1. Concepts 2. Solar System Origin 3. Planetary Processes 4. Earth Processes 5. Meteorites
6. Our Moon 7. Remote Sensing 8. Mercury 9. Mars 10. Venus, Our Twin
11. Jupiter & Jovian Moons 12. Saturn, Rings & Moons 13. Uranus 14. Neptune 15. Pluto, Charon & Comets


by Kari Hetcher

Please Read:
Chapter 21, Triton, Pluto and Charon
Chapter 22, Midsize Icy Satellites
Review: Chapters 14, 15, and 16.

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

Feathery white clouds fill the boundary between the dark and light blue regions on the Great Dark Spot. The spiral shape of both the dark boundary and the white cirrus suggests a storm system rotating counterclockwise. Periodic small-scale patterns in the white cloud, possibly waves, are short-lived and do not persist from one Neptunian rotation to the next. Depicted here is the last face-on view of the Great Dark Spot that Voyager 2 made with its narrow-angle camera. The image was shuttered 45 hours before closest approach.great dark spot

Visit 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 - Neptune's Captured Moon

The pink hue of Neptune's largest moon, Triton, is thought to result from a slowly evaporating layer of nitrogen ice. Triton is an oddity among moons in that its orbit is highly tilted to the plane of Neptune's equator, and it is in a retrograde orbit. These facts have led scientists to believe that Triton formed independently of Neptune and was later captured by Neptune's gravity.

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.


Image and text from (JPL/NASA

Triton-Polar Projection

polar projection

This polar projection of Triton's southern hemisphere provides a view of the southern polar cap and bright equatorial fringe. The margin of the cap is scalloped and ranges in latitude from +10 degrees to -30 degrees.


The bright fringe is closely associated with the cap's margin; from it, diffuse bright rays extend north-northeast for hundreds of kilometers. The bright fringe probably consists of very fresh nitrogen frost or snow, and the rays consist of bright-fringe materials that were redistributed by north-moving Coriolis-deflected winds.

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Triton's South Polar Ice Cap

south polar ice cap

Triton's surface is covered with nitrogen and methane ice. Its surface temperature is a mere 38 degrees Celsius above absolute zero. Dark streaks across the south polar cap may be the result of recent geyser-like eruptions of gas, dust, and ice venting from beneath the cap into the satellite's near-vacuum atmosphere. The diameter of Triton is 2700 km (1674 mi).

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.


Photo courtesy of JPL/NASA

Triton Surface Features

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 weirdest terrain on Triton is the so-called Cantaloupe Terrain, or fractured plains, characterized by closed depressions 30 to 50 kilometers wide, separated by ridges. It has now been shown that these depressions resemble those formed by diapirs, which are blobs of material that rise from depth and penetrate through a surface layer. This suggests that Triton's crust is layered. Some of the smooth deposits at right may be volcanic in origin.

A long graben, 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.


Geysers on Triton

In August of 1989 NASA's Voyager 2 spacecraft passed by Neptune, the most distant of the solar system's gas giant planets. Its encounter with Neptune climaxed with its closest approach to Neptune's largest moon Triton. From a distance of about 24,000 miles the robot space probe surveyed Triton's surface, whose temperature averages nearly -400 degrees Fahrenheit, and discovered surprising evidence of a complex and active world. For example, the prominent dark streaks in this image seem to come from small volcanoes and may consist of nitrogen frost mixed with organic compounds ejected during geyser-like eruption.

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 (moon)

Copy of NEREID1989N1.jpg (10077 bytes)


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.




Nereid, another Neptune moon, 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.

Sites with lots of information about Neptune and its moons'

Neptune fact sheet NASA educator's site
The Weather on Neptune Astronomy picture of the day for Oct. 28, 1996
StarChild: A Learning Center for Young Astron... from NASA, a page designed for use by elementary students.
Neptune from Voyager as Voyager leaves the system (NASA/Voyager photo)

Assignment: Answer these study questions.

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?

End Of The Module
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