Earth does not always spin on an axis running through its poles. Instead, it wobbles irregularly over time, drifting toward North America throughout most of the 20th Century (green arrow).
That direction has changed drastically due to changes in water mass on Earth.
Credit: NASA/JPL-Caltech
From BBC by Nic Fleming
The ground we stand on is not as steady as it seems.
There are a host of
factors that cause the entire Earth to judder and topple
The Earth beneath our feet seems reassuringly solid and unchanging
most of the time.
But this is an illusion, born of our limited
perspective.
Our planet rotates on its axis once every 23 hours,
56 minutes and 4 seconds.
It also orbits the Sun, while our Solar System
dashes around the centre of the Milky Way, which is itself hurtling
across the Universe towards a region of space called the Great
Attractor.
The speeds involved are frankly dizzying.
Even if you
ignore all that, the Earth is far from stable.
Beneath us, enormous
chunks of rock are constantly grinding past each other to make valleys,
pushing together to form mountains, or dragging apart to create rivers
and oceans.
The ground under us is forever shifting, stretching and
wobbling.
Most of the time, this is nothing to worry about.
However, our growing understanding of these phenomena is driving a
better understanding of the inner workings of our planet.
It is also
handy for anyone trying to track and land spacecraft.
Here, then, are
seven things that make the Earth move for us.
Before about 2000, Earth's spin axis was drifting toward Canada (green arrow, left globe).
JPL scientists calculated the effect of changes in water mass in different regions (center globe) in pulling the direction of drift eastward and speeding the rate (right globe).
Credit: NASA/JPL-Caltech
Under pressure
A desktop
globe is a perfect sphere, so it spins smoothly around a fixed axis.
However, the Earth is not spherical, and the mass within it is both
unevenly distributed and prone to moving around.
As a result, the axis
around which Earth spins, and the north and south rotational poles at
each end of the axis, move about.
What's more, because the
rotation axis is different to the figure axis around which its mass is
balanced, the Earth wobbles as it spins.
This wobble was predicted by scientists as far back as Isaac Newton.
To be more precise, it is made up of a number of distinct wobbles.
The one that has the greatest impact is known as the Chandler Wobble, first observed by American astronomer
Seth Chandler Jr in 1891.
It causes movements of the poles of around 26ft (9m) and takes some 14 months to complete a full cycle.
During
the 20th Century scientists suggested a wide variety of causes,
including changes in continental water storage, atmospheric pressure,
earthquakes, and interactions at the boundary of the Earth's core and
mantle.
Geophysicist
Richard Gross
of NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California
solved the mystery in 2000.
He applied new weather and oceanic models to
observations on the
Chandler Wobble from 1985-1995.
Gross calculated
that
two-thirds of the Wobble was caused by fluctuating seabed pressure, and one-third by changes in atmospheric pressure.
"Their
relative importance varies with time," says Gross, "but the cause is
now widely accepted to be the combination of changes in atmospheric and
oceanic pressures."
The relationship between continental water mass and the east-west wobble in Earth's spin axis. Losses of water from Eurasia correspond to eastward swings in the general direction of the spin axis (top), and Eurasian gains push the spin axis westward (bottom).
Credit: NASA/JPL-Caltech
Water works
The seasons are
the second largest influence on the Earth's wobble.
That is because they
cause geographical variations in the amount of rain, snow and humidity.
Scientists
have been able to pinpoint the poles using the relative positions of
the stars since 1899, and using satellites since the 1970s.
But even
after removing the impact of the Chandler and seasonal wobbles, the
north and south rotational poles still move about with respect to the
Earth's crust.
In
a study published in April 2016,
Surendra Adhikari and
Erik Ivins, also at JPL, identified two more important pieces of the Earth wobble jigsaw.
Before
the year 2000, the Earth's spin axis was drifting towards Canada, a few
inches per year.
But then measurements show the spin axis changed tack,
heading instead towards the British Isles. Some scientists suggested
this could be the result of the loss of ice caused by the rapid melting
of Greenland's and Antarctica's ice sheets.
Adhikari and Ivins set out to test this idea.
They compared GPS measurements of the positions of the poles with data from
GRACE, a study that uses satellites to measure changes in mass around the Earth.
They
found that the melting of the Greenland and Antarctic ice sheets only
explains around two-thirds of the recent shift in the direction of the
poles.
The remainder, they concluded, is down to the loss of water held
on continents, mostly the Eurasian land mass.
This region has been affected by aquifer depletion
and drought.
Then they factored in the position of the areas affected. "From the
fundamental physics of rotating objects, we know that movement of the
poles is highly sensitive to changes at [around] +/- 45 degree
latitudes," says Adhikari. That is exactly where Eurasia had lost water.
The study also identified continental water storage as a plausible explanation for another wobble in the Earth's rotation.
Throughout
the 20th Century, researchers were puzzled because the spin axis
shifted every six to 14 years, heading 0.5-1.5m east or west of its
overall drift. Adhikari and Ivins found that, between 2002 and 2015, dry
years in Eurasia corresponded to the eastward swings and wet years
corresponded to westward movements.
"We found a perfect match,"
says Adhikari. "It's the first time anyone successfully identified a
one-to-one match between the global-scale inter-annual wet-dry
variability and inter-annual polar motion."
This video explains what's happening to Earth
Man-made meandering
While
these movements of water and ice are caused by a combination of natural
processes and human actions, other changes that impact the Earth's
wobbling are all our own doing.
In
a 2009 study Felix Landerer,
also of the JPL, calculated that, if carbon dioxide levels double
between 2000 and 2100, the oceans will warm and expand in such a way
that
the north pole will shift around 1.5cm per year towards Alaska and Hawai'i over the next century.
Similarly, in
a 2007 study
Landerer modelled the effects of the ocean warming caused by the same
carbon dioxide increase on ocean bottom pressures and circulation.
He
found that the changes would shift mass to higher latitudes, and that
this would shorten the day by a little over 0.1 milliseconds: 1/10,000th
of a second.
Quake shake
It is not just
large volumes of water and ice that affect the Earth's rotation if they
move around. Shifting rocks have the same effect, if they are big
enough.
Earthquakes occur when the tectonic plates that make up
the Earth's surface slip past each other suddenly. In theory, that could
make a difference.
However, at first the amount of water involved seemed too
small to have such an impact.
For example, Gross studied the massive 8.8-magnitude quake that hit the coast of Chile in 2010.
In
as as-yet-unpublished study, he calculated that the plate movements shifted Earth's axis of mass balance by around 8cm.
However,
this was only a model-based estimate.
Gross and others have since
attempted to observe real shifts in the way the Earth is spinning, by
following earthquakes in GPS satellite data.
So far this has
proved unsuccessful, because it is tricky to remove all the other things
that influence how the Earth rotates.
"The models are not perfect and
there is residual noise masking the smaller earthquake signals," says
Gross.
The movements of mass that take place when tectonic plates
slip past each other also affect the length of days.
This is a little
bit like an ice skater spinning on one spot: she can speed up by drawing
her arms in and thus shifting her mass closer to her body, or slow down
by doing the opposite.
For example, Gross calculated that
the magnitude-9.1 earthquake that hit Japan in 2011
shortened the length of the day by 1.8 microseconds.
Storm force wobble
When an earthquake happens, it triggers seismic waves that carry its energy through the interior of the Earth.
There are two kinds. "P-waves" repeatedly squeeze and expand the
material they pass through, with the vibrations travelling in the same
direction as the wave.
Slower "S-waves" wobble rock from side to side,
with the vibrations occurring at right angles to their direction of
travel.
Intense storms can also create faint seismic waves like
those triggered by earthquakes.
These waves are called microseisms.
Until recently, scientists have been unable to determine the sources of
S-waves from microseisms.
In a study published in August 2016,
Kiwamu Nishida of the University of Tokyo and
Ryota Takagi
of Tohoku University reported that they had used a network of 202
detectors in southern Japan to track both P- and S-waves.
They
traced the waves' origins to a severe North Atlantic storm called a "weather bomb": a storm in which
the atmospheric pressure at the centre drops unusually rapidly.
Tracking microseisms in this way will
help researchers to better understand the internal structure of the Earth.
Lunar influence
It is not
just Earth-bound phenomena that influence our planet's movements.
Recent
research suggests that large earthquakes are more likely around full
and new moons.
That could be because the Sun, Moon and Earth are
aligned, increasing the gravitational force acting on our planet.
In a study published in September 2016,
Satoshi Ide
of the University of Tokyo and his colleagues analysed the tidal
stresses in the two-week periods prior to large earthquakes in the last
two decades. Of the largest 12 earthquakes, all of which had a magnitude
of 8.2 or higher,
nine happened close to full or new moons.
No such relationship was found for smaller quakes.
Ide
concluded that the extra gravitational force exerted at these times
could increase the forces acting on tectonic plates.
The changes would
be small, but if the plates were under stress anyway, the extra force
could be enough to turn small rock failures into larger ruptures.
While this may seem plausible, many scientists are sceptical because Ide's study only looked at 12 earthquakes.
Sun shakes
Even more
controversial is the idea that vibrations originating deep within the
Sun could help explain a number of shaking phenomena on Earth.
When
gases move around inside the Sun, they produce two different types of
waves.
Those generated by changes in pressure are called p-modes, while
those that form when dense material is pulled downwards by gravity are
called g-modes.
A p-mode takes a few minutes to complete a full
vibrational cycle, while a g-mode takes between tens of minutes and
several hours.
This amount of time is the mode's "period".
In 1995, a group led by
David Thomson
of Queen's University in Kingston, Canada analysed patterns exhibited
by the solar wind – a stream of charged particles that flows out from
the Sun – between 1992 and 1994.
They reported
fluctuations that had the same periods as p-modes and g-modes, suggesting these solar vibrations were somehow influencing the solar wind.
In
2007, Thomson went on to report that unexplained fluctuations in the
voltages of undersea communications cables, seismic measurements on
Earth and even mobile phone call dropouts also had
frequency patterns that matched the waves inside the Sun.
However,
other scientists believe Thomson's claims are on shaky ground.
According to simulations, these solar vibrations, especially the
g-modes, should be so weak by the time they get to the Sun's surface
that they could not affect the solar wind.
Even if that is not the case,
the patterns should be destroyed by turbulence in the interplanetary
medium long before they get to Earth.
"When we looked at different
time periods, the frequencies he had identified were shifting around,
when to be g-modes in particular they should remain fairly constant,"
says
Pete Riley of Predictive Science in San Diego, California.
Back in 1996 he published
a study
questioning Thomson's original results.
"We looked at the same data
Dave Thomson looked at and applied the same analysis, and couldn't find
any evidence for p-modes or g-modes."
Clearly, Thomson's idea might not pan out.
But there are plenty of other reasons why our planet wobbles and shakes.
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