Selenography

1 03 2010

We looked once more to the sky.

A team from Alpha-1 had made the elevator trip to Port Mayflower. From the observation deck they enjoyed the vertiginous view of Fram spread across the floor of space, and part of the ring slicing toward the horizon. They were looking west, and Amundsen had hours before risen from that horizon. Now it lay above the curve of Fram’s atmosphere, strung along the ring like a bead on a rust-red string.

Fram was a small world; only five and a half thousand kilometres across, not much more than forty percent the size of Earth. But like Earth, Fram had a relatively large moon. Amundsen’s irregular shape was about one thousand two hundred kilometres across, similar to the size of Quaoar or Charon. As Luna was to Earth, so Amundsen was approximately twenty percent the size of Fram. The entire Fram system, planet, ring and moons, would fit easily in the gulf between Earth and Moon.

The experts on Port Mayflower delicately pointed out that we had yet to visit any of the moons. Much of our knowledge of the satellites of Fram had been garnered from observation – by the probes which had visited the system before we had even left Sol, from the Quoqasi as we arrived, or by the orbiters since Planetfall. Some of our knowledge came from the ring system itself. The components of Fram’s ring were pieces of the disintegrating Amundsen, and we had spent weeks crawling over the surfaces of the larger pieces and moving them into safer orbits. There had been some room for science.

“The Fram ring was a good place to start our study of the three moons,” one selenographer explained, and then launched enthusiastically into an explanation.

It was a thin ring; it was about a meter thick, and it was not very opaque. The ring extended from a thousand to six thousand kilometres above Fram’s equator. Our orbiters had spent most of their time in the thousand kilometres beneath the inner ring, that part of the disk closest to Fram. The particles which composed the inner ring were no more than a meter in size, although there were some larger pieces, all of which had been tagged with transponders during the first three months after Planetfall. These larger pieces came from closer to the orbit of Amundsen.

The inner ring was divided from the outer ring by the Sverdrup Division, a sharply defined gap of around four hundred kilometres. The Division was caused by the orbit of Sverdrup, the closest of Fram’s moons, which orbited entirely within the Amundsen ring. Sverdrup was the first of Fram’s shepherd moons; as Sverdrup swam through the ring material on its orbit, its gravity cleared a path through the ring.  Material that drifted into this gap was deflected back into the ring by Sverdrup’s orbit, or accreted onto the mass of the moon.

The Sverdrup Division was not entirely empty. There was a thin, central ringlet which shared Sverdrup’s orbit. This ringlet was knotted by spiral density waves: short, horseshoe-shaped oscillations caused by the resonances of the orbits of Sverdrup and Amundsen.

The selenographer continued. “We really learned a lot about the composition of Amundsen while we were wrapping reflective blankets and installing rockets on the surfaces of the ring bodies.”

The pieces themselves were mostly silicates, a much higher proportion of silicate to iron than we had expected. What iron had been found was in the form of oxides, which explained the reddish colour of the ring. The ring possessed its own sparse atmosphere, only microns thick, of molecular oxygen, hydrogen and even hydroxide. The existence of this atmosphere suggested that the ring once contained quantities of water ice; ultraviolet light from the twin stars had broken down the water ice into its constituent elements.

“And that’s representative of Amundsen?”

“Yes and no.”

When we looked at Amundsen, much of its composition suggested an early bounty of water ice. Spectroscopic studies had shown a rather typical proportion of iron to silicate; moreover, the iron was contained within oxides. UV spectra showed more detail. There were hydrated silicates, which bore magnesium and iron; hydrogen sulphates; sodium sulphates; cyanogens; and carbon dioxide and sulphur dioxide, frozen onto the surface. But the only ices which now existed on Amundsen’s surface were carbon dioxide and volatile ammonia ices; the early water ices suggested by the ring and by the presence of so many sulphates and oxides had long since gone.

The selenographer explained how the water ice might have vanished. It really depended upon which model proved correct to explain the formation of Amundsen. If Amundsen had accreted in orbit of Fram, as the Galilean moons seem to have in the orbit of Jupiter, then it was possible that the surface water ice had simple sublimed under billions of years of ultraviolet light from Alphas A and B. Amundsen lacked any magnetosphere or atmosphere to protect the water ice from the stellar wind.

“There are a number of problems with this hypothesis,” the selenographer continued. “First, if the ice had sublimed, we’d expect to find traces of an atmosphere on Amundsen like we do in the rings. We don’t. A lot of it would escape into space, but not all. The presence of molecular hydrogen and oxygen in the rings suggests that we would find some evidence of the effect of this breakdown. Second, the fact that we find evidence of water ice in the rings suggests that there was water ice on Amundsen when it started to break-up. We think the impact which precipitated that break-up was relatively recent, only tens of millions of years.”

The program director from Alpha-1 nodded. “Not enough time for the ice on Amundsen’s surface to completely break down. I see.”

Another big problem with the accretion model, the selenographer clarified, is that it was highly unlikely a moon so relatively large and massive to the planet it orbited could accrete in so close an orbit. If it did, we would expect each body to share a mutual barycentre; that is, to rotate around a common point external to each body, like the dwarf planets Pluto and Charon. Amundsen was very large, relative to Fram, and orbited very closely, relative to their radii. The accretion model simply didn’t account for these facts.

“But Charon did not accrete around Pluto,” the director protested.

The selenographer winced. “A poorly chosen example. But we have only two star systems to draw upon for examples.”

The most likely model, perhaps unimaginatively, was the same which accounted for the formation of Earth’s moon: the giant impact theory. A large object had smashed into Fram in the distant past, spewing material into orbit. The remnants of this object accreted together with the displaced material from Fram, coalescing into Amundsen. Such an impact could explain the angular momentum of Fram and Amundsen; that they did not share a mutual barycentre, and that Fram should continue to rotate so quickly. In the absence of such an impact, Fram might well have become gravitationally locked to Amundsen. This had happened to Pluto and Charon: both dwarf planets kept the same face to each other.

There was other evidence supporting the impact model. Fram’s crust was highly anorthositic, and basalt sheets a common feature of the fractured bedrock; this suggested a catastrophic period of global magmatic activity, of a scale far out of proportion to Fram’s size and geological inactivity. It was likely that Amundsen had also once had a magma ocean; that evidence would only be found by a mission to the moon.

“Of course, if this model is proven, then Amundsen is a particularly unlucky celestial body,” the selenographer said.

Because it had been struck again, catastrophically, tens of millions of years ago. The most recent impact had been the one that shattered Amundsen’s crust, and probably accounted for the lack of water ice. It had been a traumatic experience for Amundsen – Fram’s ring demonstrated that – and while it had not led to the creation of another magma ocean, the impact would have melted away exposed water ice, then lost to space.

“But how did it get the ice, between the impact that formed it and the impact that destroyed it?”

The selenographer explained the benefits of having a moon so massive and close to Fram. It acted, in a local way, like Jupiter acted to the Solar System – is spun along its orbit and scooped up other nearby bodies, effectively protecting the inner system. In the billions of years between major events, Amundsen had been bombarded by asteroids and comets, the latter of which had deposited water and volatile ices on Amundsen’s surface. These deposits were built up laboriously over a period of some four billion years; what the suns stole through ultraviolet sublimation had been replenished by further impacts. Until the latest, most dramatic impact.

The selenographer explained that the break-up of Amundsen was probably not entirely a result of the impact; rather, that whatever hit the moon had fractured its crust and then pushed the moon beyond its Roche limit.

The farthest of the moons from Fram was Nansen. Like Sverdrup, Nansen was a small shepherd moon, regulating the ring which surrounded Fram. Nansen orbited at the far edge of the ring, defining its outer edge. At first we thought both Sverdrup and Nansen were larger pieces of Amundsen, but spectroscopic studies suggested otherwise. Both were captured asteroids, probably carbonaceous chondrites, caught up in the interacting gravities of Fram and Amundsen.

The selenographer pointed out Nansen to the team from Alpha-1. It was a small lump, highlighted by the suns, a grey potato tinted terracotta by the colour of the fuzzy ring. Nansen was the larger of the captured asteroids, just under fifty kilometres on its longest side. Both Nansen and Sverdrup were much larger than the chunks which made up the ring, and were easily distinguished from the surface.

“Far down the line,” the selenographer continued, “we want to capture a third body and use it to stabilise the inner ring, like Nansen stabilises the outer edge. Because Wilbur is far too small.”

But there would be problems. The three moons were already in a precise Laplace resonance: for every one orbit that Sverdrup completed, Amundsen completed two, and Nansen four. Moreover, any shepherd moon placed underneath the ring would be at risk of being pulled apart by the tidal forces of Fram. One solution to this would be to shape an orbit below the synchronous orbit radius. Such a fast orbit, faster than Fram’s own rotation, would stave off the risk of tidal deceleration – for a few million years, at least.

“And there are any number of asteroids out there,” the program director waved an arm in the microgravity out beyond the Fram system, “from which to choose.”

“Indeed. The moons first, however.”

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24 08 2012
Epilogue « Orbital Shipyards: Alpha Centauri System

[…] on that moon. And, if only in the spirit of the age of exploration, small missions were sent to the shepherd moons of Sverdrup and […]

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