IF YOU’RE planning a spot of polar exploration, here’s something you might want to consider. When, after that arduous trek across the ice sheet, you pose at the pole for a snapshot, you might be standing in the wrong place. In fact, the pole could be up to 10 metres from where it was only six months before.
Implausible as it sounds, the Earth’s surface is constantly shifting relative to its axis so that the geographical poles wander all over the place. It may only be a few metres a year, and it’s certainly not noticeable from day to day, but this polar wobble has been enough to puzzle stargazers for more than 100 years.
To add to the confusion, there are actually three types of wobble. Two of them were discovered and measured in the 19th century, though only explained in the last decade: the Chandler wobble takes 14 months to complete a cycle, while the annual wobble, as the name suggests, takes exactly a year. Both are driven largely by weather and ocean currents (see “Seasonal swings”).
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In 1960, William Markowitz of the US Naval Observatory in Washington DC noticed what appeared to be another, subtler form of polar wander, which moved the poles by about a metre on what seemed to be a 24-year cycle. Nobody could explain it.
Now a young geologist at the UK’s University of Leeds says he has found an explanation for the Markowitz wobble. If he’s right, studying the wobble could reveal some of the secrets of the most mysterious part of the Earth – its inscrutable inner core.
The first clear evidence that the Earth’s poles were moving came in the 1890s, when astronomers at the International Latitude Service began to monitor the position of six small observatories around the Earth, by watching the movements of the stars overhead. They found that the latitude of all these stations was changing very slightly, by just a few metres a year. The only way to explain it was if the entire crust of the Earth was slipping relative to its axis, moving some observatories to higher latitudes and others lower. The result was the discovery of the Chandler and annual wobbles and, much later, the wobble identified by Markowitz and named after him.
From the 1970s onwards, the evidence for the Markowitz wobble became less and less convincing. Advances in technology allowed the Earth’s rotation to be measured in relation to quasars billions of light years away and in relation to artificial satellites, and when GPS with its fleet of satellites came along it made measuring ground position relatively simple. All the new measurements conflicted with the old ones, and many researchers wrote off the Markowitz wobble as an illusion caused by unreliable methods of measuring the Earth’s spin. “It was kind of dismissed 10 to 15 years ago,” says David Crossley, a geophysicist at Saint Louis University in Missouri.
Even though the Markowitz wobble as he described it was later shown to be a mistake, as further space-based readings came in, something very interesting emerged. There is indeed some kind of long-term movement of the pole – not the periodic wobble that Markowitz saw, but something less easy to pin down, some kind of motion that may go back and forth, or may change speed on timescales of a few decades.
“The wobble could reveal the secrets of the Earth’s inner core”
“We still call it the Markowitz wobble to honour him,” says Richard Gross of NASA’s Jet Propulsion Laboratory in Pasadena, California. Crossley, however, is less keen on the name. “Nobody knew what that was in the first place. I’m surprised to see the term come back.” Whatever you like to call it, it’s a rather puzzling phenomenon. Something must be dragging the planet’s crust and mantle around. But what?
Perhaps deep forces are at work, emanating from the Earth’s inner core. That’s the idea behind a new explanation for the Markowitz wobble, developed by Mathieu Dumberry at the University of Leeds in the UK. Beneath the Earth’s crust lies the solid, rocky mantle. Beneath that is a molten metal outer core, and at the centre of the Earth is the inner core: a sphere 2500 kilometres across that is thought to be made of solid iron. Despite the core being isolated from the mantle and crust, and so well lubricated by the liquid outer core that surrounds it, Dumberry thinks he has found a way for it to get a grip on the outer parts of the Earth.
Inner turmoil
Although the mantle and the inner core are solid, their high temperature softens them so that they become more like the consistency of very stiff Silly Putty. If you were to hit mantle rock with a hammer it would crack, but if you squeezed it patiently for thousands of years it would ooze. In fact the mantle is convecting, like a pan of water in ultra-slow motion: over millions of years warm plumes of rock rise from hotspots on the core, and cool slabs of ocean crust fall down from subduction zones on the Earth’s surface. It is these plumes and slabs that Dumberry thinks are responsible for the Markowitz wobble.
“The viscosity of Earth’s inner core has to be just right: not exactly rock-hard, but far from soft”
Cold slabs are denser than their surroundings, so compared to the mantle, they exert a stronger gravitational pull on the core. Assuming the inner core is soft enough to be deformed by this pull, Dumberry says the effect of this gravity is to slowly cause a hump to rise in the surface of the inner core. “It’s like a permanent tide, as if the moon were fixed overhead at one place, raising a permanent hump in the ocean,” he says. Likewise, where plumes rise from the edge of the outer core, there should be a corresponding dip in the inner core’s surface, since the plume exerts less gravity than the surrounding mantle rock.
This effect would have no impact on the poles, except that the inner core is not fixed in place. Seismic studies show that it is spinning slightly faster than the rest of the planet, rotating under our feet by up to half a degree a year. As a result, those bumps and dips should slowly move out of alignment with the slabs and plumes, so that a bump might find itself tugged at in a different direction by a different mantle slab. Some of the extra gravitational attraction would then be acting from the side rather than vertically, exerting a torque on the mantle. The net result of many such mismatches could be a tug on the mantle and crust that is sufficient to change the axis of rotation, and shift the poles.
So far the numbers seem to add up. When Dumberry plugged seismic readings showing the density of different areas of the mantle into his model, they produced about the same amount of tilt as the Markowitz wobble – a maximum polar motion of about a metre.
The timescale of the Markowitz wobble also fits Dumberry’s model. The magnetic field deep within the Earth twists slowly back and forth with an irregular beat of a few decades, oscillations that show up in measurements of the field at the Earth’s surface. Dumberry thinks that this twisting magnetic field drags the inner core along with it. If he is right, the inner core’s rotation should also change on a timescale of decades. And if this rotation in turn drives the Markowitz wobble, then you’d expect the wobble to have a similar timescale, as space-based observations of the past 30 years seem to show.
So, wobble solved? Dumberry is confident. Since he presented his ideas in November 2005 at a meeting in London, reactions from the earth-science community have been positive. “It is probably one of the best explanations we’ve got,” says Gross.
One fly in the ointment is the orientation of the wobble. The measured motion is a gentle side-to-side rocking, and it is aligned along a bearing roughly 30 degrees east/150 degrees west, nodding alternately towards Egypt and Alaska. Dumberry’s model does produces a side-to-side motion, but it is closer to 90 degrees east/90 degrees west (Bangladesh and New Orleans). This is clearly a huge discrepancy, but Dumberry says that the orientation depends on exactly where in the mantle the plumes and slabs are found, and that’s something seismologists still disagree on. Dumberry plans to plug different published maps of the mantle into his model, to see if any of them provide a better fit.
Dumberry’s is not the only possibility. Recent polar motions could have been generated by melting ice, which would change the weight distribution of the crust, rebalancing the Earth. That would mean it is not a wobble at all, but more of a glitch. We don’t know if ice mass has changed enough to do this, but over the next few years satellite measurements of gravity over Antarctica and other ice-bound areas could settle the question.
For direct confirmation of Dumberry’s theory we might have to wait a while longer, to look for a correlation between the wobble and the small variations in Earth’s magnetic field over decades that are thought to be linked to inner-core motion. The wobble, magnetic field fluctuation and core rotation should all follow one another; if one were to reverse, so should the other two.
If the figures do add up and Dumberry’s model turns out to be correct, it could tell us something profound about the Earth’s inner core. Because of its rather inconvenient location, the inner core is the least understood part of the Earth, and although scientists have a fairly good idea that it is mostly made of crystalline iron, they don’t know what properties the metal has at the high pressures nearer the centre of the Earth. In fact that is an understatement: calculations of the inner core’s viscosity – the extent to which it can be deformed – vary by a factor of a trillion. The stuff could feel like toffee or, well, like iron.
One measure of this is relaxation time: if you were to put a dent in the stuff, would it smooth out in minutes or millions of years? “We don’t have a clue,” says Dumberry.
Experiments are no help. By squeezing material on an anvil made of diamond, or sending explosive shock waves through it, it is just possible to reproduce the kind of pressure felt in the mantle, but no deeper. The inner core is out of reach.
Dumberry’s model only works if the inner core’s viscosity is within a rather narrow range. If it is too fluid, the tidal bumps would stay in the same position as the core rotates; too rigid, and the repeated gravitational tugs would make the inner core wobble wildly with a period of about seven years. That seven-year wobble just isn’t seen.
So if the Markowitz wobble is indeed generated as Dumberry describes, then the viscosity of the inner core has to be just right, with a relaxation time of about 10 years. That’s not exactly rock-hard, but it is far from soft. The question is, will we all find the Earth’s core as exciting and mysterious if it turns out to be the consistency of Silly Putty?
Seasonal swings
The Earth has two major wobbles which act over relatively short timescales. The Chandler wobble, which moves the poles by 3 to 6 metres over a period of 14 months, was predicted in the 18th century, and discovered in the 1890s by American astronomer Seth Carlo Chandler. The Earth’s rotation squashes the planet slightly, making it into flattened ellipsoid which bulges at the equator. As this spins, it will wobble with its own particular period, much like a pendulum or swing. In the case of the Earth the period is 14 months. But this wobble will stop if it isn’t given a regular nudge. So what is pushing the swing? “We’re pretty confident that it is meteorological – something to do with the atmosphere and oceans and water on land,” says Richard Gross of NASA’s Jet Propulsion Laboratory in Pasadena, California. The friction of winds and ocean currents can twist the Earth one way or another, and moving air and water around changes the mass balance. “But we don’t yet know exactly what the contribution of each of those components is,” he says.
The second polar movement is the annual wobble, which causes a yearly gyration of about 3 metres. This is also caused by the weather. Each winter, a high-pressure system settles over Siberia, and this mass of air piled up on one side of the planet is enough to unbalance the it, generating most of the wobble. Other weather systems and shifts in ocean circulation make up the rest, although the details are not yet clear.
But not all of the Earth’s moves are wobbles. There is also a slow drift of the poles, left over from the last glacial period. When the ice caps of North America and Europe melted at the end of the ice age, the land beneath them bounced up, and is still rising today. As a result, the shape and balance of the Earth are still changing, with the North Pole heading towards Toronto at about 10 centimetres per year, and the South Pole moving towards Bangkok.
