Neptune’s Moon Triton

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. June 23, 2010. Credit: NASA
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 book Astronomie 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
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
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!

We have many interesting articles on Triton, Neptune, and the outer planets of the Solar System here at Universe Today.

Here’s one about the New Map of Triton, and one about the Underground Ocean it might be hiding, and 40 Years of Summer on Triton. And here’s Why You Shouldn’t Buy Real Estate on Triton.

In the Observatory also has an interview with Emily Lakdawalla, the senior editor and planetary evangelist for the Planetary Society, titled “Where Should We Look for Life in the Solar System?

Sources:

Red-faced Pluto Full of Surprises

New Horizons scientists combined the latest black and white map of Pluto’s surface features (left) with a map of the planet’s colors (right) to produce a detailed color portrait of the planet’s northern hemisphere (center). Credits: NASA/JHUAPL/SWRI

Hey, Mars, you’ve got company. Looks like there’s a second “red planet” in the Solar System — Pluto. Color images returned from NASA’s New Horizons spacecraft, now just 10 days from its encounter with the dwarf planet, show a distinctly ruddy surface with patchy markings that strongly resemble Mars’ appearance in a small telescope.

Animation of Pluto rotating from photos taken by New Horizons two weeks before the flyby. Credit:
Animation of Pluto’s rotation from photos taken by New Horizons two weeks before the flyby. What are those four nearly parallel dark streaks? Credit: NASA/JHUAPL/SWRI

On Mars, iron oxide or rust colors the planet’s soil, while Pluto’s coloration is likely caused by hydrocarbon molecules called tholins that are formed when cosmic rays and solar ultraviolet light interact with methane in Pluto’s atmosphere and on its surface. Airborne tholins fall out of the atmosphere and coat the surface with a reddish gunk.

Scientists at Johns Hopkins University’s Hörst Laboratory have produced complex chemical compounds called tholins, which may give Pluto its reddish hue. Credits: Chao He, Xinting Yu, Sydney Riemer, and Sarah Hörst, Johns Hopkins University
Scientists at Johns Hopkins University’s Hörst Laboratory have produced complex chemical compounds called tholins, which may give Pluto its reddish hue.
Credits: Chao He, Xinting Yu, Sydney Riemer, and Sarah Hörst, Johns Hopkins University

A particular color or wavelength of UV light called Lyman-alpha is most effective at stimulating the chemical reactions that build hydrocarbons at Pluto. Recent measurements with New Horizons’ Alice instrument reveal the diffuse glow of Lyman-alpha light all around the dwarf planet coming from all directions of space, not just the Sun.

Since one of the main sources of Lyman-alpha light besides the Sun are regions of vigorous star formation in young galaxies, Pluto’s cosmetic rouge may originate in events happening millions of light years away.

Triton's pink too! Montage of Neptune's largest moon, Triton (1,683 miles in diameter) and the planet Neptune showing the moon's sublimating south polar cap (bottom) and enigmatic "cantaloupe terrain". Credit: NASA
Triton’s pink too! Montage of Neptune’s largest moon, Triton (1,683 miles in diameter) and the planet Neptune showing the moon’s sublimating south polar cap (bottom) and enigmatic “cantaloupe terrain”. Photo taken by Voyager 2 in 1989. Credit: NASA

“Pluto’s reddish color has been known for decades, but New Horizons is now allowing us to correlate the color of different places on the surface with their geology and soon, with their compositions,” said New Horizons principal investigator Alan Stern of the Southwest Research Institute, Boulder, Colorado.

Tholins have been found on other bodies in the outer Solar System, including Titan and Triton, the largest moons of Saturn and Neptune, respectively, and made in laboratory experiments that simulate the atmospheres of those bodies.

True color photos showing the two hemispheres of Pluto. At right, you can clearly see the four streaks in a row. New Horizons will approach fly by the hemisphere on the left side.
True color photos showing the two hemispheres of Pluto photographed on June 27, 2015. At left, a large, dark red patch is visible. The four streaks in a row are seen at right. New Horizons will fly by the hemisphere in the left image. Credit:  NASA/JHUAPL/SWRI

As you study the photos and animation, you’ll notice that Pluto’s largest dark spot is redder than the most of the surface; you also can’ help but wonder what’s going on with those four evenly-spaced dark streaks in the equatorial zone. When I first saw them, my reaction was “no way!” They look so neatly lined up I assumed it was an image artifact, but after seeing the rotating movie, maybe not. It’s more likely that low resolution enhances the appearance of alignment.

Dark streaks on Triton formed by deposits from ice or cryovolcanos. Credit: NASA
Dark streaks on Triton deposited downwind from ice or cryovolcanos. Credit: NASA

But what are they? Located as they are on the Charon-facing side of Pluto, they may be related to long-ago tidal stresses induced by each body on the other as they slowly settled into their current tidally-locked embrace or something as current as seasonal change.

Voyager 2 photographed cyrovolcanos at Triton during its 1989 flyby of the Neptune system. Nitrogen geysers and plumes of gas and ice as high as 5 miles (8 km) were seen erupting from active volcanoes, leaving dark streaks on its icy surface.

Images showing the increase in detail from late June through July 1 as New Horizons homes in on Pluto. Credit:
Images showing the increase in detail from late June through July 1 as New Horizons homes in on Pluto. That possible big crater (seen in bottom middle photo) now looks more like a large, dark patch, BUT we still don’t know for sure what it is. Credit: NASA / JHUAPL / SwRI / Björn Jónsson
It's instructive to compare these images based on observations with the Hubble Space Telescope made well before New Horizons's arrival. They appear to record the large dark spot and possible the multiple streaks. Credit: NASA/ESA
It’s instructive to compare these images, based on observations with the Hubble Space Telescope made well before New Horizons’s arrival, with current photos. They appear to record the large dark spot and possibly the multiple streaks. Credit: NASA/ESA

Seasonal heating from the Sun is the most likely cause for Triton’s eruptions; Pluto’s dark streaks may have a similar origin.

Animation of Pluto and Charon from images taken between June 23 and June 29. Credit:
Animation of Pluto and Charon from images taken between June 23 and June 29. Credit: NASA/JHUAPL/SWR
To give you a better picture in your head how big these small bodies are, Pluto and Charon would both fit within the United States with room to spare. Credit: Laboratory for Atmospheric and Space Physics (LASP)
To better picture in your head how big these small bodies really are, Pluto and Charon would both fit within the United States with room to spare. Credit: Laboratory for Atmospheric and Space Physics (LASP)

Today, New Horizons lies just 7.4 million miles (11.9 million km) from its target. Sharpness and detail visible will rapidly improve in just a few days.

“Even at this resolution, Pluto looks like no other world in our Solar System,” said mission scientist Marc Buie of the Southwest Research Institute, Boulder in a recent press release.

Indeed!

‘Death Star’ Ocean? Seven Moons That Could Host Huge Hidden Liquid Reservoirs

A view of Mimas from the Cassini spacecraft. Credit: NASA/JPL/Space Science Institute

Could there be an ocean hidden somewhere in that Death Star-like picture? This is an image of Mimas, a moon of Saturn, and just yesterday (Oct. 15) newly released data from the Cassini spacecraft suggests there are big liquid reservoirs underneath its surface.

“The amount of the to-and-fro motion indicates that Mimas’ interior is not uniform. These wobbles can be produced if the moon contains a weirdly shaped, rocky core or if a sub-surface ocean exists beneath its icy shell,” said Cornell University in a press release. More flybys with the Cassini spacecraft will be required to learn more about what lies beneath.

You can read more about the study (led by Cornell astronomy research associate Radwan Tajeddine) in Science, where it was published. Below, learn more about other worlds in the Solar System that could host oceans under their surface.

Enceladus

Recent Cassini images of Saturn's moon Enceladus backlit by the sun show the fountain-like sources of the fine spray of material that towers over the south polar region. This image was taken looking more or less broadside at the "tiger stripe" fractures observed in earlier Enceladus images. It shows discrete plumes of a variety of apparent sizes above the limb (edge) of the moon. This image was acquired on Nov. 27, 2005.   Image Credit:   NASA/JPL/Space Science Institute
Cassini images of Saturn’s moon Enceladus backlit by the sun show the fountain-like sources of the fine spray of material that towers over the south polar region. This image was taken looking more or less broadside at the “tiger stripe” fractures observed in earlier Enceladus images. It shows discrete plumes of a variety of apparent sizes above the limb (edge) of the moon. This image was acquired on Nov. 27, 2005. Image Credit: NASA/JPL/Space Science Institute

After nearly a decade of speculation, this year the Cassini spacecraft returned gravity data suggesting Enceladus (another moon of Saturn) does have a large subsurface ocean near its south pole, if not a global ocean. If confirmed, that could help explain why scientists see water gushing out of fractures in that area. As this recent paper by Cassini scientists shows, Enceladus is a promising location for habitability.

Titan

A halo of light surrounds Saturn's moon Titan in this  backlit picture, showing its atmosphere. Credit: NASA/JPL/Space Science Institute
A halo of light surrounds Saturn’s moon Titan in this backlit picture, showing its atmosphere. Credit: NASA/JPL/Space Science Institute

By the way, anyone noticed that we still haven’t even left Saturn’s system? Titan is usually high on astrobiology wish lists for researchers because its hydrocarbon chemistry could be precursors to how life evolved. What’s not talked about as much, though, is at least two research findings pointing to evidence of a hidden ocean. Evidence comes from Titan’s tidal flexing from interacting with Saturn — which is 10 times more than what would be expected with a solid core — and the way that it moves on its own axis as well as around Saturn.

Europa

Rendering showing the location and size of water vapor plumes coming from Europa's south pole. Credit: NASA/ESA/L. Roth/SWRI/University of Cologne
Rendering showing the location and size of water vapor plumes coming from Europa’s south pole. Credit: NASA/ESA/L. Roth/SWRI/University of Cologne

That Minecraft-looking object floating beside Europa there is a rendering showing where water vapor erupted from the Jovian moon, spotted by the Hubble Space Telescope in 2013. We were lucky enough to have a close-up view of Europa in the 1990s and early 2000s courtesy of NASA’s Galileo spacecraft. What we know for sure is there’s thick ice on Europa. What’s underneath is not known, but there’s long been speculation that it could be a subsurface ocean that may have more water than our own planet.

Io

Jupiter's volcanic moon Io , imaged by the Galileo spacecraft in 1997. Credit: NASA/JPL/University of Arizona
Jupiter’s volcanic moon Io , imaged by the Galileo spacecraft in 1997. Credit: NASA/JPL/University of Arizona

Still flying around Jupiter here, we now turn our attention to Io — a place that is often remarked upon because of its blotchy appearance as well as all of the volcanoes on its surface. A newer analysis of Galileo data in 2011 — looking at some of the lesser-understood magnetic field data signatures — led one research team to conclude there could be a magma ocean lurking underneath that violence.

Triton

A glimpse of Triton from the Voyager 2 spacecraft, which flew by the Neptunian moon in August 1989. Credit: NASA/JPL
A glimpse of Triton from the Voyager 2 spacecraft, which flew by the Neptunian moon in August 1989. Credit: NASA/JPL

Little is known about Triton because only one spacecraft whizzed by it — Voyager 2, which took a running pass through the Neptune system in August 1989. An Icarus paper two years ago speculated that the world could host a subsurface ocean, but more data is needed. The energy of Neptune (which captured Triton long ago) could have melted its interior through tidal heating, possibly creating water from the ice in its crust.

Charon

Hubble image of Pluto and some of its moons, Charon, Nix and Hydra. Image Credit: NASA, ESA, H. Weaver (JHU/APL), A. Stern (SwRI), and the HST Pluto Companion Search Team
Hubble image of Pluto and some of its moons, Charon, Nix and Hydra. Image Credit: NASA, ESA, H. Weaver (JHU/APL), A. Stern (SwRI), and the HST Pluto Companion Search Team

We don’t have any close-up pictures of this moon of Pluto yet, but just wait a year. The New Horizons spacecraft will zoom past Charon and the rest of the system in July 2015. In the meantime, however, findings based on a model came out this summer in Icarus suggesting Charon — despite being so far from the Sun — might have had a subsurface ocean in the past. Or even now. The key is its once eccentric orbit, which would have produced tidal heating while interacting with Pluto. The science team plans to look for cracks that could be indicative of “the structure of the moon’s interior and how easily it deforms, and how its orbit evolved,” stated Alyssa Rhoden of NASA’s Goddard Space Flight Center in Maryland, who led the research.

Pluto Spacecraft Planning? New Map Of Neptune’s Icy Triton Could Prepare For 2015 Encounter

The southern hemisphere of Neptune's moon Triton, at a resolution of 600 meters (1,969 feet) per pixel. Credit: Paul Schenk (LPI, Houston) from Voyager 2 images acquired August 1989

Talk about recycling! Twenty-five years after Voyager 2 zinged past Neptune’s moon Triton, scientists have put together a new map of the icy moon’s surface using the old data. The information has special relevance right now because the New Horizons spacecraft is approaching Pluto fast, getting to the dwarf planet in less than a year. And it’s quite possible that Pluto and Triton will look similar.

Triton has an exciting history. Scientists believed it used to be a lone wanderer until Neptune captured it, causing tidal heating that in turn created fractures, volcanoes and other features on the surface. While Triton and Pluto aren’t twins — this certainly didn’t happen to Pluto — Pluto also has frozen volatiles on its surface such as carbon monoxide, methane and nitrogen.

What you see in the map is a slightly enhanced version of Triton’s natural colors, bearing in mind that Voyager’s sensors are a little different from the human eye. Voyager 2 only did a brief flyby, so only about half the planet has been imaged. Nonetheless, the encounter was an exciting time for Paul Schenk, a planetary scientist at the Lunar and Planetary Institute in Houston. He led the creation of the new Triton map, and wrote about the experience of Voyager 2 in a blog post.

“Triton is a near twin of Pluto,” wrote Schenk. “Triton and Pluto are both slightly smaller than Earth’s Moon, have very thin nitrogen atmospheres, frozen ices on the surface (carbon monoxide, carbon dioxide, methane and nitrogen), and similar bulk composition (a mixture of ices, including water ice, and rock.  Triton however was captured by Neptune long time ago and has been wracked by intense heating ever since.  This has remade its surface into a tortured landscape of overturned layers, volcanism, and erupting geysers.”

He also added speculation about what will be seen at Pluto. Will it be a dead planet, or will geology still be affecting its surface? How close will Triton be to Pluto, particularly regarding its volcanoes? Only a year until we know for sure.

Sources: NASA, Lunar and Planetary Institute, Paul Schenk

Why You Shouldn’t ‘Buy Real Estate’ On Neptune’s Moon Triton

Neptune's largest Moon, Triton. Astronomers think that Triton is a captured Kuiper Belt Object. Credit: NASA/JPL

Leaving aside the complications of space treaties, a new video lays out another case for why you wouldn’t want to purchase property on Triton — at least, if you were buying for the ultra-long term, over millions of years. The moon is being slowed down by Neptune and will eventually crash or break up into a ring system.

All joking aside, the video also puts forward an interesting hypothesis: that Triton was once a dwarf planet, with a companion, and that Neptune captured Triton and flung the companion away when the giant gas planet moved further out into the solar system, billions of years ago.

Checking into the theory’s credentials, it’s worth noting that the author — Kurzgesagt — represents a startup company that has posted other videos about the solar system. They’re cutely done, although the company’s website does not appear to list any names, at least yet; they describe themselves as a “team of designers, journalists and musicians.”  (That might be because they’re operating in “stealth mode”, a term describing startups that aren’t quite ready to make their idea or founders public yet.)

The theory Kurzgesagt cites is peer-reviewed, however. A 2006 Nature paper called “Neptune’s capture of its moon Triton in a binary–planet gravitational encounter” describes Triton as being part of a binary system in the past, somewhat similar to Pluto and Charon.

NASA’s web page about Triton doesn’t mention the binary system or dwarf planet hypothesis, but says “scientists think Triton is a Kuiper Belt Object captured by Neptune’s gravity millions of years ago.” (The Kuiper Belt is a collection of objects near Neptune’s orbit.)

Some of the reasons include its strange orbital motion that is opposite to Neptune’ s rotation, and the fact that Triton is overwhelmingly the largest moon in the system — suggesting it ejected other ones when it was captured.

Makes you want to send another spacecraft to Neptune, doesn’t it? The first and only visitor there, Voyager 2, flew past there in 1989.

Is Triton Hiding an Underground Ocean?

Voyager 2 mosaic of Neptune’s largest moon, Triton (NASA)

At 1,680 miles (2,700 km) across, the frigid and wrinkled Triton is Neptune’s largest moon and the seventh largest in the Solar System. It orbits the planet backwards – that is, in the opposite direction that Neptune rotates – and is the only large moon to do so, leading astronomers to believe that Triton is actually a captured Kuiper Belt Object that fell into orbit around Neptune at some point in our solar system’s nearly 4.7-billion-year history.

Briefly visited by Voyager 2 in late August 1989, Triton was found to have a curiously mottled and rather reflective surface nearly half-covered with a bumpy “cantaloupe terrain” and a crust made up of mostly water ice, wrapped around a dense core of metallic rock. But researchers from the University of Maryland are suggesting that between the ice and rock may lie a hidden ocean of water, kept liquid despite estimated temperatures of  -97°C (-143°F), making Triton yet another moon that could have a subsurface sea.

How could such a chilly world maintain an ocean of liquid water for any length of time? For one thing, the presence of ammonia inside Triton would help to significantly lower the freezing point of water, making for a very cold — not to mention nasty-tasting — subsurface ocean that refrains from freezing solid.

In addition to this, Triton may have a source of internal heat — if not several. When Triton was first captured by Neptune’s gravity its orbit would have initially been highly elliptical, subjecting the new moon to intense tidal flexing that would have generated quite a bit of heat due to friction (not unlike what happens on Jupiter’s volcanic moon Io.) Although over time Triton’s orbit has become very nearly circular around Neptune due to the energy loss caused by such tidal forces, the heat could have been enough to melt a considerable amount of water ice trapped beneath Triton’s crust.

Related: Titan’s Tides Suggest a Subsurface Sea

Another possible source of heat is the decay of radioactive isotopes, an ongoing process which can heat a planet internally for billions of years. Although not alone enough to defrost an entire ocean, combine this radiogenic heating with tidal heating and Triton could very well have enough warmth to harbor a thin, ammonia-rich ocean beneath an insulating “blanket” of frozen crust for a very long time — although eventually it too will cool and freeze solid like the rest of the moon. Whether this has already happened or still has yet to happen remains to be seen, as several unknowns are still part of the equation.

“I think it is extremely likely that a subsurface ammonia-rich ocean exists in Triton,” said Saswata Hier-Majumder at the University of Maryland’s Department of Geology, whose team’s paper was recently published in the August edition of the journal Icarus. “[Yet] there are a number of uncertainties in our knowledge of Triton’s interior and past which makes it difficult to predict with absolute certainty.”

Still, any promise of liquid water existing elsewhere in large amounts should make us take notice, as it’s within such environments that scientists believe lie our best chances of locating any extraterrestrial life. Even in the farthest reaches of the Solar System, from the planets to their moons, into the Kuiper Belt and even beyond, if there’s heat, liquid water and the right elements — all of which seem to be popping up in the most surprising of places — the stage can be set for life to take hold.

Read more about this here on Astrobiology.net.

Inset image: Voyager 2 portrait of Neptune and Triton taken on August 28, 1989. (NASA)

“Pluto-Killer” Sets Sights on Neptune

Infrared image of Neptune from Keck Observatory in Hawaii. Credit: Mike Brown/CalTech

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The confessed (and remorseless) “Pluto Killer” Mike Brown has turned his gaze – and the 10-meter telescope at the Keck Observatory in Hawaii – on Neptune, our solar system’s furthest “official” planet. But no worries for Neptune – Mike isn’t after its planetary status… he’s taken some beautiful infrared images instead!

Normally only visible as a featureless blue speck in telescopes, Brown’s image of Neptune — along with its largest moon Triton —  shows the icy gas giant in infrared light, glowing bright red and orange.

Neptune and Triton in infrared. Credit: Mike Brown/CalTech.

Brown’s initial intention was not just to get some pretty pictures of planets. The target of the imaging mission was Triton and to learn more about the placement of its methane, nitrogen and seasonal frosts, and this sort of research required infrared imaging. Of course, Neptune turned out to be quite photogenic itself.

“The big difference is doing the imaging in the infrared where methane absorbs most of the photons,” said Brown. “So the bright places are high clouds where the sunlight reflects off of them before it had a chance to pass through much of the atmosphere. Dark is clear atmosphere full of methane absorption.

“I just thought it was so spectacular that I should post it.”

No argument here, Mike!

Neptune, now officially the outermost planet in our solar system, is the fourth largest planet and boasts the highest wind speeds yet discovered — 1,250 mph winds scream around its frigid skies! Like the other gas giants Neptune has a system of rings, although nowhere near as extravagant as Saturn’s. It has 13 known moons, of which Triton is the largest.

With its retrograde orbit, Triton is believed to be a captured Kuiper Belt Object now in orbit around Neptune. Kuiper Belt Objects are Mike Brown’s specialty, as he is the astronomer most well-known for beginning the whole process that got Pluto demoted from the official planet list back in 2006.

Read more on Skymania.com here.

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Jason Major is a graphic designer, photo enthusiast and space blogger. Visit his website Lights in the Dark and follow him on Twitter @JPMajor or on Facebook for the most up-to-date astronomy awesomeness!

Guest Post: Drifting on Alien Winds: Exploring the Skies and Weather of Other Worlds

Triton Probe: Neptune’s blue skies may be visited by beachball-sized methane raindrops. (painting ©Michael Carroll)

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Editor’s note: We all want to explore other worlds in our solar system, but perhaps you haven’t considered the bizarre weather you’d encounter — from the blistering hurricane-force winds of Venus to the gentle methane rain showers of Saturn’s giant moon Titan. Science journalist Michael Carroll has written a guest post for Universe Today which provides peek at the subject matter for his new book, “Drifting on Alien Winds: Exploring the Skies and Weather of Other Worlds.

It’s been a dramatic year for weather on Earth. Blizzards have blanketed the east coast, crippling traffic and power grids. Cyclone Tasha drenched Queensland, Australia as rainfall swelled the mighty Mississippi, flooding the southern US. Eastern Europe and Asia broke high temperature records. But despite these meteorological theatrics, the Earth’s conditions are a calm echo of the weather on other worlds in our solar system.


Take our nearest planetary neighbor, Venus. Nearly a twin of Earth in size, Venus displays truly alien weather. The hurricane-force Venusian winds are ruled not by water (as on Earth), but by battery acid. Sunlight tears carbon dioxide molecules (CO2) apart in a process called photodissociation. Leftover bits of molecules frantically try to combine with sulfur and water to become chemically stable, resulting acid hazes. Temperatures soar to 900ºF at the surface, where air is as dense as the Earthly oceans at a depth of X feet.

Venus is the poster child of comparative planetology, the study of other planets to help us understand our own. Earth’s simmering sibling has taught us about greenhouse gases, and gave us an even more immediate cautionary tale in 1978. The Pioneer Venus orbiter discovered that Venus naturally generates chlorofluorocarbons (CFCs) in its atmosphere. These CFCs were tearing holes in the planet’s ozone. At the same time, a wide variety of industries were preparing to use CFCs in insecticides, spray paints, and other aerosol products. Venus presented us with a warning that may have averted a planet-wide crisis.

In the same way, Mars has provided insights into long-term climate change. Its weather is a simplified version of our own. Locked within its rocks and polar caps lie records of changing climate over eons.

Jupiter’s Great Red Spot is a cyclone larger than two Earths. (photomontage ©Michael Carroll)

But fans of really extreme weather must venture further out, to the outer planets. Jupiter and Saturn are giant balls of gas with no solid surface, and are known as the “gas giants.” They are truly gigantic: over a thousand Earths could fit within Jupiter itself.

The skies of Jupiter and Saturn are dominated by hydrogen and helium, the ancient building blocks of the solar system. Ammonia mixes in to produce a rich brew of complex chemistry, painting the clouds of Jupiter and Saturn in tans and grays. Lightning bolts sizzle through the clouds, powerful enough to electrify a small city for weeks. Ammonia forms rain and snow in the frigid skies. Jupiter’s Great Red Spot is a centuries-old cyclone large enough to swallow three Earths. Saturn has its own bizarre storms: a vast hexagon-shaped trough of clouds races across the northern hemisphere. Over the south pole, a vast whirlpool gazes from concentric clouds like a Cyclops.

Clouds tower into a twilight sky on Saturn. The planet’s glowing rings seem to bend at the horizon because of the dense air. (painting ©Michael Carroll)

Beyond Jupiter and Saturn lie the “ice giants”, Uranus and Neptune. These behemoths host atmospheres of poisonous brews chilled to cryogenic temperatures. Methane tints Uranus and Neptune blue. Neptune’s clear air reveals a teal cloud deck. Hydrocarbon hazes tinge Uranus to a paler shade of blue-green. Neptune’s clear air is somewhat of a mystery to scientists. This may be because cloud-forming particles can’t stay airborne long enough to become visible clouds. Some scientists propose that Neptune’s abundant methane rains may condense so rapidly that within a few seconds tiny methane raindrops swell to something the size of a beachball. There are no clouds adrift, because methane rains out of the atmosphere too quickly.

One of the strangest cases of bizarre weather comes to us from Neptune’s moon Triton. Triton’s meager nitrogen air is tied to the freezing and thawing of polar ices, also composed of nitrogen. Triton’s entire atmosphere collapses twice a year, when it’s winter on one of the poles. At that time of year, all of Triton’s air migrates to the winter pole, where it freezes to the ground. The moon only has “weather” during the spring and fall; its atmosphere exists only during those seasons.

So, the next time you contemplate complaining about the heat, think of Venus. And if it’s blizzards you worry about, find comfort in Triton: at least our atmosphere doesn’t disappear in winter!

For more on the subject, see Michael Carroll’s newest book, Drifting on Alien Winds: Exploring the Skies and Weather of Other Worlds from Springer.

New Horizons Mission Practices Telescopic Imager on Pluto’s Twin

New Horizons image of Neptune and its largest moon, Triton. June 23, 2010. Credit: NASA

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This summer, the New Horizons spacecraft was awoken for its annual systems checkout, and took the opportunity to exercise the long range camera by snapping pictures of Neptune, which at the time, was 3.5 billion km (2.15 billion miles) away. The Long Range Reconnaissance Imager (LORRI) snapped several photos of the gas giant, but Neptune was not alone! The moon Triton made a cameo appearance. And the New Horizons team said that since Triton is often called Pluto’s “twin” it was perfect target practice for imaging its ultimate target, Pluto.

This image gets us excited for 2015 when New Horizons will approach and make the closest flyby ever of Pluto.

“That we were able to see Triton so close to Neptune, which is approximately 100 times brighter, shows us that the camera is working exactly as designed,” said New Horizons Project Scientist Hal Weaver, of the Johns Hopkins Applied Physics Laboratory. “This was a good test for LORRI.”

Weaver pointed out that the solar phase angle (the spacecraft-planet-Sun angle) was 34 degrees and the solar elongation angle (planet-spacecraft-Sun angle) was 95 degrees. Only New Horizons can observe Neptune at such large solar phase angles, which he says is key to studying the light-scattering properties of Neptune’s and Triton’s atmospheres.

“As New Horizons has traveled outward across the solar system, we’ve been using our imagers to make just such special-purpose studies of the giant planets and their moons because this is a small but completely unique contribution that New Horizons can make — because of our position out among the giant planets,” said New Horizons Principal Investigator Alan Stern.

Triton is slightly larger than Pluto, 2,700 kilometers (1,700 miles) in diameter compared to Pluto’s 2,400 kilometers (1,500 miles). Both objects have atmospheres composed mostly of nitrogen gas with a surface pressure only 1/70,000th of Earth’s, and comparably cold surface temperatures approaching minus-400 degrees Fahrenheit. Triton is widely believed to have been a member of the Kuiper Belt (as Pluto still is) that was captured into orbit around Neptune, probably during a collision early in the solar system’s history.

Source: New Horizons

40 Years of Summer on Triton

Artist’s impression of how Triton, Neptune’s largest moon, might look from high above its surface. Credit: ESO/L. Calçada

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If you’re planning a trip to Neptune’s moon Triton, you’ll want to head to the southern hemisphere where it’s now just past mid-summer. Yes, distant Triton actually does have seasons, astronomers at ESO’s Very Large Telescope recently determined. “We have found real evidence that the Sun still makes its presence felt on Triton, even from so far away,” said astronomer Emmanuel Lellouch in an ESO press release. “This icy moon actually has seasons just as we do on Earth, but they change far more slowly.” According to the first ever infrared analysis of Triton’s atmosphere, the seasons last about 40 Earth years. But while summer is in full swing in Triton’s southern hemisphere, there’s no need to pack your bikini. The average surface temperature is about minus 235 degrees Celsius.

Oh, and you’ll also want to bring along a little breathable air. The ESO team also – unexpectedly – discovered carbon monoxide in Triton’s thin atmosphere, mixed in with methane and nitrogen.

The astronomer’s observations revealed that Triton’s thin atmosphere varies seasonally, thickening when warmed. When the distant sun’s rays hits Triton at their best summer angle, a thin layer of frozen nitrogen, methane, and carbon monoxide on Triton’s surface sublimates into gas, thickening the icy atmosphere as the season progresses during Neptune’s 165-year orbit around the Sun. Triton passed the southern summer solstice in 2000.

Voyager 2's view of Triton. Credit: NASA

So, while this action increases the thickness of the atmosphere, thus increasing the atmospheric pressure, you’ll still need a pressure suit as well for your visit. Based on the amount of gas measured, Lellouch and his colleagues estimate that Triton’s atmospheric pressure may have risen by a factor of four compared to the measurements made by Voyager 2 in 1989, when it was still spring on the giant moon. The Voyager data indicated the atmosphere of nitrogen and methane had a pressure of 14 microbars, 70,000 times less dense than the atmosphere on Earth. The data from ESO shows the atmospheric pressure is now between 40 and 65 microbars — 20,000 times less than on Earth.

Carbon monoxide was known to be present as ice on the surface, but Lellouch and his team discovered that Triton’s upper surface layer is enriched with carbon monoxide ice by about a factor of ten compared to the deeper layers, and that it is this upper “film” that feeds the atmosphere. While the majority of Triton’s atmosphere is nitrogen (much like on Earth), the methane in the atmosphere, first detected by Voyager 2, and only now confirmed in this study from Earth, plays an important role as well.

“Climate and atmospheric models of Triton have to be revisited now, now that we have found carbon monoxide and re-measured the methane,” said co-author Catherine de Bergh. The team’s results are published in Astronomy & Astrophysics

If we could actually visit Triton, it would likely be a very interesting destination as we know it has geologic activity and a changing surface – plus its unique retrograde motion would offer a unique view of the solar system.

While Triton is the seventh largest moon in our solar system, its distance and position from Earth makes it difficult to observe, and ground-based observations since Voyager 2 have been limited. Observations of stellar occultations (a phenomenon that occurs when a Solar System body passes in front of a star and blocks its light) indicated that Triton’s surface pressure was increasing in the 1990’s. But a new instrument on the VLT, the Cryogenic High-Resolution Infrared Echelle Spectrograph (CRIRES) has provided the chance to perform a more detailed study of Triton’s atmosphere. “We needed the sensitivity and capability of CRIRES to take very detailed spectra to look at the very tenuous atmosphere,” said co-author Ulli Käufl.

These observations are just the beginning for the CRIRES instrument, which will be extremely helpful in studying other distant bodies in our solar system, such as Pluto and other Kuiper Belt Objects. Pluto is often considered a cousin of Triton with similar conditions, and in the light of the carbon monoxide discovery on Triton, astronomers are racing to find this chemical on the even more distant Pluto.

Read the team’s paper.

Source: ESO