The Galileo Project | Science | Satellites of Jupiter Jupiter's moons Satellites of Jupiter Jupiter has a large number of satellites. Of these, four are comparable to the Earth's Moon in size; the rest are orders of magnitude smaller. When Jupiter is at opposition and closest to the Earth, the stellar magnitude of its four large moons is between 5 and 6. [1] This means that, were it not for the shielding brightness of Jupiter, these bodies would be visible with the naked eye. The aperture of the telescope used by Galileo in 1610 and its magnification thus brought these four "Galilean" satellites within his grasp. But first Galileo had to make adjustments to the instruments. When viewing bodies that are very bright and very small, the optical defects of the telescope can be crippling. By trial and error Galileo learned to stop down the aperture of his instrument until he could begin to make useful observations. At the end of 1609, as he was finishing his series of observations of the Moon , Jupiter was at opposition and the brightest object in the evening sky (not counting the Moon). When he had made the new adjustment to his instrument, he turned his attention to Jupiter.

On 7 January 1610 he observed the planet and saw what he thought were three fixed stars near it, strung out on a line through the planet. This formation caught his attention, and he returned to it the following evening. Galileo's expectation was that Jupiter, which was then in its retrograde loop, [2] would have moved from east to west and had left the three little stars behind. Instead, he saw all three stars to the west of Jupiter. It appeared as though Jupiter had not moved to the west but rather to the east. This was an anomaly, and Galileo returned to this formation again and again. Over the next week he found out several things. First, the little stars never left Jupiter; they appeared to be carried along with the planet. Second, as they were carried along, they changed their position with respect to each other and Jupiter. Third, there were not three but four of these little stars. By the 15th of January he had figured it out: these were not fixed stars but rather planetary bodies that revolved around Jupiter. Jupiter had four moons.

His book, Sidereus Nuncius, in which his discovery was described, came off the press in Venice in the middle of March 1610 and made Galileo famous. Galileo's observations of Jupiter's moons (from manuscripts) [click for larger image] The moons of Jupiter had a major impact on cosmology. In 1610 the traditional Aristotelian cosmology had come under attacks from Copernican astronomers. Aristotelians had a number of arguments against the Copernican System, one of which was now made obsolete. In traditional cosmology, there was only one center of motion, the center of the universe which was the place of the Earth. The motions of all heavenly bodies centered on the Earth. But according to the Copernican theory, the Earth went around the Sun while the Moon went around the Earth. There were thus two centers of motion, which seemed an absurdity. Moreover, if the Earth was a planet, like Mercury, Venus, Mars, Jupiter, and Saturn, why was it the only planet to have a Moon? Galileo's discovery answered this question. The Earth was, in fact, not the only planet to have a moon, Jupiter had four.

And no matter what cosmological system one believed in, there were now at least two centers of motion in the universe, the Earth or Sun and Jupiter. Thus, although the satellites (the term was first used by Johannes Kepler ) of Jupiter were by no means proof of the truth of the Copernican system, they certainly added ammunition on that side of the argument. In the purely astronomical realm, the satellites of Jupiter posed a new problem for astronomers. It had taken centuries in Antiquity to arrive at adequate geometrical modes for the motions of the known planets. Now there was a new system of planetary bodies in miniature, and astronomers had to develop models that could predict their motions. There was a great incentive to come up with good mathematical models, for the satellites offered some hope for the solution of the problem of longitude at sea . It took almost two centuries, however, before the models and tables based on them reached satisfactory accuracy. The naming of the satellites provides an interesting example of how such matters were handled before the foundation of the International Astronomical Union in the twentieth century. As their discoverer, Galileo claimed the right to name the satellites.

He wanted to name them after his patrons and asked whether they would prefer "Cosmic Stars" (after Cosimo II ) or "Medicean Stars." They opted for the latter, and through much of the seventeenth century they were known by that name. In his notebooks, Galileo referred to them individually by number, starting with the satellite closest to Jupiter, but he never had occasion to refer to them in this way in print. In Provence, Nicholas Claude Fabri de Peiresc tried to differentiate between the Medicean Stars by assigning them the names of individual members of the family, but this system was not published and thus was never used by others. In his Mundus Iovialis ("Jovian World") of 1614, Simon Marius went into the naming problem in some depth. First, he himself used the numerical system beginning with the satellite closest to Jupiter. Second, he thought that he might call them after his patron, the Duke of Brandenburg -- a suggestion followed by no one. Third, he suggested naming the farthest satellite the Saturn of Jupiter, the next one the Jupiter of Jupiter, the third one the Venus of Jupiter, and the one nearest the planet the Mercury of Jupiter.

This cumbersome system never caught on. Finally, Marius related a suggestion by Kepler : Jupiter is much blamed by the poets on account of his irregular loves. Three maidens are especially mentioned as having been clandestinely courted by Jupiter with success. Io, daughter of the River, Inachus, Callisto of Lycaon, Europa of Agenor. Then there was Ganymede, the handsome son of King Tros, whom Jupiter, having taken the form of an eagle, transported to heaven on his back, as poets fabulously tell . . . . I think, therefore, that I shall not have done amiss if the First is called by me Io, the Second Europa, the Third, on account of its majesty of light, Ganymede, the Fourth Callisto . . . . This fancy, and the particular names given, were suggested to me by Kepler, Imperial Astronomer, when we met at Ratisbon fair in October 1613. So if, as a jest, and in memory of our friendship then begun, I hail him as joint father of these four stars, again I shall not be doing wrong.

[3] None of these suggestion caught on because with Jupiter's satellites , there was no confusion in the numbering system. Following Galileo and Marius, astronomers simply referred to them by number. With the satellites of Saturn, however, a problem developed. In 1655 Huygens discovered the first and largest; then in 1671-72 Giandomenico Cassini discovered two more, and in 1684 yet another two. These five satellites were numbered like their Galilean counterparts. But when in 1789 William Herschel discovered two additional satellites internal to the first, confusion followed. Did one now renumber them all (thus causing confusion for those who consulted older works), refer to the two new ones as nos. 6 and 7 (thus making the order of the satellites confusing), or refer to them by order of discovery (equally confusing as to order)? Herschel's son, John Frederick William, suggested in 1847 that Saturn's satellites be given individual names of mythological figures associated with Saturn after the suggestion made by Marius for Jupiter's satellites.

When, the following year, William Lassel and George Bond independently discovered an eighth satellite of Saturn, they agreed to adopt the naming system proposed by Herschel, in which Saturn's satellites were named after his brothers and sisters, the Titans. This system and the now revived suggestion by Kepler and Marius for Jupiter quickly became the convention for naming the satellites of the superior planets. Modern Images of the Galilean Satellites Io and Europa Jupiter, Jupiter Information, Facts, News, Photos -- National Geographic This requires the latest version of Flash Player. Click here to download. Jovian Giant The most massive planet in our solar system, with four planet-size moons and many smaller satellites, Jupiter forms a kind of miniature solar system. Jupiter resembles a star in composition. In fact, if it had been about eighty times more massive, it would have become a star rather than a planet. On January 7, 1610, using his primitive telescope, astronomer Galileo Galilei saw four small "stars" near Jupiter. He had discovered Jupiter's four largest moons, now called Io, Europa, Ganymede, and Callisto. Collectively, these four moons are known today as the Galilean satellites.

Galileo would be astonished at what we have learned about Jupiter and its moons in the last 30 years. Io is the most volcanically active body in our solar system. Ganymede is the largest planetary moon and is the only moon in the solar system known to have its own magnetic field. A liquid ocean may lie beneath the frozen crust of Europa. Icy oceans may also lie deep beneath the crusts of Callisto and Ganymede. In 2003 alone, astronomers discovered 23 new moons orbiting the giant planet, giving Jupiter a total moon count of 49, the most in the solar system. The numerous small outer moons may be asteroids captured by the giant planet's gravity. Jupiter's appearance is a tapestry of beautiful colors and atmospheric features. Most visible clouds are composed of ammonia. Water exists deep below and can sometimes be seen through clear spots in the clouds. The planet's "stripes" are dark belts and light zones created by strong east-west winds in Jupiter's upper atmosphere. Within these belts and zones are storm systems that have raged for years. The Great Red Spot, a giant spinning storm, has been observed for more than 300 years.

Atmosphere The composition of Jupiter's atmosphere is similar to that of the sun—mostly hydrogen and helium. Deep in the atmosphere, the pressure and temperature increase, compressing the hydrogen gas into a liquid. At depths about a third of the way down, the hydrogen becomes metallic and electrically conducting. In this metallic layer, Jupiter's powerful magnetic field is generated by electrical currents driven by Jupiter's fast rotation. At the center, the immense pressure may support a solid core of ice-rock about the size of Earth. Jupiter's enormous magnetic field is nearly 20,000 times as powerful as Earth's. Trapped within Jupiter's magnetosphere (the area in which magnetic field lines encircle the planet from pole to pole) are swarms of charged particles. Jupiter's rings and moons are embedded in an intense radiation belt of electrons and ions trapped in the magnetic field. The Jovian magnetosphere, composed of these particles and fields, balloons 600,000 to 2 million miles (1 million to 3 million kilometers) toward the sun and tapers into a windsock-shaped tail extending more than 600 million miles (1 billion kilometers) behind Jupiter, as far as Saturn's orbit.

Discovered in 1979 by NASA's Voyager 1 spacecraft, Jupiter's rings were a surprise: a flattened main ring and an inner cloudlike ring, called the halo, are both composed of small, dark particles. A third ring, known as the gossamer ring because of its transparency, is actually three rings of microscopic debris from three small moons: Amalthea, Thebe, and Adrastea. Jupiter's ring system may be formed by dust kicked up as interplanetary meteoroids smash into the giant planet's four small inner moons. The main ring probably comes from the moon Metis. Jupiter's rings are only visible when backlit by the sun. In December 1995, NASA's Galileo spacecraft dropped a probe into Jupiter's atmosphere, which collected the first direct measurements of the atmosphere. Following the release of the probe, the Galileo spacecraft began a multiyear study of Jupiter and its largest moons. As Galileo began its 29th orbit, the Cassini-Huygens spacecraft was nearing Jupiter for a gravity-assist maneuver on the way to Saturn. The two spacecraft made simultaneous observations of the magnetosphere, solar wind, rings, and Jupiter's auroras.

—Text courtesy NASA/JPL Galileo: Discovering Jupiter's Moons | Social Studies, Science | Video | PBS LearningMedia Average rating: 4 stars - 1 rating Galileo: Discovering Jupiter's Moons The telescope forever changed astronomy by providing more detailed views of distant objects than was previously possible. Galileo pioneered astronomy as the first person to study celestial objects through a telescope. His observations, including the discovery of moons around Jupiter, helped revolutionize the way people think about the universe. This video segment adapted from NOVA describes some of Galileo's first discoveries with the telescope. For more information visit NOVA . Background Essay In 1610, when Galileo first pointed his newly built telescope toward the sky, no one had ever had the capability to see distant objects in any detail, and he had no idea what to expect. Focusing on the Moon first, he discovered that its surface was not smooth as people had previously thought, but covered with craters and mountains not unlike those on Earth. Next, Galileo turned his attention to another bright object in the night sky — the planet Jupiter. Today, we know quite a bit about Jupiter.

With a diameter about 11 times greater than Earth's, Jupiter is the largest planet around our Sun. It is so enormous that its mass is more than twice the combined mass of all the other planets in our solar system. Composed mostly of gaseous hydrogen and helium, Jupiter is a gas giant. It has a trademark banded look that results from its turbulent atmosphere, and its Great Red Spot is a giant storm that has been raging for centuries. Jupiter also has a faint ring system and, as of May 2005, 63 known moons surround it. In a very short time, Galileo discovered Jupiter's four largest moons — Io, Europa, Ganymede, and Callisto, now called the Galilean moons. Each moon has unique characteristics that make it stand out in the solar system. For example, Io is the most volcanically active object in the solar system. Europa may have an ocean of liquid water beneath its icy surface and may be the best candidate for another habitable world. Callisto is one of the most heavily cratered bodies in the solar system, and Ganymede, which is even larger than the planet Mercury, is the largest moon.

The four Galilean moons were the first celestial bodies to be discovered with a telescope. Although Galileo at first thought they were fixed stars, his continued observations showed that their positions changed on a nightly basis. Galileo soon recognized that these new objects were satellites of Jupiter. The discovery that celestial objects could revolve around something other than Earth provided strong evidence in support of the heliocentric theory. How did Galileo surmise that Jupiter had moons that revolved around it? How did the discovery of Jupiter's moons support the heliocentric theory? What was so shocking about this discovery in Galileo's time? Why did he rush to publish his discovery? Explain the importance of Galileo's work on the telescope and its effect on the world at that time and today. Discuss the role of technology in science. JUPITER'S Moons - EnchantedLearning.com Jupiter's Moons Activities, Web Links JUPITER'S MOONS Jupiter has four large moons and dozens of smaller ones (there are about 67 known moons so far).

Galileo first discovered the four largest moons of Jupiter, Io, Europa, Ganymede, and Callisto in 1610, using a 20-power telescope; these moons are known as the Galilean moons. The moons of Jupiter are (in order by their distance from Jupiter): Metis, Adrastea, Amalthea, Thebe, Io, Europa, Ganymede (the biggest), Callisto (the second biggest), Leda (the smallest), Himalia, Lysithea, Elara, Ananke, Carme, Pasiphae, Sinope, and many newly-discovered moons that haven't been named yet. METIS Metis is the closest moon to Jupiter. Metis is 25 miles (40 km) in diameter and orbits 79,500 miles (128,000 km) from Jupiter, within its main ring. Metis and the next moon, Adrastea, are probably the source of the dust in this ring. Metis has a mass of 9 x 1016kg.

It orbits Jupiter in 0.294780 (Earth) days; this is faster than Jupiter rotates on its axis. Metis was discovered by Stephen Synnott (Voyager 2) in 1980. ADRASTEA Adrastea is the second-closest moon to Jupiter. Adrastea is 12 miles (20 km) in diameter and orbits 80,000 miles (129,000 km) from Jupiter, within its main ring. Adrastea and the first moon, Metis, are probably the source of the dust in this ring. Adrastea has a mass of 1.91 x 1016kg. It orbits Jupiter in 0.29826 (Earth) days; this is faster than Jupiter rotates on its axis. Adrastea was discovered by D. Jewitt and E. Danielson (Voyager 2) in 1979. AMALTHEA Amalthea is the third-closest moon to Jupiter and the reddest object in our solar system .

Amalthea is 145 x 91 x 83 miles (232 x146 x134 km) in diameter and orbits 112,700 miles (181,300 km) from Jupiter, within the faint Gossamer ring. Amalthea and Thebe likely provide the dust for the Gossamer ring. Amalthea has a mass of 7.2 x 1021kg. It orbits Jupiter in 0.49817905 (Earth) days and is in synchronous rotation (always keeping the same side facing Jupiter). Amalthea gives off more heat than it receives from the Sun. Amalthea was discovered by Edward Emerson Barnard in 1892. THEBE Thebe is the fourth-closest moon to Jupiter. Thebe is 68 x 56 miles (110 x 90 km) in diameter and orbits 138,000 miles (222,000 km) from Jupiter. Amalthea and Thebe likely provide the dust for the Gossamer ring.

Thebe has a mass of 8 x 1017kg. It orbits Jupiter in 0.6745 day (Earth) days and is in synchronous rotation (always keeping the same side facing Jupiter). Thebe was discovered by Stephen Synnott (Voyager 1) in 1980. THE GALILEAN MOONS Two sulfurous eruptions on Io. Pillan Patera, a volcanic caldera at the left, spews sulfur 86 miles above the surface. IO Io is a large, rocky, volcanically active moon of Jupiter. Its volcanoes spew out molten sulfur, making Io a very colorful moon. It is the innermost of Jupiter's four large moons and the third largest. It has a diameter of 1,942 miles (3,636 km), very close in size to our moon . Io's mean distance from Jupiter is 220,000 miles (422,000 km). It has a mass of 8.93x1022 kg. It takes Io 1.77 days to orbit Jupiter.

There is a doughnut-shaped plasma cloud around Jupiter near Io's orbit (known as the "Io plasma torus") This torus is caused by Jupiter's strong magnetic field, which strips ions from Io as it rotates; Io acts like an electrical generator. Io was discovered by Galileo and Marius (independently) in 1610. Ole Romer (1644-1710) was a Danish astronomer who, in 1675-1676, was the first person to demonstrate that the speed of light is finite. Romer did this by observing eclipses of Jupiter's moon Io as Jupiter's distance from Earth varied through the year. He noticed that the observed period of Io's orbit differed by about 20 minutes; he concluded that this difference was due to the extra distance that the light had to travel to Earth. His calculations put the speed of light at about 225,000 kilometers per second (it is really a bit faster, at 299,792 kilometers per second). EUROPA Europa is a large, dense, icy moon of Jupiter. Europa is the smoothest object in our Solar System .

Its surface is covered with long, crisscrossing trackways (but few craters) on water ice. Frozen sulfuric acid has been found on its surface. Europa's diameter is less than 2,000 miles (3,138 km), smaller than the Earth's moon . It takes Europa 3.55 days to orbit Jupiter (in a synchronous orbit). Its mean distance from Jupiter is about 420,000 miles (670,900 km). Its mass is 4.80x1022 kg. It was discovered by Galileo and S. Marius (independently) in 1610. GANYMEDE Ganymede is the largest moon of Jupiter, a large, icy, outer moon that is scarred with impact craters and many parallel faults. It has a diameter of about 3,400 miles (5,268 km) and orbits Jupiter at a mean distance of 664,000 miles (1,070,000 km). It has a magnetic field and probably has a molten iron core.

It takes Ganymede 171.75 hours (7.15 Earth days) to orbit Jupiter. Its mass is 1.48x1023 kg. It was discovered by Galileo and S. Marius (independently) in 1610. Ganymede is the largest moon in the solar system; it is also larger than the planets Mercury and Pluto. CALLISTO Callisto is a large, icy, dark-colored, low-density outer moon of Jupiter that is scarred with impact craters and ejecta. It has a diameter of about 3,000 miles (4800 km), the second-largest moon of Jupiter; it is roughly the size of Mercury . Callisto has the largest-known impact crater in the Solar System, Valhalla, which has a bright patch 600 km across and rings that go out to almost 3000 km. Callisto orbits Jupiter at a mean distance of 1,170,000 miles (1,883,000 km). Its mass is 1.08x1023 kg.

It takes Callisto 400.8 hours = 16.7 days to orbit Jupiter (in a synchronous orbit). Callisto was discovered by Galileo and S. Marius (independently) in 1610. LEDA Leda is Jupiter's ninth and smallest moon. Leda is 9.9 miles (16 km km) in diameter and orbits at an average of 6,900,000 miles (11,094,000 km) from Jupiter. Leda has a mass of 5.68 x 1015kg. It orbits Jupiter in 238.72 (Earth) days. Very little is known about Leda. Leda was discovered by Charles Kowal in 1974. HIMALIA Himalia is Jupiter's tenth moon. Himalia is 110 miles (170 km) in diameter and orbits 7,000,000 miles (11,480,000 km) from Jupiter. Himalia has a mass of 9.5 x 1018kg.

It orbits Jupiter in 250.5662 (Earth) days. Very little is known about Himalia. Himalia was discovered by C. Perrine in 1904. LYSITHEA Lysithea is Jupiter's eleventh moon. Lysithea is 15 miles (24 km) in diameter and orbits 7,200,000 miles (11,720,000 km) from Jupiter. Lysithea has a mass of 8 x 1016kg. It orbits Jupiter in 259.22 (Earth) days. Very little is known about Lysithea. Lysithea was discovered by S. Nicholson in 1938. ELARA Elara is Jupiter's twelfth moon. Elara is 50 miles (80 km) in diameter and orbits 7,250,000 miles (11,737,000 km) from Jupiter. Elara has a mass of 8 x 1017kg. It orbits Jupiter in 259.6528 (Earth) days.

Very little is known about Elara. Elara was discovered by C. Perrine in 1905. ANANKE Ananke is Jupiter's thirteenth moon. Ananke is 12.5 miles (20 km) in diameter and orbits 13,100,000 miles (21,200,000 km) from Jupiter. Ananke has a mass of 4 x 1016kg. It orbits Jupiter in 631 (Earth) days and is in a retrograde orbit (orbiting opposite to the direction of Jupiter). Very little is known about Ananke. Ananke was discovered by S. Nicholson in 1951. CARME Carme is Jupiter's fourteenth moon. Carme is 18.5 miles (30 km) in diameter and orbits 13,800,000 miles (22,600,000 km) from Jupiter. Carme has a mass of 9 x 1016kg. It orbits Jupiter in 692 (Earth) days and is in a retrograde orbit (orbiting opposite to the direction of Jupiter).

Very little is known about Carme. Carme was discovered by S. Nicholson in 1938. PASIPHAE Pasiphae is Jupiter's fifteenth moon. Pasiphae is 22 miles (36 km) in diameter and orbits 14,600,000 miles (23,500,000 km) from Jupiter. Pasiphae has a mass of 2 x 1023kg. It orbits Jupiter in 735 (Earth) days and is in a retrograde orbit (orbiting opposite to the direction of Jupiter). Very little is known about Pasiphae. Pasiphae was discovered by P. Melotte in 1908. SINOPE Sinope is Jupiter's sixteenth moon. Sinope is 17.5 miles (28 km) in diameter and orbits 14,700,000 miles (23,700,000 km) from Jupiter. Sinope has a mass of 8 x 1016kg.

It orbits Jupiter in 758 (Earth) days and is in a retrograde orbit (orbiting opposite to the direction of Jupiter). Very little is known about Sinope. Sinope was discovered by S. Nicholson in 1914. S/1999 J 1 (a provisional name) The seventeenth and outermost moon; S/1999 J 1 is the smallest-known moon orbiting a major planet. This moon is 3 miles (5 km) in diameter and has an irregular orbit roughly 15 million miles (24 million km) from Jupiter. It orbits Jupiter in 774 (Earth) days and is in a retrograde orbit (orbiting opposite to the direction of Jupiter). It was discovered by Robert S. McMillan et al (at the Spacewatch program at the University of Arizona ) in 2000. The Galilean Moons of Jupiter The Galilean Moons of Jupiter Summary: Jupiter has more than 60 known moons, but understanding the geology of its four largest will hopefully lead to some groundbreaking discoveries.

Sections: The Galilean Moons Each of the Jovian planets has a number of moons, although Jupiter has the most with more than 60 catalogued to date. Jupiter's 4 largest moons exhibit some of the most interesting geology in the solar system. They were discovered by Galileo Galilei and are known as the Galilean moons. Jupiter's largest moon, Ganymede, is larger than Mercury while the other three are larger than Pluto. Outward from Jupiter: Io: Volcanoes and All Jupiter's Moon Io The Voyager spacecraft took the first close-up images of Io more than 300 years after the moon's discovery. The images showed a surface with no signs of craters from past impacts. What we saw instead was a surface almost entirely covered with large volcanoes. Cameras on Voyager actually captured volcanic eruptions in progress. The frequency of these sulfuric eruptions has filled in almost all of the impact craters and left Io with one of the youngest looking surfaces in the solar system. Io Close-Up (click to enlarge) Close-up photos of eruptions in progress show powerfully hot lava glowing orange and red.

Photos taken on the night side of Io show not only the hot volcanic vents, but also a thin sulfur dioxide atmosphere produced by constant outgassing. Io's unusual red and orange colors come primarily from sulfur, which condenses on the surface after being outgassed by the volcanoes. Although there is no direct evidence of tectonic activity on Io, scientists feel confident it exists since the processes that fuel volcanism also fuel tectonics. The volcanic eruptions are so frequent and cover the surface so thoroughly that any clear evidence of tectonic activity is likely to be buried. Europa: What Lies Beneath? Europa Europa's surface and crust are made almost entirely of water ice, and its bizarre, fractured appearance is proof enough that tidal heating has acted there. The icy surface is nearly devoid of impact craters and may be only a few million years old. Europa's Interior Observations made by the Galileo spacecraft show that Europa has a metallic core and a rocky mantle. Surrounding the rocky interior appears to be an icy layer 100 kilometers thick, the top few kilometers of which seem to be frozen solid.

The stretching and squeezing of tidal friction should provide enough heat to melt some of this into liquid water beneath a thin ice shell. If it does, then Europa may have an ocean with more than twice as much liquid water as all of Earth's oceans combined. Analyzing Europa's Cracked Surface Close-up photos of the surface of Europa support the idea of a liquid ocean beneath the surface. These photos, taken by the Galileo spacecraft, show what appears to be icebergs stuck in a layer of ice. Other evidence comes from double-ridged cracks on the surface. Tidal flexing that allows water to well up and build ridges may create these cracks. Ganymede: Largest Moon in the Solar System Two Regions of Ganymede's Surface (click to enlarge) The surface of Ganymede shares many similarities with Europa. Ganymede's surface is also made of water ice, but unlike Europa's surface, it shows signs of varying age. The darker regions are heavily cratered, suggesting they are billions of years old. The lighter regions show no signs of craters and it is thought that eruptions of water covered the surface before freezing over. These areas are geologically younger than the darker regions.

If liquid water occasionally makes its way to the surface to fill in craters, could that suggest a liquid ocean similar to the one that might exist on Europa? Callisto: The Outermost Galilean Moon Callisto Callisto is the stereotypical outer solar system satellite. It is one of the largest and most heavily cratered satellites in the solar system. The surface is very icy and dates back four billion years. Beneath the icy crust is possibly a salty ocean supported by a deeper rocky interior. Callisto's Surface Callisto doesn't have any large mountains, show evidence of volcanic or tectonic activity or have any appreciable level of internal heat. Nonetheless, observations of Callisto's magnetic field may cause scientists to add the large moon to the list of possible worlds with subsurface salty oceans. Steve's Astro corner: 1610 Gallileo And His Earth Shattering Discovery of Moons Orbiting Jupiter This is a fun place where I share my adventures and love of Astronomy. Enjoy!!! Thursday, January 14, 2010 1610 Gallileo And His Earth Shattering Discovery of Moons Orbiting Jupiter I have been very ill as of late and did not get a blog out last week.

A sinus+ an ear infection will do that to a blogger. I feel Oh so much better. So with all that aside, let's get cracking !!! 400 years ago in Italy there was a discovery made that rocked the foundations of the church, science, even the calendar industry! Jupiter had moons and they were in orbit around Jupiter. Now I know you might be thinking OK so what Why is that a big deal? 400 years ago for most folks on this planet everything and I do mean everything revolved around the Earth. Sun all planets you name it, it had a path that traveled around the third rock from the Sun. Earth had a fixed position in the heavens. There had been many scientists of their day that did measurements of the whole geocentric system but kept coming up with problems like retrograde motion (planets that seem to move backwards for a time then continue their orbit) or changing of brightness at opposition. To answer these big ole head exploding problems, Tycho Brahe came up with a crazily complex set of epicycles for each offending item in the geocentric system This system was even more complex than Ptolemy's original Geocentric system complete with epicycles.

The only thing that really did was just present more questions. Here is the deal folks. The church had the geocentric system firmly entrenched in there core beliefs so it pretty much boiled down to "Never let the facts get in the way of what you believe" And that was the way the church looked at the universe. It would take a miracle to change that way of thinking. Say like if Someone discovered moons orbiting something other than the Earth... Ah you start to get the picture now of what a pivotal moment it was for Science. Now if you are Galileo what do you do after making this discovery? Before you answer that Let me point out the church had just burned someone at the stake for spreading that Heretical idea of a Heliocentric (Sun centered system). Even Copernicus was excommunicated from the church for proposing it many years earlier. So now with full knowledge of what might happen . Galileo published the findings anyway! The real question here is did the church take notice of the discovery? As a matter of fact they did and they were less than pleased with the findings the point that the church said Galileo's observations are wrong. He is using a wildly inaccurate instrument.

The church deep down knew it was only a matter of time until the science would come to the fore front. Scientist like Copernicus would be vindicated and scientists like Tycho Brahe would be left in the dust with their decades of incorrect calculations. Wow, that had to be devastating to Tycho. The only solace he might have had was to tell folks that guy over there Johannes Kepler was a student of mine. That leaves one question for this week unanswered Why did the Calendar industries world get rocked? Here is a really good bit of trivia for you. Simon Marius discovered the moons of Jupiter on December 29th 1609 and Galileo discovered the moons on January 7th 1610. So why did Galileo get the credit? They did not use the same calendar! Galileo used the Gregorian Calendar while Marius used the Julian calendar. The Julian date would coincide with the 8th of Jan.1610. Simon wrote in his book that he had actually looked at Jupiter in November of 1609 and had seen the moons but did not draw anything Unfortunately for Simon, that observation was unrecorded. Do you think Simon Marius had any regrets?

Until next time, When Did Galileo Discover Jupiter? Ganymede Callisto Galileo began seriously observing the planet in 1609. This discovery of Jupiter’s moons occurred on January 7, 1610. Galileo wrote that there were “three fixed stars, totally invisible by their smallness” that were extremely close to Jupiter. However, when Galileo observed them on a different night, the “fixed stars” had changed positions. Galileo soon realized that the fixed stars were actually orbiting Jupiter. On January 10, 1610 Galileo discovered another one of Jupiter’s moons. Galileo continued to observe the moons for over a year. He attempted to estimate their periods. Impressively, Galileo’s estimations were quite close to the actual periods of the moons. Consequence of Galileo’s Discovery At the time, the majority of Europe still endorsed a theory that all the planets orbited around the Earth. According to this theory (the geocentric model of the solar system), the Earth was at the center of the Universe. However, Galileo identified smaller fixed points that were orbiting Jupiter. This went against the geocentric model that stated all heavenly bodies should orbit the Earth.

Galileo’s discovery paved the way for the heliocentric model of the solar system, in which the planets orbit around the Sun. All About Jupiter Jupiter is the fifth planet from the Sun. It is also the largest planet in the solar system, and has a greater mass than all the other planets combined. It is one of the Gas Giants. The planets Saturn, Uranus and Neptune are also considered Gas Giants. The composition of Jupiter is primarily hydrogen , as well as helium. Hydrogen makes up the majority of the composition, and helium makes up approximately one fourth of the composition. It is unknown what the core of the planet is composed of, and the core may be composed of solid elements. Jupiter has an orbital period of a little more than four hundred and thirty-three days. Currently, there are sixty three moons that orbit the planet. The largest moon that orbits Jupiter is Ganymede. Ganymede is so large, in fact, that its diameter is greater than Mercury. One of the defining features of the planet Jupiter is a giant red storm that can be seen in pictures of the planet. The storm is actually larger than the planet Earth.

It’s an anticyclonic storm, and it’s located close to the equator of the planet. The storm has possibly existed since 1665, and the storm definitely has existed since 1831. Observing the Planet Jupiter Since Jupiter is so large, it can be spotted by the naked eye. It is usually the third brightest object in the sky, behind the moon and Venus. Thus, individuals have observed the planet for years. The Romans named the planet after their king of the Gods, Jupiter. One of the earliest recorded observations of Jupiter dates back to the fifth century, and can be found in an Indian astronomical text called the Surya Siddhanta. The text attempts to calculate the diameter of the planet. Then, in the eighth century there is another attempt to estimate the diameter of the planet by an Islamic astronomer. NOVA | Galileo: Discovering Jupiter'... Log in to Teachers' Domain to download, share, rate, save, and match to state standards.

Source: NOVA: "Galileo's Battle for the Heavens" This resource was adapted from NOVA: "Galileo's Battle for the Heavens." Background In 1610, when Galileo first pointed his newly built telescope toward the sky, no one had ever had the capability to see distant objects in any detail, and he had no idea what to expect. Focusing on the Moon first, he discovered that its surface was not smooth as people had previously thought, but covered with craters and mountains not unlike those on Earth. Next, Galileo turned his attention to another bright object in the night sky — the planet Jupiter. Today, we know quite a bit about Jupiter. With a diameter about 11 times greater than Earth's, Jupiter is the largest planet around our Sun. It is so enormous that its mass is more than twice the combined mass of all the other planets in our solar system. Composed mostly of gaseous hydrogen and helium, Jupiter is a gas giant. It has a trademark banded look that results from its turbulent atmosphere, and its Great Red Spot is a giant storm that has been raging for centuries.

Jupiter also has a faint ring system and, as of May 2005, 63 known moons surround it. In a very short time, Galileo discovered Jupiter's four largest moons — Io, Europa, Ganymede, and Callisto, now called the Galilean moons. Each moon has unique characteristics that make it stand out in the solar system. For example, Io is the most volcanically active object in the solar system. Europa may have an ocean of liquid water beneath its icy surface and may be the best candidate for another habitable world. Callisto is one of the most heavily cratered bodies in the solar system, and Ganymede, which is even larger than the planet Mercury, is the largest moon. The four Galilean moons were the first celestial bodies to be discovered with a telescope. Although Galileo at first thought they were fixed stars, his continued observations showed that their positions changed on a nightly basis. Galileo soon recognized that these new objects were satellites of Jupiter. The discovery that celestial objects could revolve around something other than Earth provided strong evidence in support of the heliocentric theory. To learn more about early telescopes, check out The Beginnings of the Telescope and Galileo's Telescope .

To learn more about how Galileo's observations changed astronomy, check out Galileo: Sun-Centered System . To learn more about another of Galileo's astronomical discoveries, check out Galileo: Sunspots . Questions for Discussion How did Galileo surmise that Jupiter had moons that revolved around it? How did the discovery of Jupiter's moons support the heliocentric theory? What was so shocking about this discovery in Galileo's time? Why did he rush to publish his discovery? Explain the importance of Galileo's work on the telescope and its effect on the world at that time and today. Discuss the role of technology in science.

Question: Galileo discovered the four largest moons of which planet in 1610?
Answer: Jupiter