Steve Campbell March 2016
Neptune was the first planet to be discovered by mathematical means. After the discovery of Uranus and subsequent observations of the Seventh Planet, it was observed that its orbit was not meeting expectations of Kepler’s Laws. It was determined that there must be another planet -farther away – that was influencing the orbit. That planet was later discovered and quickly thereafter found to have a large satellite. (1)
Neptune is the farthest Giant Planet from the Sun and not surprisingly the last to be visited by a spacecraft. In the old days, when your author was young, they called Jupiter, Saturn, Uranus and Neptune “Gas Giants”. Now they reserve that title only for Jupiter and Saturn and call Uranus and Neptune “Ice Giants”. As you may know the now call Pluto and Eris “Dwarf” Planets (I can’t agree with that). As of today, Mercury, Venus, Earth and Mars are still “Terrestrial Planets”. But soon, it seems to me, each planetary body will have its own unique category.
Quoting from my two-part series “Sneaking Up on Pluto”:
“One thing that might puzzle the average student might be why we had images of all the Outer planets by the 1970s and 80s and nothing but a dot or smudge for Pluto. That all relates to what was called at the time “The Grand Tour”. As it happened, there was an alignment of the outer planets in the 70’s and 80’s such that it would be possible to use gravity assisted orbital adjustments (“the slingshot effect”) to make it possible for a space probe to visit Jupiter, Saturn, Uranus and Neptune in one long and carefully managed trajectory.”
Any path that could have slung Voyager 2 from Neptune to Pluto would have crashed the probe into Neptune itself. Knowing that, the mission planers arranged to take a good close look at Neptune’s large moon, Triton (not to be confused with Saturn’s big moon, Titan). It was thought at the time that Triton might be a lot like Pluto because Triton is in a highly inclined and retrograde orbit around Neptune. That indicates that Triton may well be a “captured” moon that was similar to Pluto. Triton is actually a bit larger than Pluto (1680 km vs. 1464 miles in diameter). Now that we have seen Pluto, it turns out that the two are quite similar.
The Voyager II Spacecraft
Again from Sneaking Up on Pluto (Please see link below):
“The Voyager probes (one of which actually made the complete “Grand Tour”) each had a main antenna that was capable of constant communications with the Earth. This necessitated what is called a “scan platform” that held the instruments that need precise pointing, that moved independently of the antenna. That configuration had proven troublesome on one of the Voyager probes at Saturn and data were lost. That is because data storage was actually on a ½ inch, 8 track magnetic tape with a total capacity of about ½ Megabyte and a top baud rate of 56 kilobits per second (2). That’s what I said – “Stone Knives and Bear Skins!” – so, real-time transmission was required for image data.” Voyager was – despite my demeaning reference – quite advanced at the time and some of its imagery is still quite impressive.
The image below depicts the identical Voyager 1 and Voyager 2 Spacecraft. (4) The dish antenna is 3.7 meters in diameter (12 feet, 2 inches) across. The arm extending to the right contains the main experiments and the imaging “scan platform”. The left arm holds the three radioisotope thermoelectric generators that power the probe. The gold disk on the “body” is the famous record* with messages and images of Earth for anyone “out there”. I made the mistake of calling it a CD earlier. It is a conventional phonograph record with grooves of recorded sound and digital pictures. It is different only in that it is not vinyl but metal and it is recorded at half speed. A stylus (the “needle”) is packed with it.
This was a project assigned to Carl Sagan. Carl has been inserted into the picture at the proper size to give it a sense of scale. This photo is from his brief and little-known “Fonzarelli” period. Raise your thumbs, Carl!
Figure A: The Voyager Spacecraft NASA/NASA website
About time we got around to the planet, I hear you thinking. I have a table of planetary statistics (3) that serves as a good introduction for any planet. You may expect to see this table in future posts. Please see Figure B, below.
Figure B: Table of Planetary Statistics NASA
You will notice that Neptune has 17 times the mass of the Earth and about 3.9 times the diameter. That only works out that way because the mean density of Neptune is 30% that of Earth. If it were as dense as the Earth, Neptune (of the same diameter) would have 60 times the Earth’s mass. All four Giant Planets are low-density like that, but Neptune is the densest of them. Likewise, all Giant Planets are fast spinning and Neptune is slower than most, rotating in 16 hours. The Navy has adopted a 16 hour rotation of duties and sleep aboard out nuclear subs, so submariners would be right at home on Neptune. Just a small tangent, I’ll get back on track, now.
Figure C: Neptune as seen by Voyager 2 NASA/JPL
The clouds were somewhat of a surprise after the Voyager’s views of Uranus – which was almost featureless. The big dark spot (named rather predictably, the “Great Dark Spot”) was another surprise as were the winds (1500 mph) stirring these features around. These are the fastest winds in the all the Planets (5) and unexpected out in the cold dark zones of the outer Solar System.
Neptune takes 164 years to orbit the Sun. It’s a long wait for Summer, eh? Neptune, like all Giant Planets (plus Mercury and Earth) does have a magnetic field and in fact, it is much stronger than the Earth’s. That would seem to indicate that it has an iron inner core. But it cannot be very large, or the overall density would be larger. It is in fact estimated that the core part of Neptune at its center is about Earth-sized. Most of what is above is water, ammonia and methane (CH4) ice (estimates vary for thickness). That is considered to be its “mantle”.
The atmosphere above that is hydrogen, helium and methane. The white clouds you see vary in composition depending on pressure. The higher clouds where pressures are about Earth-like (1 bar) are probably methane vapor. Lower down and at higher pressures are clouds of ammonia, hydrogen sulfate and even water vapor, like the clouds on Earth.
How thick each of these layers might be is still open to interpretation and you can find many differing diagrams, most with no dimensions mentioned. So, having looked at those, I will guess that the core is 4000 miles in Radius (about the same radius as Earth), the icy mantle extends another 10,000 miles above that and the gaseous atmosphere another 1400.
Later photos of Neptune by the Hubble Space Telescope have shown considerable changes in Neptune’s atmosphere, since Voyager.
Triton is the largest moon of Neptune and by no coincidence, the first discovered (17 days after the discovery of Neptune, itself). It is unique in several respects. It is the only “large” satellite to orbit in a “retrograde” sense. By large, I mean to say that it is near to the size of our own Moon. By retrograde, I mean that it orbits in a direction opposite to the rotation of its planet. That and the high inclination of Triton’s orbit seem to indicate that it was captured. For reasons we won’t go into, it is easier for a moon to be captured in a retrograde orbit than otherwise. Jupiter and Saturn have lots of former asteroids as moons, but they tend to be small and far away. Triton is so close that it is being slowly pulled closer to Neptune and in several billion years will be shattered into a ring like Saturn’s. You might expect a captured moon to be in an eccentric orbit that varies in distance from its planet, but Triton’s orbit is so close to exactly circular that the difference is not worth mentioning. It stays at about 220,483 miles from Neptune which, coincidentally is about the same distance from the Earth to our own moon. It orbits Neptune in 5.8 days and rotates in the same time. That is to say, it keeps the same side toward Neptune, just as our Moon does to Earth. Now, some of my readers are sharp enough to notice that our moon takes 28 days to orbit. Why so different if the distances are near the same? The difference, of course is that Neptune is 17 times as massive as the Earth, as I mentioned a few paragraphs ago. This will be on the test! 😉 Figure D, below is a Voyager 2 image of Triton
Figure D: Triton NASA/JPL
All other large satellites orbit the same way and are therefore by definition, prograde. Triton also looks quite distinctively different from most other planetary satellites, which tend to be rather uniform and crater covered (admittedly with many exceptions). It has an atmosphere that, while very thin, has detectable clouds. It shares the much modified and differentiated characteristics that we now know of on Pluto. That tends to confirm the “capture” hypothesis.
You may ask, “Just how does a passing object become “captured”? “. There are several ways. One would be for Triton to have collided with a smaller moon, as it passed near Neptune. That might slow it just enough to wind up in an orbit. As it would have collided with a prograde moon, that would be especially effective since that would almost double the velocity difference between the two and quadruple the energy delivered to the passing Triton. That should have left a mighty crater on Triton. While nonesuch was seen by Voyager, such a crater could have since been covered by the glacier-like deposits of Nitrogen ice (the part that looks like cantaloupe peel) that are visible in Figure D. Likewise, that crater might have been in the darkened part of Triton, that was not visible when the Voyager went zooming by at the greatest velocity ever given to a man-made object (at the time)
A second possibility would be “gas drag” as Triton passed through the upper atmosphere. That would seem unlikely, unless Neptune had a more extensive atmosphere at the time. Since it may have been captured billions of years ago, that is entirely possible, but still just speculation.
Another possibility was detailed in a paper by Craig Agnor (University of California, Santa Cruz) and Douglas Hamilton (university of Maryland) in 2006. (6) First, I should explain that Pluto and all the other Smaller Planets out past Neptune have been designated as Kuiper Belt Objects (KBOs). If Triton had been one of a co-orbiting binary pair of KBOs, it is possible that a pass near Neptune would have captured it, while at the same time, ejecting its companion to a more distant orbit of the Sun. It all has to do with relative motion of the three bodies. This hypothesis has the virtue of not relying on chance collisions or hypothesized “greater atmospheres”. This idea was made more believable by the discovery that many KBOs are indeed, binary. Not the least of these is Pluto, who’s biggest satellite (Charon) is about one half its own size. It has been estimated that 15% of KBOs may be binary in nature. That a KBO could have come near to Neptune is not unlikely since Pluto itself comes nearer the Sun than Neptune as it was during the late years, last Century. Having said that, I must also remark that Pluto is now in a resonance with Neptune that keeps the two safely apart. I mean to say, that when Pluto comes nearer to the Sun, it is still very far from Neptune and always will be.
But, all in all, I think these guys are very near the mark with their hypothesis.
Neptune is another fascinating member of the Solar System and I learned a lot by researching to write this article. I hope you find it interesting as well. You may wonder why I do this. Well, those of you who know me know that I suddenly have time on my hands. It is a blessing…and a curse. ;-). Also, I have always had a fascination with the Solar System that goes back to my days in Elementary School.
You and I are truly fortunate to live in a time when these mysterious dots of light in the sky that were the Planets are now becoming known as Great Worlds, many that dwarf the Earth in size and complexity and others that are revealing the secrets of Nature that have been heretofore unknowable.
- Voyager Data Rate: http://itknowledgeexchange.techtarget.com/storage-disaster-recovery/nasas-voyager-used-8-track-tape-to-go-into-space/
- Table of Planetary Statistics: http://nssdc.gsfc.nasa.gov/planetary/factsheet/planet_table_british.html
- Voyager details: https://en.wikipedia.org/wiki/Voyager_2#