Why do Uranus and Neptune Have Magnetic Fields? Hot ice

The outer “ice giant” planets, Neptune and Uranus, have plenty of mysteries.  One of the biggest is where exactly they got their magnetic fields.  They are strong at that, with Neptune’s being twenty-seven times more powerful than Earth’s, while Uranus’ varies between ?  and four times Earth’s strength.  Chaos rules in these electromagnetic environments, making them exceptionally hard to both understand and model.  Now a team of researchers led by Dr. Vitali Prakpenka of the University of Chicago thinks they might have found the underlying cause of both the field’s strength and its randomness – “hot ice.”

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Giant Balls of Mush Made From Ammonia and Water Form in the Atmospheres of Uranus and Neptune

One advantage to planetary science is that insights from one planet could explain phenomena on another.  We understand Venus’ greenhouse gas effect from our own experience on the Earth, and Jupiter and Saturn share some characteristics.  But Jupiter also provides insight into other, farther out systems, such as Uranus and Neptune.  Now, a discovery from a spacecraft orbiting Jupiter might have solved a long-standing mystery about Uranus and Neptune – where has all the ammonia gone?

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Hubble Releases a New Image of Neptune, Revealing a Rapidly Shifting Storm

Storms on Neptune seem to follow a pattern of forming, strengthening and then dissipating over the course of about two Earth years. But a Neptunian storm spotted in the planet’s atmosphere over two years ago has done something quite different: it has reversed course and is still going strong.

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Neptune & Triton – August 31, 1989.

Image-processor extraordinaire Kevin Gill has reached back in time to give us a new image of Neptune and its moon Triton.

When NASA’s Voyager 2 spacecraft flew past Neptune and Triton in August 1989, its cameras were very busy. Kevin has taken separate color-filtered images from that visit and calibrated and combined them to give us a new, almost haunting look at the planet and its largest moon.

Continue reading “Neptune & Triton – August 31, 1989.”

NASA Thinks it’s Time to Return to Neptune With its Trident Mission

Is it time to head back to Neptune and its moon Triton? It might be. After all, we have some unfinished business there.

It’s been 30 years since NASA’s Voyager 2 spacecraft flew past the gas giant and its largest moon, and that flyby posed more questions than it answered. Maybe we’ll get some answers in 2038, when the positions of Jupiter, Neptune, and Triton will be just right for a mission.

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Say Hello to Hippocamp! The New Moon Discovered at Neptune, Which Could Have Broken off from the Larger Moon Proteus

An artist's illustration of the tiny Neptunian moon Hippocamp. Image Credit: NASA, ESA and J. Olmsted (STScI)

Moons have the coolest names, don’t they? Proteus, Titan, and Callisto. Phobos, Deimos, and Encephalitis. But not Io. That’s a stupid name for a moon. There’s only two ways to pronounce it and we still get it wrong. Anyway, now we have another cool one: Hippocamp!

Okay, maybe the new name isn’t that cool. It sounds like a summer camp for overweight artiodactyls. But whatever. It’s not every day our Solar System gets a new moon.

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Exploring the Ice Giants: Neptune and Uranus at Opposition for 2018

The existence of Neptune was inferred by its gravitational effect on other bodies long before it was ever observed. Image Credi: NASA/JPL

Have you seen the outer ice giant planets for yourself?

This week is a good time to check the most difficult of the major planets off of your life list, as Neptune reaches opposition for 2018 on Friday, September 7th at at ~18:00 Universal Time (UT)/2:00 PM EDT. And while it may not look like much more than a gray-blue dot at the eyepiece, the outermost ice giant world has a fascinating tale to tell. Continue reading “Exploring the Ice Giants: Neptune and Uranus at Opposition for 2018”

Neptune’s Huge Storm Is Shrinking Away In New Images From Hubble

Back in the late 1980’s, Voyager 2 was the first spacecraft to capture images of the giant storms in Neptune’s atmosphere. Before then, little was known about the deep winds cycling through Neptune’s atmosphere. But Hubble has been turning its sharp eye towards Neptune over the years to study these storms, and over the past couple of years, it’s watched one enormous storm petering out of existence.

“It looks like we’re capturing the demise of this dark vortex, and it’s different from what well-known studies led us to expect.” – Michael H. Wong, University of California at Berkeley.

When we think of storms on the other planets in our Solar System, we automatically think of Jupiter. Jupiter’s Great Red Spot is a fixture in our Solar System, and has lasted 200 years or more. But the storms on Neptune are different: they’re transient.

Voyager 2 captured this image of Neptune in 1982, when it was over 7 million km (4.4 million miles) away from the planet. The Great Dark Spot in the middle of the image was the first storm ever seen on Neptune. Image: By NASA (JPL image) [Public domain], via Wikimedia Commons

The storm on Neptune moves in an anti-cyclonic direction, and if it were on Earth, it would span from Boston to Portugal. Neptune has a much deeper atmosphere than Earth—in fact it’s all atmosphere—and this storm brings up material from deep inside. This gives scientists a chance to study the depths of Neptune’s atmosphere without sending a spacecraft there.

The first question facing scientists is ‘What is the storm made of?’ The best candidate is a chemical called hydrogen sulfide (H2S). H2S is a toxic chemical that stinks like rotten eggs. But particles of H2S are not actually dark, they’re reflective. Joshua Tollefson from the University of California at Berkeley, explains: “The particles themselves are still highly reflective; they are just slightly darker than the particles in the surrounding atmosphere.”

“We have no evidence of how these vortices are formed or how fast they rotate.” – Agustín Sánchez-Lavega, University of the Basque Country in Spain.

But beyond guessing what chemical the spot might me made of, scientists don’t know much else. “We have no evidence of how these vortices are formed or how fast they rotate,” said Agustín Sánchez-Lavega from the University of the Basque Country in Spain. “It is most likely that they arise from an instability in the sheared eastward and westward winds.”

There’ve been predictions about how storms on Neptune should behave, based on work done in the past. The expectation was that storms like this would drift toward the equator, then break up in a burst of activity. But this dark storm is on its own path, and is defying expectations.

“We thought that once the vortex got too close to the equator, it would break up and perhaps create a spectacular outburst of cloud activity.” – Michael H. Wong, University of California at Berkeley.

“It looks like we’re capturing the demise of this dark vortex, and it’s different from what well-known studies led us to expect,” said Michael H. Wong of the University of California at Berkeley, referring to work by Ray LeBeau (now at St. Louis University) and Tim Dowling’s team at the University of Louisville. “Their dynamical simulations said that anticyclones under Neptune’s wind shear would probably drift toward the equator. We thought that once the vortex got too close to the equator, it would break up and perhaps create a spectacular outburst of cloud activity.”

Rather than going out in some kind of notable burst of activity, this storm is just fading away. And it’s also not drifting toward the equator as expected, but is making its way toward the south pole. Again, the inevitable comparison is with Jupiter’s Great Red Spot (GRS).

The GRS is held in place by the prominent storm bands in Jupiter’s atmosphere. And those bands move in alternating directions, constraining the movement of the GRS. Neptune doesn’t have those bands, so it’s thought that storms on Neptune would tend to drift to the equator, rather than toward the south pole.

Jupiter’s prominent storm, the Great Red Spot, is held in place by the alternating storm bands in Jupiter’s atmosphere. Image: By NASA, ESA, and A. Simon (Goddard Space Flight Center) [Public domain], via Wikimedia Commons

This isn’t the first time that Hubble has been keeping an eye on Neptune’s storms. The Space Telescope has also looked at storms on Neptune in 1994 and 1996. The video below tells the story of Hubble’s storm watching mission.

The images of Neptune’s storms are from the Hubble Outer Planets Atmosphere Legacy (OPAL) program. OPAL gathers long-term baseline images of the outer planets to help us understand the evolution and atmospheres of the gas giants. Images of Jupiter, Saturn, Uranus and Neptune are being taken with a variety of filters to form a kind of time-lapse database of atmospheric activity on the four gas planets.

Triton’s Arrival was Chaos for the Rest of Neptune’s Moons

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.

Further Reading: The Astronomical Journal, USRA