The Gas Giants Astronomy Lesson 13 The four outer planets: Jupiter, Saturn, Uranus, and Neptune, are much larger and more massive than Earth, and they do not have solid surfaces. Because these planets are so large and composed of gases, they are commonly referred to as the gas giants. Like the sun, the gas giants are comprised of mainly hydrogen and helium. Because they are so massive, the gas giants have a much larger gravitational force than the terrestrial planets. This increased gravity prevents the gases of the planet from escaping, so they have thick atmospheres. Because of the gravitational pull and thick atmospheres, the pressure inside the planet is enormous. Thus, the hydrogen and helium comprising the planet is in liquid form. Because the gas giants are so far from the sun, their outer layers are extremely cold. But, due to the increased pressure inside the planet, temperature increased greatly within the planet. Each gas giant has many moons and is surrounded by a set of rings. A ring is a thin disk of small particles of ice and rock. Jupiter Jupiter is the largest and most massive of the planets. In fact, it is 2 ½ times larger than all of the other planets combined! Scientists think that Jupiter, like the other gas giants, probably has a dense core of rock and iron. This inner core is overlaid by a deep layer of metallic hydrogen. Jupiter also has four moons: Io, Europa, Genymede, and Callisto. The outer atmosphere is visibly segregated into several bands at different latitudes, resulting in turbulence and storms along their interacting boundaries. A prominent result is the Great Red Spot, a giant storm that is known to have existed since at least the seventeenth century. Saturn The second-largest planet in the solar system is Saturn. Saturn also has a thick atmosphere made up of helium and hydrogen. Saturn's interior is similar to that of Jupiter, having a small rocky core surrounded mostly by hydrogen and helium. The rocky core is similar in composition to the Earth, but denser. Above this, there is a thicker liquid metallic hydrogen layer, followed by a layer of liquid hydrogen and helium, and in the outermost 1,000 kilometers (600 mi) a gaseous atmosphere. Saturn has the most spectacular rings of any planet. These rings are made up of chunks of ice and rock, each orbiting the planet. Saturn s rings are very thin and broad, like a CD with a planet in the hole. Saturn s largest moon, Titan, is larger than the planet Mercury. Saturn has at
least 60 moons, but NASA and astronomers are discovering new moons all the time! Uranus Although the gas giant Uranus is about four times the diameter (what s that?) of Earth, it is still much smaller that Jupiter or Saturn. Uranus is twice as far from the sun as Saturn, so it is much colder. Uranus looks blue-green because of methane in its atmosphere. Uranus axis of rotation is tilted about 90 in the vertical, so from Earth it looks like it s rotating on its side. Compared to Earth, it takes Uranus 30,799 days or 84 years to orbit the sun! Finally, Uranus has at least 27 moons that we ve been able to detect to date and, like Saturn, we re always looking for more! Neptune Neptune is even farther from the sun than Uranus. Neptune is a cold, blue planet. Scientists think that Neptune is slowly shrinking, causing the interior to heat up. Astronomers have discovered at least thirteen moons orbiting Neptune, the largest of which is Triton. Neptune s orbit around the sun takes approximately 60,327 days. How many years is that? (~165 years)
Images used in this lesson: Jupiter http://en.wikipedia.org/wiki/file:jupiter.jpg Jupiter to Earth Comparison http://en.wikipedia.org/wiki/file:jupiter-earth- Spot_comparison.jpg Jupiter s Interior http://en.wikipedia.org/wiki/file:jupiter_interior.png Giant Red Spot http://en.wikipedia.org/wiki/file:great_red_spot_from_voyager_ 1.jpg Jupiter Seen Cassini http://en.wikipedia.org/wiki/file:pia04866_modest.jpg Saturn as Seen by Cassini http://en.wikipedia.org/wiki/file:saturn_from_cassi ni_orbiter_(2004-10-06).jpg Saturn to Earth Comparison http://en.wikipedia.org/wiki/file:saturn,_earth_size_compariso n.jpg Titan http://en.wikipedia.org/wiki/file:titan_in_natural_color_cassin i.jpg
Saturn with Moons Titan and Tethys http://www.titanexploration.com/titan%20images/titanimages 2008.htm Uranus http://en.wikipedia.org/wiki/file:uranus2.jpg Uranus Rings http://en.wikipedia.org/wiki/file:uranusandrings.jpg Uranus with Clouds and Rings http://en.wikipedia.org/wiki/file:uranus_clouds.jpg Uranus to Earth Comparison http://en.wikipedia.org/wiki/file:uranus,_earth_size_comparis on.jpg Uranus Moons (A montage of Uranus' large moons and one smaller moon: from left to right Puck, Miranda, Ariel, Umbriel, Titania and Oberon. Other moons are not yet photographed in detail. Size proportions are correct.) http://commons.wikimedia.org/wiki/file:uranus_moons.jpg Neptune http://en.wikipedia.org/wiki/file:neptune.jpg
Neptune to Earth size comparison http://en.wikipedia.org/wiki/file:neptune,_earth_size_comparis on.jpg The Rings of Neptune http://en.wikipedia.org/wiki/file:neptune_ring_arcs.jpg Neptune and Triton http://en.wikipedia.org/wiki/file:voyager_2_neptune_and_trito n.jpg