Mysterious Neptune and why we need to put an orbiter around it

Most of our limited knowledge of Neptune comes from a single flyby mission in 1989

Neptune’s existence was a triumph of the laws of celestial mechanics, as it was the first planet to be discovered using mathematical prediction rather than empirical observation. As with Uranus, most of our limited knowledge of Neptune comes from a single flyby mission by the Voyager 2 spacecraft in 1989.

Neptune compared to Earth. Source: Wikipedia

Last time we discussed the various mysteries of Uranus and why we need to put an orbiter around it. Time for Neptune to show some of its intriguing features:

#1: Neptune has incomplete rings

The outermost ring of Neptune (called Adams) is actually broken up into 5 individual arcs. These arcs shouldn’t exist since the ring particles in them should spread out to fill the gaps quickly and yet they have remained stable since their discovery in the 1980’s.

Something’s wrong with Neptune’s rings. Source: Planet Pailly

Neptune’s moon Galatea is thought to be keeping the arcs stable because of its gravity via what is called orbital resonances. But the mathematical models for the same don’t match with the observations completely. There could be another undiscovered moon there helping to keep the ring-arcs stable, which an orbiter probably will be able to observe.

#2: Neptune’s atmospheric color is bluer than expected

Uranus and Neptune are the Ice Giants of the Solar System and both of them derive their blue color from the red-light-absorbing methane in their atmospheres. Despite each having roughly the same amount of methane, Neptune’s vivid purplish blue is quite different than Uranus’ cyan blue.

Uranus & Neptune. Note the stark difference in their blue colors. Credit: Me, using Source 1 and Source 2.

The bluer than expected color for Neptune hints that some other unknown component is present in its atmosphere which will be interesting to find out.

#3: Neptune has surprising storms and windy weather

See the central dark thing in the image of Neptune above? That’s an anti-cyclonic storm about half the size of the Earth. It was discovered by Voyager 2 during the 1989 flyby, among other similar discoveries.

Neptune’s Great Dark Spot compared to the Earth. Source: Flickr

Surprisingly, this storm was nowhere to be found 5 years later, when Neptune was imaged by the Hubble Space Telescope in 1994. Later in 2016 when Hubble imaged Neptune again, a new giant vortex was found. With a surprising diversity of storms and with wind speeds reaching as high as 2000 kilometers/hour, an orbiter around Neptune would have a lot to monitor and understand the physics of.

#4: Neptune’s moon Triton may have a subsurface ocean

Triton is interesting in a lot of ways:

Global color mosaic of Neptune’s moon Triton taken using the Voyager 2 spacecraft in 1989. Source: NASA. Caption: Me.

A capture means that Triton’s orbit was highly elliptical at some point which would cause large tides on the moon. The friction from these tides heats the icy interior and can form an ocean beneath the icy shell, just like in the case of Jupiter’s moon Europa:

An artist’s illustration showing the internal structure of Jupiter’s moon Europa. Beneath the thick ice covering, lies an ocean of liquid water caused by the tidal heating effects of Jupiter’s gravity. Source: Source: NASA

The presence of ammonia in Triton’s interior further lowers the melting point of water making the prospect more feasible. An orbiter around Neptune might be able to unlock the mysteries hidden beneath Triton’s icy shell.

Why we need an orbiter around Neptune?

A few-days flyby mission can only tell you so much about a planet. Moreover, remote observations are not sufficient to help understand the various mysteries surrounding Neptune. Putting an orbiter around it, like the ones we have put around Jupiter and Saturn, will help us not just understand Neptune but also the past of our Solar System. The fruitfulness of understanding Ice Giants like Uranus/Neptune goes beyond that. Have a look at this graph showing the number of exoplanets discovered of each type:

A histogram showing the number of exoplanets discovered by each type. The blue bars represent previously verified exoplanets and the orange bars represent Kepler’s newly verified planets as of May 2016. Source: Wikipedia

The category of planets called Sub/Mini-Neptunes are the most common type as per our observations. Understanding Uranus & Neptune is thus the key to understanding how a giant fraction of all planets form and behave.

The good news is that a Neptune orbiter+probe is proposed to be launched in the 2030s, which includes a Uranus flyby. The bad news is that even though the best case scenario is having both Uranus and Neptune orbiters, budget constraints in science mean that we might end up doing only one of them.