Neptune, The Other Blue Planet

Neptune with a Great Dark Spot prominent in the center

Image Credit:

NASA

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.

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.

Titan

Cassini Image of Titan showing the atmosphere of the moon

Image Credit:

NASA/Cassini

 

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.