An international mission to Neptune and Triton has long been overdue; such an expedition could shed more light on what these ice giants represent.
Like Jupiter, Uranus and Neptune are ice giants; planets with only trace amounts of hydrogen or helium present. But unlike Jupiter, their orbital periods are short enough for missions to Uranus or Neptune using gravity assistance from Saturn as an effective cost-cutting measure and to speed travel time.
The Voyager 2 Mission
Voyager 2’s encounters with Uranus and Neptune altered astronomers’ understandings of these two gas planets. Astronomers learned that Neptune had an intricate system of rings with numerous shepherding moons; moreover, its winds exceeded 680 miles (1,100 kilometers per hour).
Voyager team made several strategic choices that ensured its mission would yield maximum science on its Neptune flyby. For instance, its slow-scan color TV camera allowed for photo taking; and instruments were installed that collected magnetic, plasma, atmospheric and lunar data.
Engineering teams took great care in positioning Voyager so it would pass close enough to Neptune’s largest moon, Triton. Close inspection revealed its surface is not covered in an even layer of ice; rather it features active geysers and even a nitrogen ice “volcano”. Both Voyagers continue transmitting messages almost 40 years after launch – both have crossed through what’s known as the heliopause boundary between solar wind and interstellar space – an event known as crossing “over”.
The Rings
Astronomers had long anticipated discovering rings on Uranus’ twin, Neptune. Although many partial structures had been reported before Voyager 2 reached Neptune, definitive ring structures were eventually revealed by Voyager 2.
The craft discovered two rings featuring arcs of particles that were five to ten times brighter than previous images taken during its approach. These wide-angle views were obtained using a technique which took advantage of how microscopic particles tend to forward-scatter light more readily.
Voyager 2’s observations included rings similar to Jupiter’s Great Red Spot, rapid winds with much faster gusts than expected and an unexpectedly large moon called Triton. Astronomers are eager to witness how Neptune’s rings and weather evolve with the launch of JWST in 2021 – its $10 billion telescope that will primarily explore distant corners of space but will also observe Neptune and its moons in greater detail.
The Weather
Voyager 2 took close-up images of Neptune in 1989 and observed its many cloud and haze features, many resembling Jupiter’s Great Red Spot or Saturn’s Great White Spots.
But within years, our planet’s weather dramatically shifted: Voyager 2 had identified large dark patches in the southern hemisphere which promptly vanished while its observations of warmer conditions in polar regions indicated warming conditions as a whole.
Planetary scientists were puzzled by this sudden shift and began creating computer simulations in an attempt to comprehend what was taking place.
Researchers discovered that temperature variations caused by changing winds on Neptune had an impactful impact on its gravitational field and were able to precisely calculate Neptune’s internal rotation rate by studying its wind-induced gravity signature; it marked the first time such information had ever been obtained for any outer planet and will help support future spacecraft designs to Neptune and Triton.
Triton
Triton stands out due to its retrograde motion and geological activity, but also features peculiar geysers. Voyager 2 captured images of bubbling nitrogen gas and dark material spewing from its south polar region – and scientists speculate these eruptions may be drawing water from an underground ocean beneath Triton’s surface.
Another oddity about Triton is that its axis of rotation is offset 157 degrees relative to Neptune’s, making its polar and equatorial regions alternate in facing the Sun – leading to dramatic seasonal shifts on Triton.
Astronomers don’t yet fully understand how Triton managed to arrive on its current orbit so quickly, though they believe it likely originated in the Kuiper Belt and was captured by Neptune billions of years ago. Rufu and Canup conducted research modeling this capture scenario for Triton and found that it needed to crash into other irregular satellites like Nereid, Ananke, Carme and Sinope in order to settle into its present one; collision energy could explain its extended orbital period.