Jupiter. Saturn. Uranus. Neptune. Each of these giant planets is the center of its own miniature solar system. Each is spectacularly beautiful and scientifically fascinating, which are reasons enough to explore them. But by studying the giant planets and their rings and moons, we can also learn about the forces that operated during the formation of our own solar system, as well as the origins of the hundreds of new extrasolar planetary systems that we discover every year.
And their moons are worlds in their own right. There are at least 16 outer planetary moons that would be called dwarf planets if they orbited the Sun rather than a planet. Two (Jupiter's Ganymede and Saturn's Titan) are larger than the planet Mercury, and one (Triton) is probably a captured Kuiper belt object.
But it is challenging and expensive to explore the outer planets, and missions to the outer planets take a very long time to develop, fly, and operate. Cassini will be orbiting Saturn until 2017, and Juno will operate at Jupiter from 2016 to 2017. After that, it's not clear if anyone will be sending a followup mission to Saturn or Jupiter or its moons, or an orbiter to survey the Uranus or Neptune systems. And there is a critical shortage of the isotope of plutonium that is needed to generate power for outer planetary missions.
Remember the amazing photo of Saturn's north pole that I posted yesterday? Now, thanks to an amateur image processor, it moves, and the motions of the individual clouds within the belts are mesmerizing.
A sharp-eyed amateur noticed two images of Titan taken 20 months apart from nearly exactly the same perspective, and they illustrate how the shifting of Saturn's seasons has brought change to Titan's atmosphere.
New ground-based images of Uranus show more finely detailed structure than any photos I have ever seen.
A summary of just one talk from the Division for Planetary Sciences meeting, by Lindy Elkins-Tanton, which provided a neat explanation for how asteroids can be melted and layered on the inside yet have a primitive-looking exterior.
In the first full day of the annual meeting of the Division of Planetary Sciences of the American Astronomical Society, I listened to scientific sessions on icy worlds and on an exoplanet in a four-star system.
It's a quarterly feast day for me: the day that the Cassini mission delivers three months' worth of data to NASA's Planetary Data System. Here's a few images processed from the October 1, 2012 data release.