Our Wacky Universe

Voyager Project, JPL, and NASA

It is an icy body, much like the objects in the Kuiper Belt and Oort Cloud. There is another Kuiper Belt objects that have a similar composition to Pluto, but it actually larger than it.

28 October 2014

The Great Dark Spots

Neptune's First Great Dark Spot
Image Credit:

The Great Dark Spot is actually a series of storms that were first discovered by Voyager 2 in 1989. They are anticyclonic (weather systems with high pressure eyes) much like the Great Red Spot on Jupiter. However, unlike the Great Red Spot, these storms are generally cloud-free and only last a few months to few years.

The winds associated with the Great Dark Spots are the fastest known in the solar system, topping out at 2400 kilometers per hour (about 1500 miles per hour). They are thought to be holes in the methane cloud deck, occuring in the troposphere at lower altitudes than the clouds. The first spot to be discovered varied in size and shape as it was viewed from Earth and there had been a plan to photograph the storm with Hubble space telescope in 1994. By the time, Hubble was in position to take an image, the storm had dissipated. However, the spots do reappear on Neptune, and a new storm appeared in the northern hemisphere of Neptune. Unlike the Great Red Spot which is a single storm, the Great Dark Spots is a series of storms with similar appearances and properties to each other.

One of the prevailing theories of what happens to the storms is that as the storms migrate towards the equator of Neptune, the storms break up and disappear. Also, clouds usually appear outside the storms so may indicate that a storm has just dissipated or may appear soon.

The storms themselves are relatively stable because they are vortexes, but again, don't last as long as the Great Red Spot.

27 October 2014

Neptune's Rings

Image of Neptune's Rings taken from Voyager 2 in 1989.

Three rings are easily seen in this image: Adams (outermost), Le Verrier (middle) and Galle (inner ring). To the left of the image, in the Adams Ring, Galatea is visible. Between Adams and Le Verrier, two faint rings can be made out: Arago and Lassell.

Image Credit:

NASA/JPL/Voyager

Neptune has five main rings: Galle, Le Verrier, Lassell, Arago, and Adams, named after five important astronomers in Neptune's history. Much like the rings of Uranus, the rings of Neptune are made up of dust particles, and kept in place by shepherd moons, which include Galatea (Adams Ring) and Despina (Le Verrier Ring). It is believed that Neptune has other moons which help keep the rings narrow and stable (so to speak), but they have yet to be discovered.

The Adams Ring is unique in that it contains arcs, which are caused by gravity from Galatea. The arcs were discovered when Neptune occulted a star and where the rings should have been, the star shone through the rings. Close ups by Voyager confirmed that the Adams Ring contained arcs.

Much like the rings around Jupiter, Saturn, and Uranus, the material in the rings of Neptune are not permanent. They are continuously replenished by collisions with the moons of Neptune and are kept in orbit by shepherd satellites. The rings themselves are mostly dust with some ice particles and are covered in organic material (carbon compounds) that make them dark.

Image Credit:

Sky and Telescope

23 October 2014

Triton and the Other Moons of Neptune

Neptune has two moons that can easily be seen from Earth with the aid of a telescope. One of those moons is one of the few moons in our Solar System with an atmosphere, Triton. The other moon Nereid is smaller, but still visible in a large enough telescope. Proteus is actually larger than Nereid, but not as easily seen as it does not reflect as much light.

Triton with Neptune in the Background

Image Credit:

Triton is the largest moon orbiting Neptune, though its size is only 78% that of the Moon. It is one of the few moons (and the largest of all solar system moons) to have a retrograde orbit around its planet. It is believed that some interaction early in the formation of the Neptunian system caused it to reverse its orbit around Neptune. It also may be a Kuiper Belt object, which we will discuss later. Despite having a retrograde orbit, however, it does have a very circular orbit. It is very cold and because it is cold, it has an atmosphere. Our Moon, despite being larger, does not have any atmosphere as it is much closer to the Sun than Neptune. The atmosphere is dominated by molecular nitrogen and methane, but is 10 million times less dense than the Earth's atmosphere. Because it is so cold, there are no clouds in Triton's atmosphere and is transparent, allowing us to see the surface. Triton has an icy surface of nitrogen ice, methane ice, carbon monoxide ice, and carbon dioxide ice. It was found by William Lassell in 1846, just after the discovery of Neptune itself.

Triton has few craters, which tells us that its surface is relatively young, though it does have dark spots on the surface. Those dark regions are from organic molecules, created when methane is exposed to light. And because of the methane on its surface, there are regions near its southern pole where methane geysers can erupt through cracks in the surface. Around these cracks, dark smudges are found which are created in the same way as the dark spots on Triton.

Fuzzy Image of Nereid seen from Voyager 2

Image Credit:

Nereid is unique as it has a very eccentric orbit, which may mean that it is a captured comet, based on its composition.. However, it does orbit prograde, and despite being smaller than Proteus, was actually the second moon discovered around Neptune in 1949 by Gerard Kuiper.

Proteus

Image Credit:

Proteus is the second largest moon but was not discovered until Voyager 2 spotted it in 1989, which is odd considering Nereid was discovered by an Earth-based telescope

22 October 2014

The Discovery of Neptune

Neptune was observed before its "official" discovery, starting with Galileo in the 1610s. However, for every observation of Neptune, the astronomer thought he was seeing a star, so would record it as such. After almost two centuries of observation, Neptune's position did not seem to fit what scientists felt should be a planetary orbit.

John Couch Adams

In 1643, Englishman John Couch Adams was a new astronomer who wanted to reconcile the observational data on Neptune with gravitational perturbations in the orbit of Uranus. Using the relatively new invention of calculus (Isaac Newton "discovered" calculus in the 1700s), Adams was able to show that there should be an object at the relative location of Neptune's orbit causing these variations of Uranus' orbit. Showing his calculations to the Astronomer Royal, George Airy, Adams felt that there could be a planet out there. George Airy did not take the calculations seriously, and so did not actively look for Neptune.

Urbain Le Verrier

However, at the same time Adams was performing his calculations, Frenchman Urbain Le Verrier also made the same calculations as Adams. Unlike Adams, his calculations were taken seriously, and Johann Galle at the Berlin Observatory used the predictions given him by Le Verrier to look for Neptune. On September 23, 1846, within half an hour of looking, Galle was the first to see Neptune and realize that it was a planet and not a star. The position of Neptune compared to Le Verrier's calculations was within 1° and 12° of Adams.

Johann Galle

When it was discovered by George Airy that a Frenchman had done the same calculation as Adams and a German had found Neptune, Airy wanted to make sure that an Englishman had made the same calculations. At first, Le Verrier thought that Adams had plagarised his work, but soon it was realized that Adams and Le Verrier had never interacted and that both work independently coming up with the same prediction. As a result, Adams and Le Verrier were considered co-discoverers of Neptune, until the 1990s, when the Royal Observatory decided that since Adams calculations were not used to find Neptune, he would no longer be considered a discoverer.

Neptune was named for the Roman god of the sea because of its blue appearance. It also stayed in line with the naming of the planets after Roman and Greek gods.

21 October 2014

Neptune, The Other Blue Planet

Neptune with a Great Dark Spot prominent in the center

Image Credit:

Earth is not the only blue planet in our Solar System. The planet farthest from the Sun (and Pluto is still not a planet, nor should it be), is also blue. However, Neptune's color does not come from water but from the atmosphere composed mainly of methane. Neptune is also the fourth largest planet in diameter (smaller than Uranus and larger than Earth), but the third largest in mass (between Saturn and Uranus). It orbits at a semi-major axis distance of 30 AU which gives it a orbital period of 165 Earth years. Based on its eccentricity of 0.0087, it is by far the planet with the most circular orbit.

Neptune is also known as the first object to be predicted to exist before it was discovered. In the 1840s, two different astronomers predicted another planet orbited outside the orbit of Uranus and those predictions were confirmed in 1846. Since its discovery, Neptune has only completed one full orbit, way back in 2011.

Neptune also is home to one of the larger moons in the Solar System, Triton. It also has 13 confirmed moons, and may have more awaiting discovery. And just like the other gas planets, Neptune does have a ring system.

As shown in the above image, Neptune has a series of prominent storms on its surface that have persisted since their discovery in 1989 by Voyager 2. Much like Jupiter's Great Red Spot, they are a fascinating set of storms that we are still learning about.

20 October 2014

The Unusual Inclination of Uranus

Remember way back in June when we talked about the strange inclination of Venus? Believe it or not, there is one planet that has a stranger inclination. Here, the Sun does not rise in the west and set in the east. But then, it doesn't really rise in the east or set in the west all year long. For a portion of its year, Uranus has a hemisphere completely illuminated and another hemisphere completely in the dark.

Uranus inclination is about 98°, which means that its rotational axis is nearly perpendicular to its orbital axis. Another way of saying this, is that Uranus rotates on its side. This means that for a portion of its year, the north pole of Uranus is almost pointed straight at the Sun and the northern hemisphere is almost completely sunlight while the south pole is pointed away from the Sun and therefore, the southern hemisphere is dark. Vice versa, halfway around its orbit, the south pole is pointed towards the Sun and the north pole away.

This phenomena happens on Earth, but not as extreme. There are two latitudes, one north of the equator and one south of the equator, where in the northern summer, at any location north of the northern latitude the Sun will never set and in the northern winter, the Sun will never rise. The same thing happens in the southern latitudes near Antarctica. These latitudes are called the Arctic Circle and the Antarctic Circle. If you have heard of "The Land of the Midnight Sun", this is in reference to the Arctic Circle. On Earth, these circles are at 66.5° North for the Arctic Circle and 66.5° South for the Antarctic Circle. If you recall from science class, that the Earth is tilted 23.5° with respect to the Earth's orbital plane, those two angles equal 90°. On Uranus, it is a little more complicated than that.

When the north pole of Uranus is pointed towards the Sun (or the northern "summer" on Uranus), any latitude north of 8°N will never see the Sun set and any latitude south of 8°S on Uranus will never see the Sun rise. Vice versa, in the southern "summer" on Uranus, any latitude south of 8°S will never see the Sun set and any latitude north of 8°N will never see the Sun rise.

Why does Uranus have such an extreme inclination? Much like Venus, it is believed that soon after formation, Uranus was hit by a large object that knocked it on its side, but because it has a much larger mass than Venus, it did not flip upside down, but rather just onto its side.

18 October 2014

Solar Eclipse of 23 October 2014

On October 23, 2014, there will be a partial solar eclipse visible to most of North America and extreme eastern Siberia.

from Time and Date

Event	UTC Time
First location to see partial eclipse begin	Oct 23 at 7:38 PM
Maximum Eclipse	Oct 23 at 9:45 PM
Last location to see partial Eclipse end	Oct 23 at 11:52 PM

Adjust time accordingly for your time zone.

Eastern Daylight Time = -4 hours
Central Daylight Time = -5 hours
Mountain Daylight Time = -6 hours
Pacific Daylight Time = - 7 hours

Here is a nice map from NASA.

17 October 2014

The Moons of Uranus

The Moons of Uranus from closest to farthest starting from the left

Image Credit:

Uranus has 27 known moons which are mostly icy bodies similar to comets. Most of them might be captured comets from the Kuiper Belt or the Oort Cloud. However, there are a few that are considered planetary mass bodies, which means they have enough mass to be differentiated and spherical in shape. Most likely, these five major moons were probably formed at the same time as Uranus.

Titania

Image Credit:

Titania has the largest diameter of all of the moons of Uranus, but it is still smaller than our Moon. Its diameter is 1600 km, compared to 3500 km for the Earth. Even though it has the largest diameter, it is not the most massive. That honor goes to Oberon which will be discussed later. Of the five major moons, it is the fourth out from Uranus and has an inclination of 0.34°, meaning it orbits along the equatorial plane of Uranus. William Herschel discovered Titania along with Oberon in January of 1787. Observations of Titania with telescopes and with the Voyager probes tell us a lot of Titania. We know that the moon has two main layers, and icy crust/mantle and a rocky core. The surface is heavily cratered, though not as much as Oberon which also tells us that it was more active in the past than Oberon. Titania was named after the Queen of the Fairies in William Shakespeare's A Midsummer's Night Dream.

Oberon

Image Credit:

Oberon has the largest mass, and is second to Titania in diameter. It is also the farthest of the major moons. Discovered at the same time as Titania, Oberon was named after the King of the Fairies in A Midsummer's Night Dream. It is the most heavily cratered moon orbiting Uranus and much like Titania, it has a icy crust/mantle with a rocky core. With an inclination of only 0.058°, it is almost completely aligned with Uranus' equator.

Umbriel

Image Credit:

Umbriel is the third largest in mass and diameter of the moons, but was not discovered until 1851 by William Lassell. Like Titania and Oberon, it is an icy and rocky body with an inclination of 0.128°. It is the second most cratered body but went under internal geologic processes in its past to give it the surface it has now. It was named after a character in Alexander Pope's The Rape of the Lock.

Ariel

Image Credit:

Ariel is the fourth largest moon and second closest. It was discovered with Umbriel by William Lassell. Again, it is an icy and rocky body that formed in the accretion disk around Uranus based on its inclination of 0.260°. It is also tidally locked in the same way the Moon is tidally locked to Earth. Its name comes from two sources: Pope's The Rape of the Lock and Shakespeare's The Tempest.

Miranda
Image Credit:
NOAA
The last and smallest major moon is Miranda, which is also the closest to Uranus. It is also tidally locked to Uranus and was the last one discovered. Gerard Kuiper (after whom the Kuiper Belt is named - we'll discuss the Kuiper Belt later) found Miranda in 1948. Voyager imaged Miranda as it passed by the Uranus system and discovered that it is the most geologically active because it is the closest and tidal forces keep the interior warm enough. It has the highest inclination at 4.232° and was named after a character in Shakespeare's The Tempest.

16 October 2014

The Rings of Uranus

Image Credit:

Image Credit:

Uranus has a faint ring system around the planet. William Herschel thought that he had discovered rings around the planet when he first spotted Uranus, but based on how faint they are, it seems unlikely that what he saw were the rings.

The rings are oriented parallel to the equator of Uranus and much like those of Jupiter and Saturn, are continually replenished by collisions by objects colliding with the moons of Uranus. They are very dark, though they contain icy particles because they are covered with dust from the moons and the lack of sunlight reaching the region around Uranus.

The rings themselves are very narrow, made up of meter-sized particles and smaller. The majority of the rings are no more than 10km wide, with the widest ring only 100km (less than 0.2% the diameter of Uranus). Much like Pandora and Prometheus keeping the F-Ring around Saturn narrow, the rings of Uranus are kept narrow because of shepherd satellites, like Cordelia and Ophelia maintain the ε-Ring (epsilon ring). The ε-Ring is actually very eccentric because of the highly eccentric orbits of Cordelia and Ophelia.

The ring around Uranus were first indirectly discovered in 1977 when Uranus occulted a star, i.e. passed in front of the star. Just before the disk of Uranus passed in front of the star, the star dimmed a little, and just after the Uranus passed in front, the star did not return to its original brightness. Based on the dimming, astronomers were able to conclude, correctly, that Uranus possibly had a ring system. It wasn't unitl Voyager 2 passed Uranus that we were able to image the rings directly, and show that Uranus did have rings.

15 October 2014

The Discovery of Uranus

Uranus is the first planet that was not known to ancient astronomers and was not discovered until more than 180 years after Galileo first turned his telescope to the sky and discovered the Galilean satellites. Uranus is bright enough to have been seen by Galileo with his telescope, but if he had seen it (and there is evidence that he might have), he would have only assumed it was a star as his telescope did not have enough resolution to see Uranus as a disk rather than a pinpoint of light. Also, that pinpoint of light would not have moved fast enough to distinguish itself as a planet or some other object rather than a star.

In 1690, John Flamsteed actually saw Uranus with his telescope. But much like Galileo, he did not see it moving fast enough that in the six times that he observed it, he thought it was a star in Taurus and even labeled it as 34 Tauri. The 34 means that to him, Uranus was the 34th star discovered in Taurus. Since he did not observe it long enough, he was not able to see it moving faster compared to the actual stars in Taurus.

Pierre Lemonnier also spotted Uranus between 1750 and 1769. However, he also was not able to distinguish it as a planet as he did not see it moving among the stars.

Finally, in 1781, William Herschel made careful observations of Uranus and was able to tell that it moved faster than the farther stars. Not only that, his telescope had good enough resolution to see Uranus was a fuzzy disk rather than a pinpoint like a star would appear in a telescope. By watching Uranus over time, he was able to tell whether or not it was a comet or a planet. A comet would have been moving much faster than a planet as a comet would start to come in towards the inner solar system, but Uranus did not. By following the trajectory of Uranus, he was able to determine how far away it was.

Herschel wanted to name the planet after the King George III of his adopted country, England. But the name was argued back and forth, British astronomers decided to name it Uranus, who was Zeus' grandfather in Greek mythology. It became the first planet, other than Earth, not be named after a Roman god.

14 October 2014

Uranus

Image Credit:

NASA/Voyager 2

No planet or celestial object causes as much snickering as the seventh planet from the Sun. Depending on whom you ask, it can be pronounced one of two ways. The first way is the one that causes the snickering, especially among high school and college students (I know, I taught college astronomy). The other way, the way that I prefer to use, does not make it sound so humorous. Of course, I am talking about the planet, Uranus.

I pronounce Uranus as if it sounds like "You're A Nus" or "You're a Nis". Trust me, it helps people from giggling when you say its name. The name Uranus itself comes from Greek mythology as Uranus was the father of the first Titans (including Cronus) and the grandfather of the Olympians, the Greek gods. It is the first planet (other than Earth) not named for a Roman god. If the tradition had held using Roman names, Uranus should have been called Caelus, the father of Saturn and in turn, the grandfather of Jupiter.

Uranus is also the first planet discovered with a telescope. Up to its discovery in 1781 by William Herschel, only the six planets were known (Mercury, Venus, Earth, Mars, Jupiter, and Saturn). Of course, before the heliocentric model of the Solar System, Earth was not considered a planet. There is evidence that Galileo saw Uranus, but mistook it for a star. Two other astronomers also observed Uranus, but did not identify it as a planet.

Like Jupiter and Saturn, Uranus is a Jovian planet, i.e. a gas giant planet. It does not have a solid surface, but contains hydrogen, helium, methane, ammonia, and water in the outer layers. Its "mantle" is a mixture of ice and rock and has a heavy element core. The density of Uranus is 1.29 g/cm³, making it the seventh densest planet, only ahead of Saturn.

Uranus is the third largest planet in terms of diameter, but the fourth largest in mass (Neptune is larger in mass, but has a smaller diameter). It orbits 19.18 AU from the Sun, taking just over 84 years to orbit the Sun. Since its discovery in 1781, it has only completed two orbits, its third orbit won't be complete until 2033.

Since it is a Jovian planet, it also exhibits two properties that Jupiter and Saturn display: a ring system and multiple moons. The rings are more similar to Jupiter's rings than Saturn's rings as they are very faint and were not confirmed until Voyager 2 imaged them directly. Uranus has 27 confirmed moons which are all icy bodies. Five are considered to have planetary mass which means that they are spherical. Titania and Oberon were actually first discovered by William Herschel in 1787, after his discovery of Uranus.

Image Credit:

W. M. Keck Observatory

Lastly, the most amazing thing about Uranus is its day. Recall that Venus has an inclination of almost 180°. Uranus' inclination is not as severe, but may considered stranger. Its inclination is 98° which means that its axis of rotation is almost parallel to Uranus' orbital plane.

13 October 2014

The Composition of Saturn

Like Jupiter, Saturn contains hydrogen, helium, ammonia, and methane. However, 88% of the mass of Saturn is composed of hydrogen with 11% helium, the two lightest elements on the periodic table.

Saturn has a diameter of 9.42 times that of Earth and a mass that is 95.15 times that of Earth. Despite being so much larger than Earth, these properties lead to a strange phenomena. Saturn's density is only 0.69 g/cm³. Water has a density of 1.0 g/cm³. Yes, Saturn has a lower density than water. What this means that if you could construct a large enough tank and filled it with water, Saturn would float in that tank. No other planet has a density of less than 1.0 g/cm³, though there are some satellites that have low densities.

Saturn is also banded, but not to the extent as Jupiter. Since it is farther away, the clouds are not as brightly illuminated as they are on Jupiter. Also, the ammonia ice crystals in the atmosphere of Saturn are above the cloud layers, preventing light from reaching the clouds, and help make the clouds darker than otherwise. Of course, Saturn's gorgeous rings make up for the blandness of its atmosphere.

Westinghouse Atom Smasher

Living in Pittsburgh, I live near some cool physics and astronomy landmarks. I mentioned before, I used to work at Allegheny Observatory in Riverview Park. As a graduate student in physics, I spent a lot of time at the Observatory. However, today, I am going to talk a little bit about the Westinghouse Atom Smasher, located in the eastern suburb of Forest Hills. I am lucky enough that I live near there and got a chance to visit.

The property where the Atom Smasher sits was purchased recently with plans to develop the land. However, the owner would like to be able to preserve the dome but cannot due to costs to move the dome from the site. The dome is in danger of being demolished and saved for scrap.

10 October 2014

The Hexagonal Storm on Saturn

Saturn's Hexago

Image Credit:

NASA/JPL-Caltech/Space Science Institute

North Polar Hexagon on Saturn

Image Credit:

One of the strangest features on Saturn is the hexagon-shaped storm in its northern hemisphere. The storm was first discovered by Voyager in 1981-1982 and has been persistent since. It is located at 78° N latitude, but does not change longitude over time, unlike other storms on Saturn. The sides of the storm are about 13,800 km, longer than the Earth's diameter of abotu 13,500 km.

The Cassini mission also has taken images of the storm since its arrival in 2006. And if the conditions are right, i.e. the storm is in daylight, fuzzy images of the storm can be seen from Earth-based telescopes, even by amateur astronomers.

Oxford University astronomers proposed a hypothesis for the formation of the storm. In the lab, regular shpaes were created in a circular tank of liquid that had different rotation rates at the center and at the edges. Squares, hexagons, and octagons were all created, with hexagons being the most common shape. These latitudinal gradients in the rotation are one probable cause for the hexagon on Saturn.

09 October 2014

Orion's First Flight

NASA is allowing people to virtually board the Orion Test Flight in December. All you have to do is sign up here:

Orion First Flight Boarding Pass

08 October 2014

Other Saturn Moons

Moons of Saturn 2007

From top left to right: Mimas, Enceladus, Tethys, Dione, Rhea, Titan in the background, Hyperion in bottom right, Iapetus, and Phoebe

NASA/JPL/Space Science

Wikipedia

Saturn has more moons than any other planet, including Jupiter. Besides Titan, it has many satellites that are strange in there own way.

The second largest moon of Saturn is Rhea, which is less than half the size of the Earth's Moon. It was discovered by Giovanni Cassini in 1672. Of all bodies in the Solar System in hydrostatic equilibrium, it is the smallest, i.e. gravity and fluid pressure are in balance. It is mainly ice, and much like Titan and the Moon, it is tidally locked to Saturn. It is also the largest moon without an atmosphere.

Rhea

Image Credit:

The third largest moon of Saturn is Iapetus. It was also discovered by Giovanni Cassini in 1671 and is the largest body in the Solar System not in hydrostatic equilibrium. It is two-toned with the leading face of the moon dark with a low albedo and the back end brighter, like the color of dingy snow. It almost resembles the yin-yang symbol. It is believed that the dark face comes from Iapetus colliding with dark carbon-rich or silicate-rich materials. It is also tidally locked with Saturn.

Iapetus

Image Credit:

Dione is the fourth largest moon of Saturn, discovered by Cassini in 1684. Like Rhea and Iapetus (and all the moons in this post), it is an icy body. Again, it is tidally locked to Saturn and shares an orbital resonance with Enceladus of 1:2. It also has two co-orbiting satellites, Heleneand Polydeuces, located at the L4 and L5 Lagrange points for Dione. Though it is mainly water ice, its density leads us to believe that it has a rocky interior.

Dione

Image Credit:

Tethys is the fifth largest moon of Saturn, and was discovered by Cassini in 1684. It is spherical and has a density very nearly that of water. We also know that Tethys is not very active as it is heavily crated and contains a long, deep ice crack about 500 km long and 3 km deep.

Tethys with Odysseus crater

Image Credit:

Enceladus is the sixth largest moon of Saturn, but was not discovered until 1789 by William Herschel. As an icy body, it also has a very high albedo, making it very reflective. Not only is it an icy body, it is also an active body. The interior of Enceladus is kept hot by tidal forces between itself, Saturn, and Dione. Waper vapor geysers have been discovered erupting from the surface from the Cassini spacecraft. Some of the water vapor does fall back down as snow, but most escapes due to low gravity on Enceladus. Material in the E Ring is replenished by the erupting geysers. Under the surface ice, Enceladus is believed to have a liquid ocean which may contain organic molecules (molecules with carbon).

Enceladus

Image Credit:

Mimas is the seventh largest of Saturn's moon, and the one that many people might recognize. It was discovered by William Herschel in 1789 and much like the other moons listed here, it is a small icy body, with many craters. It has a deep 2 km crack that was probably created early in its history. Mimas is the smallest body in the Solar System that is spherical due to self-gravity. It has a 2:1 resonance with the Cassini Division, which helps keep it clear of particles. Mimas has one really distinguishing feature, and that is a large crater in the northern hemisphere.

Mimas with Herschel Crater in upper left

Image Credit:

Solar Views

Death Star from Star Wars. I wonder where George Lucas got his inspiration?

Hyperion is the eighth largest of the major moons of Saturn, and was discovered by William Lassell, William Cranch Bond, and George Phillips Bond, all in 1848. It is the largest of the irregularly shaped moons and as seen below, looks like a potato. Its shape is best determined by the diameters along its three axes, 410 km by 260 km by 220 km.

Irregularly shaped Hyperion

Image Credit:

07 October 2014

Titan

Cassini Image of Titan showing the atmosphere of the moon

Image Credit:

Titan is the largest Saturnian moon and the second largest in the Solar System behind Jupiter's moon Ganymede. Like Ganymede, it is also larger than Mercury. Christian Huygens, who discerned the rings of Saturn, also discovered Titan in 1655, making it the fifth satellite discovered with the telescope.

It is the only moon known to have a dense atmosphere, where atmospheric pressure is measurable on the surface. Though Europa, Ganymede, and Callisto may have liquid oceans below their outer crusts, Titan is the only body in the Solar System to have surface liquid besides the Earth. However, you wouldn't want to swim in those oceans as they are bodies of methane.

Because it has liquid methane oceans and lakes, much like Earth has a water cycle, Titan experiences a methane cycle. Surface methane evaporates and forms clouds in the Titan sky. The rain Titan experiences is methane.

Although Titan is smaller than Earth, its atmosphere is dense enough to create higher surface pressure than on Earth, at about 1.45 atmospheres (146.7 kPa). One atmosphere on Earth is the normal pressure at sea level which is 101.5 kPa (kilopascals). As shown in the above image, the atmosphere is extended with a composition of 98.4% Nitrogren (N₂), 1.4% methane (CH₄), and trace other molecules including water in the stratosphere (higher levels) and 95% N₂, 4.9% CH₄, and other molecules in the troposphere. Because of the methane clouds, the sky is very hazy on Titan so would have poor visibility on the surface when we make our first crewed mission to Titan sometime in the future.

The gravity on Titan is only 85% of that on the Moon even though it is larger because of its smaller density. If you were to stand on Titan, your weight would only be 15% of that on Earth.

Much like the Moon and Earth are tidally locked, Titan is tidally locked to Saturn, so for every orbit around Saturn, which is almost 16 Earth days, it only rotates once on its axis.

06 October 2014

F Ring and Shepherd Satellites

The F-Ring, Prometheus (inner moon), and Pandora (outer moon). The A-Ring fills up the bottom half of the image with the Keepler Gap easily visible

Image Credit:

NASA/JPL/Voyager

Saturn's F-Ring is the outermost of the discrete rings discovered in 1979 by Pioneer 11. Compared to the other rings, it is very active with features changing in the structure of the ring on a timescale of hours. The F-Ring is 3,000 km from the A-Ring and is separated from the A-Ring by the Roche Division. Compared to the other discrete rings, the F-Ring is very narrow, only a few hundred kilometers thick. So how exactly does the F-Ring maintain its shape?

The ring is between the orbits of two satellites, Prometheus and Pandora. Prometheus orbits just inside the inner edge of the F-Ring and Pandor just outside the outer edge. These two satellites are able to use their gravitational influence on the ring to keep it stationary and in place. If these moons were not there, the F-Ring would have dissipated long ago.

Prometheus also creates kinks and knots in the ring from its orbit which show up in the ring when Prometheus is at apoapis (farthest distance from Saturn). Because Prometheus does have an elliptical orbit, at each successive apoapis, the knots and kinks are 3.2° ahead of the previous section.

Prometheus creating knots and streamers in the inner F-Ring

Image Credit:

Cassini/ISS/JPL/EPA/NASA

03 October 2014

Saturn's Roche Division

Close up of the Roche Division. Visible from the bottom left to the upper right, the A Ring with the Encke Gap and the Keeler Gap, Atlas in the center, and the F Ring

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