GEOLOGY 422/522 . . .by Kari Hetcher
Reading: Chapter 21, Triton, Pluto and Charon, and
Chapter 22, Midsize Icy Satellites.
Review: Chapters 14, 15, and 16.
Visit the site: Welcome to the Planets by California Institute of Technology
Image courtesy of JPL/NASA
Neptune
is the 8th planet from the sun. Hydrogen and helium dominate the
interior of Neptune. The
Methane gas in its cloudy atmosphere gives Neptune a brilliant blue glow. White
clouds can be seen swirling around the planet, and at times giant storm systems
cover parts of the atmosphere. At first glance, Neptune resembles a smaller,
more blue, Jupiter. Visit Neptune at the Nine
Planets for a detailed overview of the planet and it's moons.
The Great Dark Spot
Click this title for an update on the Great Dark Spot.
Image courtesy of JPL/NASA
Visit Voyager Neptune Science Summary from Planetary Data System or Voyager Neptune Science Summary from the Jet Propulsion Laboratory for an overview of Voyager discoveries of Neptune. When comparing sizes, rotation rates, magnetic fields, and atmospheres, Neptune is often called Uranus’ sister planet. The pole of Neptune’s magnetic field is tilted from its revolutionary plane and is off center of the planet, similar to Uranus. Like Earth and Venus, however, these sisters’ differences become apparent upon closer examination. Neptune emits heat due to compressional heating caused by convection in its atmosphere. It is also denser and colder than Uranus (a chilling 69 K!). Neptune has at least 9 satellites, but only one of them, Triton, (this Triton site has animations, very cool!) is over 500 km in diameter. The diagram of the interior shows Neptune to be similar to Uranus, though the higher density suggests the presence of more rocky material.
Like the other gas giants, Neptune has rings. Visit Neptune's Ring System to learn more about Neptune's rings.
Triton is the largest of Neptune’s satellites and is the innermost of the two largest moons. It is about 2700 km in diameter (similar to the Earth’s Moon, Europa, and Io). It is made up of a mixture of methane (CH4) and nitrogen (N2) ices and rock. Scientists believe Triton was gravitationally captured after it formed. Evidence of this is its retrograde motion, the fact that it keeps the same face toward the planet, and its 21 degree tilt of the orbital plane from the equatorial plane. This tilt, along with the tilt of Neptune itself, results in very interesting atmospheric characteristics on Triton.
Click the picture to see a larger image and more information.
Click on the picture to the left to see a larger image and text about Triton's south pole.
(Photo courtesy of JPL/NASA)
Click on the picture to the right for a larger image and more information about Triton's south pole.
(Image courtesy of NASA).
Methane and nitrogen are the dominant ices of Triton’s surface, and its extremely cold temperatures (37K) have resulted in the formation of large polar ice caps. Due to the tilt of both Triton and Neptune, Triton’s South Pole is currently pointed almost directly toward the sun. Seasons on Triton are about 600 years long, so estimates suggest the North Pole will not start facing the sun for almost 300 more years. A thick cap of frozen nitrogen can be seen on Triton’s South Pole, but evidence of sublimation along the edges suggests the ice is melting, and is possibly being transferred to the north polar region in the form of a thin haze of gas. The North Pole was in shadows at the time of the Voyager 2 mission, so it was not photographed.
Three major types of terrain appear to exist on the surface of Triton. These are the old fractured plains, the volcanic plains, and the young polar ice caps. What is missing? You might notice that there is no heavily cratered terrain seen on Triton. Tidal heating caused the eruption of icy lavas onto the surface, covering up any old crater features. If Triton was gravitationally captured by Neptune’s orbit, incredible tidal pressures were probably exerted on Triton in order to make its orbit as nearly circular as it is today. This heat probably remained with Triton for millions of years after capture, resulting in the surface features we see today
The fractured plains, also known as the Cantaloupe Terrain, are caused by rifting of the lithosphere during extension. Long graben, often over 1000 km in length, can be seen crisscrossing the dark plains of Triton. In the middle of most of the graben are raised ridges that probably consist of ice extruded from below. At the extremely cold temperatures on Triton’s surface, why would we see extrusion of this viscous, icy lava? When methane or ammonia are mixed with water ice, the melting temperature is lowered. Perhaps this allowed sufficient melting to produce the volcanism seen among Triton’s fractured plains. This terrain is called the Cantaloupe Terrain because of the complex series of ridges and depressions that look like the skin of a cantaloupe.
The Volcanic Plains of Triton 
The flooded plains of Triton are made up of large sub-circular depressions resembling lave lakes seen on Earth, Mars, and Io. They are made by less viscous lavas than the Cantaloupe terrain, and are formed when the lava accumulates within depressions. As the lava cooled, the surface became a solid lid. A portion of the underlying lava then drained out, causing the hardened surface to crack and collapse. Several periods of cooling and draining may be evident within a lava lake. The old levels of the lava leave scarps along the edges of the depressions.
The third type of terrain seen of Triton is the polar ice cap. This is obviously the youngest of the three types of terrain, and is one of the most reflective surfaces in our solar system. The edges of the ice cap have strange dark streaks that seem to trend in a northeasterly direction. Close examination of these streaks shows them to be the eruption plumes of active volcanoes.
As the methane and nitrogen ice within the ice cap reaches its melting point below the surface of the ice cap, it builds up pressure and changes volume. This causes the liquid to eventually reach its boiling point. When this occurs, it explodes through the ice surface as a geyser-like eruption. The northeasterly trend of the dark streaks might be caused by preferential winds going from the cold regions of the ice cap toward the warmer equator.
Proteus
-is the second largest moon of Neptune. It orbits very close to Neptune
and it was discovered by Voyager 2. Proteus
is a small, dark, irregular body.
Proteus
Poor Nereid is trapped in a dramatic boomerang orbit around Neptune! Its orbit
is so irregular that at times it is 1.3 million km from the planet, and at others
it is a whopping 9.7 million km away. It takes Nereid 359 days to complete each
one of its eccentric orbits. No close-up pictures have been taken of Nereid,
Neptune’s outermost satellite, but Voyager 2 photographs did show that
it has an irregular shape and a diameter (340km) a bit smaller than Miranda
or Mimas. It seems this moon was probably captured by Neptune’s gravitational
field, or was thrown into this strange orbit when Triton was captured. Perhaps
several of the irregular-shaped satellites orbiting Neptune were modified by
the dramatic capture of Triton.
Nereid
Sites with lots of information about Neptune and its moons'
http://pds.jpl.nasa.gov/planets/welcome/neptune.htm
Neptune from Voyager as Voyager leaves the system (NASA/Voyager
photo)
1. Find an example of a lava lake on Earth or Io. Describe the similarities and differences when comparing that lake to one seen on Triton. Besides Triton, what are the only other two bodies in the solar system with known active volcanoes?
2. Create a bar graph showing the densities of the moons of Jupiter, Saturn, Uranus, and Neptune. Is there a trend in the densities of these gas giants? If you see a trend, describe it. Explain the implications of the results. Create another graph of the gas giant planets. Graph the densities of each planet against their distance from the sun. Describe the trend(s) you see. Include the graphs and explanations with your e-mailed assignment.
3. Neptune emits heat. What is the mechanism by which Neptune emits heat?
4. Neptune's moon, Triton, orbits in retrograde motion. Explain what retrograde motion is and speculate on why Triton orbits Neptune in this manner. Nereid also has an anomalous orbit. Describe Nereid's orbit, what makes Nereid's orbit anomalous, and give some ideas about why you think Nereid has this orbit.
7. What do Triton and Pluto have in common?