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Thursday, March 29, 2018

Poles of Beauty

An image recently published by one of my favorite blogs, Astronomy Picture of the Day, made me realize that our planet earth posses perhaps one of the dullest looking poles of the solar system. Just a white, irregular and featureless patch of snow and ice. In comparison, this is what the north pole of Mars looks like.
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The latte-like spirals around the pole are frozen carbon dioxide that precipitates out of the thin Martian atmosphere every winter and is deposited over the frozen pole cap made up of water-ice. The carbon dioxide layer is about a meter thick. The spiral troughs in the layer are formed by strong katabatic winds that swirl around the pole due to the planet’s rotation. This image is a mosaic generated earlier this year from numerous images taken by the ESA’s and NASA’s missions, and covers an area of around a million square kilometers.
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This image is from NASA’s Mars Global Surveyor.
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A mosaic of the north pole of Mars captured by the Viking spacecraft.
Other planets of the solar system too have interesting looking poles. In late May of this year, NASA released a spectacular picture of Jupiter’s south pole taken by the the Juno spacecraft that arrived at the gas giant last summer. The image (shown below) shows multiple storms the size of earth swirling around the pole, densely clustered and rubbing together. The image was taken from an altitude of 52,000 kilometers. Each cyclone is about a 1,000 kilometers in diameter.
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“We're puzzled as to how they could be formed, how stable the configuration is, and why Jupiter’s north pole doesn't look like the south pole,” said Diane Brown, Juno program executive. “We're questioning whether this is a dynamic system, and are we seeing just one stage, and over the next year, we're going to watch it disappear, or is this a stable configuration and these storms are circulating around one another?”
Saturn is another remarkable planet. The poles of this ringed planet are nothing like anything seen on any other planets of the solar system. Around the north pole of Saturn there is a persistent hexagonal cloud pattern with sides longer than the diameter of Earth. The cloud is also rotating with a period of roughly ten and half hours. Within the hexagonal pattern a variety of cloud structures can be seen. At the center of the hexagon is a massive hurricane with an eye about fifty times larger than the average hurricane eye on earth. Swirling around the central hurricane are several smaller hurricane and vortices. The biggest of these vortices spans about 3,500 kilometers.
The hexagon is seen only on Saturn’s north pole.
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Saturn’s hexagon captured by the Cassini spacecraft.
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Saturn’s hexagon captured by the Cassini spacecraft.
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A true color image of Saturn’s hexagon.
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In this animated image, different wavelengths of light from ultraviolet to visible to infrared have been assigned colors, show a distinct contrast between the types of atmospheric particles inside and outside the hexagon.
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In recent years, the hexagon was also observed to change color from blue to golden. Astronomers are not sure why the color had changed, but they feel it has something to do with the way sunlight strikes the planet and the presence of smog particles.
Our final stop today will be Neptune. While a photograph of the planet in the visible spectrum doesn’t reveal anything out of ordinary, when astronomers switched to infrared, they noticed a curious oddity. The planet’s south pole is warmer than the rest of the planet, as much as by 10 degrees centigrade. The average temperature on Neptune is about minus 200 degrees Celsius, so the south pole is still frigid but warm enough for methane gas to escape from the poles and into the planet’s atmosphere. It is this abundance of methane in the atmosphere that gives the planet its characteristic blue color.
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On the left is the thermal image of Neptune. The bright spot at the bottom is the warmest region. On the right is the true color image of Neptune.

Neptune has a tilted axis (about 28° compared to earth’s 23°), so it experiences seasons just like earth as the poles take turn being in the sunlight and in the shadow. However, Neptune takes roughly 165 years to orbit the Sun once, which means that each season on Neptune lasts more than 40 earth years. It has been summer on Neptune’s southern hemisphere for the last few decades. The south pole is the only place on the planet that has been receiving sunlight continuously all throughout the summer, and so it is the warmest place on the planet. The rest of the planet doesn’t get enough time to warm as it goes in and out of sunlight every 8 hours (Neptune’s day lasts 16 hours).

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