Orbiting the planet further out are the irregular moons, including Triton. Scientists suspect that Triton migrated from somewhere outside, and that its arrival disrupted Neptune’s moon system. For example, it may have forced Nereid, the third-largest moon, into its now very eccentric orbit.
Triton is easily the leader in terms of mass. It accounts for more than 99 percent of the total mass of the moons. Triton likely threw the original inner moons out of their orbits, and when they collided, the fragments formed the rings and today’s inner moons.
For Neptune, there is an advantage to being so far out in the solar system—the planet can keep its moons on a longer leash without needing to worry about them being stolen away by the competition. The distance between Neptune and its two moons Psamathe and Neso, probably created by the break-up of an earlier celestial body, is 125 times greater than the distance between the Earth and our moon.
Nereid—the Eccentric Moon
Before the Voyager 2 spacecraft made its trip, there was knowledge of only two Neptunian moons, Nereid, which was discovered in 1949, and the mighty Triton. Nereid owes its discovery to its very eccentric orbit, which brings it almost as close as a million kilometers from Neptune before it swings out to a distance of 10 million kilometers. The moon was either captured by Neptune from the Kuiper belt or thrown into this orbit by Triton.
Nereid has a radius of at least 170 kilometers. To this day, astronomers don’t know its precise shape because Voyager 2 didn’t come close enough. Its spectrum suggests that there is water ice on the surface, while its composition appears to indicate that it originated quite near Neptune. Nereid may be in a kind of chaotic rotation, stumbling along its way as if it’s had too much to drink. This would explain why its brightness sometimes fluctuates significantly in the images that have been taken from Earth only to then remain constant.
Proteus—the Angular Moon
Proteus is Neptune’s second-largest moon. With a radius of 420 kilometers, it is significantly larger than Nereid. Because it is so close to Neptune, telescopes on Earth can’t see it, though Voyager 2 sent images. These images show that, despite its size, Proteus does not have a spherical shape, which is unusual. It is probably just at the threshold. Celestial bodies that are slightly larger are forced—by their own gravity—into spheres.
The surface of Proteus is covered by numerous craters that range up to 260 kilometers across. The collisions have led to large cracks and warpages that draw their way around the moon. The moon’s interior is likely comprised of rocks with significant amounts of water ice. However, this is not evident on the moon’s surface, which is very dark.
Triton—the Geyser Moon
Just 17 days after astronomers had identified Neptune, the British beer brewer William Lassell discovered the moon of Triton. Thanks to his successful business, Lassell could afford an expensive observatory and also found moons of Saturn and Uranus. He found Triton using a 61-cm telescope he had made himself.
The name, which comes from a Greek god of the sea frequently depicted as the son of Poseidon, i.e. Neptune, was suggested in 1880 by Camille Flammarion. For a long time, prior to the discovery of the other moons, Triton was simply referred to as the ‘moon of Neptune.’
Triton’s orbit exhibits a peculiarity. It follows an orbit that is almost perfectly circular, but also retrograde to its planet, and thus in the direction opposite to Neptune’s rotation. Triton takes 5 days and 21 hours to complete its orbit. According to the current model, which represents the moons as having been formed from a rotating disk of dust, Triton could not have originated in its present location. Presumably, the seventh-most-massive moon in the solar system—its mass is greater than the combined mass of all the moons smaller than itself—migrated from far outside, where it may have long been a minor planet. Researchers therefore suspect that its structure allows for insights about the structure of other objects from the outer solar system such as Pluto or its companion Charon, which is slightly smaller than Triton.
With a radius of 2,700 kilometers, Triton always faces its planet with the same side. It is currently much closer to Neptune than Earth’s moon is to the Earth, which means that it is heavily influenced by Neptune’s gravitational forces, since Neptune is 19 times heavier than Earth. This invites comparison with Saturn’s moon Enceladus, which is in a similar relationship. Although Triton’s surface, at minus 237.6 degrees, represents the lowest temperature ever recorded in the solar system, it’s possible there is a liquid ocean below the moon’s surface, as is the case with Enceladus.
Scientists suspect that there is a solid core of rock and iron inside the moon. Covering this would be a frozen mantle, likely made of water ice, which makes up between 15 and 35 percent of Triton’s mass. The crust that is located over the predicted ocean would also be of ice, with a layer of nitrogen over it. Images from Voyager 2 show fascinating surface structures, all of which must still be relatively young. Therefore, Triton must still be geologically active. The core is large enough that heat is likely produced from radioactive decay.
On the surface, this is evidenced by numerous forms that are clearly volcanic or tectonic. In contrast to Earth, however, the volcanoes on Triton don’t spew lava, but instead water and ammonia. Moreover, at hotspots in the crust, Voyager 2 found geysers emitting nitrogen gas mixed with dust. The sun was probably responsible for their formation, with its radiation penetrating a thin ice crust and heating the underlying nitrogen ice until enough pressure built up for it to escape through an opening.
These eruptions do not last for a few hours, as with geysers on Earth, but rather for up to a year. They emit an enormous amount of gas that is borne to heights of up to 150 kilometers. The dust that is carried along with the gas settles into characteristic streaks. This process is evidenced by observations from the Gemini South Observatory, where researchers have managed to collect signatures that clearly come from a mixture of nitrogen ice and carbon monoxide ice, not the two types of ice alone. The combination could also have spread across the surface of Triton over the course of the seasons, depending on the amount of sunlight.
“Despite Triton’s distance from the Sun and the cold temperatures, the weak sunlight is enough to drive strong seasonal changes on Triton’s surface and atmosphere,” explained Henry Roe of the Gemini research team. Since Neptune takes 165 years to travel around the Sun, Triton’s seasons are, compared to Earth’s, incredibly long. Only humans who are now at least 60 years old were alive at the same time as the end of Triton’s last winter. Right now it is summer. Fall will arrive at the end of the 2030s, and winter will return in 2080. If the geysers are indeed created by the heat of the sun in the summer, tourists to Neptune should be sure to reserve their tickets for Triton prior to 2080.
Now that we have beheld these impressive geysers, I invite you to take a flight with me, over the caps that cover both poles—at the North Pole you can’t be sure there is a cap, as it was not visible to Voyager 2. They consist of frozen nitrogen and methane. You will come across geysers again, and then more geysers. Further towards the equator, we will fly over broad, flat plains with height differences of no more than 200 meters, pocked with older craters that are only indistinctly perceptible. These plains were created by ammonia and water lava outflows. The volcanic springs can still be recognized in the form of small dots. In the eastern half, the plains are decorated with maculae—spots with dark centers and bright halos—that are 20 to 30 km in diameter. How they came about is unclear.
In some places, the plains are interrupted by ditches and valleys that are of tectonic origin. In the western half, the Cantaloupe terrain is worth a visit. You can’t view such a sight on any other known celestial body. From afar, the surface really does look like the outside of a melon from the cantaloupe family. The strange indentations of dirty water ice are among the oldest structures on Triton. Again, the exact origin is unknown, though diapirism is possibly the cause—that is, material that either has a lower density than the adjacent material
, or is heated below and rises up, similar to the functioning of a lava lamp, from the inside of the celestial body.
Incidentally, Triton’s future is dark. The effects of the tides are slowing it down. As a result, it is getting closer and closer to Neptune and will keep doing so until it falls below a theoretical limit, the Roche boundary, in about 3.6 billion years, but possibly as little as 100 million years. At this time, it will either collide with Neptune or break apart. If you intend to visit this moon and its nitrogen geysers, don’t wait too long. After that, instead of the moon, there may be a more substantial ring system that forms and would undoubtedly be worth the visit. Or perhaps Triton will fire its engines and leave the solar system again.
At the moment, there are no concrete plans to visit Triton or even Neptune. In the spring of 2019, a team of researchers proposed the ‘Trident mission’ as part of NASA’s Discovery program, which costs $500 million. The probe could launch in the spring of 2026 and reach Neptune’s system in 2038 after passing Venus and Jupiter. In a single flyby of Triton in the New Horizons style, the probe would map large parts of its surface. It would approach to approximately 500 kilometers to investigate the atmosphere up close. Whether Trident makes it on NASA’s list will be determined in early 2020.
Tip: You can receive A Guided Tour of Neptune as an illustrated PDF, as always, by going to hard-sf.com/subscribe.
Excerpt: Mars Nation
Sol 3, NASA base
The sun floated right above the horizon in a rosy sky. Lance squinted at it. It looked much smaller from here than it did from Earth, but its light could still blind him if he gazed right into it. After a few more arc minutes around the sun, the sky shifted to blue. Sharon, the pilot whose studies had also included meteorology, had told him about this, but he hadn’t believed her. He needed to apologize to her, even though the others knew by now that he was one of those who had to see such things with his own eyes.
“Everything alright?” he heard Mike’s voice say over his helmet radio.
“Yes, Commander. It’s very romantic.”
“You have things to do.”
Thanks, Mike, he thought. Like I didn’t know that already. Lance took his time anyway. It was to be his first step onto the new planet that would be his home for the next six months. Until now, all he had done was travel across the Mars surface from the landing site of the Endeavour to their quarters inside the Rover’s pressurized cabin, which he reached via a pressurized tube.
He squinted again at the distant sun. It looked white from here, not yellow like from Earth, which might explain why it felt less warm. Of course, that wasn’t the fault of the color, but rather the distance. “We need to optimize the disembarking process,” he said. “If it takes us six hours every time, we’ll never get any work done.”
“Don’t worry,” Mike replied. “Even after the sun sets, it will stay light for a while. The dust in the atmosphere will keep reflecting the sunlight even after the sun has dropped below the horizon.”
“No shit,” slipped out before Lance could catch it.
The commander’s know-it-all commentaries always rubbed Lance the wrong way. They had received more than enough information about what a day was like on Mars during their intensive training. However, it never paid to let himself get annoyed. He was lucky Mike was always insensitive to stuff like this. And, there were the times when no one except Mike had a handy answer for something. It wasn’t a surprise to find out that the man had finished his college physics degree by the age of nineteen.
Lance braced his hands against the roof to their quarters and pushed downward. Ninety percent of the base was buried under the Martian surface to protect its residents from radiation. The roof was composed of the now unneeded heat shields that had once belonged to the explorer robots that had built the base prior to the humans’ arrival. A thick layer of Martian dirt was spread across the metal. Lance finished pushing his torso up through the hatch. He pulled his legs out, knelt on the roof, and finally stood up. His breathing was heavy. After so many months of weightlessness, every activity seemed astonishingly difficult. What had been the point of pedaling like crazy on that exercise bike all the time?
“Okay, I’m out,” Lance said.
“Looking good,” Mike answered. “At least from what I can see on the screen. But your heart rate! You’re really out of shape, aren’t you?”
“Watch it, boy, or I’ll show you what shape I’m in at arm wrestling later on.”
Diplomatically speaking, Mike was notably underwhelming in terms of physique. It was nothing short of a miracle that he had made it through the training process. His intellect probably compensated for everything else, but he didn’t stand a chance beating Lance at arm wrestling. Lance took a cautious step away from the hatch toward the sunset. He expected to hear a hollow tone as he moved forward, but the air was probably too thin and the sensitivity of the external microphone too low for that.
“You know you’re supposed to wait, right?” Mike asked.
Of course, he knew that. The airlock was so narrow that only one person in a spacesuit could fit through at a time. Once outside, they were always supposed to move around in pairs, which was why Sarah, his buddy for this walk, would emerge through the hatch in a minute or two. But it wouldn’t hurt if he looked around a little, would it?
He walked over to the edge of the roof. From here, it was only about twenty centimeters down to the Mars surface. Should he? He had yet to feel like he was actually standing on the planet. What would happen? The ground looked dry and stable. The area right around the base had been reinforced during the construction process. Lance glanced back toward the airlock, but Sarah was still nowhere in sight. He slowly extended his right leg, scooted it forward, and lowered it—and himself—toward the ground. He gingerly placed his foot down, beginning with his heel, and had to burst out laughing. The situation was hardly humorous, so the outburst had to be a subliminal overreaction. He was the first person to ever set foot on Mars! Although he didn’t feel up to celebrating, he couldn’t stop the laugh. Lance shifted his weight onto his right leg before bringing the left one down to the ground. He stood there then, and laughed again.
“Are you feeling okay?” Mike asked. “I assume that, despite our agreement, you didn’t wait for Sarah. We’re lucky we’re the only ones here. How would it look to the bigwigs if they knew you disregarded my direct orders about the exiting process?”
“But I didn’t, Mike, promise,” he replied.
“Earth is asking how everything’s going with the suit,” Mike said.
By this point the first images had been transmitted home. That meant twenty minutes had gone by, assuming that Earth had responded immediately. Lance gazed at the glowing numerals on his wrist. Exactly. However, that also meant that one-twelfth of their maximum shift of four hours was already gone. Sarah should be here any minute. The mission’s doctor was supposed to help him clear the dust from the top of the base so that everything would look in tip-top shape for the TV transmission. Mission Control never left anything to chance. The international mission had cost too much to open it up to criticism.
Lance looked down. The suit was very form-fitting, and it looked quite good on him if he did say so himself. But that was definitely not what Mission Control wanted to hear. Although... maybe. Compared to the older models, which had ballooned around the astronauts’ bodies, the new suits were significantly more photogenic. The only part still pressurized was the inside of the helmet, while the suit’s elastic material was wrapped tightly around his body. He had noticed how agile he felt when he climbed out of the hatch. During the training on Earth, he had gotten to know—and to hate—the old, stiff spacesuits.
“I really like it. Kudos to the developers!” he said.
He then tapped the glowing numerals on his arm, thus activating the projection screen inside his helmet. Earth would soon be bombarded with data, the extent of which blew his mind. Locational and directional specs, temperature, pressure, wind
direction, the composition of the stone on which he was standing. It was too much, within the confines of a space helmet. He tapped his arm again, and the digits and arrows gradually vanished. He really didn’t need to know anything else besides the cardinal direction in which he was looking at the moment.
“The data stream on my screen is a little much,” he said.
“Oh, the suit is tracking your position. Don’t worry,” Mike replied.
“Lance?”
He turned around at the sound of Sarah’s voice. Her head was just emerging from the round hole in the roof. He couldn’t make out her face due to the reflection of the setting sun in her visor.
“Need some help?” he asked.
“I’ve got it, thanks,” she assured him.
Her English was spoken with a faint accent. Sarah hailed from Switzerland. Nobody on board had yet managed to pronounce her last name correctly. Jaeggli sounded a little like Jacqueline, but in Sarah’s mouth, it was something completely different. Lance enjoyed teasing her that she didn’t look anything like the stereotypes about her country. After all, people always imagined Swedes as tall and blonde, but she was short and dark-haired. She calmly kept reminding him that she came from Switzerland, not Sweden, which she knew were hard for an American to keep straight.
As a precaution, he climbed back onto the roof. They didn’t know yet how well Sarah would adjust to the unfamiliar gravitational pull.
“Are you both ready?” Mike asked from inside.
“Looks like it,” Lance said.
The Triton Disaster: Hard Science Fiction (Solar System Series Book 4) Page 28
