Visions of the Cosmos

Planetary Science

The Solar System

The Moons of Neptune

The Moons of Neptune Statistics

Moon Discoverer Diameter in kilometres     Mean Distance from Neptune in kilometres
Naiad Voyager 2, 1989 60 48,000
Thalassa Voyager 2, 1989 80 50,000
Despina Voyager 2, 1989 150 52,500
Galatea Voyager 2, 1989 160 62,00
Larissa Voyager 2, 1989 190 73,600
Proteus Voyager 2, 1989 420 117,600
Triton Lassell, 1846 2,700 354,800
Nereid Kuiper, 1949                340 5,513400

Up until 1989 only two moons of Neptune were known namely Triton first observed by Lassell in 1846 and Nereid discovered by Kuiper in 1949.  In 1898 six small moons were discovered by the cameras of the Voyager Spacecraft.  Altogether thirteen moons were reported on 8 December by Marylin Lindstrom on the NASA Solar System Exploration web-site  Five of them are recent discoveries of very small bodies.

Triton is by far the largest moon of Neptune and the only really planet-sized moon.

Until the Voyager Mission only one other satellite of Neptune was known - tiny Nereid with a diameter of around 340 kilometres. (non-spherical).   Voyager discovered six more, which were given the names Proteus, Larissa, Galatea, Despina, Thulassa and Naaiad.  Very little is known of Nereid and the newly discovered small satellites. 

Triton is undoubtedly one of the coldest and darkest spots in the Solar System.    The Sun will be even less brilliant than it is from Miranda and the temperature during a warm summer's day may hit around -233º Celsius (40 degrees above absolute zero).   During summer nights in the warmest time of the long year, the thin nitrogen atmosphere will deposit solid nitrogen frost upon the surface of Triton.  Occasionally geysers of liquid nitrogen under high pressure will erupt into the thin atmosphere before the nitrogen changes to a gas and then sublimes to scatter nitrogen snow upon the frozen ground

In Greek mythology, Triton is a god of the sea, the son of Poseidon (Neptune); usually portrayed as having the head and trunk of a man and the tail of a fish.

Triton has been visited by only one spacecraft, Voyager 2 on Aug 25 1989. Almost everything we know about it comes from this encounter.

 Triton is different from all other icy satellites Voyager has studied. About three-quarters the size of Earth's Moon, Triton circles Neptune in a tilted, circular, retrograde orbit (opposite to the direction of the planet's rotation), completing an orbit in 5.875 days at an average distance of 330,000 kilometers above Neptune's cloud tops. Triton shows evidence of a remarkable geologic history, and Voyager 2 images show active geyser-like eruptions spewing invisible nitrogen gas and dark dust particles several kilometers into space.

Up until the historic encounter of Voyager 2 in August 1989 with Neptune and its satellites very little was known of the Neptune system and the various observations and calculations carried out on Triton gave confusing results.  Measurements of its radius and density were particularly difficult to calculate with even a moderate degree of accuracy.  Voyager 2 altered all that when it passed within about 40,000 kilometres of the surface of Triton and resolved features as small as one kilometre across.   It gave us a tantalising glimpse into the chemistry and physics of that far distant world. Illustration Credit NASA/JPL

Triton is considerably smaller than the other large satellites of the major planets.   Voyager 2 measured its radius as 1,355  ± 7 kilometres (diameter 2,710 ± 14 kilometres) and its density as 2,054  ± 0.032 grams per cc.  It has a mass of about one third as great as the Earth's Moon.  .

The picture of Triton was taken in 1989 by Voyager 2, the only spacecraft ever to pass near the little planet.  Voyagerfound a fascinating terrain, a thin atmosphere, and even evidence for ice volcanoes on this world of peculiar orbit and spin.

Topographically Triton exhibits two major types of surface.  The western equatorial region, named Bumembe Regio, has a peculiar wrinkled surface covered by closely spaced depressions and ridges that resemble the skin of a cantaloupe melon.  Hence it is referred to as a CANTELOUPE TERRAIN.     This may have been caused by repeated episodes of local melting followed by collapse.  The eastern-hemisphere has several caldera-like basins filled with smooth ice Acknowledgement NASA/JPL

            During its history Triton has experienced CRYO-VOLCANISM.   During these episodes water-ice and probably ammonia-ice have melted under the surface and erupted to give lava flows which have then resolidified to produce smooth frozen lakes.  This is analogous to the melting of silicate rocks to give magma, which flows to produce igneous rocks.  It is possible that other cold planets such as the moons of Jupiter and Saturn (see Enceladus) may have also had episodes of cryovolcanic activity.

Triton orbits around Neptune in a retrograde direction.  Most moons in the Solar System go round their 'parent' planets in the same direction as the planets themselves rotate.

The fact that Triton has a retrograde motion suggests that Neptune is not its true ‘parent’ but that at some time it was 'captured' by Neptune.   One strong piece of evidence for this is that it shows a very marked lack of impact craters that indicates a very young surface.  Early in the history of the Solar System the planets were subjected to very heavy bombardments by meteorites, asteroids and comets.  This period lasted until about 3,900 million years ago.  The approximate ‘age’ of a rock surface is roughly indicated by the number of craters on the surface.   The Earth has very few craters since geological processes have covered the whole surface - geologically speaking most of our rocks are very 'young'.  Many of the surfaces on the Moon, Mercury and Callisto however are very 'old'

It seems that, at some time after the heavy bombardments, Triton suffered some kind of catastrophic event and the old surface was completely covered.  One large cryovolcanic episode may have been initiated by the capture of Triton by Neptune.   A possible explanation advanced by Professor Lewis of the University of Arizona is that the change in kinetic energy at the time of the capture by Neptune was so immense as to cause melting of all the ices.  Such an event would have released liquid water and ammonia and turned all the methane and nitrogen into gases giving a temporary thick atmosphere.  He suggests that this could have happened in as short a period of time as a single Earth day.  This would have resulted in a rapid obliteration of all the early craters produced during the great bombardments.   It is believed that the initial orbit was strongly elliptical.   Over the period since the capture, it has gradually grown more circular and has finally become locked in a synchronous orbit around Neptune (it always turns the same face to Neptune which is similar to the Earth-Moon System.)  Because its orbit is retrograde, it is slowly getting closer to Neptune.   This means that at some time in the future it is likely to either break up into a ring system, when it reaches a certain distance called the Roche limit or to undergo catastrophic collision with Neptune.  The possible time in the future for this to happen is a matter of strong controversy - estimates vary between 10 million and a few billion years!

Composition of Triton

It is possible that the basic composition of Triton is similar to Titan with a silica-rock core overlaid by ices and a final surface of methane and nitrogen.

Dale Cruikshank and Peter Silvaggio of the University of Hawaii have measured visible and near infrared spectra between 0.8-2.5 m m in the visible and near infrared.   The 2.16 m m feature in the spectrum of surface of Triton suggests the presence of condensed nitrogen.  

Much of the surface is covered with fresh ice or frost, which appears to be a mixture of nitrogen, methane and H2O. 

The surface of Triton is pinkish in colour, which is reminiscent of Titan.  It appears that photochemical reactions followed by free radical reactions occur on Triton are analogous to those which take place on Titan but at a very much slower rate. The pink colour is thought to be due to the presence of hydrocarbon and organic nitrogen material.  Very slow photolysis of methane by the ultraviolet radiation from the distant Sun produces methyl free radicals which then react further to give stable compounds such as low molecular weight products including ethane and hydrogen cyanide (from nitrogen).  This will be followed by the production of more complex molecules including the formation of polymers..    Because of the exceptionally cold conditions, hydrocarbon compounds such as ethane have not been found in the gas phase.  They would be expected to condense very rapidly and their presence in the vapour phase would be too low for our present instrumentation to detect. 

The atmospheric pressure on Triton has been measured as 16 ± 3 microbars (0.016 ± 0.003 millibars).  This is about 100 thousandth as high as the pressure on Titan.  Most of the vapour phase material is N2.     The amount of methane is only small.  The photochemical lifetime of methane in the atmosphere of Triton would only last about 16 Earth years at the calculated rate of photolysis, so there must be a very large reservoir of solid methane, which is continuously replenishing methane in the vapour phase.  The temperature of the surface has been measured as -236 º Celsius (38 degrees above absolute zero).  Triton is the coldest place in the Solar System so far measured.  

            The nitrogen that dominates Triton's tenuous atmosphere is in vapour-pressure equilibrium with the nitrogen ice on its surface.  This means that nitrogen molecules are subliming from the surface of one hemisphere (the ‘day’ side) into the atmosphere at the same rate that they are freezing onto the surface of the other hemisphere (the ‘night’ side).

For a given temperature there is only one pressure for which this equilibrium holds.  The equilibrium relationship between temperature and pressure is very steep - an increase of only one degree Celsius will approximately double the atmospheric pressure.  Triton's atmosphere transports heat from the equator to the poles and from the sunlit day-side to the night-side using the latent heat effect..  When the molecules sublime from the sunlit side from solid to vapour heat is removed and the surface cools.   The molecules then migrate in the gas phase to the night side and resublime onto the ground releasing the same amount of heat that was removed during sublimation to vapour on the day side.  This causes all the surface frost on the planet to be at almost the same temperature throughout.

The Seasons on Triton.

Despite its intense cold, Triton does experience very pronounced seasonal variations.  Neptune takes almost 165 years to orbit the Sun.  However, the plane of Triton's orbit precesses with a period of about 640 years.  At the present time, the sub-solar point is at about 40 degrees North leaving the South Pole in perpetual darkness for about 100 years.  There must be a global temperature minimum near the pole, which is in mid-winter.    This will create a cold trap where most of the planetary volatiles will tend to accumulate. At the moment, the Northern Hemisphere is basking in mid-summer!

 The Nitrogen Geysers

Voyager observed two dark plumes rising vertically over eight kilometres through the thin atmosphere before flattening out into horizontal wind blown trails about 150 kilometres in length.  They were in fact NITROGEN GEYSERS.  The most probable explanation advanced for this phenomenon is that it is due to a kind of 'greenhouse effect'. Weak sunlight (visible and infrared) is thought to pass through a transparent layer of nitrogen ice and be absorbed by the dark hydrocarbon layer beneath. As this layer warms up it melts the nitrogen immediately above it, which builds up a pressure and blasts through the crust. A mixture of nitrogen gas and nitrogen snow is ejected up to an altitude of about eight kilometres at speeds of up to 150 metres per second (540 kilometres per hour).

A number of black wind streaks less than 1000 years old have been seen on the surface of Triton.  It is thought that they were produced by the nitrogen geysers.

 

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