Saturday, December 12, 2015

We're learning a lot about Titan!

Titan is the largest moon of Saturn, and the Cassini space probe has taught us a huge amount of the planetoid. What we're discovering is amazing... Cassini's gravity sensors indicate that Titan has a far more complex interior that we ever thought. On top is a shell 30 to 120 miles thick. It's mostly water, which surprised scientists. We know that because measurements of Titan's mountains indicated they were areas of weak gravity. Usually, mountains are areas of strong gravity. After much analysis, planetary geologists realized that Titan's mountains are actually water-icebergs, trapped in a highly rigid shell. They have to be ice, because ice rises; mountains made of methane or other solidified gases would sink. Water-ice, however, displaces a lot of water, so that there would be less water beneath an iceberg mountain. Thus, less gravity.

So what is the rest of the outer shell made of? Clathrate. Clathrates are lattices of molecules. Water molecules are trapped inside the lattice, which help to make the lattice-work heavy and rigid.

Below the outer shell is a subsurface ocean. Cassini discovered this by observing how Titan's shaped was affected when it was closer to Saturn. Amazingly, Titan became elongated like a football when close to the gas giant, and more sperical further away. Scientists believe this could only happen if the clathrate outer shell was decoupled from the interior by the presence of a subsurface ocean. This ocean, they believe, is very salty, which helps to prevent it from freezing. The tidal forces exerted by Saturn also help to generate friction, which keeps the subsurface ocean from freezing.

Below the subsurface ocean is another shell of water ice under high pressure. Water can take on a wide variety of ice forms when under pressure. On Earth, water molecules at 32 degrees Fahrenheit form hexagonal structures we call "ice". Between -63 and -225 degrees below zero (Fahrenheit), ice takes on a cubic form. But, between -81 and -117, at 2,000 atmospheres of pressure, we get a rhombohedral form of ice, called "ice II". At 3,000 atmospheres, at +80F, we get tetragonal ice, or "ice III". (Note the markedly higher temperature at which ice III forms.) At 8,100 atmospheres, we can get ice IV. Ice IV begins with what is called "amorphous ice", ice which forms without a crystalline structure. Beginning at -198F and heating very, very slowly, a second rhombohedral form of ice (ice IV) is created. By applying 5,000 atmospheres to water until it reaches -4F, we can get ice V. It has a monoclinic structure (a rectangular prism with a parallelogram as its base). At 11,000 atmospheres, water that is cooled to 26F forms a tetragonal crystal, known as ice VI.

Scientists now believe, given calculations of the pressure and temperature on Titan, that this inner shell is composed of ice VI.

Deep inside Titan is a solid core about 1,250 miles across. This solid core is made up of hydrous silicate. That's not unusual, as silicon is one of the most abundant elements in the universe.


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