Iapetus, the Dinosaur Moon

Iapetus is a moon of Saturn with a funny name and a funny geological feature. I always think of it as the "dinosaur moon", because it has a distinct ridge feature on its surface that reminds me of the back of a dinosaur. Orbiting at 2.2 million miles from Saturn's surface, it's farther away from Saturn than Titan is. The surface of Iapetus was imaged by the Cassini mission in 2004, and the images revealed the equatorial ridge, a 6 mile high mountain range. It's a bit unclear how this band of mountains ended up on Iapetus. One theory is that a long time ago, Iapetus had a ring similar to Saturn's ring. As the moon evolved, the ring began to collapse onto the surface, and this ridge is where all the material collected. A second idea is that the ridge formed during a time when Iapetus was spinning on its axis much faster than it does today. Bodies in space, such as the Sun and the Earth, spin on their axis. Because of this, they bulge just a little bit in the middle. So Earth and the Sun are not perfect spheres, but rather balls that are slightly wider at the center. If Iapetus was spinning really fast some time in the past, this ridge might be the result of the moon bulging in in the middle. Astronomers will have to take a closer look at the composition and orbital properties of this moon before they can determine exactly how the ridge formed.

Image Credit: NASA/JPL/Cassini

Tidal Locking

You've probably seen a full moon many times during your life, but have you ever noticed that it always looks exactly the same?  The Earth and the moon are tidally locked to each other, which means that the same side of the moon always face Earth. This can happen when you have a small body close to a large body, and gravitational interactions cause the small objects orbit and rotation to synchronize. Let me explain. Intuition tells most people that if you always see the same side of the moon, then the moon must not be rotating on it's axis. But this is not true! You can convince yourself of this by doing a little demo with your hands. Hold up your right hand and make a fist, then point the fingers of your left hand toward it. Now move your left hand around your right, such that the tips of your fingers always point to your right fist. You'll quickly find that you have to rotate your left hand to do that! The moon goes around the Earth once every ~28 days, and it rotates on its axis once every ~28 days as well. This causes the same side of the moon to always face Earth! Planets that are close to their host stars can be tidally locked in the same fashion, and so can two stars. Many moons in our solar system are known to be tidally locked to their host planet, and astronomers speculate that many known exoplanets are tidally locked to their host stars.

Acceleration Due To Gravity

We experience gravity everyday. It's the force that keeps our feet on the ground, makes apples fall from trees, and is occasionally cursed when you drop and break something. Newton may have "discovered" the theory of gravity, but it was Galileo (back in the 16th century) who claimed that all objects, regardless of their weight, shape, and size, fall at the same rate. In other words, the acceleration due to gravity is the same for all objects! Gravitational acceleration is governed by the size of the object causing the gravity. So here on Earth, gravitational acceleration (g) is 9.8m/s. So if I drop a hammer and a feather from the same height, they should reach the ground at the same time… right? Well scientifically, yes, but that often doesn't happen on Earth. Why? Well that's because we have a thick atmosphere and wind which causes air resistance. Lighter objects are more easily affected by wind, where as heavy objects are much less affected. So If do the hammer-feather experiment on Earth, the feather gets caught up in the wind and falls much slower that the hammer. But what happens in a place like the moon where there is  less gravity (1.8m/s) but no atmosphere and thus no air resistance. Do the hammer and the feather fall at the same rate? Let's find out…..

Video Credit:Apollo 15 Crew, NASA

GRAIL

Artists impression of GRAIL

Even though the space shuttle program has ceased, NASA is still alive and active with eyes towards the moon. On Saturday, September 10th around 9am EST, NASA launched the Gravity Recovery and Interior Laboratory (GRAIL) spacecraft from Cape Canaveral, FL. GRAIL is a set of two satellites that will orbit the moon and map its gravitational field. The moon has about 15% the amount of gravity we experience here on Earth, mainly because it is only about a quarter of the size of Earth. Understanding the moons gravitational field will help scientists understand the moons interior structure reconstruct its past. We will be able to answer questions like: Does the moon have a molten core? Why do the craters on the light side of the moon appear to be filled with solid magma, while those on the dark side do not? One interesting thing about the launch of GRAIL is its journey to the moon. A trip to the moon for the space shuttle took about 3 days, but GRAIL is going to take 3.5 months! The rocket was actually aimed in the direction of the sun, and with the help of small thrusters the trajectory will be corrected so that the satellites will reach the moon. This is called a low-energy trajectory and was done intentionally to save fuel, and to leave time for the satellites and equipment to adjust for space conditions. Think about it like this: if we hurtle towards the moon in three days, it will take a lot of fuel and energy to slow the satellites descent so they don't crash into the moon. But if instead we go the "long way around" and leisurely approach the moon, we can just float into orbit using very little fuel only to adjust trajectory. In these hard economic times, NASA is trying to save every penny they can!

Image credit: NASA/JPL/MIT

Apollo Moon Landings

I'm sure many of you remember that glorious day when man first stepped on the moon. The date was July 16, 1969, the mission: Apollo 11. Neil Armstrong, Buzz Aldrin, and Michael Collins left the Earth that day, and five days later made history by putting the mark of a human being on another astronomical body.  This is the day most people think of when someone mentions the moon landing, but we've actually sent man to the moon a total of 7 times. Missions Apollo 11 through Apollo 17 all rocketed towards the moon, and all but Apollo 13 landed successfully on the moon. During each mission astronauts set foot on the moon, and the Apollo 15 mission was the first to drive a vehicle on the moon, called the lunar rover (pictured above). After Apollo 17, manned missions to the moon were canceled due to lack of funding. NASA needed to make some budget cuts, and decided to put money towards Skylab instead. Man has not set foot on the moon since. Many orbiters and robotic landers have studied the moon since then, and the next mission, a set of orbiters called GRAIL, is set to launch from Cape Canaveral on September 8th, 2011.

The movie Apollo 18 comes out this Friday and it looks like it's going to be a great sci-fi thriller! It's based on the idea that NASA actually did send astronauts to the moon again as an Apollo 18 mission, but never told the world because of what they found…. aliens on the moon! Of course these creatures are hostile and live on the dark side of the moon, what sci-fi movie would be complete without killer aliens? I'm surprised it took 40 years to come out with a film like this, and I hope it's a good one!

Image Credit: NASA

Fourth Moon Discovered Around Pluto

NASA announced the discovery of a fourth moon around Pluto last week! Astronomers were using the Hubble Space Telescope to look for rings around the dwarf planet when they discovered the new moon. It's currently designated P4 until they come up with a better name for it. The Hubble image above shows Pluto and its four moons. The big black bar down the center is used to block most of the light Pluto reflects so faint objects around it, like the moons, can be seen. P4 is only 8-21 miles wide, whereas Charon is 648 miles wide and Nix and Hydra are more like 20-70 miles wide. Pluto is a member of the Kuiper belt, along with 70,000+ other objects, so it's not surprising that Pluto has another moon that went undetected for years. It's difficult to see such small objects in space, even with Hubble! NASA's new horizon mission is currently on its way to Pluto to study the planet and its moons. It will be another ~5 years before New Horizons makes it there, but once it has arrived it will no doubt send back unprecedented information about Pluto and other outer solar system objects

Image Credit: NASA, ESA, and M. Showalter (SETI institute)

Week of Moons: Triton

Today we have a guest post by Laura Arnold about Neptune's moon Triton!

The largest moon of Neptune, and the 7th largest moon in the solar system, has a peculiar orbit. It orbits opposite the rotation of Neptune. Moons are theorized to form in the same way solar systems are, from a rotating flattened disk of gas and dust. And the fact that all of the planets orbit the Sun in the same direction as the sun rotates supports this theory. Scientists think that Triton's backward orbit indicates that it once orbited the Sun and was captured by Neptune when they came close to each other. In order for a massive body to enter into orbit around Neptune and not just whizz right by it would have to lose a lot orbital energy, or angular momentum. One theory states, that if Triton had a moon itself or was in a planetary binary system, Triton could be captured if the excess orbital energy had been carried off by its satellite which with all this energy would be ejected from orbit around Triton and Neptune

 

 Voyager 2 returned pictures of Triton in 1989, revealing an icy surface with very few impact craters. The surface of Triton smooth in some places and crinkled, like the skin of a cantaloupe, in other places. Because of the lack of cratering, it is thought that Triton's got a face-lift around 10-100 million years ago. This resurfacing could be done if ice in Triton's interior was heated due to tidal interactions with Neptune or heat from radioactive decay. This would provide for a liquid ocean under the icy surface of Triton, similar to Jupiter's moon Europa and Saturn's moon Enceladus. If pressure built up enough, ice volcanoes (cryovolcanism) could form spewing liquid, likely water mixed with ammonia and methane, onto Triton's dreadfully cold surface (colder than -382 F) which would then freeze. Voyager 2 saw two active geysers on Triton which spewed nitrogen gas as high as 8 km into the air. The energy powering these geysers is thought to come from sunlight, as these arose around the point on Triton which receives the most sunlight (though this is still about 1 thousandth of what Earth receives.)

 Image credit: NASA/JPL/USGS