Artist's impression of what the surface of Triton may look like. Credit: ESO
The study of the Solar System’s many moons has revealed a wealth of information over the past few decades. These include the moons of Jupiter – 69 of which have been identified and named – Saturn (which has 62) and Uranus (27). In all three cases, the satellites that orbit these gas giants have prograde, low-inclination orbits. However, within the Neptunian system, astronomers noted that the situation was quite different.
Compared to the other gas giants, Neptune has far fewer satellites, and most of the system’s mass is concentrated within a single satellite that is believed to have been captured (i.e. Triton). According to a new study by a team from the Weizmann Institute of Science in Israel and the Southwest Research Institute (SwRI) in Boulder, Colorado, Neptune may have once had a more massive systems of satellites, which the arrival of Triton may have disrupted.
The study, titled “Triton’s Evolution with a Primordial Neptunian Satellite System“, recently appeared in The Astrophysical Journal. The research team consisted of Raluca Rufu, an astrophysicist and geophysicist from the Weizmann Institute, and Robin M. Canup – the Associate VP of the SwRI. Together, they considered models of a primordial Neptunian system, and how it may have changed thanks to the arrival of Triton.
Neptune and its large moon Triton as seen by Voyager 2 on August 28th, 1989. Credit: NASA
For many years, astronomers have been of the opinion that Triton was once a dwarf planet that was kicked out of the Kuiper Belt and captured by Neptune’s gravity. This is based on its retrograde and highly-inclined orbit (156.885° to Neptune’s equator), which contradicts current models of how gas giants and their satellites form. These models suggest that as giant planets accrete gas, their moons form from a surrounding debris disk.
Consistent with the other gas giants, the largest of these satellites would have prograde, regular orbits that are not particularly inclined relative to their planet’s equator (typically less than 1°). In this respect, Triton is believed to have once been part of a binary made up of two Trans-Neptunian Objects (TNOs). When they swung past Neptune, Triton would have been captured by its gravity and gradually fell into its current orbit.
As Dr. Rufu and Dr. Canup state in their study, the arrival of this massive satellite would have likely caused a lot of disruption in the Neptunian system and affected its evolution. This consisted of them exploring how interactions – like scattering or collisions – between Triton and Neptune’s prior satellites would have modified Triton’s orbit and mass, as well as the system at large. As they explain:
“We evaluate whether the collisions among the primordial satellites are disruptive enough to create a debris disk that would accelerate Triton’s circularization, or whether Triton would experience a disrupting impact first. We seek to find the mass of the primordial satellite system that would yield the current architecture of the Neptunian system.”
Montage of Neptune’s largest moon, Triton and the planet Neptune showing the moon’s sublimating south polar cap (bottom) and enigmatic “cantaloupe terrain”. Credit: NASA
To test how the Neptunian system could have evolved, they considered different types of primordial satellite systems. This included one that was consistent with Uranus’ current system, made up of prograde satellites with a similar mass ration as Uranus’ largest moons – Ariel, Umbriel, Titania and Oberon – as well as one that was either more or less massive. They then conducted simulations to determine how Triton’s arrival would have altered these systems.
These simulations were based on disruption scaling laws which considered how non-hit-and-run impacts between Triton and other bodies would have led to a redistribution of matter in the system. What they found, after 200 simulations, was that a system that had a mass ratio that was similar to the current Uranian system (or smaller) would have been most likely to produce the current Neptunian system. As they state:
“We find that a prior satellite system with a mass ratio similar to the Uranian system or smaller has a substantial likelihood of reproducing the current Neptunian system, while a more massive system has a low probability of leading to the current configuration.”
They also found that the interaction of Triton with an earlier satellite system also offers a potential explanation for how its initial orbit could have been decreased fast enough to preserve the orbits of small irregular satellites. These Nereid-like bodies would have otherwise been kicked out of their orbits as tidal forces between Neptune and Triton caused Triton to assume its current orbit.
The moons of Uranus and Neptune as imaged during the 2011 opposition season. Credit: Rolf Wahl Olsen.
Ultimately, this study not only offers a possible explanation as to why Neptune’s system of satellites differs from those of other gas giants; it also indicates that Neptune’s proximity to the Kuiper Belt is what is responsible. At one time, Neptune may have had a system of moons that were very much like those of Jupiter, Saturn, and Uranus. But since it is well-situated to pick up dwarf planet-sized objects that were kicked out of the Kuiper Belt, this changed.
Looking to the future, Rufu and Canup indicate that additional studies are needed in order to shed light on Triton’s early evolution as a Neptunian satellite. Essentially, there are still unanswered questions concerning the effects the system of pre-existing satellites had on Triton, and how stable its irregular prograde satellites were.
These findings were also presented by Dr, Rufu and Dr. Canup during the 48th Lunar and Planetary Science Conference, which took place in The Woodlands, Texas, this past March.
Artist's impression of New Horizons' close encounter with the Pluto–Charon system. Credit: NASA/JHU APL/SwRI/Steve Gribben
The New Horizons probe made history in July of 2015, being the first mission to ever conduct a close flyby of Pluto. In so doing, the mission revealed some never-before-seen things about this distant world. This included information about its many surface features, its atmosphere, magnetic environment, and its system of moons. It also provided images that allowed for the first detailed maps of the planet.
Having completed its rendezvous with Pluto, the probe has since been making its way towards its first encounter with a Kuiper Belt Object (KBO) – known as 2014 MU69. And in the meantime, it has been given a special task to keep it busy. Using archival data from the probe’s Long Range Reconnaissance Imager (LORRI), a team of scientists is taking advantage of New Horizon‘s position to conduct measurements of the Cosmic Optical Background (COB).
A partial solar eclipse rising over the VAB. Image by author.
It’s that time of year again… time to look ahead at the top 101 astronomical events for the coming year.
And this year ’round, we finally took the plunge. After years of considering it, we took the next logical step in 2017 and expanded our yearly 101 Astronomical Events for the coming year into a full-fledged guide book, soon to be offered here for free download on Universe Today in the coming weeks. Hard to believe, we’ve been doing this look ahead in one form or another now since 2009.
This “blog post that takes six months to write” will be expanded into a full-fledged book. But the core idea is the same: the year in astronomy, distilled down into the very 101 best events worldwide. You will find the best occultations, bright comets, eclipses and much more. Each event will be interspersed with not only the ‘whens’ and ‘wheres,’ but fun facts, astronomical history, and heck, even a dash of astronomical poetry here and there.
It was our goal to take this beyond the realm of a simple almanac or Top 10 listicle, to something unique and special. Think of it as a cross between two classics we loved as a kid, Burnham’s Celestial Handbook and Guy Ottewell’s Astronomical Calendar, done up in as guide to the coming year in chronological format. Both references still reside on our desk, even in this age of digitization.
And we’ve incorporated reader feedback from over the years to make this forthcoming guide something special. Events will be laid out in chronological order, along with a quick-list for reference at the end. Each event is listed as a one- or two-page standalone entry, ready to be individually printed off as needed. We will also include 10 feature stories and true tales of astronomy. Some of these were culled from the Universe Today archives, while others are new astronomical tales written just for the guide.
Don’t miss 2017’s only total solar eclipse, crossing the United States! Image credit: Michael Zeiler/The Great American Eclipse.
The Best of the Best
Here’s a preview of some of the highlights for 2017:
-Solar cycle #24 begins to ebb in 2017. Are we heading towards yet another profound solar minimum?
-Brilliant Venus reaches greatest elongation in January and rules the dusk sky.
-45P/Honda-Mrkos-Pajdusakova passes 0.08 AU from Earth on February 11th, its closest passage for the remainder of the century.
-An annular solar eclipse spanning Africa and South America occurs on February 26th.
A sample occultation map from the book. Image credit: Occult 4.1.2.
-A fine occultation of Aldebaran by the Moon on March 5th for North America… plus more occultations of the star worldwide during each lunation.
-A complex grouping of Mercury, Venus, Mars and the Moon in mid-September.
-Saturn’s rings at their widest for the decade.
Getting wider… the changing face of Saturn’s rings. Image credit and copyright: Andrew Symes (@FailedProtostar).
-A fine occultation of Regulus for North America on October 15th, with occultations of the star by the Moon during every lunation for 2017.
-Asteroid 335 Roberta occults a +3rd magnitude star for northern Australia…
And that’s just for starters. Entries also cover greatest elongations for the inner planets and oppositions for the outer worlds, the very best asteroid occultations of bright stars, along with a brief look ahead at 2018.
Get ready for another great year of skywatching!
And as another teaser, here’s a link to a Google Calendar download of said events, complied by Chris Becke (@BeckePhysics). Thanks Chris!
Artist's concept of Trojan asteroids, small bodies that dominate our solar system. Credit: NASA
The Solar System is filled with what are known as Trojan Asteroids – objects that share the orbit of a planet or larger moon. Whereas the best-known Trojans orbit with Jupiter (over 6000), there are also well-known Trojans orbiting within Saturn’s systems of moons, around Earth, Mars, Uranus, and even Neptune.
Until recently, Neptune was thought to have 12 Trojans. But thanks to a new study by an international team of astronomers – led by Hsing-Wen Lin of the National Central University in Taiwan – five new Neptune Trojans (NTs) have been identified. In addition, the new discoveries raise some interesting questions about where Neptune’s Trojans may come from.
The PS1 telescope at dawn, with the mountain of Mauna Kea visible in the distance. Credit: pan-starrs.ifa.hawaii.edu
The team used data obtained by the PS-1 survey, which ran from 2010 to 2014 and utilized the first Pan-STARR telescope on Mount Haleakala, Hawaii. From this, they observed seven Trojan asteroids around Neptune, five of which were previously undiscovered. Four of the TNs were observed orbiting within Neptune’s L4 point, and one within its L5 point.
The newly detected objects have sizes ranging from 100 to 200 kilometers in diameter, and in the case of the L4 Trojans, the team concluded from the stability of their orbits that they were likely primordial in origin. Meanwhile, the lone L5 Trojan was more unstable than the other four, which led them to hypothesize that it was a recent addition.
As Professor Lin explained to Universe Today via email:
“The 2 of the 4 currently known L5 Neptune Trojans, included the one L5 we found in this work, are dynamically unstable and should be temporary captured into Trojan cloud. On the other hand, the known L4 Neptune Trojans are all stable. Does that mean the L5 has higher faction of temporary captured Trojans? It could be, but we need more evidence.”
In addition, the results of their simulation survey showed that the newly-discovered NT’s had unexpected orbital inclinations. In previous surveys, NTs typically had high inclinations of over 20 degrees. However, in the PS1 survey, only one of the newly discovered NTs did, whereas the others had average inclinations of about 10 degrees.
Animation showing the path of six of Neptune’s L4 trojans in a rotating frame with a period equal to Neptune’s orbital period.. Credit: Tony Dunn/Wikipedia Commons
From this, said Lin, they derived two possible explanations:
“The L4 “Trojan Cloud” is wide in orbital inclination space. If it is not as wide as we thought before, the two observational results are statistically possible to generate from the same intrinsic inclination distribution. The previous study suggested >11 degrees width of inclination, and most likely is ~20 degrees. Our study suggested that it should be 7 to 27 degrees, and the most likely is ~ 10 degrees.”
“[Or], the previous surveys were used larger aperture telescopes and detected fainter NT than we found in PS1. If the fainter (smaller) NTs have wider inclination distribution than the larger ones, which means the smaller NTs are dynamically “hotter” than the larger NTs, the disagreement can be explained.”
According to Lin, this difference is significant because the inclination distribution of NTs is related to their formation mechanism and environment. Those that have low orbital inclinations could have formed at Neptune’s Lagrange Points and eventually grew large enough to become Trojans asteroids.
Illustration of the Sun-Earth Lagrange Points. Credit: NASA
On the other hand, wide inclinations would serve as an indication that the Trojans were captured into the Lagrange Points, most likely during Neptune’s planetary migration when it was still young. And as for those that have wide inclinations, the degree to which they are inclined could indicate how and where they would have been captured.
“If the width is ~ 10 degrees,” he said, “the Trojans can be captured from a thin (dynamically cold) planetesimal disk. On the other hand, if the Trojan cloud is very wide (~ 20 degrees), they have to be captured from a thick (dynamically hot) disk. Therefore, the inclination distribution give us an idea of how early Solar system looks like.”
In the meantime, Li and his research team hope to use the Pan-STARR facility to observe more NTs and hundreds of other Centaurs, Trans-Neptunian Objects (TNOs) and other distant Solar System objects. In time, they hope that further analysis of other Trojans will shed light on whether there truly are two families of Neptune Trojans.
This was all made possible thanks to the PS1 survey. Unlike most of the deep surveys, which are only ale to observe small areas of the sky, the PS1 is able to monitor the whole visible sky in the Northern Hemisphere, and with considerable depth. Because of this, it is expected to help astronomers spot objects that could teach us a great deal about the history of the early Solar System.
Reconstruction of Voyager 2 images showing the Great Black spot (top left), Scooter (middle), and the Small Black Spot (lower right). Credit: NASA/JPL
Our Solar System is a fascinating place. Between its eight planets and many dwarf planets, there are some serious differences in terms of orbit, composition, and temperature. Whereas conditions within the inner Solar System, where planets are terrestrial in nature, can get pretty hot, planets that orbit beyond the Frost Line – where it is cold enough that volatiles (i.e. water, ammonia, methane, CO and CO²) condense into solids – can get mighty cold!
It is in this environment that we find Neptune, the Solar System’s most distance (and hence most cold) planet. While this gas/ice giant has no “surface” to speak of, Earth-based research and flybys have been conducted that have managed to obtain accurate measurements of the temperature in the planet’s upper atmosphere. All told, the planet experiences temperatures that range from approximately 55 K (-218 °C; -360 °F) to 72 K (-200 °C; -328 °F), making it the coldest planet in the Solar System.
Orbital Characteristics:
Of all the planets in the Solar System, Neptune orbits the Sun at the greatest average distance. With a very minor eccentricity (0.0086), it orbits the Sun at an semi-major axis of approximately 30.11 AU (4,504,450,000,000 km), ranging from 29.81 AU (4.459 x 109 km) at perihelion and 30.33 AU (4.537 x 109 km) at aphelion.
Neptune and the icy-asteroid-rich Kuiper Belt that lies beyond its orbit. Credit: NASA
Neptune takes 16 hours 6 minutes and 36 seconds (0.6713 days) to complete a single sidereal rotation, and 164.8 Earth years to complete a single orbit around the Sun. This means that a single day lasts 67% as long on Neptune, whereas a year is the equivalent of approximately 60,190 Earth days (or 89,666 Neptunian days).
Because Neptune’s axial tilt (28.32°) is similar to that of Earth (~23°) and Mars (~25°), the planet experiences similar seasonal changes. Combined with its long orbital period, this means that the seasons last for forty Earth years. In addition, the planets axial tilt also leads to variations in the length of its day, as well as variations in temperature between the northern and southern hemispheres (see below).
“Surface” Temperature:
Due to their composition, determining a surface temperature on gas or ice giants (compared to terrestrial planets or moons) is technically impossible. As a result, astronomers have relied on measurements obtained at altitudes where the atmospheric pressure is equal to 1 bar (or 100 kilo Pascals), the equivalent of air pressure here on Earth at sea level.
Color-contrasted photo showing Neptune’s atmospheric features. Credit: Erich Karkoschka
It is here on Neptune, just below the upper level clouds, that pressures reach between 1 and 5 bars (100 – 500 kPa). It is also at this level that temperatures reach their recorded high of 72 K (-201.15 °C; -330 °F). At this temperature, conditions are suitable for methane to condense, and clouds of ammonia and hydrogen sulfide are thought to form (which is what gives Neptune its characteristically dark cyan coloring).
But as with all gas and ice giants, temperatures vary on Neptune due to depth and pressure. In short, the deeper one goes into Neptune, the hotter it becomes. At its core, Neptune reaches temperatures of up to 7273 K (7000 °C; 12632 °F), which is comparable to the surface of the Sun. The huge temperature differences between Neptune’s center and its surface create huge wind storms, which can reach as high as 2,100 km/hour, making them the fastest in the Solar System.
Temperature Anomalies and Variations:
Whereas Neptune averages the coldest temperatures in the Solar System, a strange anomaly is the planet’s south pole. Here, it is 10 degrees K warmer than the rest of planet. This “hot spot” occurs because Neptune’s south pole is currently exposed to the Sun. As Neptune continues its journey around the Sun, the position of the poles will reverse. Then the northern pole will become the warmer one, and the south pole will cool down.
Neptune’s more varied weather when compared to Uranus is due in part to its higher internal heating, which is particularly perplexing for scientists. Despite the fact that Neptune is located over 50% further from the Sun than Uranus, and receives only 40% its amount of sunlight, the two planets’ surface temperatures are roughly equal.
Four images of Neptune taken a few hours apart by the Hubble Space Telescope on June 25-26, 2011. Credit: NASA, ESA and the Hubble Heritage Team (STScI/AURA)
Deeper inside the layers of gas, the temperature rises steadily. This is consistent with Uranus, but oddly enough, the discrepancy is larger. Uranus only radiates 1.1 times as much energy as it receives from the Sun, whereas Neptune radiates about 2.61 times as much. Neptune is the farthest planet from the Sun, yet its internal energy is sufficient to drive the fastest planetary winds seen in the Solar System. The mechanism for this remains unknown.
And while temperatures on Pluto have been recorded as reaching lower – down to 33 K (-240 °C; -400 °F) – Pluto’s status as a dwarf planet mean that it is no longer in the same class as the others. As such, Neptune remains the coldest planet of the eight.
Neptune photographed by Voyager 2. Image credit: NASA/JPL
Neptune is the eight planet from our Sun, one of the four gas giants, and one of the four outer planets in our Solar System. Since the “demotion” of Pluto by the IAU to the status of a dwarf planet – and/or Plutoid and Kuiper Belt Object (KBO) – Neptune is now considered to be the farthest planet in our Solar System.
As one of the planets that cannot be seen with the naked eye, Neptune was not discovered until relatively recently. And given its distance, it has only been observed up close on one occasion – in 1989 by the Voyager 2 spaceprobe. Nevertheless, what we’ve come to know about this gas (and ice) giant in that time has taught us much about the outer Solar System and the history of its formation.
Discovery and Naming:
Neptune’s discovery did not take place until the 19th century, though there are indications that it was observed before long that. For instance, Galileo’s drawings from December 28th, 1612, and January 27th, 1613, contained plotted points which are now known to match up with the positions of Neptune on those dates. However, in both cases, Galileo appeared to have mistaken it for a star.
1821, French astronomer Alexis Bouvard published astronomical tables for the orbit of Uranus. Subsequent observations revealed substantial deviations from the tables, which led Bouvard to hypothesize that an unknown body was perturbing Uranus’ orbit through gravitational interaction.
New Berlin Observatory at Linden Street, where Neptune was discovered observationally. Credit: Leibniz-Institute for Astrophysics Potsdam
In 1843, English astronomer John Couch Adams began work on the orbit of Uranus using the data he had and produced several different estimates in the following years of the planet’s orbit. In 1845–46, Urbain Le Verrier, independently of Adams, developed his own calculations, which he shared with Johann Gottfried Galle of the Berlin Observatory. Galle confirmed the presence of a planet at the coordinates specified by Le Verrier on September 23rd, 1846.
The announcement of the discovery was met with controversy, as both Le Verrier and Adams claimed responsibility. Eventually, an international consensus emerged that both Le Verrier and Adams jointly deserved credit. However, a re-evaluation by historians in 1998 of the relevant historical documents led to the conclusion that Le Verrier was more directly responsible for the discovery and deserves the greater share of the credit.
Claiming the right of discovery, Le Verrier suggested the planet be named after himself, but this met with stiff resistance outside of France. He also suggested the name Neptune, which was gradually accepted by the international community. This was largely because it was consistent with the nomenclature of the other planets, all of which were named after deities from Greco-Roman mythology.
Neptune’s Size, Mass and Orbit:
With a mean radius of 24,622 ± 19 km, Neptune is the fourth largest planet in the Solar System and four times as large as Earth. But with a mass of 1.0243×1026 kg – which is roughly 17 times that of Earth – it is the third most massive, outranking Uranus. The planet has a very minor eccentricity of 0.0086, and orbits the Sun at a distance of 29.81 AU (4.459 x 109 km) at perihelion and 30.33 AU (4.537 x 109 km) at aphelion.
A size comparison of Neptune and Earth. Credit: NASA
Neptune takes 16 h 6 min 36 s (0.6713 days) to complete a single sidereal rotation, and 164.8 Earth years to complete a single orbit around the Sun. This means that a single day lasts 67% as long on Neptune, whereas a year is the equivalent of approximately 60,190 Earth days (or 89,666 Neptunian days).
Because Neptune’s axial tilt (28.32°) is similar to that of Earth (~23°) and Mars (~25°), the planet experiences similar seasonal changes. Combined with its long orbital period, this means that the seasons last for forty Earth years. Also owing to its axial tilt being comparable to Earth’s is the fact that the variation in the length of its day over the course of the year is not any more extreme than it on Earth.
Neptune’s orbit also has a profound impact on the region directly beyond it, known as the Kuiper Belt (otherwise known as the “Trans-Neptunian Region”). Much in the same way that Jupiter’s gravity dominates the Asteroid Belt, shaping its structure, so Neptune’s gravity dominates the Kuiper Belt. Over the age of the Solar System, certain regions of the Kuiper belt became destabilised by Neptune’s gravity, creating gaps in the Kuiper belt’s structure.
There also exists orbits within these empty regions where objects can survive for the age of the Solar System. These resonances occur when Neptune’s orbital period is a precise fraction of that of the object – meaning they complete a fraction of an orbit for every orbit made by Neptune. The most heavily populated resonance in the Kuiper belt, with over 200 known objects, is the 2:3 resonance.
Objects in this resonance complete 2 orbits for every 3 of Neptune, and are known as plutinos because the largest of the known Kuiper belt objects, Pluto, is among them. Although Pluto crosses Neptune’s orbit regularly, the 2:3 resonance ensures they can never collide.
Neptune has a number of known trojan objects occupying both the Sun–Neptune L4 and L5Lagrangian Points – regions of gravitational stability leading and trailing Neptune in its orbit. Some Neptune trojans are remarkably stable in their orbits, and are likely to have formed alongside Neptune rather than being captured.
Neptune’s Composition:
Due to its smaller size and higher concentrations of volatiles relative to Jupiter and Saturn, Neptune (much like Uranus) is often referred to as an “ice giant” – a subclass of a giant planet. Also like Uranus, Neptune’s internal structure is differentiated between a rocky core consisting of silicates and metals; a mantle consisting of water, ammonia and methane ices; and an atmosphere consisting of hydrogen, helium and methane gas.
The core of Neptune is composed of iron, nickel and silicates, with an interior model giving it a mass about 1.2 times that of Earth. The pressure at the center is estimated to be 7 Mbar (700 GPa), about twice as high as that at the center of Earth, and with temperatures as high as 5,400 K. At a depth of 7000 km, the conditions may be such that methane decomposes into diamond crystals that rain downwards like hailstones.
The mantle is equivalent to 10 – 15 Earth masses and is rich in water, ammonia and methane. This mixture is referred to as icy even though it is a hot, dense fluid, and is sometimes called a “water-ammonia ocean”. Meanwhile, the atmosphere forms about 5% to 10% of its mass and extends perhaps 10% to 20% of the way towards the core, where it reaches pressures of about 10 GPa – or about 100,000 times that of Earth’s atmosphere.
Composition of Neptune. Image credit: NASA
Increasing concentrations of methane, ammonia and water are found in the lower regions of the atmosphere. Unlike Uranus, Neptune’s composition has a higher volume of ocean, whereas Uranus has a smaller mantle.
Neptune’s Atmosphere:
At high altitudes, Neptune’s atmosphere is 80% hydrogen and 19% helium, with a trace amount of methane. As with Uranus, this absorption of red light by the atmospheric methane is part of what gives Neptune its blue hue, although Neptune’s is darker and more vivid. Because Neptune’s atmospheric methane content is similar to that of Uranus, some unknown atmospheric constituent is thought to contribute to Neptune’s more intense coloring.
Neptune’s atmosphere is subdivided into two main regions: the lower troposphere (where temperature decreases with altitude), and the stratosphere (where temperature increases with altitude). The boundary between the two, the tropopause, lies at a pressure of 0.1 bars (10 kPa). The stratosphere then gives way to the thermosphere at a pressure lower than 10-5 to 10-4 microbars (1 to 10 Pa), which gradually transitions to the exosphere.
Neptune’s spectra suggest that its lower stratosphere is hazy due to condensation of products caused by the interaction of ultraviolet radiation and methane (i.e. photolysis), which produces compounds such as ethane and ethyne. The stratosphere is also home to trace amounts of carbon monoxide and hydrogen cyanide, which are responsible for Neptune’s stratosphere being warmer than that of Uranus.
A modified color/contrast image emphasizing Neptune’s atmospheric features, including wind speed. Credit Erich Karkoschka)
For reasons that remain obscure, the planet’s thermosphere experiences unusually high temperatures of about 750 K (476.85 °C/890 °F). The planet is too far from the Sun for this heat to be generated by ultraviolet radiation, which means another heating mechanism is involved – which could be the atmosphere’s interaction with ion’s in the planet’s magnetic field, or gravity waves from the planet’s interior that dissipate in the atmosphere.
Because Neptune is not a solid body, its atmosphere undergoes differential rotation. The wide equatorial zone rotates with a period of about 18 hours, which is slower than the 16.1-hour rotation of the planet’s magnetic field. By contrast, the reverse is true for the polar regions where the rotation period is 12 hours.
This differential rotation is the most pronounced of any planet in the Solar System, and results in strong latitudinal wind shear and violent storms. The three most impressive were all spotted in 1989 by the Voyager 2 space probe, and then named based on their appearances.
The first to be spotted was a massive anticyclonic storm measuring 13,000 x 6,600 km and resembling the Great Red Spot of Jupiter. Known as the Great Dark Spot, this storm was not spotted five later (Nov. 2nd, 1994) when the Hubble Space Telescope looked for it. Instead, a new storm that was very similar in appearance was found in the planet’s northern hemisphere, suggesting that these storms have a shorter life span than Jupiter’s.
Reconstruction of Voyager 2 images showing the Great Black spot (top left), Scooter (middle), and the Small Black Spot (lower right). Credit: NASA/JPL
The Scooter is another storm, a white cloud group located farther south than the Great Dark Spot. This nickname first arose during the months leading up to the Voyager 2 encounter in 1989, when the cloud group was observed moving at speeds faster than the Great Dark Spot.
The Small Dark Spot, a southern cyclonic storm, was the second-most-intense storm observed during the 1989 encounter. It was initially completely dark; but as Voyager 2 approached the planet, a bright core developed and could be seen in most of the highest-resolution images.
Neptune’s Moons:
Neptune has 14 known satellites, all but one of which are named after Greek and Roman deities of the sea (S/2004 N 1 is currently unnamed). These moons are divided into two groups – the regular and irregular moons – based on their orbit and proximity to Neptune. Neptune’s Regular Moons – Naiad, Thalassa, Despina, Galatea, Larissa, S/2004 N 1, and Proteus – are those that are closest to the planet and which follow circular, prograde orbits that lie in the planet’s equatorial plane.
They range in distance from 48,227 km (Naiad) to 117,646 km (Proteus) from Neptune, and all but the outermost two (S/2004 N 1, and Proteus) orbit Neptune slower than its orbital period of 0.6713 days. Based on observational data and assumed densities, these moons range in size and mass from 96 x 60 x 52 km and 1.9 x 1017 kg (Naiad) to 436 x 416 x 402 km and 50.35 x 1017 kg (Proteus).
This composite Hubble Space Telescope picture shows the location of a newly discovered moon, designated S/2004 N 1, orbiting the giant planet Neptune, nearly 4.8 billion km (3 billion miles) from Earth. Credit: NASA, ESA, and M. Showalter (SETI Institute).
With the exception of Larissa and Proteus (which are largely rounded) all of Neptune’s inner moons are believed to be elongated in shape. Their spectra also indicates that they are made from water ice contaminated by some very dark material, probably organic compounds. In this respect, the inner Neptunian moons are similar to the inner moons of Uranus.
Neptune’s irregular moons consist of the planet’s remaining satellites (including Triton). They generally follow inclined eccentric and often retrograde orbits far from Neptune. The only exception is Triton, which orbits close to the planet, following a circular orbit, though retrograde and inclined.
In order of their distance from the planet, the irregular moons are Triton, Nereid, Halimede, Sao, Laomedeia, Neso and Psamathe – a group that includes both prograde and retrograde objects. With the exception of Triton and Nereid, Neptune’s irregular moons are similar to those of other giant planets and are believed to have been gravitationally captured by Neptune.
In terms of size and mass, the irregular moons are relatively consistent, ranging from approximately 40 km in diameter and 4 x 1016 kg in mass (Psamathe) to 62 km and 16 x 1016 kg for Halimede. Triton and Nereid are unusual irregular satellites and are thus treated separately from the other five irregular Neptunian moons. Between these two and the other irregular moons, four major differences have been noted.
First of all, they are the largest two known irregular moons in the Solar System. Triton itself is almost an order of magnitude larger than all other known irregular moons and comprises more than 99.5% of all the mass known to orbit Neptune (including the planet’s rings and thirteen other known moons).
Global Color Mosaic of Triton, taken by Voyager 2 in 1989. Credit: NASA/JPL/USGS
Secondly, they both have atypically small semi-major axes, with Triton’s being over an order of magnitude smaller than those of all other known irregular moons. Thirdly, they both have unusual orbital eccentricities: Nereid has one of the most eccentric orbits of any known irregular satellite, and Triton’s orbit is a nearly perfect circle. Finally, Nereid also has the lowest inclination of any known irregular satellite
With a mean diameter of around 2700 km and a mass of 214080 ± 520 x 1017 kg, Triton is the largest of Neptune’s moons, and the only one large enough to achieve hydrostatic equilibrium (i.e. is spherical in shape). At a distance of 354,759 km from Neptune, it also sits between the planet’s inner and outer moons.
Triton follows a retrograde and quasi-circular orbit, and is composed largely of nitrogen, methane, carbon dioxide and water ices. With a geometric albedo of more than 70% and a Bond albedo as high as 90%, it is also one of the brightest objects in the Solar System. The surface has a reddish tint, owning to the interaction of ultraviolet radiation and methane, causing tholins.
Triton is also one of the coldest moons in the Solar System, with surface temperature of about 38 K (-235.2 °C). However, owing to the moon being geologically active (which results in cryovolcanism) and surface temperature variations that cause sublimation, Triton is one of only two moons in the Solar System that has a substantial atmosphere. Much like it’s surface, this atmosphere is composed primarily of nitrogen with small amounts of methane and carbon monoxide, and with an estimated pressure of about 14 nanobar.
Triton has a relatively high density of about 2 g/cm3 indicating that rocks constitute about two thirds of its mass, and ices (mainly water ice) the remaining one third. There also may be a layer of liquid water deep inside Triton, forming a subterranean ocean. Surface features include the large southern polar cap, older cratered planes cross-cut by graben and scarps, as well as youthful features caused by endogenic resurfacing.
Because of its retrograde orbit and relative proximity to Neptune (closer than the Moon is to Earth), Triton is grouped with the planet’s irregular moons (see below). In addition, it is believed to be a captured object, possibly a dwarf planet that was once part of the Kuiper Belt. At the same time, these orbital characteristics are the reason why Triton experiences tidal deceleration. and will eventually spiral inward and collide with the planet in about 3.6 billion years.
Nereid is the third-largest moon of Neptune. It has a prograde but very eccentric orbit and is believed to be a former regular satellite that was scattered to its current orbit through gravitational interactions during Triton’s capture. Water ice has been spectroscopically detected on its surface. Nereid shows large, irregular variations in its visible magnitude, which are probably caused by forced precession or chaotic rotation combined with an elongated shape and bright or dark spots on the surface.
Neptune’s Ring System:
Neptune has five rings, all of which are named after astronomers who made important discoveries about the planet – Galle, Le Verrier, Lassell, Arago, and Adams. The rings are composed of at least 20% dust (with some containing as much as 70%) while the rest of the material consists of small rocks. The planet’s rings are difficult to see because they are dark and vary in density and size.
The Galle ring was named after Johann Gottfried Galle, the first person to see the planet using a telescope; and at 41,000–43,000 km, it is the nearest of Neptune’s rings. The La Verrier ring – which is very narrow at 113 km in width – is named after French astronomer Urbain Le Verrier, the planet’s co-founder.
At a distance of between 53,200 and 57,200 km from Neptune (giving it a width of 4,000 km) the Lassell ring is the widest of Neptune’s rings. This ring is named after William Lassell, the English astronomer who discovered Triton just seventeen days after Neptune was discovered. The Arago ring is 57,200 kilometers from the planet and less than 100 kilometers wide. This ring section is named after Francois Arago, Le Verrier’s mentor and the astronomer who played an active role in the dispute over who deserved credit for discovering Neptune.
The outer Adams ring was named after John Couch Adams, who is credited with the co-discovery of Neptune. Although the ring is narrow at only 35 kilometers wide, it is the most famous of the five due to its arcs. These arcs accord with areas in the ring system where the material of the rings is grouped together in a clump, and are the brightest and most easily observed parts of the ring system.
Although the Adams ring has five arcs, the three most famous are the “Liberty”, “Equality”, and “Fraternity” arcs. Scientists have been traditionally unable to explain the existence of these arcs because, according to the laws of motion, they should distribute the material uniformly throughout the rings. However, stronomers now estimate that the arcs are corralled into their current form by the gravitational effects of Galatea, which sits just inward from the ring.
The rings of Neptune as seen from Voyager 2 during the 1989 flyby. Credit: NASA/JPL
The rings of Neptune are very dark, and probably made of organic compounds that have been altered due to exposition to cosmic radiation. This is similar to the rings of Uranus, but very different to the icy rings around Saturn. They seem to contain a large quantity of micrometer-sized dust, similar in size to the particles in the rings of Jupiter.
It’s believed that the rings of Neptune are relatively young – much younger than the age of the Solar System, and much younger than the age of Uranus’ rings. Consistent with the theory that Triton was a KBO that was seized, by Neptune’s gravity, they are believed to be the result of a collision between some of the planet’s original moons.
Exploration:
The Voyager 2 probe is the only spacecraft to have ever visited Neptune. The spacecraft’s closest approach to the planet occurred on August 25th, 1989, which took place at a distance of 4,800 km (3,000 miles) above Neptune’s north pole. Because this was the last major planet the spacecraft could visit, it was decided to make a close flyby of the moon Triton – similar to what had been done for Voyager 1‘s encounter with Saturn and its moon Titan.
The spacecraft performed a near-encounter with the moon Nereid before it came to within 4,400 km of Neptune’s atmosphere on August 25th, then passed close to the planet’s largest moon Triton later the same day. The spacecraft verified the existence of a magnetic field surrounding the planet and discovered that the field was offset from the center and tilted in a manner similar to the field around Uranus.
Neptune’s rotation period was determined using measurements of radio emissions and Voyager 2 also showed that Neptune had a surprisingly active weather system. Six new moons were discovered during the flyby, and the planet was shown to have more than one ring.
While no missions to Neptune are currently being planned, some hypothetical missions have been suggested. For instance, a possible Flagship Mission has been envisioned by NASA to take place sometime during the late 2020s or early 2030s. Other proposals include a possible Cassini-Huygens-style “Neptune Orbiter with Probes”, which was suggested back in 2003.
Another, more recent proposal by NASA was for Argo – a flyby spacecraft that would be launched in 2019, which would visit Jupiter, Saturn, Neptune, and a Kuiper belt object. The focus would be on Neptune and its largest moon Triton, which would be investigated around 2029.
With its icy-blue color, liquid surface, and wavy weather patterns, Neptune was appropriately named after the Roman god of the sea. And given its distance from our planet, there is still a great deal that remains to be learned about it. In the coming decades, one can only hope that a mission to the outer Solar System and/or Kuiper Belt includes a flyby of Neptune.
We have many interesting articles about Neptune here at Universe Today. Below is a comprehensive list for your viewing (and possibly researching) pleasure!
Neptune, that icy gas giant that is the eighth planet from our Sun, was discovered in 1846 by two astronomers – Urbain Le Verrier and Johann Galle. In keeping with the convention of planetary nomenclature, Neptune was named after the Roman god of the sea (the equivalent to the Greek Poseidon). And just seventeen days after it was discovered, astronomers began to notice that it too had a system of moons.
Initially, only Triton – Neptune’s largest moon – could be observed. But by the mid-20th century and after, thanks to improvements in ground-based telescopes and the development of robotic space probes, many more moons would be discovered. Neptune now has 14 recognized satellites, and in honor of of their parent planet, all are named for minor water deities in Greek mythology.
Discovery and Naming:
Triton, being the largest and most massive of Neptune’s moons, was the first to be discovered. It was observed by William Lassell on October 10th, 1846, just seventeen days after Neptune was discovered. It would be almost a century before any other moons would be discovered.
The first was Nereid, Neptune’s second largest and most massive moon, which was discovered on May 1st, 1949, by Gerard P. Kuiper (for whom the Kuiper Belt is named) using photographic plates from the McDonald Observatory in Fort Davis, Texas. The third moon, later named Larissa, was first observed by Harold J. Reitsema, William B. Hubbard, Larry A. Lebofsky and David J. Tholen on May 24th, 1981.
Hubble Space Telescope composite picture showing the location of a newly discovered moon, designated S/2004 N 1. Credit: NASA, ESA, and M. Showalter (SETI Institute).
The discovery of this moon was purely fortuitous, and occurred as a result of the ongoing search for rings similar to those discovered around Uranus four years earlier. If rings were in fact present, the star’s luminosity would decrease slightly just before the planet’s closest approach. While observing a star’s close approach to Neptune, the star’s luminosity dipped, but only for several seconds. This indicated the presence of a moon rather than a ring.
No further moons were found until Voyager 2 flew by Neptune in 1989. In the course of passing through the system, the space probe rediscovered Larissa and discovered five additional inner moons: Naiad, Thalassa, Despina, Galatea and Proteus.
In 2001, two surveys using large ground-based telescopes – the Cerro Tololo Inter-American Observatory and the Canada-France-Hawaii telescopes – found five additional outer moons bringing the total to thirteen. Follow-up surveys by two teams in 2002 and 2003 respectively re-observed all five of these moons – which were Halimede, Sao, Psamathe, Laomedeia, and Neso.
And then on July 15th, 2013, a team of astronomers led by Mark R. Showalter of the SETI Institute revealed that they had discovered a previously unknown fourteenth moon in images taken by the Hubble Space Telescope from 2004–2009. The as yet unnamed fourteenth moon, currently identified as S/2004 N 1, is thought to measure no more than 16–20 km in diameter.
In keeping with astronomical convention, Neptune’s moons are all taken from Greek and Roman mythology. In this case, all are named for gods of the sea, or for the children of Poseidon (which include Triton, Proteus, Depsina and Thalassa), minor Greek water dieties (Naiad and Nereid) or Nereids , the water nymphs in Greek mythology (Halimede, Galatea, Neso, Sao, Laomedeia and Psamathe).
However, many of the moons were not officially named until the 20th century. The name Triton, which was originally suggested by Camille Flammarion in his 1880 book Astronomie Populaire, but not into common usage until at least the 1930s.
Inner (Regular) Moons:
Neptune’s Regular Moons are those located closest to the planet and which follow circular prograde orbits that lie in the planet’s equatorial plane. They are, in order of distance from Neptune: Naiad (48,227 km), Thalassa (50,074 km), Despina (52,526 km), Galatea (61,953 km), Larissa (73,548 km), S/2004 N 1 (105,300 ± 50 km), and Proteus (117,646 km). All but the outer two are within Neptune-synchronous orbit (meaning that orbit Neptune slower than it’s orbital period (0.6713 days) and thus are being tidally decelerated.
The inner moons are closely associated with Neptune’s narrow ring system. The two innermost satellites, Naiad and Thalassa, orbit between the Galle and LeVerrier rings, whereas Despina orbits just inside the LeVerrier ring. The next moon, Galatea, orbits just inside the most prominent Adams ring and its gravity helps maintaining the ring by containing its particles.
Based on observational data and assumed densities, Naiad measures 96 × 60 × 52 km and weighs approximately 1.9 x 1017 kg. Meanwhile, Thalassa measures 108 x 100 × 52 km and weighs 3.5 x 1017 kg; Despina measures 180 x 148 x 128 and weighs 21 x 1017 kg; Galatea measures 204 x 184 x 144 and weighs 37.5 x 1017 kg; Larissa measures 216 x 204 x 168 and weighs 49.5 x 1017 kg; S/2004 N1 measures 16-20 km in diameter and weighs 0.5 ± 0.4 x 1017 kg; and Proteus measures 436 x 416 x 402 and weighs 50.35 x 1017 kg.
Only the two largest regular moons have been imaged with a resolution sufficient to discern their shapes and surface features. Nevertheless, with the exception of Larissa and Proteus (which are largely rounded) all of Neptune’s inner moons are believed to be elongated in shape. In addition, all the inner moons dark objects, with geometric albedo ranging from 7 to 10%.
Their spectra also indicated that they are made from water ice contaminated by some very dark material, probably organic compounds. In this respect, the inner Neptunian moons are similar to the inner moons of Uranus.
Outer (Irregular) Moons:
Neptune’s irregular moons consist of the planet’s remaining satellites (including Triton). They generally follow inclined eccentric and often retrograde orbits far from Neptune; the only exception is Triton, which orbits close to the planet following a circular orbit, though retrograde and inclined.
In order of their distance from the planet, the irregular moons are Triton, Nereid, Halimede, Sao, Laomedeia, Neso and Psamathe, a group that includes both prograde and retrograde objects. With the exception of Triton and Nereid, Neptune’s irregular moons are similar to those of other giant planets and are believed to have been gravitationally captured by Neptune.
In terms of size and mass, the irregular moons are relatively consistent, ranging from approximately 40 km in diameter and 4 x 1016 kg in mass (Psamathe) to 62 km and 16 x 1016 kg for Halimede.
Triton and Nereid:
Triton and Nereid are unusual irregular satellites and are thus treated separately from the other five irregular Neptunian moons. Between these two and the other irregular moons, four major differences have been noted.
First of all, they are the largest two known irregular moons in the Solar System. Triton itself is almost an order of magnitude larger than all other known irregular moons and comprises more than 99.5% of all the mass known to orbit Neptune (including the planet’s rings and thirteen other known moons).
Global Color Mosaic of Triton, taken by Voyager 2 in 1989. Credit: NASA/JPL/USGS
Secondly, they both have atypically small semi-major axes, with Triton’s being over an order of magnitude smaller than those of all other known irregular moons. Thirdly, they both have unusual orbital eccentricities: Nereid has one of the most eccentric orbits of any known irregular satellite, and Triton’s orbit is a nearly perfect circle. Finally, Nereid also has the lowest inclination of any known irregular satellite
With a mean diameter of around 2700 km and a mass of 214080 ± 520 x 1017 kg, Triton is the largest of Neptune’s moons, and the only one large enough to achieve hydrostatic equilibrium (i.e. is spherical in shape). At a distance of 354,759 km from Neptune, it also sits between the planet’s inner and outer moons.
Triton follows a retrograde and quasi-circular orbit, and is composed largely of nitrogen, methane, carbon dioxide and water ices. With a geometric albedo of more than 70% and a Bond albedo as high as 90%, it is also one of the brightest objects in the Solar System. The surface has a reddish tint, owning to the interaction of ultraviolet radiation and methane, causing tholins.
Triton is also one of the coldest moons in the Solar System, with surface temperature of about 38 K (?235.2 °C). However, owing to the moon being geologically active (which results in cryovolcanism) and surface temperature variations that cause sublimation, Triton is one of only two moons in the Solar System that has a substantial atmosphere. Much like it’s surface, this atmosphere is composed primarily of nitrogen with small amounts of methane and carbon monoxide, and with an estimated pressure of about 14 ?bar.
Using the CRIRES instrument on ESO’s Very Large Telescope, a team of astronomers has been able to see that the summer is in full swing in Triton’s southern hemisphere. Credit: ESO
Triton has a relatively high density of about 2 g/cm3 indicating that rocks constitute about two thirds of its mass, and ices (mainly water ice) the remaining one third. There also may be a layer of liquid water deep inside Triton, forming a subterranean ocean. Surface features include the large southern polar cap, older cratered planes cross-cut by graben and scarps, as well as youthful features caused by endogenic resurfacing.
Because of its retrograde orbit and relative proximity to Neptune (closer than the Moon is to Earth), Triton is grouped with the planet’s irregular moons (see below). In addition, it is believed to be a captured object, possibly a dwarf planet that was once part of the Kuiper Belt. At the same time, these orbital characteristics are the reason why Triton experiences tidal deceleration. and will eventually spiral inward and collide with the planet in about 3.6 billion years.
Nereid is the third-largest moon of Neptune. It has a prograde but very eccentric orbit and is believed to be a former regular satellite that was scattered to its current orbit through gravitational interactions during Triton’s capture. Water ice has been spectroscopically detected on its surface. Nereid shows large, irregular variations in its visible magnitude, which are probably caused by forced precession or chaotic rotation combined with an elongated shape and bright or dark spots on the surface.
Formation:
Given the lopsided distribution of mass in its moons, it is widely believed that Triton was captured after the formation of Neptune’s original satellite system – much of which would have been destroyed in the process of capture. Many theories have been offered regarding the mechanisms of its capture over the years.
The most widely-accepted is that Triton is a surviving member of a binary Kuiper Belt Object that was disrupted with an encounter with Neptune. In this scenario, Triton’s captured was the result of a three-body encounter, where it fell into a retrograde orbit while the other object was either destroyed or ejected in the process.
Triton’s orbit upon capture would have been highly eccentric, and would have caused chaotic perturbations in the orbits of the original inner Neptunian satellites, causing them to collide and reduce to a disc of rubble. Only after Triton’s orbit became circular again could some of the rubble re-accrete into the present-day regular moons. This means it is likely that Neptune’s present inner satellites are not the original bodies that formed with Neptune.
Numerical simulations show that there is a 0.41 probability that the moon Halimede collided with Nereid at some time in the past. Although it is not known whether any collision has taken place, both moons appear to have similar (“grey”) colors, implying that Halimede could be a fragment of Nereid.
Given its distance from the Sun, the only mission to ever study Neptune and its moons up close was the Voyager 2 mission. And though no missions are currently being planned, several proposals have been made that would see a robotic probe dispatched to the system sometime in the late 2020s or early 2030s.
We have many interesting articles on Neptune, Neptune’s Moons, and the Trans-Neptunian region here at Universe Today. Here’s a full article about Neptune’s Moon Triton, Naiad and Nereid and S/2004 N 1.
It seems as if the planets are fleeing the evening sky, just as the Fall school star party season is getting underway. Venus and Mars have entered the morning sky, and Jupiter reaches solar conjunction this week. Even glorious Saturn has passed eastern quadrature, and will soon depart evening skies.
Enter the ice giants, Uranus and Neptune. Both reach opposition for 2015 over the next two months, and the time to cross these two out solar system planets off your life list is now.
Looking east at dusk in late August, as Uranus and Neptune rise. Image credit: Stellarium
First up, the planet Neptune reaches opposition next week in the constellation Aquarius on the night of August 31st/September 1st. Shining at magnitude +7.8, Neptune spends the remainder of 2015 about three degrees southwest of the +3.7 magnitude star Lambda Aquarii. It’s possible to spot Neptune using binoculars, and about x100 magnification in a telescope eyepiece will just resolve the blue-grey 2.3 arc second disc of the planet. Though Neptune has 14 known moons, just one, Triton, is within reach of a backyard telescope. Triton shines at magnitude +13.5 (comparable to Pluto), and orbits Neptune in a retrograde path once every 6 days, getting a maximum of 15” from the disk of the planet.
The path of Neptune from late August through early November 2015. Inset: the position of Neptune’s moon Triton on the evening of August 31st: Image credit: Starry Night Education software
Uranus reaches opposition on October 11th in the adjacent constellation Pisces. Keep an eye on Uranus, as it nears the bright +5.2 magnitude star Zeta Piscium towards the end on 2015. Shining at magnitude +5.7 with a 3.6 arc second disk, Uranus hovers just on the edge of naked eye visibility from a dark sky site.
Uranus, left of the eclipsed Moon last October. Image credit and copyright: A Nartist
It’ll be worth hunting for Uranus on the night of September 27th/28th, when it sits 15 degrees east of the eclipsed Moon. Uranus turned up in many images of last Fall’s total lunar eclipse. This will be the final total lunar eclipse of the current tetrad, and the Moon will occult Uranus the evening after for the South Atlantic. This is part of a series of 19 ongoing occultations of Uranus by the Moon worldwide, which started in August 2014, and end on December 20th, 2015. After that, the Moon will move on and begin occulting Neptune next year in June through the end of 2017.
The visibility footprint of the September 29th occultation of Uranus by the Moon. Image credit: Occult 4.0.
Uranus has 27 known moons, four of which (Oberon, Ariel, Umbriel and Titania) are visible in a large backyard telescope. See our extensive article on hunting the moons of the solar system for more info, and the JPL/PDS rings node for corkscrew finder charts.
The path of Uranus, from late August through early December 2015. Inset: the position of the moons of Uranus on the evening of October 12th. Image credit: Starry Night Education software
The two outermost worlds have a fascinating entwined history. William Herschel discovered Uranus on the night of March 13th, 1781. We can be thankful that the proposed name ‘George’ after William’s benefactor King George the III didn’t stick. Herschel initially thought he’d discovered a comet, until he followed the slow motion of Uranus over several nights and realized that it had to be something large orbiting at a great distance from the Sun. Keep in mind, Uranus and Neptune both crept onto star charts unnoticed pre-1781. Galileo even famously sketched Neptune near Jupiter in 1612! Early astronomers simply considered the classical solar system out to Saturn as complete, end of story.
A classic 7″ Merz refractor at the Quito observatory, nearly identical to the instrument that first spied Neptune. Image Credit: Dave Dickinson
And the hunt was on. Astronomers soon realized that Uranus wasn’t staying put: something farther still from the Sun was tugging at its orbit. Mathematician Urbain Le Verrier predicted the position of the unseen planet, and on and on the night of September 23rd, 1846, astronomers at the Berlin observatory spied Neptune.
In a way, those early 19th century astronomers were lucky. Neptune and Uranus had just passed each other during a close encounter in 1821. Otherwise, Neptune might’ve remained hidden for several more decades. The synodic period of the two planets—that is, the time it takes the planets to return to opposition—differ by about 2-3 days. The very first documented conjunction of Neptune and Uranus occurred back in 1993, and won’t occur again until 2164. Heck, In 2010, Neptune completed its first orbit since discovery!
To date, only one mission, Voyager 2, has given us a close-up look at Uranus and Neptune during brief flybys. The final planetary encounter for Voyager 2 occurred in late August in 1989, when the spacecraft passed 4,800 kilometres (3,000 miles) above the north pole of Neptune.
All thoughts to ponder as you hunt for the outer ice giants. Sure, they’re tiny dots, but as with many nighttime treats, the ‘wow’ factor comes with just what you’re seeing, and the amazing story behind it.
Need an easy way to remember the order of the planets in our Solar System? The technique used most often to remember such a list is a mnemonic device. This uses the first letter of each planet as the first letter of each word in a sentence. Supposedly, experts say, the sillier the sentence, the easier it is to remember.
So by using the first letters of the planets, (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune), create a silly but memorable sentence.
Here are a few examples:
My Very Excellent Mother Just Served Us Noodles (or Nachos)
Mercury’s Volcanoes Erupt Mulberry Jam Sandwiches Until Noon
Very Elderly Men Just Snooze Under Newspapers
My Very Efficient Memory Just Summed Up Nine
My Very Easy Method Just Speeds Up Names
My Very Expensive Malamute Jumped Ship Up North
The Sun and planets to scale. Credit: Illustration by Judy Schmidt, texture maps by Björn Jónsson
If you want to remember the planets in order of size, (Jupiter, Saturn, Uranus, Neptune, Earth, Venus Mars, Mercury) you can create a different sentence:
Just Sit Up Now Each Monday Morning
Jack Sailed Under Neath Every Metal Mooring
Rhymes are also a popular technique, albeit they require memorizing more words. But if you’re a poet (and don’t know it) try this:
Amazing Mercury is closest to the Sun,
Hot, hot Venus is the second one,
Earth comes third: it’s not too hot,
Freezing Mars awaits an astronaut,
Jupiter is bigger than all the rest,
Sixth comes Saturn, its rings look best,
Uranus sideways falls and along with Neptune, they are big gas balls.
Or songs can work too. Here are a couple of videos that use songs to remember the planets:
If sentences, rhymes or songs don’t work for you, perhaps you are more of a visual learner, as some people remember visual cues better than words. Try drawing a picture of the planets in order. You don’t have to be an accomplished artist to do this; you can simply draw different circles for each planet and label each one. Sometimes color-coding can help aid your memory. For example, use red for Mars and blue for Neptune. Whatever you decide, try to pick colors that are radically different to avoid confusing them.
Or try using Solar System flash cards or just pictures of the planets printed on a page (here are some great pictures of the planets). This works well because not only are you recalling the names of the planets but also what they look like. Memory experts say the more senses you involve in learning or storing something, the better you will be at recalling it.
Planets made from paper lanterns. Credit: TheSweetestOccasion.com
Maybe you are a hands-on learner. If so, try building a three-dimensional model of the Solar System. Kids, ask your parents or guardians to help you with this, or parents/guardians, this is a fun project to do with your children. You can buy inexpensive Styrofoam balls at your local craft store to create your model, or use paper lanterns and decorate them. Here are several ideas from Pinterest on building a 3-D Solar System Model.
If you are looking for a group project to help a class of children learn the planets, have a contest to see who comes up with the silliest sentence to remember the planets. Additionally, you can have eight children act as the planets while the rest of the class tries to line them up in order. You can find more ideas on NASA’s resources for Educators. You can use these tricks as a starting point and find more ways of remembering the planets that work for you.
Global Color Mosaic of Triton, taken by Voyager 2 in 1989. Credit: NASA/JPL/USGS
The planets of the outer Solar System are known for being strange, as are their many moons. This is especially true of Triton, Neptune’s largest moon. In addition to being the seventh-largest moon in the Solar System, it is also the only major moon that has a retrograde orbit – i.e. it revolves in the direction opposite to the planet’s rotation. This suggests that Triton did not form in orbit around Neptune, but is a cosmic visitor that passed by one day and decided to stay.
And like most moons in the outer Solar System, Triton is believed to be composed of an icy surface and a rocky core. But unlike most Solar moons, Triton is one of the few that is known to be geologically active. This results in cryovolcanism, where geysers periodically break through the crust and turn the surface Triton into what is sure to be a psychedelic experience!
Discovery and Naming:
Triton was discovered by British astronomer William Lassell on October 10th, 1846, just 17 days after the discovery of Neptune by German astronomer Johann Gottfried Galle. After learning about the discovery, John Herschel – the son of famed English astronomer William Herschel, who discovered many of Saturn’s and Uranus’ moons – wrote to Lassell and recommended he observe Neptune to see if it had any moons as well.
New Horizons image of Neptune and its largest moon, Triton, taken by the LORRI instrument on June 23, 2010. Credit: NASA
Lassell did so and discovered Neptune’s largest moon eight days later. Thirty-four years later, French astronomer Camille Flammarion named the moon Triton – after the Greek sea god and son of Poseidon (the equivalent of the Roman god Neptune) – in his 1880 bookAstronomie Populaire. It would be several decades before the name caught on however. Until the discovery of the second moon Nereid in 1949, Triton was commonly known simply as “the satellite of Neptune”.
Size, Mass and Orbit:
At 2.14 × 1022 kg, and with a diameter of approx. 2,700 kilometers (1,680 miles) km, Triton is the largest moon in the Neptunian system – comprising more than 99.5% of all the mass known to orbit the planet. In addition to being the seventh-largest moon in the Solar System, it is also more massive than all known moons in the Solar System smaller than itself combined.
With no axial tilt and an eccentricity of virtually zero, the moon orbits Neptune at a distance of 354,760 km (220,438 miles). At this distance, Triton is the farthest satellite of Neptune, and orbits the planet every 5.87685 Earth days. Unlike other moons of its size, Triton has a retrograde orbit around its host planet.
Most of the outer irregular moons of Jupiter and Saturn have retrograde orbits, as do some of Uranus’s outer moons. However, these moons are all much more distant from their primaries, and are rather small in comparison. Triton also has a synchronous orbit with Neptune, which means it keeps one face aimed towards the planet at all times.
As Neptune orbits the Sun, Triton’s polar regions take turns facing the Sun, resulting in seasonal changes as one pole, then the other, moves into the sunlight. Such changes were observed in April of 2010 by astronomers using the European Southern Observatory’s Very Large Telescope.
Another all-important aspect of Triton’s orbit is that it is decaying. Scientists estimate that in approximately 3.6 billion years, it will pass below Neptune’s Roche limit and will be torn apart.
Composition:
Triton has a radius, density (2.061 g/cm3), temperature and chemical composition similar to thatof Pluto. Because of this, and the fact that it circles Neptune in a retrograde orbit, astronomers believe that the moon originated in the Kuiper Belt and later became trapped by Neptune’s gravity.
Another theory has it that Triton was once a dwarf planet with a companion. In this scenario, Neptune captured Triton and flung its companion away when the giant gas moved further out into the solar system, billions of years ago.
Also like Pluto, 55% of Triton’s surface is covered with frozen nitrogen, with water ice comprising 15–35% and dry ice (aka. frozen carbon dioxide) forming the remaining 10–20%. Trace amounts of methane and carbon monoxide ice are believed to exist there as well, as are small amounts of ammonia (in the form of ammonia dihydrate in the lithosphere).
Triton’s density suggests that its interior is differentiated between a solid core made of rocky material and metals, a mantle composed of ice, and a crust. There is enough rock in Triton’s interior for radioactive decay to power convection in the mantle, which may even be sufficient to maintain a subterranean ocean. As with Jupiter’s moon of Europa, the proposed existence of this warm-water ocean could mean the presence of life beneath the icy crusts.
Atmosphere and Surface Features:
Triton has a considerably high albedo, reflecting 60–95% of the sunlight that reaches it. The surface is also quite young, which is an indication of the possible existence of an interior ocean and geological activity. The moon has a reddish tint, which is probably the result of the methane ice turning to carbon due to exposure to ultraviolet radiation.
Triton is considered to be one of the coldest places in the Solar System. The moon’s surface temperature is approx. -235°C while Pluto averages about -229°C. Scientists say that Pluto may drop as low as -240°C at the furthest point from the Sun in its orbit, but it also gets much warmer closer to the Sun, giving it a higher overall temperature average.
It is also one of the few moons in the Solar System that is geologically active, which means that its surface is relatively young due to resurfacing. This activity also results in cryovolcanism, where water ammonia and nitrogen gas burst forth from the surface instead of liquid rock. These nitrogen geysers can send plumes of liquid nitrogen 8 km above the surface of the moon.
Triton (lower left) compared to the Moon (upper left) and Earth (right), to scale. Credit: NASA/JPL/USGS
Because of the geological activity constantly renewing the moon’s surface, there are very few impact craters on Triton. Like Pluto, Triton has an atmosphere that is thought to have resulted from the evaporation of ices from its surface. Like its surface ices, Triton’s tenuous atmosphere is made up of nitrogen with trace amounts of carbon monoxide and small amounts of methane near the surface.
This atmosphere consists of a troposphere rising to an altitude of 8km, where it then gives way to a thermosphere that reaches out to 950 km from the surface. The temperature of Triton’s upper atmosphere, at 95-100 K (ca.-175 °C/-283 °F) is higher than that at the surface, due to the influence of solar radiation and Neptune’s magnetosphere.
A haze permeates most of Triton’s troposphere, thought to be composed largely of hydrocarbons and nitriles created by the action of sunlight on methane. Triton’s atmosphere also has clouds of condensed nitrogen that lie between 1 and 3 km from the surface.
Observations taken from Earth and by the Voyager 2 spacecraft have shown that Triton experiences a warm summer season every few hundred years. This could be the result of a periodic change in the planet’s albedo (i.e. its gets darker and redder) which could be caused by either frost patterns or geological activity.
Using the CRIRES instrument on ESO’s Very Large Telescope, a team of astronomers has been able to see that the summer is in full swing in Triton’s southern hemisphere. Credit: ESO
This change would allow more heat to be absorbed, followed by an increase in sublimation and atmospheric pressure. Data collected between 1987 and 1999 indicated that Triton was approaching one of these warm summers.
Exploration:
When NASA’s Voyager 2 made a flyby of Neptune in August of 1989, the mission controllers also decided to conduct a flyby of Triton – similar to Voyager 1‘s encounter with Saturn and Titan. When it made its flyby, most of the northern hemisphere was in darkness and unseen by Voyager.
Because of the speed of Voyager’s visit and the slow rotation of Triton, only one hemisphere was seen clearly at close distance. The rest of the surface was either in darkness or seen as blurry markings. Nevertheless, the Voyager 2 spacecraft managed to capture several images of the moon and spotted geysers of liquid nitrogen blasting out of two distinct features on the surface.
In August of 2014, in anticipation of New Horizons impending encounter with Pluto, NASA restored these photos and used them to create the first global color map of Triton. Produced by Paul Schenk, a scientist at the Lunar and Planetary Institute in Houston, the map was also used to make a movie (shown below) that recreated the historic Voyager 2 encounter in time for the 25th anniversary of the event.
Yes, Triton is indeed an unusual moon. Aside from its rather unique characteristics (retrograde motion, geological activity) the moon’s landscape is likely to be an amazing sight. For anyone standing on the surface, surrounded by colorful ices, plumes of nitrogen and ammonia, a nitrogen haze and Neptune’s big blue disc hanging on the sky, the experience would seem like something akin to a hallucination.
In the end, it is too bad that the Solar System will one day be saying good-bye to this moon. Because of the nature of its orbit, the moon will eventually fall into Neptune’s gravity well and break up. At which point, Neptune will have a huge ring like Saturn, until those particles crash into the planet as well.
That too would be something to behold. One can only hope that humanity will still be around in 3.6 billion years to witness it!
