Space Part 1: Our Solar System: Uranus

Uranus:

Uranus is the 7th planet in our solar system.  It lies about 1.79 billion miles from the Sun, or about 215,000 planet Earths away (pretty hard to imagine that but it's true!).  Uranus has a diameter of 31,763 miles, making it about 4 times wider than Earth.  Uranus is labeled as a gas giant planet, similar to Jupiter and Saturn, however with some very distinct differences.

Uranus has a diameter that is almost exactly four times that of Earth's.

Uranus, and its nearly twin planet Neptune, are often grouped into a relatively new type of planet called ice giants.  This new distinction between normal gas giants like Jupiter and Saturn (both of which are comprised of mostly helium and hydrogen) has to do with their internal structure.

Uranus, unlike Jupiter and Saturn, has an internal structure composed of three main layers.  The inner most layer is composed of similar elements that make up the Earth (mostly silicate rocks and iron).  The middle layer or mantle is composed of an amazing combination of water, ammonia, and methane ices.  This layer is what gives Uranus and Neptune their new classifications.  Because of the extremely cold temperatures when you get really far away from the sun, coupled with the immense pressures inside of Uranus, you get crazy compounds that scientists really have no reference for on Earth.  They are described as ices by Nasa scientists however their actual structure is a bit of a mystery.  Some scientists have gone so far as to call the mantle of Uranus an ocean, however much more research is needed in order to prove such hypothesis.  The outer most layer is composed of helium and hydrogen, just like Jupiter and Saturn, however the abundant presence of methane is what differs Uranus and Neptune from the other gas giants.

Unfortunately for me and you, not all that much is known about Uranus as only one mission has ever gone to the planet, Voyager 2, way back in 1986.  This mission was also only a flyby, but it did give us the first detailed images of Uranus and enlightened the world to the coolest thing about Uranus.

Uranus with its rings and a rare surface cloud

Notice anything about the above picture? Well, despite what you may have guessed, it isn't the rings themselves, but their orientation.  Uranus is thus far the only planet discover that rotates almost completely on its side.  It also rotates the opposite way, just like Venus.  Scientists have been completely baffled by this for years, however the only real explanation is that at some point early on in the planet's history, an Earth sized object must have smashed into it, distorting Uranus' orientation and rotation.  This massive impact is believed to have taken place during the same chaotic period in our early solar system that scientists believe formed our moon (although this will probably never be confirmed).  The simple fact of the matter is that Uranus is mostly a mystery, due to its awesome distance from the Sun, making it wildly expensive to send things out to take a look.  Hopefully one day something like Cassini (the satellite currently in orbit around Saturn) will grace the Uranus system.

In case you were wondering, Uranus has 27 moons, all of which are named after characters from the writings of Shakespeare and Alexander Pope.  None of the moons are very significant and not very much is known about them.  The largest of them, Titania, has a diameter of only 981 miles.

The above image shows Uranus' biggest moons in relation to the size of their host planet.  From left to right: Puck, Miranda, Ariel, Umbriel, Titania, and Oberon.

I really wish Uranus was more interesting but until more research is done, there isn't much else we can know.

Space Part 1: Our Solar System: The Moons of Saturn

Its been a busy August and September for me, so that's the reason I'm giving for not posting as often.

The Moons of Saturn:

Much like the Jovian system, the Saturnian system is riddled with moons of all shapes and sizes.  There are currently 62 recognized moons of Saturn, meaning that although there are technically billions of objects orbiting Saturn (the rings), only a few of them are large enough to be considered moons.  Of these 62, only 53 have been named and only 13 of them have a diameter of larger than 50 (just over 31 miles) kilometers, so most of the objects orbiting Saturn are pretty puny and uninteresting.  For the purposes of this blog, I am going to focus on 7 of those moons that are larger than 50 km, as they are by far the most interesting and the most explored.  For your reading pleasure, here are bios of those 7 moons:

Titan - With a diameter of 3,200 miles, Titan is 200 miles wider than the planet Mercury, making it the second largest moon in the solar system.

The two largest moons in our Solar System pictured above next to Mercury.

Titan, apart from its size, is best known for being the only moon in our solar system with a thick, planet-like atmosphere.  Before the Voyager missions, very little was known about the solar system, let alone Titan.  However when Voyager first flew by, it captured this image below.

Titan!

This image was the first ever image of Titan taken by Voyager and for that matter was the first ever close up image of Titan ever taken.  It was also the photograph to prove that the yellowish color of Titan was in fact, an atmosphere.  This atmosphere is what makes Titan so interesting and is the reason why Titan is so far the most explored Moon in our Solar system besides our own.

First image of Titan taken by the Hubble Space Telescope (notice the thick atmosphere).

It wasn't until the Hubble Space telescope came around in 1990 that we finally got clear images of Titan and even then (as you can see) we were still unable to penetrate below Titan's thick atmosphere.  Because of this lack of serious data, NASA and other space agencies collaborated to create what would be known as the Cassini-Huygens probe.  This mission departed from Earth in 1997 and didn't arrive in the Saturnian system until 2005.  However, when it did finally arrive, it gave us everything we asked for and more.  The mission itself had two parts: The Cassini portion of the mission (which to this day is ongoing) was designed to get in orbit around Saturn and gather data from orbit.  The Huygens portion of the mission was a probe that, upon arrival in the Saturnian system, would detach and descend into the atmosphere of Titan and land on the surface.  While this mission was not designed to be a lengthy one like the Curiosity Rover on Mars, it was designed to send data and hopefully some images back to Earth.

 The first image of a body in the outer Solar System taken from the surface.  A serious scientific milestone.

Sure enough, after touching down successfully on Titan, the Huygens probe, during its 90 minutes of functionality, sent back the above image.  While this is not the exact color of Titan's surface, it's about as close as we can get without actually being there ourselves.  This was an amazing moment for NASA and honestly for the entire world; the first image taken from the surface of a body in the outer solar system, truly incredible.  Cassini however, would be the main focus of the mission and to date has provided us with amazing images and data about Titan.

One of the first images of Titan's surface taken by Cassini.  Cassini was the first time we had the opportunity to image the actual surface of Titan from orbit.

We now know intimate details about Titan, especially the surface.  Titan's surface has an average temperature of about -290 degrees F, so cold in fact, that methane is a liquid.  As a matter of fact, liquid methane shapes Titan's surface in amazing ways.  Not only are there clouds of methane and methane rain, there are also entire lakes of liquid methane dotting the surface of Titan (in case you were wondering the black splotch on the image above is not a giant methane lake but more likely a desert like region).

The blue regions (not true color) have been darkened for emphasis.  Anything blue is a concentration of liquid methane.

Here however, are images taken by Cassini of the liquid methane lakes on Titan.  These lakes tend to be concentrated near the poles of Titan, however there are deposits of methane on the entire surface.  There are numerous geologic features that on Earth would be shaped by water but on Titan are shaped by methane.  These lakes are being closely studied by NASA and the Cassini team because they all believe that Titan is an excellent example of what planet Earth looked like about 4 billion years ago.  Titan and Earth has similar chemical and physical make-ups and both have a thick atmosphere (Titan's is twice as thick as Earth's).  Scientists believe that if we can understand what it is that makes Titan tick, we might be able to understand how our Earth transformed into the life-rich planet it has become today.


Obviously, Titan still has a wealth of information that it has yet to reveal to us, but NASA's Cassini mission (which orbits the Saturnian system) has been extended until 2017, at which time it will fall into Saturn and eventually be crushed by the massive pressure.  Until that time though, let us hope that we continue to get amazing images of the Saturnian system just like the one below.

Titan imaged in three wavelengths by Cassini (natural color, near infrared, and true infrared)

Despite the fact that we know more about Titan than just about any other Moon in the Solar System, there are still 6 more moons I want to discuss.

Rhea -

Rhea taken by Cassini

Rhea is the second largest moon of Saturn and the ninth largest moon in the solar system, however with a diameter of only 949 miles, it is just over 1/3 the size of Titan.  Rhea is just about the most boring moon/body in the Solar System.  With no geologic activity, no atmosphere, no water of any kind, and no truly unique geologic features, Rhea is just another giant rock floating in space.  NASA has no plans to visit it and it one of the least imaged moons of the Cassini mission.  The only thing to look forward to on Rhea would be the sensation of being on a body that doesn't rotate.  Rhea is tidally locked to Saturn and thus always has the same side facing Saturn and the same side facing away.  How awesome it would be to freeze to death on the dark side before coming over to the light side and...freezing to death.

In case you were wondering, here is the size comparison for Earth, our Moon, and Rhea.

On to the next moon!

Iapetus -

Iapetus as imaged by Cassini

Iapetus is the third largest moon of Saturn.  With a diameter of 912 miles, it is only slightly smaller than Rhea.  Iapetus, unlike Rhea, actually has some very interesting features that make it far superior to Rhea; kind of like a twin that got more nutrients in the womb.

To start, Iapetus is composed of nearly 3/4 water ice, giving it a density barely thicker than liquid water.  Despite being nearly the same size as Rhea, it is only about half as massive.  Because of Iapetus' composition, its surface is quite dynamic and has one of the most unique features in the solar system.  One glance at the photo above, shows that some parts of the surface are black, reflecting almost 0 light. While other parts, because they are water ice, reflect an enormous amount of light out into space.  In the image above, you can also see the largest crater on Iapetus, which is just under 400 miles across.  However, the most insane feature of Iapetus, one that completely blows my mind, is shown on the image below.

Notice the ridge that runs along the equator and how some parts of the surface are much brighter than others.

Iapetus has an equatorial ridge running over halfway around it.  Scientists have a few theories as to how it got there, maybe a collapsed ring system or maybe Iapetus used to spin faster, and there is little prospect for ever discovering how it happened.  Needless to say, the mountains on the ridge are almost all over 6 miles high and they really make me wish the Earth's moon had one.  These features on Iapetus make it so much cooler than Rhea could ever be.

Dione (pronounced die-oh-nee) -

Dione as imaged by Cassini

Dione is Saturn's fourth largest Moon and has a diameter of 698 miles.  This moon marks the beginning of the extreme drop off in size for Saturn's moons as Dione is a full 250 miles narrower than Rhea.  Dione, much like Iapetus, is majorly composed of water ice, which provides us with some very interesting geologic features.  Notice in the image above, the white wispy formations that seem to streak across Dione's surface.  These white streaks are collapsing ice and rock as Saturn exudes its enormous gravitational forces onto Dione.  Besides these streaks on the surface, Dione is pretty dull and doesn't have all that much going on personally.  However because Dione is close to Enceladus and Saturn's rings, it is pretty much constantly bombarded by ice particles which, due to the extreme cold of the Saturnian system (-290 degree F), are harder than rock.  So, next time you travel to Dione, be sure to bring some form of protection from the hail storms of death.

Dione orbiting close to the rings of Saturn

Tethys -

Tethys as imaged by Cassini

Tethys has a diameter of 660 miles, making it Saturn's fifth largest moon.  Tethys is one of the most interesting moons in the Saturnian system because of its incredible geologic features.  To start, Tethys is less dense than water, therefore scientists believe Tethys to be composed almost entirely of water ice with trace amounts of silicate rock.  Tethys' composition has helped to create two of the largest geologic features in the solar system.  The first of these features is known as the Odysseus Crater and can be seen in the image above.  The Odysseus Crater is over 250 miles across, making it one of the largest in the solar system.  The asteroid that made this crater, under normal circumstances, would have destroyed Tethys.

Here is an image of Tethys.  On the left is the true color and on the right is an infrared image.  Notice just how large the Odysseus Crater is (it is nearly 2/5 the diameter of Tethys).

However, scientists believe that early on in its life, Tethys was a bit warmer and thus squishier (due to all of the liquid water) and was able to absorb the bulk of the impact.  This lucky break may have also contributed to the second geologic feature of note on Tethys, the Ithaca Chasma.

The lines you see are just part of the Chasma.

The Ithaca Chasma extends for 1200 miles across the surface of Tethys, nearly 3/4 of the circumference of Tethys, and is about 60 miles wide and 3 miles deep.  Scientists have two possible theories as to the origins of this chasma.  They believe that, as it is mostly on the opposite side of the Odysseus Crater, the impact formed from said asteroid might have formed the chasma.  The other theory is that, as Tethys was much softer and had a lot more fluid in its early life, that as it froze and expanded, it literally tore itself apart.  Both of these theories are valid explanation and we may in fact never know exactly what caused the Ithaca Chasma to form.  Tethys, a tiny moon around Saturn, has some of the largest geolgic features in the solar system, very cool.

Enceladus -

Enceladus as imaged by Cassini (blue color is only to show certain geologic features.  not true color)

Enceladus is one of the most exciting moons in the solar system.  With a diameter of about 310 miles it isn't even a tenth the diameter of Titan.  However, scientists believe that underneath the surface of Enceladus lies an ocean of liquid water.  Several questions come up when presented with this fact, such as, how can there be water when its' so cold? Or, how can we look below the surface and tell that there is water?  All of these questions, while difficult, have relatively simple answers.

Jets of water shooting out of Enceladus.

As you can see from the above image, water has been imaged shooting out of Enceladus into space.  Enceladus is one of only three bodies in the solar system where any kind of volcano or geyser has been witnessed erupting.  Water is able to remain a liquid below the surface because Enceladus is very close to Saturn, about 110,000 miles away.  While this might seem very far away, dont forget that our moon is 236,000 miles from Earth, so in fact, Enceladus is twice a close to its host planet as our moon is to us.  This proximity to Saturn not only heats up Enceladus but it also squishes it and squeezes it as it orbits, breaking up much of the sub-surface ice.  We can actually see this process going on beneath the surface because of the way the surface is cracked and jagged in parts.

Note the many cracks on the surface.  The colored portion is an infrared image taken by Cassini.  This proves that not far below the surface, there are warm spots where liquid water almost certainly exists.

The surface of Enceladus is one of the most reflective natural sources ever observed in the universe.  It reflects nearly 100% of the light that it absorbs from the Sun and other sources, pointing again to its water ice and liquid water composition.  This liquid water has NASA very interested in the possibility that life could exist below the surface of Enceladus.  In 2011, scientists from NASA said that besides the Earth, Enceladus is becoming the greatest candidate for extraterrestrial life in the Solar System.  As of right now, there are plans for a 2020 mission to Enceladus to see whether or not the Moon does in fact support life.  I am seriously giddy with excitement.

Mimas -

Mimas with Herschel crater visible.

Finally we come to the last moon in this blog, Saturn's moon Mimas.  Mimas has a diameter of 246 miles, making it quite a puny moon.  However, Mimas has two things about it that are absolutely awesome.  The first is that Mimas is the smallest body ever observed to have self-gravitationally become a sphere.  When bodies become massive enough, they gravitationally collapse on themselves, turning into sphere like shapes.  Mimas appears to be about as small as a body can get while still being able to collapse into a sphere.  This fact however doesn't even come close to being the best thing about Mimas.

Mimas and The Death Star

This, is the best thing about Mimas.  It looks uncannily like the Death Star from Star Wars.  The really cool thing is that when Star Wars first came out, we didn't have this detailed of an image of Mimas so there is no way this similarity is intentional.  Something about that makes me really happy, unless of course Mimas is in fact the death star.  In which case we should probably find a way to take down its shields and blow it up.

Thanks so much for reading this blog on the Moons of Saturn!  This is why I love space.

Space Part 1: Our Solar System: Saturn

Obligatory Lord of the Rings joke out of they.

Saturn:

Saturn is the second largest planet in our solar system with a diameter of about 74,500 miles it is nearly ten times greater in size than the Earth.  At about 891,000,000 miles (112,000 planet Earths) away from the Sun, it is the sixth planet in our Solar System.

As you can see, Earth is a lot smaller than Saturn.

 Saturn is similar to Jupiter in almost every way: both are made primarily of hydrogen and helium, both have the same internal structure (initial gaseous atmosphere, liquid metallic hydrogen interior layer, rocky/iron core), and both of them have a multitude of moons (next blog post will be on the moons of Saturn).  Saturn however has one feature that no other planet in the Solar System has, a beautiful, large, and complex system of rings.  Now, this is not to suggest that Saturn is the only planet with rings.  Jupiter, Neptune, and Uranus all boast fairly visible ring systems.  None of them however can compare to the sheer immensity and beauty of Saturn's rings.

Jupiter's Rings as imaged by an infrared camera.

As you can you, Jupiter has a well defined ring system.

Uranus' rings

As does Uranus...

Neptune's rings.  Picture taken by Voyager 2

And Neptune...

Here however, are the rings of Saturn

They really are quite incredible and dwarf the rings of every other planet in our Solar System.  Because they are so spectacular, they have been studied extensively by NASA and countless other scientists the world over.

Saturn's Rings, with enhanced color, as imaged by Voyager 2

This image above, shows the true complexity present within Saturn's rings.  As you can see, unlike the other planets, Saturn's rings have many gaps and unique features.  These gaps have caused scientists to label each ring with a letter.

The names are given based on when each ring was discovered, not how far or how close it is to Saturn.   Amazingly, these gaps, in some instances, formed naturally.  Some of the gaps however are the result of some of Saturn's moons.

Known as shepherd moons, Prometheus (top) and Pandora (bottom) literally shepherd this ring, keeping it in place.

As you can see from the above image, two of Saturn's moons (Prometheus and Pandora) are literally shepherding one of Saturn's rings, in the sense that they keep it in place and keep its size relatively constant.  Unusual features such as this are common in Saturn's ring system.

This image above shows another one of these features, the spokes that are present in Saturn's rings.  These black smudges that appear on the rings are actually caused by Saturn's magnetic field interacting with particles inside the rings.  They come and go with predictable regularity and were first discovered when Voyager first flew by Saturn.  Scientists are unsure as to why they darken the areas of the rings that they affect, but research is still being conducted by the Cassini spacecraft imaging team.

Of the billions of objects that make up Saturn's rings, the largest each individual object gets is about the size of large house, so in terms of objects in space, Saturn's rings are made up of very insignificant clumps of water ice.  Scientists have so far been unable to determine the origin of the rings, or how long they have been in existence, or how long they will be around.  This is because the number of collisions between objects is extremely hard to measure, and Saturn's rings are still being fed by outside forces.  More on this later, but Saturn's moons Enceladus is one of the main contributors of mass to the rings, perpetuating their life by countless numbers of years. One final note on the rings, which is demonstrated in the image below, is how skinny Saturn's rings are.  When you look at them directly from the side, they appear to disappear.  This is because the rings themselves are only a few hundred meters thick, minute in comparison to the amount of space the rings themselves take up.  Many people think of the rings as these large objects, and this is because most of the images of the rings are from above or below, so that the rings themselves can be visible.

Thus far, this blog has discussed the rings of Saturn, definitely Saturn's coolest feature.  This is because Saturn is pretty much a smaller clone of Jupiter.  However, there are a few things about Saturn that help to distinguish it from its much bigger brother, Jupiter.  The most obvious of these is Saturn's much less dynamic surface (when compared to Jupiter).  Now, if you read my last blog post, you would know that Jupiter's surface is covered in storms, jet streams, and other random cool features.  Saturn, for the most part, has a pretty static surface.  Jet streams are present, but there are far fewer distinguishable surface features.  However, when there are surface features, they tend to be large and powerful.

This image shows what is called a global storm.  This image was taken by Hubble and shows a storm that goes 2/3 of the way around the entire planet, most of which is hidden by the dark side.

This image shows a storm on Saturn and as you can see, it takes up a large amount of space.  Storms on Saturn have some of the fastest winds in the Solar System, some of them have been clocked at 1,100 miles per hour, nearly twice as fast as the fastest winds on Jupiter.  The simple explanation for this is that there are less things on the planet that cause friction and thus, less things to slow the winds down.  

Speaking of winds, one of the biggest unexplained mysteries of the Solar system is on a part of Saturn that you might not have ever thought about, its north pole.

Saturn's North Pole, image courtesy of Cassini

This is an image of Saturn's hexagonal cloud.  First discovered when Voyager flew by, scientists have no idea why a nearly perfect hexagon is floating on Saturn's north pole.  Even now,over thirty years since it was discovered, scientists have absolutely no idea how this cloud is formed.  It is a permanent fixture on Saturn's surface and the only explanation is that Saturn's magnetic field is interacting with something (possibly Jupiter's magnetic field) to create this amazing shape.  My personal favorite idea for why this exists is that aliens have a power station just underneath the cloud, creating it.  Whatever the explanation, it cannot be denied.  There is a hexagonal cloud on top of Saturn and scientists haven't got a clue as to why it is there (I think this is really cool).  

Saturn is many people's favorite planet, mainly because of its amazing rings.  Take away the rings though, and Saturn is a pretty boring gas giant planet.

Saturn looks kind of lame without the rings to be perfectly honest.

Now Saturn's moons on the the other hand are unbelievably awesome and I will discuss them in great detail in my next blog post.  Thanks so much for reading!!!

Space Part 1: Our Solar System: The Moons of Jupiter

Sorry again for the long breaks in between posts.  I just got a job not to long ago so it has been tough to keep up and that's the excuse I'm using so deal.


Originally I thought I would just mention the moons of Jupiter in my last post.  However after much research, I have decided they are worthy of their own post (as the moons of Saturn probably will be as well).


The Moons of Jupiter:


Currently, Jupiter has 66 known Moons, giving it more than any other planet in the solar system.  These Moons differ greatly in size, shape, mass, and in pretty much every other conceivable way.  This massive diversity within the satellites of Jupiter has caused many astronomers to dub Jupiter as a miniature solar system within our own solar system.


Here in the Western world, many of the "discoveries" of the planets that we know of are quite inaccurate.  Most of the planets had been observed long before many of the Europeans claimed to have discovered them.  However, one discovery that cannot be disputed is the discovery of Jupiter's four largest moons in 1610 by Galileo Galilea.  This is because Galileo himself made such amazing improvements to the telescopes available at the time that, for some time, Galileo was in possession of the most powerful telescopes on the planet.  These allowed him to not only discover the four largest Moons of Jupiter: Ganymede, Io, Callisto and Europa.  These four moons marked the first observation of another body orbiting something besides the Earth or the Sun.  This amazing discovery would eventually change the way people thought about the Solar system and the entirety of space.  He was able to discover this because when he first pointed his telescope at the sky, he only observed three bodies around Jupiter.  Every day, he marked where they appeared around the planet, until one day he observed a fourth body.  He continued to witness the bodies moving in and out of his view as they moved in front of and behind Jupiter.  Upon his discovery, he decided to name them after the lovers and daughters of the Roman God Jupiter.  The four large moons would become known as the Galilean Moons.

From left to right: Io, Europa, Ganymede and Callisto

Io, Europa,Ganymede, and Callisto are the four largest and most interesting Moons that orbit Jupiter.  They offer scientific discoveries abound and human beings plan to explore them much further in the near future.  For now, here are profiles of each of the Galilean Moons (going in descending order by size)

Ganymede - Ganymede is the largest Moon in the Solar system.  With a diameter of 3,273 miles it is about 200 miles wider than the Planet Mercury.  This behemoth of a moon is also the only moon known to create its own magnetic field, due to its size and the internal heat caused by convection with Ganymede's liquid iron core (very simply, hot matter rising up to the surface from within the planet and colder matter sinking gravitationally into the planet).  This is because Ganymede is also a fully differentiated body, meaning that there are distinct layers present within the entirety of the planet.  Much like here the Earth, where we have an outer crust and than layers of different substances culminating in an iron core, Ganymede follows this same pattern.  Of the Galilean Moons, Ganymede is the third out from Jupiter (665,000 miles or about 2.5 times the distance our Moon is from the Earth), taking about 7 days to make one complete orbit of Jupiter.  This Moon is pockmarked by impact craters and strange geological features.

This is an actual image of Ganymede.  While technically the color was computer generated, Scientists at NASA believe that this is how Ganymede would actually look.

Due to tidal forces imparted by Jupiter onto Ganymede, the surface of the moon has been continually changed throughout the course of its life.  Ganymede is composed of about half silicate rock and half water ice, meaning that although it is wider than the planet Mercury, it is only about half as massive.  To date, few spacecraft have explored Ganymede in detail, but all of that will change because of the recent announcement of the JUICE spacecraft.  The JUICE (Jupiter Icy Moon Explorer) is a product of the ESA (European Space Agency) and is scheduled to arrive in the Jovian system by 2020.  This spacecraft will give us detailed images of the four Galilean Moons before entering into orbit around Ganymede until it eventually loses power and dies.  JUICE will give us detailed information on pretty much anything you might want to know about the Galilean moons and will provide much more detailed images of the Galilean Moons as well as more detailed scans of Jupiter itself.  When JUICE enters orbit around Ganymede, it will radar map over 99.5% of the surface, giving us one of the most complete maps of the surface of a Moon which is not our own.

Callisto - With a diameter of 2,985 miles, Callisto is the third largest Moon in the Solar System, second largest moon of Jupiter, and is only 47 miles skinnier than the Planet Mercury.  It is the furthest Galilean Moon from Jupiter (1.2 million miles away or about 5 times the distance our moon is from the Earth) and is the only one not tidally heated by Jupiter.  This means that the surface of Callisto isn't really affected by Jupiter's massive amount of energy.  This lack of a tidal forces affecting Callisto has caused its shape to be determined by impacts of objects from space.

Callisto

As you can see from this image, Callisto is covered heavily by craters and in fact, scientists believe it may be the most heavily cratered object in the Solar System.  Due in part to its size, and in part to its proximity to Jupiter.  Because of Jupiter's massive size, it attracts many free floating objects from space.  As they near Jupiter, they often slam into Callisto because, being the furthest away from Jupiter, it is often the first thing encountered by asteroids.  There are so many craters on Callisto, that no matter where a new impact takes place, it can only cover up an already existing crater(s).  Callisto has no past evidence of volcanism or of any other serious geological events.  When scientists first sent the Voyager Spacecrafts into space, they thought that all of Jupiter's Moons would be like Callisto, dead, cold, and covered in a massive amount of craters.  When they arrived, they were surprised at the sheer diversity within the Jovian system.  Callisto, of the Galilean Moons, was the only one that lived up to the expectations.  Callisto, like Ganymede, is composed mostly of silicate rock and ice and while it might be almost the same size as Mercury, it does not even have half the mass.  Interestingly, like many of the rocky bodies in the solar system, scientists believe an ocean of liquid water might exist about 100 km below the surface of Callisto.  While the motto for finding life in space has been find the water, scientists believe that due to the lack of tidal forces from Jupiter and a lack of its own internal heat source, Callisto probably is not as good of a candidate for life as say Europa (Fourth Galilean Moon) or Enceladus (moon of Saturn).

Io - With a diameter of 2,263 miles, Io is the third largest Moon of Jupiter and the fourth largest moon in our solar system.  Io is one of the most dynamic bodies in the solar system.  Not only is Io the most volcanically active body in our solar system, but the tidal forces exerted upon it by Jupiter cause it to stretch and squeeze, constantly changing the entirety of Io's being.

Here is an actual image of Io

As you can see from the picture above, Io appears to be yellow and has a very pockmarked surface.  The yellow color you see is caused by the massive amounts of sulfur that is spewed onto the surface by the many active volcanoes on Io.  Because of the frigid temperatures away from the volcanos, about -300 degrees Fahrenheit, sulfur literally freezes into crystals, sticking to the surface of Io, making it yellow.  The sheer amount of sulfur is due to the 400+ active volcanos present on Io's surface.  To compare, each year the Earth only has about 50-70 volcanoes that could be classified as active (going to erupt within the year).

This image shows new lava flows on the surface of Io.  The two images are of the same place, except that the image on the right was taken two weeks after the image on the left.  This image and the image above were taken by the Galileo spacecraft.  Temperatures near the lava flows can reach 1200 degrees Farenheit, despite the frigid temperatures.

Not only are there more volcanoes on Io than on the Earth, but the eruptions are more frequent and more powerful.  This is due to two main factors: 1. Io has a lot less gravitational force, so lava can be shot about 190 miles into the air, meaning that it affects a larger area; 2. Io is tidally affected by Jupiter's gravitational power. Here on Earth, volcanoes erupt at points along fault lines.  These fault lines are the dividing lines of tectonic plates (pieces of the Earth's crust that are literally bouncing off of each other).  These tectonic plates are able to move so much because of the internal heat generated by the Earth.  On Io however, there is no natural internal heat source.  As I mentioned earlier, the stretching and squeezing of Io is caused by Io orbiting around Jupiter.  These forces are so strong that in places they cause land to shift by over 300 feet at times (For a quick comparison, the largest tide on the Earth is 60 feet, and that is water moving, not land).  This amazing force is what generates the internal heat of Io and as a result, generates the over 400 active volcanoes on its surface.  As if all of this wasn't enough, the Moons Ganymede and Europa add even more power to the shifting landscape of Io; just like Jupiter but on a much smaller scale.  As of now, scientists have been unable to accurately measure the age of Io.  This is because, unlike the other Jovian moons and in fact unlike many other moons in our solar system, Io is not pockmarked by impact craters.  This is because the constant resurfacing of Io by fresh lava flows causes any fresh impact crater to be covered up and lost forever, making Io practically impossible to date accurately.

An image of Io taken as Voyager flew by.  Notice, just above the top of the dark side of the moon, the large explosion of material caused by volcanos on the surface.  There is also another one erupting on the bottom of the dark side of the moon.

Io receives these massive gravitational forces from Jupiter because it is very close to the planet (262,000 miles away, only about 26,000 miles further away than our Moon is from the Earth).  What is amazing, is that because Io is so close and thus so affected by Jupiter, it produces about 400,000 volts of electricity, all of which interacts with the atmosphere and magnetosphere of Jupiter, creating permanent electrical storms in the space between Jupiter and Io.  Io truly is an amazing moon, featuring an amazing amount of unique features.


Europa - Europa has a diameter of about 1,950 miles, making it only 100 miles skinnier than our own moon.  Orbiting about 417,000 (about 1.75 times the distance of our Moon from the Earth) miles away from Jupiter, it is the second closest Galilean moon to Jupiter.  Europa is my personal favorite moon within the Jovian system due almost entirely by the fact that Europa is one of the best candidates for harboring life in the solar system (more on that later).

An image of Europa, taken by the Galileo spacecraft.

As you can see from this picture, Europa is covered in what appears to be deep gashes across the surface.  While not immediately apparent, the surface of Europa is covered entirely by water ice, making it one of the smoothest surfaces of any object in the solar system.  The covering of water ice makes Europa's surface a very dynamic, much like the ice caps here on Earth.  Europa is also almost entirely devoid of impact craters, making it difficult to determine its age.  As you might have guessed, the water ice surface indicates that liquid water could also be present on Europa and in fact, scientists have discovered that there is a heck of a lot.

An inner look at Europa, notice the massive amount of water present under the ice.

This picture is what scientists believe is below the initial layer of ice that covers Europa, an almost continuous ocean of liquid water.  In fact, as the picture below demonstrates, there is more liquid water on Europa than there is on Earth.

Looking at Earth with no water on it is weird.  However, if you couldn't already tell, each blue ball represents the amount of water on each body.  Notice how Europa (on the left) has considerably more water than the Earth.

Amazing yes? I think so.


How do scientists know this water is there? Well, if one glances at the surface of Europa, the similarities between ice floating on top of water on Earth and ice floating on top of water on Europa are quite striking.

Known as chaos features because of their unpredictable and chaotic nature, the ice on Europa's surface is cracked.  This happens because the ice on the surface floats on top of the water below, causing the surface ice to buckle and crash into each other.  The ice on Earth creates very similar features and moves in very similar ways, which in turn makes scientists very excited about the possibility of life on Europa.  Even though water is often thought of as an indicator of the possibility of life, scientists also believe that heat is necessary for life to come into being.  Normally, being so far from the Sun, one would not expect a body such as Europa to have adequate enough heat for life to form.  However scientists believe that the tidal forces imparted by Jupiter onto Europa, another culprit in the cracked surface, could be enough to sustain life.  Because there is internal heat present within Europa, scientists believe a scenario similar to the life at the bottom of our ocean could exist.

A hydrothermal vent colony on the bottom of the Atlantic Ocean.  These red tube worms thrive here at the bottom of the ocean despite being subjected to extreme temperatures, pressure, complete lack of sunlight, proximity to poisonous substances, and general lack of resources.  Scientists believe life might be able to thrive on Europa just like life does on Earth in this way.

The complexity of life found on Earth probably does not exists on Europa, however scientists are hopeful that bacterial life may thrive under the ice on Europa.  These colonies of life might live close to hydrothermal vents present under the ocean, much like the ones on Earth pictured above.  While all of this is purely hypothetical, scientists believe it is on of the best bets at finding life within our own solar system that doesn't exist on Earth.  This possibility has caused scientists on Earth, specifically in Europe, to develop the previously mentioned JUICE spacecraft.  JUICE will hopefully paint a much greater picture of all of the Galilean Moons, specifically, whether or not the moons have life.  In addition to JUICE, NASA is very interested in the possibility of life on Europa.  While the mission is currently on hold due to budgetary constraints, the plans for mission are truly epic.

Whoa!

This little guy is the prototype for the part of the JUICE spacecraft that will hopefully be able to travel in the ocean of Europa.  The plan is to melt down through the ice and deposit this guy in the water where it will have the ability to swim around, take video and pictures, analyze the water, and hopefully determine whether or not life does in fact exist on Europa.  Amazingly, the computer in charge of this thing will not be controlled from Earth at all.  It is programmed to search the water on its own, scan the ocean and rocky bottom of the moon for life, and to make all its own decisions regarding where to go and what actions to perform.  NASA scientists are calling this the most ambitious mission they have ever even thought of, let alone attempted.


The Galilean Moons are incredibly diverse, a scientific goldmine if you will.  However, Jupiter does have 62 other moons that I have yet to mention.  

Pie chart of the total mass of all of the Jovian Moons.  Can't see the color for "all other moons"?  That's because the mass of all the 62 other moons is just .0003% of the total mass orbiting Jupiter.

The other 62 moons of Jupiter are all irregular in shape, meaning that they do not possess enough mass to gravitationally collapse into a sphere.  The largest non-Galilean Moon is Amalthea, a body that is about 155 miles by 90 miles by 80 miles, much smaller than the smallest Galilean Moon Europa.  The smallest non-Galilean moons of Jupiter are even less than 1 km across.

Amalthea, the fifth largest moon of Jupiter as seen from the Galileo spacecraft.

I will now list in order from largest to smallest the rest of the moons of Jupiter.  All of them are relatively unknown to scientists and only a few of them actually have pictures that go beyond a dot of light in space.

Himalia, Thebe, Elara, Pasiphae, Metis, Carme, Sinope, Lysithea, Ananke, Adrastea, Leda, Callirrhoe, Themisto, Praxidiki, Megaclite, Kalyke, Taygete, Iocaste, Autonoe, Isonoe, Eukelade, S/2003 J 5, Chaldene, Aoede, Helike, Harpalyke, Thyone, Hermippe, Euanthe, Eurydome, Carpo, Arche, Erinome, Aitne, Hegemone, S/2003 J 23, S/2003 J 10, S/2003 J 18, S/2003 J 16, Orthosie, Thelxinoe, S/2003 J 3, Pasithee, Euporie, S/2003 J 15, S/2010 J 1, Kore, Cyllene, S/2003 J 4, S/2003 J 19, Kallichore, Kale, Herse, Mneme, S/2003 J 2, Sponde, S/2011 J 2, S/2003 J 9, S/2003 J 12, S/2011 J 1, and finally S/2010 J 2.  In case you were wondering about the numbered moons, they have not been named by their discoverers yet and are thus referred to by their catalog number, which works like this:
S/(year discovered)(around what planet)(what number object found in said year, if it says 2, it was the second object discovered in that year).

All of these moons are grouped into families, mainly by location, shape, and proximity to Jupiter.  These moons are also simply the Moons that have been discovered and classified as Moons.  There are hundreds of other even smaller objects that scientists are unable to classify and which may not even be gravitationally bound to Jupiter. There are also about 16 objects currently up for consideration as new Moons of Jupiter.  The Jovian system is a Solar System in miniature.  The incredible amount of diversity present in this system goes to show how much there is to know and how relatively little we know about our own Solar system.

Thanks for reading! Next up, Saturn!