Voyager-2’s most detailed look at Neptune’s moon Triton

Triton
Click for original image.

Today we conclude our tour of the Voyager-2 fly-bys of Uranus in 1986 and Neptune in 1989 with what is the most detailed look at the alien surface of Neptune’s moon Triton, taken on August 25, 1989 and shown to the right, cropped, rotated, reduced, and sharpened to post here.

Taken from a distance of only 25,000 miles, the frame is about 140 miles across and shows details as small as [a half mile in width]. Most of the area is covered by a peculiar landscape of roughly circular depressions separated by rugged ridges. This type of terrain, which covers large tracts of Triton’s northern hemisphere, is unlike anything seen elsewhere in the solar system. The depressions are probably not impact craters: They are too similar in size and too regularly spaced. Their origin is still unknown, but may involve local melting and collapse of the icy surface.

A conspicuous set of grooves and ridges cuts across the landscape, indicating fracturing and deformation of Triton’s surface. The rarity of impact craters suggests a young surface by solar system standards, probably less than a few billion years old.

What this photograph as well as the handful of other Voyager-2 images of Triton tell us is that we only have gotten a tiny taste of what’s there, only enough to tell us we don’t understand what we are seeing in the slightest. This is a truly alien world, cold, dark, and composed of materials far different then that found in the inner solar system. Its formation is a mystery, and its subsequent geological history a cypher. Scientists have made some guesses, but to get a real understanding we need to go back, and be there for a long time.

In fact, this is the final conclusion of all of the Voyager-2 images from both Uranus and Neptune. That probe gave humanity its first good close look at these distant worlds, but the look was still a quick and very superficial one. The images and data left us with far more questions than answers.

Unfortunately, there is at present no mission approved and under development to go to either Uranus or Neptune, though several have been proposed. Thus, it will likely be at least two decades before any mission gets there, if that soon.

Triton: Neptune’s largest moon

The southern mid-latitudes of Neptune's moon Trident
Click for original image.

Triton

Today’s cool image begins a new tour I plan on doing over the next week or so of the few close-up photographs we have of Neptune and its moons, sent back by Voyager-2 when it did its close fly-by of this distant planet on August 25, 1989. That fly-by was almost 37 years ago, and it remains our only close look. While at the time it shined a quick flashlight of new knowledge on Neptune, its moons, and its ring system, we remain generally in the dark about what’s there, despite some good imagery produced in subsequent years by Hubble and some ground-based telescopes.

The image above, cropped and enhanced to post here, shows a portion of the southern mid-latitudes of Triton, Neptune’s largest moon, as Voyager-2 made its closest pass at a distance of about 25,000 miles. The photo to the right, cropped and reduced, shows a more global view to provide some context, with the box indicating the approximate area covered by the upper image. It was taken when Voyager-2 was on approach, at a distance of about 330,000 miles. The top picture captures several dozen black plumes that appear to vent material from below. From the caption:

The plumes originate at very dark spots generally a few miles in diameter and some are more than 100 miles long. The spots which clearly mark the source of the dark material may be vents where gas has erupted from beneath the surface and carried dark particles into Triton’s nitrogen atmosphere. Southwesterly winds then transported the erupted particles, which formed gradually thinning deposits to the northeast of most vents.

It is possible that the eruptions have been driven by seasonal heating of very shallow subsurface deposits of volatiles, and the winds transporting particles similarly may be seasonal winds. The polar terrain, upon which the dark streaks have been deposited, is a region of bright materials mottled with irregular, somewhat dark patches. The pattern of irregular patches suggests that they may correspond to lag deposits of moderately dark material that cap the bright ice over the polar terrain.

As we only have a few images of this planet, and those provided views of only about 40% of its surface, any theory that tries to explain the weird geology here is certain to be wrong to some degree.

More to come in the next few days. As much as we think we know, these pictures are going instead highlight how sparse that knowledge really is.