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.

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:

The largest moon of Neptune is also likely the coldest place in the Solar System.

Voyager 2 observed active geyser-like eruptions of icy material going as high as 8km from the surface.

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

Triton orbits Neptune in a direction opposite to that of planet’s rotation. Triton was thus likely captured inwards by Neptune’s gravity at some point, after originating in the Kuiper Belt (which starts from Pluto).

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.