The Week in Space and Physics: Volcanoes on the Moon
On the far side of the Moon, the collapse of Arecibo, Uranus' odd magnetic field and the next flight of Starship
The far side of the Moon remains an oddly mysterious place. For most of history, of course, we had no way to see it - since our moon rotates exactly once per orbit, it always turns the same face towards Earth - but even after we could, even after we built rockets and spaceships and sent men to walk on the Moon, no one ever ventured to the far side. Few probes have either: the first successful landing, indeed, came only in 2019.
Most of what we know about the Moon, therefore, comes from the near side. This is the face of the Moon we easily recognise: a scarred landscape, covered by dark and vast seas of ancient lava, in the shape of which we sometimes see the appearance of a man or the outline of a rabbit.
This side we have studied intensively. Telescopes have watched it for centuries. Probes have landed there, taken samples and conducted experiments. Astronauts have left their footprints and carried rocks back to Earth. From all this we know the near side of the Moon is both ancient and surprisingly Earthlike. Its rocks look like ours would, if only they had been spared billions of years of rainfall, erosion and biology.
And yet we have good reason for believing the other side really is different. The images returned in the 1960s showed the lunar far side to be rugged, home to both high mountains and deep valleys, and missing the seas of ancient lava so widespread on our side. Analysis from orbit shows it to be geologically distinct as well, a finding that hints at a less volcanic past.
This, on the face of it, is rather strange. Why should the far side of the Moon differ at all from the side we see? Why, indeed, should one side of the Moon have experienced huge volcanic eruptions, each of which poured out vast quantities of lava, while the other half remained so apparently calm?
For answers to this puzzle we need to take a closer look at the far side of the Moon. And fortunately - after a Chinese mission earlier this year - we may finally have the samples we need to start doing that. Chang’e 6, China’s most sophisticated lunar mission to date, touched down on the lunar far side on June 1. After collecting about two kilograms of rock and soil it took off again, headed back to Earth and delivered its cargo to scientists waiting in the Gobi desert.
The first analysis of these rocks, however, has thrown up some surprises. Two independent studies found them to be far younger than expected, showing that volcanoes must have been erupting on the far side far more recently than once believed. Puzzlingly, the rocks also show little sign of the radioactive elements normally needed to power those eruptions.
That could, the authors of one of the studies say, suggest an ancient impact played an important role in forming the Moon’s two halves. That event formed a huge crater near the lunar south pole - but it probably also sent a plume of heat through the Moon. This could have pushed radioactive elements closer to the surface on the near side, heat from which then fuelled powerful eruptions.
Rocks on the lunar far side would thus appear to hold less of these elements, just as Chang’e 6 has found. But to really settle the question we need to spend more time exploring the far side of the Moon. China, at least, seems certain to do that in the years to come.
Why Did Arecibo Collapse?
For almost six decades the name Arecibo was synonymous with radio astronomy. It was from there, in the 1960s and 70s, that the first solid evidence of neutron stars came, that the first binary pulsar was spotted, and that we made, in 1974, a deliberate attempt to contact alien civilizations.
And yet today the Arecibo telescope is no more. In the latter half of 2020, after fifty-seven years of operation, the cables holding the observatory’s suspended platform in place broke. The platform fell, smashed through the telescope’s thousand foot wide dish, and left the structure in ruins. Repair or replacement, its operators concluded in 2022, would not be feasible.
Why did the telescope collapse? The sequence of events, a new report into the failure found, began with Hurricane Maria in 2017. At the time - despite the intensity of the hurricane - engineers thought the observatory had gotten off lightly. Although one cable did break, the rest of them remained in place. Neither did the hurricane put excessive stress on the rest of the telescope - the loads on all its cables, analysis found, were well within their designed limits.
Unrealised at the time, however, was a more subtle effect. Engineers have now found that the intense radio waves produced by the telescope were gradually weakening the zinc sockets holding the cables in place. Until Maria this weakness was small enough to be manageable, but afterwards - after the stress of the storm’s winds - the sockets began to fail.
In August 2020, three years after the storm, a cable pulled out of its socket and crashed down onto the dish. Alone that could have been a freak event, and the telescope should have been able to survive it. True, the loss of the cable increased the load placed on all the remaining ones, but still - engineers thought - none were under undue strain.
The weakened zinc sockets, however, could not handle the extra weight. Within weeks two more cables fell, leaving the telescope in a precarious state. In November the facility was closed, but then, on December 1, another socket gave way. It proved the end. The remaining cables could no longer hold the weight of the platform and it fell, smashing through the dish on its way down.
Such a failure mode, the report found, could not have been anticipated. But better monitoring and reporting could have spotted the failing sockets sooner, and perhaps have given some warning of the impending collapse. Management, then, is partially to blame - but so was Hurricane Maria and the unusual conditions created by one of the world’s most powerful telescopes.
Uranus: An Unfortunate Timing?
Did Voyager 2 probe Uranus at a bad time? Our one and only visit to the giant planet came in 1986, when the Voyager spacecraft flew past the planet, spotted two new ring systems, charted ten new moons, and measured an odd-looking magnetic field.
Unlike the fields around other planets, Uranus’ magnetic field seemed short on plasma, a form of matter in which charged ions and electrons move separately. Its radiation belts, too, seemed unusually intense, putting them on par with the far larger planet Saturn.
Since then astronomers have concluded that Uranus simply has, for some reason, an unusual magnetic field. The lack of plasma might, for example, be down to a collection of calm and inactive moons, none of which add particles in sufficient numbers to build up the plasma levels seen elsewhere.
If so, that would mean Uranus’ moons are rather boring, especially in comparison to those of Jupiter or Saturn. Now, however, a group of physicists have proposed a new explanation: that we simply caught Uranus at a bad time, and so got a skewed image of its magnetic field.
They reckon data from Voyager shows a solar storm sweeping past the probe a few days before it reached Uranus. That storm would have hit the planet just before Voyager arrived. It would have shrunk the planet’s magnetic field, pushed out any plasma within it and left intense belts of radiation behind, exactly as Voyager saw.
All that means we probably need to take a fresh look at Uranus, its moons and its magnetic fields. Fortunately we may, in the next decade or so, finally send a spacecraft to explore the icy giant properly: NASA is currently studying plans to put a probe in orbit sometime in the 2030s.
Starship Launch Number Six
SpaceX plans another launch of their Starship rocket this week. The launch comes less than six weeks after the last one, during which SpaceX managed to catch the rocket’s super heavy booster for the first time.
The next launch will include another attempt to do that, but will likely also include some changes to iron out the issues seen by SpaceX in the first catch. The company will also make another effort at relighting Starship’s engines in space - something that failed during an earlier test flight and has not been reattempted since.
Until now all Starship flights have been suborbital, meaning they fall back to Earth rather than travelling all the way around it. This has been by design - since SpaceX has not yet shown they can fire its engines in space, they have not yet proven they can deorbit it safely. Instead they send it on a known trajectory that brings it back to Earth over a remote part of the Indian Ocean.
If SpaceX can show the engine relight works in the next flight, future test flights are likely to stay in orbit for days or weeks at a time. SpaceX will then move on to test the ability of Starships to meet and dock, and then to transfer propellant between them. That will not be easy, but it is an essential part of SpaceX’s plans for the Moon and Mars.
The (detailed) investigation of the far side of the Moon has barely started, but it seems to me that the role played by radioactive heating is overemphasized... Anyway, I hope the (unmanned) missions to the Moon will continue...