Sciency Words: Ring Arcs

Sciency Words PHYS copy

Today’s post is part of a special series here on Planet Pailly called Sciency Words. Each week, we take a closer look at an interesting science or science-related term to help us all expand our scientific vocabularies together. Today’s word is:


Something’s wrong with Neptune’s rings.

Nv11 Neptune's Arcs

Neptune has five rings, all named after astronomers or scientists associated with significant Neptune-related discoveries. They are (in order of increasing distance from Neptune):

  • Galle: named after the guy who discovered Neptune, sort of. He had help from…
  • Le Verrier: named after the guy who calculated Neptune’s exact position, allowing Galle to “discover” it.
  • Lassell: named after the discoverer of Triton, Neptune’s largest moon.
  • Arago: named after the teacher who encouraged Le Verrier in his calculations and helped defend Le Verrier in a dispute with…
  • Adams: named after another person who calculated Neptune’s position before its discovery and started a fuss with Le Verrier over who deserved credit.

There was plenty of drama surrounding the discovery of Neptune, and that has been preserved in the names of the rings that also surround the planet.

Neptune has an unnamed sixth “ring,” if we can be generous enough to call it a ring, located between Arago and Adams. A small moon named Galatea also orbits within that gap. This unnamed ring doesn’t circle all the way around the planet, so it is better described as an “arc.”

Furthermore, a short segment of the outermost ring (Adams) is also broken up into several small arcs. These arcs were originally named Liberty, Equality, and Fraternity (bonus points to anyone who can tell me what the planet Neptune has to do with the French Revolution).

Later, two more arcs were found in the Adams ring, so the list became (in order):

  • Courage: the faintest arc.
  • Liberty: often described as the “leading arc,” even though Courage orbits ahead of it.
  • Equality 1 and Equality 2: the Equalities are so close together that they’re almost a single arc.
  • Fraternity: brings up the rear and is the largest and brightest of Neptune’s arcs.

The existence of these arcs doesn’t make a whole lot of sense. Ring particles should spread out the fill the gaps within a matter of months, yet the arcs have remained stable since their discovery in the 1980’s.

An orbital resonance with Galatea is almost certainly involved, but mathematical models of Galatea and the Adams arcs don’t always match with observations. Neptune may have an additional as-yet-undiscovered moon near its rings, or perhaps some other unknown factor is at work.

Probably aliens.

P.S.: Neptune isn’t the only planet with arcs in its rings. Saturn has them too. So Neptune: you don’t have anything to feel embarrassed about.


Neptune’s Rings and “Ring Arcs” from JPL’s Voyager Mission webpage.

Rings of Neptune from Universe Today.

Stability of Neptune’s Ring Arcs in Question from Letters to Nature.

* * *

Today’s post is part of Neptune month for the 2015 Mission to the Solar System. Click here to learn more about this series.

The Strange Story of Naiad

Neptune is really far away and difficult to observe through a telescope. Nothing emphasizes this better than the fact that we lost one of Neptune’s moons. It was there. We saw it just once, thanks to the Voyager 2 spacecraft. And then for over twenty years, we couldn’t find it again.

Nv10 Naiad Lost

That lost moon is called Naiad, and there are several reasons why it’s so hard to see.

  • It’s really freaking small, less than 100 kilometers across.
  • Naiad is Neptune’s innermost moon. It’s so close to Neptune that it gets lost in the glare of light reflecting off the planet.
  • Thalassa, Neptune’s second innermost moon, is roughly the same size and shape as Naiad, leading to potential confusion and misidentification (the Wikipedia pages for Naiad and Thalassa are currently using the exact same photo for both moons).
  • Naiad’s orbit (we now know) is unstable. This teeny-tiny moon wobbles about, sometimes speeding up, sometimes slowing down, for reasons that are not yet fully understood.

In 2004, the Hubble Space Telescope made a concerted effort to find Naiad. Hubble failed, or so it seemed at first. In 2013, new image processing techniques were applied to the 2004 data, and Naiad finally appeared.

Nv10 Naiad Found

The game of hide and seek isn’t quite over. We still don’t know why Naiad’s orbit is so erratic, so we can’t accurately predict where Naiad might turn up next.

But this is a risky game Naiad is playing. It’s rapidly approaching its Roche limit (if it hasn’t crossed it already), meaning there’s a good chance this tiny moon will soon be sheared apart by Neptune’s gravity.

But if the strange story of Naiad and the strange story of Triton aren’t strange enough for you, then there’s one more strange thing happening in the Neptune system. Something’s wrong with Neptune’s rings. More about that on Friday.


Archival Hubble Images Reveal Neptune’s “Lost” Inner Moon from the SETI Institute.

Astrophile: Lost Moon Naiad Swims Back into View from New Scientist.

Roche Limit and Radius of Roche from Astronoo.

* * *

Today’s post is part of Neptune month for the 2015 Mission to the Solar System. Click here to learn more about this series.

Why Haven’t We Returned to Neptune?

Nv09 Lonely Neptune

I feel bad for Neptune. Only one spacecraft (Voyager 2) has ever been there, and it didn’t stay long.

Neptune is the planet farthest from the Sun, so getting there is an extra special challenge. It’s not only a question of distance. It’s also a question of gravity. Spacecraft require ludicrous amounts of energy to travel that far, given that the Sun’s gravity is constantly trying to pull them back.

But right now (between 2015 and 2019), the planets are aligned to make a trip to Neptune easier. With a gravity assist from Jupiter and another from Saturn, a small spacecraft could build up enough momentum to fling itself toward Neptune, using relatively little fuel in the process.

Like Voyager 2, the Argo spacecraft (as NASA called it) wouldn’t be able to stay long. By the time it reached Neptune, Argo would have built up so much momentum that it wouldn’t be able to slow down enough to enter Neptunian orbit.

But hey, a flyby mission is better than no mission at all. As added bonuses, Argo would also have opportunities to collect data about Jupiter and Saturn while performing its gravity assist maneuvers. Mission planners also designed an orbital trajectory that would allow Argo to get some extreme close-ups of Triton, Neptune’s largest moon. Argo would also be able to visit a Kuiper belt object some time after its primary mission ended.

All of this could be done within the strict spending limits of a New Frontiers-class mission ($1 billion or less). That’s a real bargain, at least for NASA. So why aren’t we doing this?

The Great Recession hit at just the wrong time for Argo. The mission was still in early development when Congress slashed NASA’s budget. And now, even though NASA has some money again, there isn’t enough time to develop and construct a spacecraft before the 2015-2019 launch window closes.

Nv09 Neptune Understands

Sorry, Neptune. Better luck next time.

* * *

Today’s post is part of Neptune month for the 2015 Mission to the Solar System. Click here to learn more about this series.

Sciency Words: Centaur

Sciency Words MATH

Today’s post is part of a special series here on Planet Pailly called Sciency Words. Each week, we take a closer look at an interesting science or science-related term to help us all expand our scientific vocabularies together. Today’s word is:


When it comes to large, rocky objects drifting through space, naming conventions can get tricky. Many are called asteroids. Others go by stranger names. Some of these objects are called centaurs.

Nv08 Centaurs

Eh… no. It has nothing to do with horses, although these objects are sort of half one thing and half another. Most if not all centaurs could be classified as both asteroids and comets.

One definition of centaur is an object orbiting the Sun between the orbits of Jupiter and Neptune. More technical definitions involve an object’s semi-major axis, relative to the semi-major axes of Jupiter and Neptune, and non-resonant (i.e.: unstable) orbital periods. But I think a better way to describe centaurs is this: they are objects in a state of transition.

It is believed that most centaurs used to be Kuiper belt objects or objects from the scattered disk: basically, they were those large, icy things hanging out beyond Neptune. Due to gravitational interactions with Neptune and other gas giants, these objects have been pulled into increasingly eccentric (non-circular) orbits.

Eventually, most of these objects will transform into comets, with orbital paths that cut through the inner Solar System and bring them within melting distance of the Sun (allowing them to form cometary tails).

The Interanational Astronomy Union (I.A.U.) originally wanted to name all centaurs after actual centaurs from Greek mythology. However, some have been named after other hybrid or shape-shifting creatures, which I think is perfectly appropriate. Examples include Typhon (a snake or dragon hybrid), Ceto (a sea monster goddess), and Narcissus (a man who transformed into a flower).

In the future, centaurs could conceivably be named after the minotaur, the sphinx, or perhaps even Dracula. If the I.A.U. wants to have some fun, they could also use names like Ariel from The Little Mermaid, Constable Odo from Star Trek: Deep Space IX, or any of the Animagus characters from Harry Potter.

In fact, throughout fiction and mythology, there are plenty of hybrids and shape-shifters to chose from. So what names do you think the I.A.U. should give the next centaur asteroids?


Centaurs: Cross-Dressing Comets That Go as Asteroids from Discovery News.

Centaur (minor planet) from Wikipedia.

JPL Small-Body Database Search Engine: List of Centaurs from JPL Solar System Dynamics.

Neptune: Which Way Is North?

Something’s wrong with Neptune’s magnetic field.

Nv07 Neptune's Compass

Not all planets have magnetic fields. For the planets that do, we can generally expect two things:

  • Magnetic north and south should roughly correspond to geographic north and south.
  • The center of the magnetic field should run through the planet’s core.

Neither of these statements are true for Neptune (or Uranus, by the way). In 1989, when Voyager 2 visited Neptune, it found a magnetic field skewed 47º from the planet’s axis of rotation, with the source of the magnetic field offset by 0.55 Neptune radii (placing it not in the planet’s core but in a seemingly random point in the mantle).

Nv07 Neptune's Magnetic Field

More recent computer simulations suggest that the situation on Neptune could be even more complicated than Voyager 2 observed, with the magnetic field in constant flux, rotating and changing in ways that planetary magnetic fields ought not to do.

But new research, published earlier this year in Nature Communications, might help explain what’s going on. Remember those exotic forms of water ice, like ice VII, ice VIII, and ice X? Those weird “hot” ices believed to exist in the high-pressure interiors of Uranus and Neptune? Now we can add superionic ice to that icy mix.

In superionic ice, water molecules are under so much pressure that the oxygen and hydrogen atoms dissociate from each other. Oxygen forms a tight lattice structure which ionized hydrogen atoms can then move through freely. Much like free flowing electrons, these free flowing hydrogen ions (also known as protons) would generate an electric current.

Superionic ice located in the mantles of Neptune and Uranus would conveniently explain why these two planets have such wobbly, lopsided magnetic fields. Yes, that would be very convenient. Too convenient, perhaps, to be the whole story.


Scientists Predict Cool New Phase of Superionic Ice from Science Daily.

Magnetic Fields: On Other Planets from Joly Astronomy.

Neptune’s Badly Behaved Magnetic Field from Astronomy Magazine.

* * *

Today’s post is part of Neptune month for the 2015 Mission to the Solar System. Click here to learn more about this series.

Molecular Monday: Hot as Ice VII

Welcome to Molecular Mondays! Every other Monday, we examine the atoms and molecules that serve as the building blocks of our universe, both in reality and in science fiction. Today we continue our investigation of:


Water is basically everywhere in our universe. On Uranus and Neptune, most of the water is in the form of ice.

Nv06 Ice Ice Baby

But this ice isn’t cold. It’s hot. Like thousands of degrees Celsius hot.

The ice you and I are most familiar with is called ice Ih (pronounced ice one-h). The I is a Roman numeral one, and the h stands for hexagonal, because the water molecules form hexagon-shaped crystals.

Another form of ice is called ice Ic (ice one-c). In this case, the c stands for cubic.  The water molecules crystalize into cube shapes.

Nv06 Ice Cube

Ice Ic requires extremely cold temperatures (-50 to -140ºC) and is believed to form in the uppermost reaches of Earth’s atmosphere.

There are at least fifteen more crystalline forms of ice, ranging from ice II to ice XVI. Some have been created in the laboratory. Others remain purely hypothetical. Water molecules will line up to form ice crystals of wildly different shapes, sizes, and complexities depending on various combinations of pressure and temperature.

Nv06 Snowflake

Somewhere deep inside the planets Uranus and Neptune, water molecules probably “freeze” as ice VII and ice VIII. I’m not sure how to describe the geometry of these types of ice; just click the links to see some diagrams.

Pressures on Uranus and Neptune may even be great enough for ice X to form. In ice X, water molecules are so tightly packed that they lose their identities. It would be better to think of oxygen and hydrogen atoms squeezed together, not separate water molecules. For this reason, ice X is sometimes called non-molecular ice.

Of course, all of this seems very strange and exotic to us Earthlings. And yes, things like ice VII or ice X aren’t exactly common in our universe. But that doesn’t mean ice Ih is normal either.

In the next edition of Molecular Mondays (two weeks from today), we’ll meet yet another kind of ice. A kind of ice that doesn’t play by the rules and doesn’t need no Roman numerals.


Is Salt the Key to Unlocking the Interiors of Neptune and Uranus? from Science Daily.

Phase Diagram of Water from Wikipedia.


Sciency Words: Cantaloupe Terrain

Sciency Words PHYS copy

Today’s post is part of a special series here on Planet Pailly called Sciency Words. Each week, we take a closer look at an interesting science or science-related term to help us all expand our scientific vocabularies together. Today’s word is:


This is a cantaloupe.

Nv05 Not Actually a Cantaloupe

And this is Triton, Neptune’s largest moon.

Nv05 Not Actually Triton

Wait, I think I got those mixed up…

In 1989, Voyager 2 became the first (and so far the only) spacecraft to visit Triton, and it sent back some weird pictures of Triton’s surface. Pictures like this one:

Nv05 Cantaloupe Terrain

This heavily dimpled surface topography, which bears an uncanny resemblance to the skin of a cantaloupe, is unique to Triton. It may have formed due to a geologic process called diapirism, whereby blobs of warm material (called diapirs) force their way upward through layers of solid rock.

We know this process occurs on Earth and possibly a few other places in the Solar System. However, diapirism does not generally produce a cantaloupe-like appearance. That only happens on Triton, and no one’s entirely sure why.

So research continues on what scientists have officially named “cantaloupe terrain.”

Nv05 Cantaloupe Terrain vs Chaos Terrain

Today’s post is part of Neptune month for the 2015 Mission to the Solar System. Click here to learn more about this series.

And click here to find out more about Europa’s chaos terrain.

The Strange Story of Triton

Meet Triton, the largest of Neptune’s moons.

Nv04 Meet Triton

Triton is in a retrograde orbit, meaning that as Neptune rotates in one direction, Triton orbits in the opposite direction.

Nv04 Retrograde Orbit

This weird orbit strongly suggests that Triton is not one of Neptune’s “natural children” but rather an “adopted child.”

So who knows where Triton originally came from? As usual, scientists have two competing theories.

A Lunar Collision

One day, as Triton was going along minding its own business, it happened to pass a little too close to Neptune. Neptune would have has a few moons already, and one of those moons happened to be on a collision course with Triton!

The collision would have altered Triton’s momentum and trajectory through space, allowing Neptune’s gravity to capture it.

The problem with this explanation is that the hypothetical moon that hit Triton had to have been just the right size: big enough to change Triton’s orbit but not so big that it could destroy Triton in the collision.

And this just-right-sized moon would have to have been in just the right place at just the right time in order for the collision to occur. Stranger coincidences have happened in our universe, but this scenario is a little too unlikely for my taste.

A Binary Planet

Instead, let’s imagine that Triton was once part of a binary planet system, similar to the Pluto-Charon binary. Such binaries appear to be common in the Kuiper belt, as are binary asteroids in the asteroid belt and elsewhere.

One day, Triton and its binary partner strayed too close to Neptune. This resulted in a gravitational tug-of-war, with Neptune pulling in one direction and Triton’s companion pulling in the other. Neptune won, and due to Newton’s whole equal and opposite reaction thing, Triton’s companion was flung off into deep space. It may have even been ejected from the Solar System.

These sorts of gravitational tug-of-wars happen far more frequently than collisions, and this scenario does not depend on Triton’s hypothetical partner (or any other object in space) being perfectly sized or perfectly positioned.

That doesn’t prove that the binary planet hypothesis is correct, but it does sound a lot more likely than the alternative.

Life on Triton?

Whatever happened, Triton’s introduction into Neptune’s family would have had interesting effects. Triton is large for a moon (almost twice as massive as Pluto), and its initial orbit around Neptune would have crossed paths with many of Neptune’s much smaller moons. That means more gravitational tug-of-wars, creating chaos in the Neptune system.

Gradually, tidal forces pulled Triton into a more circular (and less disruptive) orbit, but those tidal forces would also have caused Triton’s interior to heat up dramatically. Today, Triton still shows evidence of active cryovulcanism and may have subsurface liquid water, just like Europa, Enceladus, or many other moons in the outer Solar System.

Which means we can add Triton to the list of places in the Solar System that may support extraterrestrial life.


Neptune’s Capture of Its Moon Triton in a Binary-Planet Gravitational Encounter from Nature.

Is Triton Hiding an Underground Ocean? from Universe Today.

Life Beyond Earth: A Day on Triton (Neptune’s Largest Moon) from Quarks to Quasars.

Sciency Words: Flagship Mission

Sciency Words BIO copy

Today’s post is part of a special series here on Planet Pailly called Sciency Words. Each week, we take a closer look at an interesting science or science-related term to help us all expand our scientific vocabularies together. Today’s word is:


There is a growing need among planetary scientists to study an ice giant up close. We keep discovering ice giant size planets orbiting distant stars, but we know next to nothing about the two ice giants in our own Solar System: Uranus and Neptune.

Nv03 Uranus and Neptune

To get to know these two planets better, NASA will have to launch a robotic mission of some kind. But which kind? There are three mission classes, defined primarily by their price tags:

  • Discovery Missions: Proposals for discovery-class missions are submitted to NASA and go through a highly competitive selection process. Approved missions must cost less than $450 million (a real bargain! Well, for NASA at least). Examples include the Mars Pathfinder Mission, the MESSENGER mission to Mercury, and the Kepler Space Telescope.
  • New Frontiers Missions: Like discovery missions, new frontiers missions go through a highly competitive selection process. Total costs (not including the launch vehicle) are capped at $1 billion. Examples include the New Horizons Mission to Pluto and the Juno Mission to Jupiter, due to arrive in 2016.
  • Flagship Missions: Unlike the other two mission classes, there is no regular submissions process for a flagship mission. Instead, NASA develops these missions internally, with costs ranging between $2 and $4 billion. NASA tends to launch only one flagship mission per decade. Examples include the Curiosity rover on Mars, the Cassini spacecraft orbiting Saturn, and the Voyager 1 and 2 probes that are currently exploring the very edges of the Solar System.

In relation to the ice giants, everyone seems to agree that a discovery-class mission could never reach Uranus or Neptune. A new frontiers mission could work, especially if it’s just a flyby mission like the recent New Horizons flyby of Pluto.

But to really get up close and personal with an ice giant, we need to send an orbiter. That will be expensive, and it will require NASA to commit to a flagship mission.

Upcoming flagship missions will focus on Mars (another Curiosity style rover) and Europa (potential home to alien fish). So despite the growing need among planetary scientists to study an ice giant up close, there probably won’t be a Uranus or Neptune orbiter any time soon.

Molecular Monday: Water on Neptune

Welcome to Molecular Mondays! Every other Monday, we examine the atoms and molecules that serve as the building blocks of our universe, both in reality and in science fiction. Today we continue our investigation of:


Meet Neptune, the other blue planet.

Nv01 Neptune Blues

Neptune may be named after the ancient god of the sea, but the planet’s striking blue color is caused by a high concentration of atmospheric methane, not water. Methane absorbs red light and reflects blue light back into space. So Neptune has nothing to do with that most precious commodity in space: water.


Okay, this sort of statement should no longer come as a surprise to me or anyone else, but the planet Neptune does in fact have water. In fact, water is sort of everywhere in space, which makes sense because the water molecule is made of two of the most common elements in the universe: hydrogen and oxygen.

Furthermore, water is the simplest possible combination of hydrogen and oxygen. So of course we’re going to find it everywhere. Even on Neptune. Why would anyone think water is somehow special to Earth? (Why did I think that for so long?)

But water on other planets rarely behaves the way it does here on Earth. As we’ve seen in two previous Molecular Monday posts (click here and here), the freezing and boiling temperatures of water change depending on pressure and salinity. At a certain point, known as the triple point, water’s freezing and boiling temperatures are the same, so water skips over its liquid phase, transitioning directly from gas to solid and vice versa.

Nv01 Water Water Everywhere

So the planet Neptune might actually live up to its sea god name. Not only does it have water, but there is a slim possibility that deep in the planet’s interior, the pressure and temperature are just right to support an ocean of liquid water. If Neptune were just a bit colder, the odds of such an ocean forming would improve dramatically.

Discovering liquid water on Neptune would be pretty cool, but there’s something even cooler (or perhaps hotter) about Neptune’s water. In the next edition of Molecular Monday (two weeks from today), we’ll take a closer look at Netune’s “icy” interior.

* * *

Today’s post is the beginning of Neptune month for the 2015 Mission to the Solar System. Click here to learn more about this series.