Banded and blue, big beautiful Neptune skirts the outer region of our Solar System, where it dwells as the smallest of the quartet of known gaseous giant planets: Jupiter, Saturn, Uranus, and itself. Here, in the dimly lit frigid outskirts of our Sun’s region of influence, Neptune is the outermost known planet, where it reigns in sapphire-blue splendor within its icy, murky, place in space. Neptune is named after the ancient Roman god of the sea, and it displays within its lovely bands of various shades of blue, dark, ominous, and immense hurricane-like storms, that whirl menacingly and mysteriously within its heavy gaseous atmosphere, playing a vanishing-act game with curious observers. In February 2018, astronomers announced that they have seen a dark, threatening storm on Neptune that was once big enough to extend across the Atlantic ocean from Connecticut to Portugal, now in the act of mysteriously vanishing out of existence as seen in images of Neptune taken by NASA’s Hubble Space Telescope (HST). Furthermore, this strange shrinking, swirling storm may be composed of hydrogen sulfide.
Neptune is three billion miles from Earth, and its immense storms were first discovered in the late 1980s by NASA’s Voyager 2 spacecraft. Since then, only HST has succeeded in possessing the necessary sharpness in blue light to track these strange and elusive features–that appear and then disappear without a trace. HST spotted a duo of dark storms that showed up back in the mid-1990s and then mysteriously vanished. This latest enormous vortex storm was first observed in 2015, but is now in the act of shrinking.
This dark Neptunian storm swirls in an anti-cyclonic direction just like Jupiter’s famous Great Red Spot (GRS). Also, like Jupiter’s gigantic crimson storm, Neptune’s dark storm is dredging up material from deep within the ice giant planet’s atmosphere. The shrinking feature provides astronomers with a rare and precious chance to study Neptune’s deep interior winds, which cannot be directly measured.
Neptune and Uranus and both classified as ice-giants, while Jupiter and Saturn are classified as gas-giants. The duo of ice-giants have heavy gaseous envelopes that are, nevertheless, much thinner than the enormous gaseous envelopes of their gas-giant siblings. Uranus and Neptune also have larger solid cores than Jupiter and Saturn, and are also smaller in size–even though the entire quartet of known outer Solar System planets are giants.
Furthermore, Neptune’s shrinking, stormy dark spot may be composed of hydrogen sulfide–a foul-smelling substance that stinks like rotten eggs. “The particles themselves are still highly reflective; they are just slightly darker than the particles in the surrounding atmosphere,” commented Dr. Joshua Tollefson in a February 15, 2018 Hubblesite Press Release. Dr. Tollefson is from the University of California at Berkeley.
Unlike Jupiter’s crimson GRS, which has been observed for at least two centuries, Neptune’s dark vortices only last a few years. This storm is the first one that has actually been caught in the act of dying. The others simply disappeared.
“We have no evidence of how these vortices are formed or how fast they rotate. It is most likely that they arise from an instability in the sheared eastward and westward winds,” noted Dr. Agustin Sanchez-Lavega in the same Hubblesite Press Release. Dr. Sanchez-Lavega is from the University of the Basque Country in Spain.
In Neptune’s Distant Realm
The great Italian polymath, Galileo Galilei (1564-1642), discovered Neptune using his primitive “spyglass”, on December 28, 1612. He observed it again on January 27, 1613. Alas, on both occasions, Galileo thought that this distant, giant planet was a fixed star, appearing near the planet Jupiter in the darkness of a clear star-speckled night sky. For this reason, Galileo is usually not credited with the discovery of Neptune.
NASA’s Voyager 2 spacecraft flew past Uranus in 1986, and later Neptune in 1989. Voyager 2 sent back to Earth enticing images of Neptune that showed a strikingly beautiful deep blue planet, that sported bands and stripes–as well as spot-like storms similar to Earth’s hurricanes. Neptune’s stripes and bands are lovely shades of blue that are created by atmospheric methane, not oxygen. Some of Neptune’s storms are white, and look like whirling marshmallows.
NASA’s Voyager 2 spacecraft was the first to detect Neptune’s Great Dark Spot back in 1989. The Great Dark Spot was an anti-cyclonic storm system that extended 12,000 X 6,600 kilometers, and astronomers immediately noticed the resemblance between this Neptunian storm and that of Jupiter’s GRS. However, on November 2, 1994, the HST did not see the vanished Great Dark Spot swirling on this big blue planet. Instead, it detected a new storm that was strikingly similar to the Great Dark Spot in Neptune’s northern hemisphere.
Another strange storm on Neptune, dubbed Scooter, is composed of a group of white clouds. Scooter was first seen situated further south than the Great Dark Spot. This playful nickname for the collection of marshmallow clouds was suggested during the months leading up to the 1989 Voyager 2 encounter with Neptune, when astronomers observed the Scooter clouds skipping along at speeds faster than the Great Dark Spot. Indeed, images obtained at a later date revealed the presence of clouds traveling even faster than those that had first been detected by Voyager 2.
The Small Dark Spot is yet another southern whirling cyclonic storm observed by Voyager 2 during its productive 1989 encounter. The Small Dark Spot proved to be the second-most-powerful storm seen during the Voyager encounter, and it was initially completely dark. However, as Voyager 2 flew closer to the planet, a bright core formed, and it can be seen in most of the highest resolution images.
Neptune’s dark spots are thought to occur in the troposphere at lower altitudes than the brighter cloud features. This is the reason why they appear to be holes in the upper cloud decks. Because the dark spots are stable features that can exist for several months, they are generally interpreted to be vortex structures. Brighter, persistent clouds of methane are frequently associated with the dark spots. These bright clouds of methane probably form around the tropopause layer of Neptune’s heavy gaseous atmosphere. The persistence of companion clouds indicates that some of the erstwhile dark spots may continue to exist as cyclones even though they are no longer visible as a dark feature. Dark spots may fall apart as they travel too close to Neptune’s equator or possibly through some other unknown mechanism.
Neptune itself is not visible to the unaided human eye and is the only planet in our Sun’s family to be discovered by mathematical prediction instead of by direct observation. Unexpected alterations in the orbit of the greenish-blue planet Uranus–Neptune’s larger ice-giant neighbor–led the French astronomer Alexis Bouvard (1767-1843) to calculate that its orbit was being influenced by the gravitational perturbations of an unknown planet. Neptune was subsequently discovered with a telescope on September 23, 1846 by the German astronomer Johann Galle (1812-1911) within a degree of the position predicted by the French mathematician and astronomer Urbain Le Verrier (1811-1877). Soon after that, Triton–Neptune’s largest moon–was discovered. However, none of the planet’s remaining known 13 moons were found telescopically until the 20th century.
Neptune’s distance from Earth gives it a small apparent size in our sky, making it difficult to study with Earth-bound telescopes. The space telescope HST, along with large ground-based telescopes using adaptive optics, recently provided a wealth of detailed observations from far away. Adaptive optics is a technique that corrects for distortions caused by wind, temperature, and mechanical stress by deforming a mirror to compensate for the distortion.
Like our Solar System’s gas-giant duo, Jupiter and Saturn, Neptune’s atmosphere is mostly made up of hydrogen and helium, with some trace quantities of hydrocarbons and possibly nitrogen mixed into this alien brew. However, unlike Jupiter and Saturn, the ice-giant Neptune contains a greater proportion of “ices”, such as water, methane, and ammonia. However, Neptune’s interior–like that of its sibling planet, Uranus–is primarily composed of ices and rock, which accounts for the distant icy duo’s classification as ice-giants, in order to distinguish them from Jupiter and Saturn.
The trace quantities of methane in Neptune’s outermost layers is partly responsible for its beautiful blue hue. In stark contrast to the hazy. relatively featureless, and somewhat bland atmosphere of greenish Uranus, Neptune’s atmosphere has visibly active weather patterns–accounting for its many vortex storms. These active weather systems are driven by the most powerful sustained winds of any planet in our Solar System. Indeed, the winds of Neptune roar and rage at speeds of 1,300 miles per hour. Because of Neptune’s immense distance from our Star–it is the eighth major planet from our Sun, while Uranus is the seventh–its outer atmosphere is one of the coldest places in our Solar System. The temperatures at Neptune’s cloud tops approach a truly frigid -218 degrees C. Temperatures at Neptune’s hidden center are thought to be about 5,100 degrees C.
Neptune’s more varied weather when compared to Uranus is thought to be partly the result of its higher internal heating. Even though Neptune is more than 50% farther from our Star than Uranus, and it receives only 40% of its amount of sunlight, the frigid ice-giant duo sport approximately equal surface temperatures. The top regions of Neptune’s troposphere plummet to a low temperature of -221.3 degrees C. At a depth where Neptune’s atmospheric presure is equivalent to 1 bar, the temperature is about -201.15 degrees C. Deep within Neptune’s hidden layers of gas, the temperature rises steadily. Like Uranus, Neptune’s source of this heating remains a mystery. Uranus radiations a mere 1.1 times as much energy as it receives from our roiling Sun, while Neptune radiations approximately 2.61 times as much energy as our Star bestows upon it. Even though Neptune is the farthest major planet from our Sun, its internal energy is adequate to drive the fastest planetary winds in our Solar System. Depending on the thermal properties of its interior, the heat still lingering from Neptune’s formation about 4.5 billion years ago, may be sufficient to explain its current flow of heat. However, it is considerably more difficult to explain Uranus’s lack of internal heat while maintaining the apparent similarity between the two ice-giants inhabiting our Solar System’s frigid twilight zone.
Neptune also has a fragmented and very faint system of delicate rings (called arcs). The arcs were first discovered back in 1982, and then ultimately confirmed by Voyager 2.
The Mystery Of Neptune’s Shrinking, Smelly, and Disappearing Storm
The dark and disappearing vortex storm on Neptune, observed in the act of vanishing, is not behaving the way planet-watchers predicted. “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,” commented Dr. Michael H. Wong in the February 15, 2018 Hubblesite Press Release, referring to research conducted by Dr. Ray LeBeau of St. Louis University (St. Louis, Missouri) and Dr. Tim Dowling’s team at the University of Louisville in Kentucky. “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,” Dr. Wong continued to explain. Dr. Wong is of the University of California at Berkeley.
However, the mysterious vanishing dark vortex storm, which was first observed at mid-southern latitudes, simply faded away–instead of dying with the anticipated spectacular rage within the Neptunian clouds. This occurrence may be related to the surprising direction of its measured drift toward the south pole, instead of wandering northward toward the equator. The Neptunian spot–in dramatic contrast to Jupiter’s GRS–is not as tightly constrained by a multitude of alternating wind jets (observed as bands in Jupiter’s heavy atmosphere). Neptune apparently has only three broad jets: a westward one at its equator; and eastward jets around both its north and south poles. The vortex should be free to change its traffic route and travel anywhere in between the jets.
“No facilities other than Hubble and Voyager have observed these vortices. For now, only Hubble can provide the data we need to understand how common or rare these fascinating Neptunian weather systems may be,” Dr. Wong added in the February 15, 2018 Hubblesite Press Release.
The first images obtained of the dark vortex are from the Outer Planet Atmospheres Legacy (OPAL) program. OPAL is a long-term HST project that annually obtains global maps of our Solar System’s quartet of outer planet behemoths. At this point, only the HST has the capability to probe these distant gigantic gaseous worlds in ultraviolet light, which provides valuable information that is unavailable using other present-day telescopes. Additional information, derived from an HST program focusing on the dark vortex, are provided by an international team of astronomers that include Wong, Tollefson, Sanchez-Lavega, Andrew Hsu, Imke de Pater, Amy Simon, Ricardo Hueso, Lawrence Stromovsky, Patrick Fry, Statia Luszcz-Cook, Heidi Hammel, Marc Delcroix, Katherine de Kleer, Glenn Orton, and Christoph Baranec.
This research is published online in the February 15, 2018 edition of The Astronomical Journal.