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Astronomers Think They Know The Reason For Uranus’s Kooky Off-Kilter Axis : ScienceAlert
Uranus marches to the beat of its own weird little drum.
Although it shares many similarities with our Solar System’s other ice giant, Neptune, it has a bunch of quirks that are all its own.
And one of these is impossible to miss: Its rotational axis is so skewed it may as well be lying down. That’s a whopping tilt of 98 degrees from the orbital plane.
And, to top it all off, it’s rotating clockwise – the opposite direction from most of the other planets in the Solar System.
A new study has found a plausible explanation for this weird behavior: A moon migrating away from the planet, resulting in Uranus being pulled over onto its side. And it wouldn’t even need to be a big moon. Something half the mass of our own Moon could have done it, although a larger moon would be the more likely contender.
The reasoning has been laid out in a paper led by astronomer Melaine Saillenfest of the National Centre for Scientific Research in France. This paper, not yet peer-reviewed, has been accepted into the journal Astronomy & Astrophysics and made available on preprint resource arXiv.
Scientists have come up with models to explain this weird behavior, such as a massive object that collided with Uranus and literally smacked it sideways, but the more favored explanation is a bunch of smaller objects.
However, this hypothesis raises issues that are even more difficult to explain: namely, those pesky similarities to Neptune.
The two planets have extremely similar masses, radii, rotation rates, atmosphere dynamics and compositions, and wacky magnetic fields. These similarities suggest that the two planets could have been born together, and they become much more difficult to reconcile when you throw planet-tipping impacts into the mix.
This has led scientists to seek other explanations, such as a wobble that could have been introduced by a giant ring system or a giant moon early in the Solar System’s history (albeit with a different mechanism).
But then, a few years ago, Saillenfest and his colleagues found something interesting about Jupiter. Thanks to its moons, the gas giant’s tilt could increase from its current slight 3 percent to around 37 percent in a few billion years, thanks to the outward migration of its moons.
Then they took a look at Saturn and found that its current tilt of 26.7 degrees could be the result of the rapid outward migration of its largest moon, Titan. This could have happened, they found, almost without having any effect on the planet’s spin rate.
Obviously, that raised questions about the most tilted planet in the Solar System. So the team performed simulations of a hypothetical Uranian system to determine whether a similar mechanism could explain its peculiarities.
It’s not unusual for moons to migrate. Our own Moon is currently moving away from Earth at a rate of about 4 centimeters (1.6 inches) per year. Bodies orbiting a mutual center of gravity exert a tidal force on each other that gradually causes their rotations to slow. In turn, this loosens gravity’s grip so that the distance between the two bodies widens.
Turning back to Uranus, the team performed simulations with a range of parameters, including the mass of the hypothetical moon. And they found that a moon with a minimum mass of around half that of Earth’s Moon could tilt Uranus towards 90 degrees if it migrated by more than 10 times the radius of Uranus at a rate higher than 6 centimeters per year.
However, a larger moon with a size comparable to Ganymede was more likely, in the simulations, to produce the tilt and spin we see in Uranus today. However, the minimum mass – about half an Earth Moon – is about four times the combined mass of the current known Uranian moons.
The work accounts for this, too. At a tilt of about 80 degrees, the moon became destabilized, triggering a chaotic phase for the spin axis that ended when the moon ultimately collided with the planet, effectively “fossilizing” Uranus’ axial tilt and spin.
“This new picture for the tilting of Uranus appears quite promising to us,” write the researchers.
“To our knowledge, this is the first time that a single mechanism is able to both tilt Uranus and fossilize its spin axis in its final state without invoking a giant impact or other external phenomena. The bulk of our successful runs peaks at Uranus’s location, which appears as a natural outcome of the dynamics,” they continue.
“This picture also seems appealing as a generic phenomenon: Jupiter today is about to begin the tilting phase, Saturn may be halfway in, and Uranus would have completed the final stage, with the destruction of its satellite.”
It’s not clear whether Uranus could have hosted a moon large enough and at a high enough migration rate to produce this scenario, and it will, the researchers say, be challenging to show with observations.
However, a better understanding of the current rate of migration for Uranus’s moons would go a significant way towards resolving these questions. If they are migrating at a high rate, this could mean that they formed from the debris of the ancient moon following its destruction many eons ago.
Bring on that Uranus probe.
The research has been accepted into Astronomy & Astrophysics and is available on arXiv.
Earth’s Days Are Mysteriously Getting Longer, Scientists Say
Atomic clocks, combined with precise astronomical measurements, have revealed that the length of a day is suddenly getting longer, and scientists don’t know why.
This has critical impacts not just on our timekeeping, but also things like GPS and other technologies that govern our modern life.
Over the past few decades, Earth’s rotation around its axis – which determines how long a day is – has been speeding up. This trend has been making our days shorter; in fact, in June 2022 we set a record for the shortest day over the past half a century or so.
But despite this record, since 2020 that steady speedup has curiously switched to a slowdown – days are getting longer again, and the reason is so far a mystery.
While the clocks in our phones indicate there are exactly 24 hours in a day, the actual time it takes for Earth to complete a single rotation varies ever so slightly. These changes occur over periods of millions of years to almost instantly – even earthquakes and storm events can play a role.
It turns out a day is very rarely exactly the magic number of 86,400 seconds.
The ever-changing planet
Over millions of years, Earth’s rotation has been slowing down due to friction effects associated with the tides driven by the Moon. That process adds about about 2.3 milliseconds to the length of each day every century. A few billion years ago an Earth day was only about 19 hours.
For the past 20,000 years, another process has been working in the opposite direction, speeding up Earth’s rotation. When the last ice age ended, melting polar ice sheets reduced surface pressure, and Earth’s mantle started steadily moving toward the poles.
Just as a ballet dancer spins faster as they bring their arms toward their body – the axis around which they spin – so our planet’s spin rate increases when this mass of mantle moves closer to Earth’s axis. And this process shortens each day by about 0.6 milliseconds each century.
Over decades and longer, the connection between Earth’s interior and surface comes into play too. Major earthquakes can change the length of day, although normally by small amounts.
For example, the Great Tōhoku Earthquake of 2011 in Japan, with a magnitude of 8.9, is believed to have sped up Earth’s rotation by a relatively tiny 1.8 microseconds.
Apart from these large-scale changes, over shorter periods weather and climate also have important impacts on Earth’s rotation, causing variations in both directions.
The fortnightly and monthly tidal cycles move mass around the planet, causing changes in the length of day by up to a millisecond in either direction. We can see tidal variations in length-of-day records over periods as long as 18.6 years.
The movement of our atmosphere has a particularly strong effect, and ocean currents also play a role. Seasonal snow cover and rainfall, or groundwater extraction, alter things further.
Why is Earth suddenly slowing down?
Since the 1960s, when operators of radio telescopes around the planet started to devise techniques to simultaneously observe cosmic objects like quasars, we have had very precise estimates of Earth’s rate of rotation.
A comparison between these estimates and an atomic clock has revealed a seemingly ever-shortening length of day over the past few years.
But there’s a surprising reveal once we take away the rotation speed fluctuations we know happen due to the tides and seasonal effects. Despite Earth reaching its shortest day on 29 June 2022, the long-term trajectory seems to have shifted from shortening to lengthening since 2020. This change is unprecedented over the past 50 years.
The reason for this change is not clear. It could be due to changes in weather systems, with back-to-back La Niña events, although these have occurred before. It could be increased melting of the ice sheets, although those have not deviated hugely from their steady rate of melt in recent years.
Could it be related to the huge volcano explosion in Tonga injecting huge amounts of water into the atmosphere? Probably not, given that occurred in January 2022.
Scientists have speculated this recent, mysterious change in the planet’s rotational speed is related to a phenomenon called the “Chandler wobble” – a small deviation in Earth’s rotation axis with a period of about 430 days.
Observations from radio telescopes also show that the wobble has diminished in recent years; the two may be linked.
One final possibility, which we think is plausible, is that nothing specific has changed inside or around Earth. It could just be long-term tidal effects working in parallel with other periodic processes to produce a temporary change in Earth’s rotation rate.
Do we need a ‘negative leap second’?
Precisely understanding Earth’s rotation rate is crucial for a host of applications – navigation systems such as GPS wouldn’t work without it. Also, every few years timekeepers insert leap seconds into our official timescales to make sure they don’t drift out of sync with our planet.
If Earth were to shift to even longer days, we may need to incorporate a “negative leap second” – this would be unprecedented, and may break the internet.
The need for negative leap seconds is regarded as unlikely right now. For now, we can welcome the news that – at least for a while – we all have a few extra milliseconds each day.
Matt King, Director of the ARC Australian Centre for Excellence in Antarctic Science, University of Tasmania and Christopher Watson, Senior Lecturer, School of Geography, Planning, and Spatial Sciences, University of Tasmania.
This article is republished from The Conversation under a Creative Commons license. Read the original article.