earth flybys, lunar mapping took place over approximately two months, in two parts. The first part consisted of a five-hour elliptical polar orbit. The scans taken by Clementine included ultraviolet and infrared maps. These observations were originally for the purposes of assessing the surface mineralogy of the Moon. However, in these maps NASA found evidence that pointed to the possible existence of ice on the Moon. The supposed ice was believed to be in the bottom of a shadowed crater near the Moon's south pole. It was also thought that other frozen chemicals, such as methane, were in the same shadowed area. The deposit was estimated to be approximately 60,000 to 120,000 cubic meters in volume. The deposit was believed to be four football fields in area and almost 16 feet deep (NASA).
The $63 million Lunar Prospector mission was led by Dr. Alan Binder of the Lunar Research Institute in Tucson, AZ, and managed by NASA's Ames Research Center in Moffett Field, CA. During its 18-month mission orbiting the Moon, Lunar Prospector used a method called neutron spectroscopy to look for lunar water. Neutrons are subatomic particles that are continually thrown from the lunar soil by cosmic rays (NASA 2). On March 5, 1998, it was announced that data returned by the Lunar Prospector spacecraft indicated that water was present at both the north and south lunar poles, in agreement with Clementine’s results reported in November 1996 (NASA 3). Scientists hoped that by crashing the spacecraft into a shadowed crater they would be able to find direct evidence of water. This controlled crash of the Lunar Prospector spacecraft into a crater near the south pole of the Moon on July 31 produced no sign of water on the surface of the Moon. Observations by the Lunar Prospector were focused primarily on the use of sensitive spectrometers tuned to record ultraviolet emission lines from the hydroxyl (OH) molecules that should indicate icy rock. There are several possibilities to explain why the spacecraft did not find any water including: the spacecraft might have missed the target area, water molecules may have been firmly bound in rocks as hydrated mineral as opposed to existing as free ice crystals, and the crash lacked enough energy to separate water from hydrated minerals (NASA 2).
The important thing out of these missions is that Clementine was not as effective as Lunar Prospector because Clementine didn’t land on the Moon.
Europa (moon of Jupiter)
The Voyager Explorers were launched separately in the summer of 1997. The Voyagers measured the atmospheres, magnetospheres, satellites and ring systems of Jupiter and Saturn. In addition, the Twin Voyagers were equipped with 10 instruments designed for experimental purposes in space. The first photos of Europa, sent back by Voyager 1, were incredibly vague. They led scientist to believe there was tectonic shifting. Pictures from Voyager 2 cleared up this misconception. Scientists concluded from the second set of pictures the hypothesis of an existing surface of ice (50 km thick) on Europa and an underground ocean.
The Galileo mission was launched in 1989 and reached Jupiter in 1995. Galileo has given scientists an extremely more detailed look at Europa then the pictures of the Twin Voyager mission. Scientists have realized, from the Galileo pictures, that many fractures in the moon’s surface have been smoothed over by new ice. This very fact is highly suggestive of an underground ocean and of past “ice volcanoes” that flowed liquid water. Galileo has discovered that Europa has a metallic core and an ionosphere, which is an electrical charge of gasses. Galileo has also lead scientist to believe that the surface of Europa is only 4 km thick.
“The Europa Orbiter” is scheduled to be launched in 2008. This new explorer has the sole purpose of going to Europa and uncovering the many wonders that lie on that moon.
Europa
It has been well established that beneath the hostile surface of Europa, the smallest of the four Galilean moons of Jupiter, there may exist a salt-water ocean, one that could provide the resources to support extraterrestrial life. Since 1979, when the Voyager 2 spacecraft collected the first reliable, high-resolution images of Europa’s mysterious surface, there has been global interest taken in Europa’s capacity to support life as we know it. Its smooth, brown-streaked terrain is in stark contrast from neighboring bodies, which are dotted with craters as a result of meteor collisions. Observations made throughout the 70s (which continue even today), from both Earth-based telescopes and space missions like the Voyager, support speculation that Europa is covered by an ice crust, constantly breaking and reforming. In addition, proposals state that beneath the frozen mantle is an ocean of water. Cracks in the surface of the moon are attributed to the stress exerted on the ice from tidal forces; as water boiled up through cracks, it would refreeze, expand, and create new locales subject to rupture. Findings and interpretations published in the early 1980s include the idea that liquid water is produced and maintained by a mingling of two sources of heat: tidal dissipation, which produces energy in the form of heat, is balanced in areas of thin ice by both thermal conduction exuded by the core and also by heat generated from radioactive elements.
In 1996, the Galileo Europa Mission, known as GEM, initiated a new trend of exploration as it set out on a 14-month study with the purpose of characterizing the crust, atmosphere, and ocean using imaging, gravity, and space physics data. Basically, the mission was successful in reaffirming tentative predictions about Europa’s tantalizing surface, and in laying the foundation for future missions. Several essential findings of Galileo’s primary mission are as follows: the presence of a metallic core; volcanic ice flow, and melting or "rafting" of ice, which supports the premise of subsurface liquid oceans; and finally, the existence of a magnetic field.
Based on findings from the aforementioned missions, as well as various others, reliable suppositions can be made regarding the presence of liquid water beneath the surface of Europa’s ice crust. However, no mission completed thus far has given any direct evidence as to the presence of extraterrestrial life or even of the exact molecular make-up of the surface; it’s essential that we develop and send out new probes/missions in order continue research and exploration.
Io (moon of Jupiter)
Since its discovery in 1610 by Galileo Galilei, Io has been observed and studied by scientists. The most recent and enlightening observations have been made by the Voyager and Galileo missions. The Voyager spacecrafts recorded in 1979 that Io is covered with volcanic calderas. This is the reason why Io is not covered by craters like most bodies in our Solar System. Due to the numerous lava flows caused by the volcanoes, the surface is ever-changing. Voyager made records concerning temperature, tides caused by the gravitational force of Jupiter and height of volcanic eruptions. The most recent observations of Io have been made by the Galileo spacecraft. It has been orbiting Jupiter and its moons since December 1995. During this time, Galileo has made new discoveries about the moon and has witnessed changes not only between its own flybys, but changes on Io since Voyager. Galileo detected more than 100 active volcanoes on Io while Voyager was only able to find nine. Record temperatures and eruptions were recorded by Galileo. Changes inside the Tvashtar Catena region near the north pole of Io have given evidence to erosion similar to that here on Earth but caused by liquid sulfur dioxide. Using measurements made by Galileo, scientists have been able to determine the size and composition of Io’s core. The changes in Io’s surface from Voyager’s mission to Galileo’s mission have given scientists great insight into the processes of the most volcanically active body in the Solar System.
Jupiter has been visited by six different spacecrafts that have been sent from Earth. Three of these spacecrafts were sent in two separate missions that were successful in gaining much information concerning Jupiter and its moons. Two were launched in 1977 (Voyager 1 and 2) and another one was in 1989 (Galileo). Both of these missions had focal points that differed, which helped to increase our knowledge about the 5th planet from the sun.
The Voyager mission made it to Jupiter in 2 years, which is rather quick compared to the 6 years that it took Galileo to get there. Once there, Voyager made several discoveries that had never been observed about Jupiter before. For one thing, the Voyagers were able to study the Great Red Spot, and classify it as a massive, complex storm. Also there were many more storms and lightning that was witnessed by the Voyagers going on in between the bands of clouds. This was the first time that the intense amount of lightening was seen on Jupiter. Voyager also made some interesting discoveries pertaining to Jupiter's moons. They located 9 eruptions on the moon Io. Until then, volcanoes had never been observes anywhere but Earth. The Moons Callisto, Ganymede, and Europa were also examined by the Voyager spacecrafts. It was also during this mission that a faint, dusty ring was found around Jupiter.
As for Galileo, it was originally scheduled to be launched during 1986. But due to the Challenger disaster, it was halted until 1989. This mission took advantage of the "gravity assist" technique, which was used to propel the 2.5 ton Galileo all the way to Jupiter. Once there, Galileo made a plethora of examinations and discoveries. A probe was launched from Galileo that was able to successfully record information concerning Jupiter's atmosphere and clouds. After 73 minutes of free falling at 115,000 mph the probe was finally crushed by Jupiter's growing pressure. Aside from the probe, Jupiter was able to witness and record many images of the Shoemaker-Levy 9 comet that collided with Jupiter in 1994. And Galileo also witnessed an asteroid, Ida, which had a moon. This had never been seen before until the Galileo mission. Galileo has made 36 passes by Jupiter's major moons and numerous trips around the huge, mysterious planet. It is currently using its final power and has positioned itself to collide with Jupiter in only a few short months. Both of these missions were very successful and helped us learn much about the 5th planet from the sun. These missions have paved the way to future Jupiter missions which are destined to learn even more about the gaseous planet.
Jupiter
Building on the knowledge and experience gained from previous spacecraft missions, NASA was able to construct the first spacecraft to ever orbit a gaseous planet. The information and images that the Galileo spacecraft sent back to Earth were more specific and clearer than those of the Pioneer 10. The Galileo mission consisted of two spacecraft: an atmospheric probe and an orbiter, which is still circling the planet today (Bell). Galileo was launched on October 18, 1989, with the use of a two-stage Inertial Upper Stage ("Galileo Mission to Jupiter"). The Galileo was originally designed to use a three-stage IUS like the Pioneer 10, but NASA decided to use a two-stage IUS after the Challenger accident. This new two-stage IUS was safer and allowed the spacecraft to view other objects in space before its main mission to Jupiter ("Galileo Mission to Jupiter"). Galileo flew by planets close enough to allow their gravity to "slingshot" the spacecraft to Jupiter ("Galileo Mission to Jupiter"). The spacecraft took images and sent data about Venus, Earth, Gaspra (an asteroid), and Comet Shoemaker-Levy, before entering the atmosphere of Jupiter (Bell). The spacecraft released the probe in July 1995, and the orbiter entered the atmosphere in December 1995. The first day that the orbiter entered the atmosphere it passed by Europa and Io and started receiving data from the probe. The "wok-shaped probe" transmitted data for an hour as it parachuted through Jupiter's atmosphere at a speed of 106,000 mph and traveled through temperatures exceeding 150 degrees Celsius ("Galileo Mission to Jupiter"). The probe relayed information about Jupiter's "sunlight, heat flux, pressure, temperature, winds, lightning, and compression of atmosphere," before the electronics stopped working due to temperature and pressure ("Galileo Mission to Jupiter"). This data was extremely more accurate than that taken by the Pioneer 10 twenty years prior. After receiving the data from the probe, Galileo's engine was fired to get in place for orbit. Galileo collected data at a distance from Jupiter that was about 1,000 times closer than the Pioneer 10. The spacecraft was also able to collect detailed information about Jupiter's moons since it orbited those as well. The main mission of the Galileo ended in December 1997, but scientists have taken advantage of the spacecraft's ability to continue working ("Galileo Mission to Jupiter"). Since 1997 the spacecraft has been able to orbit the moons and confirm information about Europa's icy surface, thunderstorms of Jupiter, and many other facts. In 2000, it was approved for the orbiter to get close enough to Jupiter to study its magnetosphere and the moons Io and Ganymede ("Galileo Millennium Mission Status"). In September 2003, NASA plans for the orbiter to fly through the planet's rings and then crash into Jupiter ("Galileo Mission to Jupiter"). The data gained from the Galileo is far more detailed, and NASA actually has control of the spacecraft's journey with engines. This is why the Galileo proves to be an enhanced and better version of the Pioneer 10 spacecraft that simply flew by Jupiter years before.
In 1996, NASA launched both the Mars Pathfinder and the Mars Global Surveyor. The Pathfinder was to land on the red planet and collect information pertaining to the atmosphere and terrain of Mars. Scientists also hoped for new information concerning the possible presence of life on Mars. The Pathfinder succeeded in landing on Mars and collecting informative data for scientists about the planet's atmosphere and terrain. However, because of an extreme drop in the planet's temperatures, the rover's battery was depleted and all contact was lost with the Pathfinder. The Pathfinder mission was more of a success than past landing missions because the last successful landing mission on Mars was Viking 2 in 1975, however it was a failure because of the obvious fact that it did not return to earth or uncover any information as to the presence of life on Mars.
The Mars Global Surveyor successfully achieved orbit around Mars when it was launched in 1996 and was sent to acquire the same type of scientific data as the Pathfinder. It was also the first successful orbiting mission since Viking 2 in 1975. The Global Surveyor is planned to return to earth at the end of April if all goes well. It has been extremely helpful in providing more information about the atmosphere and terrain of Mars. But, the Surveyor's biggest breakthrough came in 1999 when scientists discovered pictures suggesting the possible presence of liquid water on Mars. Thus, because of this gigantic scientific discovery, and the fact that the Mars Pathfinder did not return to earth, the Mars Global Surveyor has proved to be a larger success for the scientific community than the Mars Pathfinder mission.
Mars
July 26,1976, marked the beginning of the continual exploration of Mars when the Viking 1 Lander touched down on the Chryse Planitia, offering the people of Earth images of the planet’s surface. The Viking Mission consisting of the Viking 1 and 2 landers and orbiters was the first mission ever to successfully reach Mars and operate on the surface of the planet. The primary objectives of the mission were to transmit pictures of the Martian surface to Earth, analyze the structure and make-up of the atmosphere and surface, and finally search of life. The Viking mission successfully sent back over 56,500 images of the planet and mapped 97 percent of the planet’s surface. The mission proved very beneficial to the studies of Mars, but it cost about a billion dollars.
Twenty years after the Viking Missions, NASA launched the Mars Pathfinder to demonstrate the ability of a low-cost successful mission to explore Mars. This mission featured a rover, named Sojourner, able to navigate around the surface. The Pathfinder proved to another success for NASA as it sent back 16,000 lander images and 550 rover images. The Pathfinder provided chemical analyses of rocks and soil on the surface, pictures of the Martian red sky with blue clouds, and other evidence indicating Mars was once warmer and wetter suggesting the possibility of life in the past or even now. The Mars Pathfinder was very successful for scientists and most importantly only cost $265 million including the building, testing and operation of the lander and rover.
Mercury, the closest planet to the sun, has the most extreme temperature variations in the solar system, which range from 90 Kelvin to 700 Kelvin. It is the second smallest planet and has been visited by only one spacecraft, Mariner 10. In 1974 and 1975, 45% of the surface was mapped. The rest of Mercury has yet to be mapped because of its proximity to the sun.
The Mariner 10 spacecraft revealed that Mercury has a mass much greater than thought, which could possibly mean that it has an iron core, which makes up 75% of the planet. Mercury's iron core interior is 1800 to 1900 km. The silicate outer shell is 500 to 600 km thick. At least some of the core is probably molten. Mercury, with its heavily cratered surface, is similar to the Moon. Mercury is very old as well; its surface has not changed much since its formation. Mercury actually has a very thin atmosphere consisting of atoms blown off its surface by the solar wind. This solar wind is the low-density stream of charged particles emitted by the sun. Because Mercury is so hot, these atoms quickly escape into space. Thus in contrast to the Earth and Venus whose atmospheres are stable, Mercury's atmosphere is constantly beingreplenished (Arnet).
Mercury is the second densest major body in the solar system after Earth, only because the Earth’s density is due to gravitational compression, had it not been for that, Mercury would be the densest body (Arnet). There are many craters on Mercury’s surface, but there is also evidence of smooth plains as well. These smooth plains may be explained by ancient volcanic activity on the planet or the deposition of waste from cratering impacts. The surface of Mercury exhibits enormous escarpments (dictionary.com); steep slopes or long cliffs that results from erosion or faulting and separates two relatively level areas of differing elevations. Some are up to hundreds of kilometers in length and as much as three kilometers high. A number of them cut through the rings of craters and other features in such a way as to indicate that they were formed by compression. It is estimated that the surface area of Mercury shrank by about 0.1%. Another spacecraft has received permission to visit Mercury. This discovery-class mission, executed by the Messenger, will launch in 2004 and orbit Mercury starting 2009 (Arnet). The Messenger may be able to elaborate on the things that scientist already know about Mercury.
In 1846, Johann Gottfried Galle and Heinrich Louis d’Arrest discovered the planet Neptune through mathematical observations. In 1977, NASA launched the space probe Voyager 2. Its mission was to explore the four gas planets: Jupiter, Saturn, Uranus, and Neptune. Voyager 2 reached Neptune in 1989. It explored Neptune and its surrounds for six months. Voyager 2 discovered that Neptune’s atmosphere has large amounts of methane, which helps give Neptune a blue color. Its surface is mostly rock and ice; the ice comes from water, methane, and ammonia. Neptune’s wind speeds can reach 1300 mph, which is higher than any other planet in the solar system. Neptune’s average temperature is close to - 400oF. Voyager 2 found that Neptune has eight moons and four rings in its orbit. The largest moon, Triton, has active geysers and may even have a surface similar the North Pole. This has brought the idea that life may exist on Triton. On average, Neptune is the eighth farthest planet from the Sun, but for a few years at a time, it is the farthest. This happens because Pluto sometimes travels inside of Neptune’s orbit around the Sun. Many “spots” have been seen on Neptune. These “spots” may be giant storms or holes in Neptune’s atmosphere. In 1994, NASA placed the Hubble Space Telescope (HST) into the Earth’s orbit. The HST has observed Neptune and found that the many “spots” on Neptune disappear and reappear constantly. Voyager 2 is the only space probe to visit Neptune. Since its exploration of Neptune, the HST has made any further observations.
On April 5, 1973, the Pioneer 11 was launched into space and preceded to its topic of destination: Jupiter and Saturn. In September of 1979, the Pioneer 11 came in contact with Saturn, the sixth and second largest planet in the solar system. This unmanned mission was deemed as a “low-cost” mission developed by scientists in order to explore the planets of Saturn more thoroughly. The probe consisted of two basic antennas, each used to gather information, such as data measurements and photographs from the atmosphere of Saturn. Pictures from the Pioneer 11 indicated the discovery of an additional ring encircling the equator of Saturn and scientific instruments located two previously undiscovered rings in Saturn’s orbit. Instruments also observed and charted that magnetosphere of Saturn’s largest moon, Titan, and found the moon to be too cold to sustain life. The mission ended on September 30, 1995, when the last transmission of data was received and contact with the spacecraft was lost.
Today, scientists working space organizations worldwide, have developed “the best instrumented spacecraft ever sent to another planet”, otherwise known as the Cassini spacecraft, created to explore the planet of Saturn, and Saturn alone. The spacecraft was launched in 1997 and will reach Saturn in the year of 2004. This spacecraft carries the Huygens probe, which will enter the atmosphere of Titan in December of 2004. Once the Huygens probe has been released, the Cassini will continue to orbit Saturn for four more years, using high-tech instruments to study the planetary features of Saturn, such as its rings, moon, internal structure and atmosphere, as well as the structure of Titan and other Saturnian moons. The Cassini spacecraft will inevitably conduct more extensive research and investigations of the Saturn which will greatly benefit scientists and astronomers.
Saturn
Saturn is the second largest planet in the solar system. It is approximately 95 times as massive as Earth. It is also the sixth farthest from the sun. Earth rotates the sun 30 times before Saturn completes one revolution. Saturn is part of the outer, gas planets along with Jupiter, Uranus, and Neptune. Outside it’s small rocky core, Saturn contains 88% hydrogen and 11% helium. Saturn has 18 observed moons and the biggest of these moons, Titan, is the second largest in the solar system. Saturn is also surround by massive rings of rock and interstellar dust. These rings are no more then 10 meters thick in some areas. The cloud formations on Saturn are determined by the extreme winds blowing in zones across the surface of the planet.
There have been only three spacecrafts to study the planet. Pioneer passed by in April of 1979 and was responsible for the first ever detailed pictures of the planet. It also discovered an outer ring and two additional moons around the planet. Voyager 1 and 2 passed the planet in November of 1980 and August of 1981 respectively. These two probes studied the rings of the planet in greater detail and provided even higher resolution pictures then Pioneer 11. It was discovered that Saturn has an incredibly low density. It is the only planet in the solar systems whose density is less then that of water.
Currently the spacecraft Cassini is on route to the planet. When it arrives in 2004, it will be the first fly by in more then 20 years. Cassini promises the scientific community very exciting opportunity to study Saturn in greater detail.
Titan (moon of Saturn)
Due to Titan’s thick nitrogen atmosphere neither of the Voyager space probes (launched in 1977) have provided much information for scientists. A majority of the information was gathered from The Hubble Space Telescope and other instruments like spectrascopes. Titan’s mass (1.35e^23 kg), surface area (5,150 kilometers), atmospheric pressure (1.6 bars), and other such data like temperature (-178 degrees Celsius, which means that the water on Titan is in solid form) was determined by mathematical calculations from the data collected from these instruments. The Voyager probes did however provide photographs of “dark hoods” near Titan’s poles. These ended up being the seasonal changes on the surface of Titan. Data on the actual surface of Titan has yet to be collected, but many astronomers theorize about the amino acids (the building blocks of life) that could be found here due to combination through photochemistry with the thick nitrogen atmosphere and the vast seas of ethane. The Cassini space probe will drop the lander, Huygens, on to the surface of Titan in 2004 to collect this data. This data is critical in understanding the history of our planet, because it is almost exactly how early Earth was before the introduction of oxygen.
The Mariner 2 and the Magellan provided scientists with information about Venus. The Mariner 2, a fly-by, was launched on August 27, 1962. The Magellan was launched on May 4, 1989 and fell in Venus’ orbit. The Magellan was equipped with seven experiments and a SAR, Synthetic Aperture Radar, which enabled the spacecraft to send vast amounts of information on Venus’ atmosphere and surface to Earth. Although the Magellan cost $1 million, the information gained has far outweighed its costs. Most of the information known about Venus comes from the Magellan mission.
Venus
Venus is the second planet from the sun in our solar system and is the closest planet to earth. Venus has been visited by many space probes and satellites over the past several decades and two particular missions are the VEGA Space Mission and the Pioneer Venus Mission. The Pioneer mission was launched in 1978 and consisted of two separate missions launched within months of one another. One of the missions was a satellite and the second consisted of four probes sent into the atmosphere of the planet. The purpose of this mission was to answer questions dealing with the clouds on Venus that often hide it from Earth’s view. The VEGA Space mission was a combined spacecraft mission to Venus and the Halley comet, launched in 1984 from Russia. The mission consisted of data recording balloons and spacecrafts that landed on the surface. The mission achieved many of its goals, including the first direct tracing of Venus’ atmospheric properties, identification of atmospheric components, and the first study of the environment. The Pioneer mission also used balloons for a majority of the recorded data.
Venus
The Pioneer Venus Orbiter was an orbiter -probe combination designed to conduct a comprehensive investigation on the atmosphere of Venus. Pioneer orbited Venus from December 1978 to July 1980 and was reactivated in 1991 until 1992 to gain final measurements of Venus. The spacecraft was a solar-powered cylinder about 250 cm in diameter, which cost $125 million to build and operate (NASA). The Pioneer Venus Orbiter measured the detailed structure of the upper atmosphere and ionosphere of Venus, investigated the interaction of the solar wind with the ionosphere and the magnetic field in the vicinity of Venus. The orbiter was able to determine the characteristics of the atmosphere and surface of Venus on a planetary scale and determine the planet's gravitational field (NASA). over 99% of Venus's surface has been mapped with a resolution ten times better than obtained by the earlier Soviet Venera 15 and 16 missions. The total cost of the mission was $551 million. The study of Magellan's global images is providing evidence to understanding the role of impacts, volcanism, and tectonics in the formation of Venusian's surface structures. The gravitational field of Venus is highly correlated with the surface topography, which is completely different from the Earth. The differences in surface topography are still unresolved and because of it, scientists do not completely understand the Venus's global tectonics.Magellan showed us that an Earth-size planet like Venus could have a different kind of surface that Earth. At least 85% of Venus's surface is covered with volcanic flows; the remainder has highly deformed mountain belts. The volcanic materials include vast lava plains, fields of small lava domes and large shield volcanoes. The presence of lava channels over 6,000 kilometers long suggests river-like flows of extremely low-viscosity lava that probably erupted at a high rate. Large pancake-shaped volcanic domes suggest the presence of a type of lava produced by extensive evolution of crustal rocks. The typical signs of terrestrial plate tectonics - continental drift and basin floor spreading - are not in evidence on Venus. The planet's tectonics is dominated by a system of global rift zones and numerous broad, low domical structures called coronae, produced by the upwelling and subsidence of magma from the mantle.