Archive of Items of Interest

Bristol Geology

 

Earth's other 'moon' and its crazy orbit could reveal mysteries of the solar system

By Duncan Forgan, University of St Andrews

We all know and love the moon. We’re so assured that we only have one that we don’t even give it a specific name. It is the brightest object in the night sky, and amateur astronomers take great delight in mapping its craters and seas. To date, it is the only other heavenly body with human footprints.

What you might not know is that the moon is not the Earth’s only natural satellite. As recently as 1997, we discovered that another body, 3753 Cruithne, is what’s called a quasi-orbital satellite of Earth. This simply means that Cruithne doesn’t loop around the Earth in a nice ellipse in the same way as the moon, or indeed the artificial satellites we loft into orbit. Instead, Cruithne scuttles around the inner solar system in what’s called a “horseshoe” orbit.

Cruithne’s orbit

To help understand why it’s called a horseshoe orbit, let’s imagine we’re looking down at the solar system, rotating at the same rate as the Earth goes round the sun. From our viewpoint, the Earth looks stationary. A body on a simple horseshoe orbit around the Earth moves toward it, then turns round and moves away. Once it’s moved so far away it’s approaching Earth from the other side, it turns around and moves away again.

Cruithne from a stationary Earth position

Horseshoe orbits are actually quite common for moons in the solar system. Saturn has a couple of moons in this configuration, for instance.

What’s unique about Cruithne is how it wobbles and sways along its horseshoe. If you look at Cruithne’s motion in the solar system, it makes a messy ring around Earth’s orbit, swinging so wide that it comes into the neighbourhood of both Venus and Mars. Cruithne orbits the sun about once a year, but it takes nearly 800 years to complete this messy ring shape around the Earth’s orbit.

Cruithne close up

So Cruithne is our second moon. What’s it like there? Well, we don’t really know. It’s only about five kilometres across, which is not dissimilar to the dimensions of the comet 67P/Churyumov-Gerasimenko, which is currently playing host to the Rosetta orbiter and the Philae lander.

The surface gravity of 67P is very weak – walking at a spirited pace is probably enough to send you strolling into the wider cosmos. This is why it was so crucial that Philae was able to use its harpoons to tether itself to the surface, and why their failure meant that the lander bounced so far away from its landing site.

Given that Cruithne isn’t much more to us at this point than a few blurry pixels on an image, it’s safe to say that it sits firmly in the middling size range for non-planetary bodies in the solar system, and any human or machine explorers would face similar challenges as Rosetta and Philae did on 67P.

Possible clash: Venus J.Gabás Esteban, CC BY-SA

If Cruithne struck the Earth, though, that would be an extinction-level event, similar to what is believed to have occurred at the end of the Cretaceous period. Luckily it’s not going to hit us anytime soon – its orbit is tilted out of the plane of the solar system, and astrophysicists have shown using simulations that while it can come quite close, it is extremely unlikely to hit us. The point where it is predicted to get closest is about 2,750 years away.

Cruithne is expected to undergo a rather close encounter with Venus in about 8,000 years, however. There’s a good chance that that will put paid to our erstwhile spare moon, flinging it out of harm’s way, and out of the Terran family.

It’s not just Cruithne

The story doesn’t end there. Like a good foster home, the Earth plays host to many wayward lumps of rock looking for a gravitational well to hang around near. Astronomers have actually detected several other quasi-orbital satellites that belong to the Earth, all here for a little while before caroming on to pastures new.

Secrets: solar system Tashal

So what can we learn about the solar system from Cruithne? Quite a lot. Like the many other asteroids and comets, it contains forensic evidence about how the planets were assembled. Its kooky orbit is an ideal testing ground for our understanding of how the solar system evolves under gravity.

As I said before, it wasn’t until the end of the 20th century that we even realised that bodies would enter such weird horseshoe orbits and stay there for such a long time. The fact they do shows us that such interactions will have occurred while the solar system was forming. Because we think terrestrial planets grow via collisions of bodies of Cruithne-size and above, this is a big new variable.

One day, Cruithne could be a practice site for landing humans on asteroids, and perhaps even mining them for the rare-earth metals our new technologies desperately crave. Most importantly of all, Cruithne teaches us that the solar system isn’t eternal – and by extension, neither are we.

The Conversation

This article was originally published on The Conversation. Read the original article.

The Earth's inner core gives up more of its magnetic secrets

By Simon Redfern, University of Cambridge

 

The planet Earth’s inner core is not a single solid mass but comprised of two layers, and new evidence about the core’s composition from a team of US and Chinese geophysicists suggests that the innermost core is rotated on its side compared to its outer layer.

 

            The new research, in a paper published in the journal Nature Geoscience, provides new clues about how the inner core formed and began to solidify. Sitting inside a shell of molten iron alloy, the inner core grows at about half a millimetre a year. Probing deeper into the solid inner core is like tracing back through time, to the beginnings of its formation.

 

            The problem is that this part of Earth is farthest away from us, and is difficult to analyse as the seismic waves used to inspect it have to pass through the rest of the planet on their journey to the core and back. Decoding what is going on in the centre of the Earth is one of the most challenging problems in geophysics.

 

            Despite this, Earth’s magnetic field and the existence of the core may have been essential to the development of life on Earth. The core’s magnetic field acts like a shield to the magnetic storms that the sun continually throws at us.

 

Hard core

            It isn’t clear when the inner core first started to solidify, but estimates suggest somewhere between half a billion years ago and one and a half billion years ago. This is well into the mature years of the planet, which is more than four and a half billion years old.

 

            People have noticed differences in the way seismic waves travel through the outer parts of the inner core and its innermost reaches before, but never before have they suggested that the alignment of the crystalline iron that its composed of is set at a different angle to the outermost parts.

 

            If true, this would imply that something very substantial happened to flip the orientation of the core fairly early after it formed, turning the alignment of crystals from the original east-west orientation found in the inner core to the north-south alignment seen today in its outer parts. Interestingly, some researchers who measure the palaeomagnetic field in old rocks from Earth’s surface have previously suggested that the Earth’s magnetic field switched between equatorial axes and polar axes more than half a billion years ago.

 

            It could be that the strange alignment that these researchers see in the innermost core explains the strange palaeomagnetic signatures from ancient rocks that may have been present near the equator half a billion years ago. Some people have correlated this with a sudden increase in the speed of evolution of new life, termed the Cambrian Explosion.

 

            For the moment, however, the model proposed in this paper needs testing against other ways of reading the seismic data from Earth’s innermost core, especially since no other researchers have suggested evidence for the same conclusions in previous studies.

 

            This article was originally published on The Conversation.

            Read the original article.

9th February 2015

 

 

The sun won't die for 5 billion years, so why do humans have only 1 billion years left on Earth?

 

 By Jillian Scudder   University of Sussex

 

            In a few billion years, the sun will become a red giant so large that it will engulf our planet. But the Earth will become uninhabitable much sooner than that. After about a billion years the sun will become hot enough to boil our oceans.

 

            The sun is currently classified as a “main sequence” star. This means that it is in the most stable part of its life, converting the hydrogen present in its core into helium. For a star the size of ours, this phase lasts a little over 8 billion years. Our solar system is just over 4.5 billion years old, so the sun is slightly more than halfway through its stable lifetime.

 

Even stars die

 

            After 8 billion years of happily burning hydrogen into helium are over, the sun’s life gets a little more interesting. Things change because the sun will have run out of hydrogen in its core – all that’s left is the helium. The trouble is that the sun’s core is not hot or dense enough to burn helium.

 

            In a star, gravitational force pulls all the gases towards the centre. When the star has hydrogen to burn, the creation of helium produces enough outward pressure to balance out the gravitational pull. But when the star has nothing left in the core to burn, gravitational forces take over.

 

            Eventually that force compresses the centre of the star to such a degree that it will start burning hydrogen in a small shell around the dead core, which is still full of helium. As soon as the sun begins to burn more hydrogen, it would be considered a “red giant”.

 

            The process of compression in the centre allows the outer regions of the star to expand outwards. The burning hydrogen in the shell around the core significantly increases the brightness of the sun. Because the size of the star has expanded, the surface cools down and goes from white-hot to red-hot. Because the star is brighter, redder and physically larger than before, we dub these stars “red giants".

 

Earth’s fiery demise

 

            It is widely understood that the Earth as a planet will not survive the sun’s expansion into a full-blown red giant star. The surface of the sun will probably reach the current orbit of Mars – and, while the Earth’s orbit may also have expanded outwards slightly, it won’t be enough to save it from being dragged into the surface of the sun, whereupon our planet will rapidly disintegrate.

 

            Life on the planet will run into trouble well before the planet itself disintegrates. Even before the sun finishes burning hydrogen, it will have changed from its present state. The sun has been increasing its brightness by about 10% every billion years it spends burning hydrogen. Increased brightness means an increase in the amount of heat our planet receives. As the planet heats up, the water on the surface of our planet will begin to evaporate.

 

            An increase of the sun’s luminosity by 10% over the current level doesn’t sound like a whole lot, but this small change in our star’s brightness will be pretty catastrophic for our planet. This change is a sufficient increase in energy to change the location of the habitable zone around our star. The habitable zone is defined as the range of distances away from any given star where liquid water can be stable on the surface of a planet.

 

 

            With a 10% increase of brightness from our star, the Earth will no longer be within the habitable zone. This will mark the beginning of the evaporation of our oceans.  By the time the sun stops burning hydrogen in its core, Mars will be in the habitable zone, and the Earth will be much too hot to maintain water on its surface.

 

Uncertain models

 

            This 10% increase in the sun’s brightness, triggering the evaporation of our oceans, will occur over the next billion years or so. Predictions of exactly how rapidly this process will unfold depend on who you talk to. Most models suggest that as the oceans evaporate, more and more water will be present in the atmosphere instead of on the surface. This will act as a greenhouse gas, trapping even more heat and causing more and more of the oceans to evaporate, until the ground is mostly dry and the atmosphere holds the water, but at an extremely high temperature.

 

            As the atmosphere saturates with water, the water held in the highest parts of our atmosphere will be bombarded by high energy light from the sun, which will split apart the molecules and allow the water to escape as hydrogen and oxygen, eventually bleeding the Earth dry of water.

 

            Where the models differ is on the speed with which the earth reaches this point of no return. Some suggest that the Earth will become inhospitable before the 1 billion year mark, since the interactions between the heating planet and the rocks, oceans, and plate tectonics will dry out the planet even faster. Others suggest that life may be able to hold on a little longer than 1 billion years, due to the different requirements of different life forms and periodic releases of critical chemicals by plate tectonics.

 

            The Earth is a complex system – and no model is perfect. However, it seems likely that we have no more than a billion years left for life to thrive on our planet.

 

This article was originally published on The Conversation.

      Read the original article.

12th February 2015

 

 

 

 

The Ancient Ocean of Mars

A video from Nasa which is well worth watching

Newly discovered layer in Earth's mantle can affect surface dwellers too

Hauke Marquardt, Bayreuth University

Sinking tectonic plates get jammed in a newly discovered layer of the Earth’s mantle – and could be causing earthquakes on the surface.

It was previously thought that Earth’s lower mantle, which begins at a depth of around 700 km and forms the major part of the mantle, is fairly uniform and varies only gradually as it goes deeper.

However, our new study points towards a layer in the mantle characterised by a strong increase in viscosity – a finding which has strong implications for our understanding of what’s going on deep down below our feet.

The deep unknown

The Earth’s mantle is the largest shell inside our planet. Ranging from about 50 km to 3000 km depth, it links the hot liquid outer core – with temperatures higher than 5,000K – to the Earth’s surface.

The movement of materials within the Earth’s mantle is thought to drive plate tectonic movements on the surface, ultimately leading to earthquakes and volcanoes. The mantle is also the Earth’s largest reservoir for many elements stored in mantle minerals. Throughout Earth’s history, substantial amounts of material have been exchanged between the deep mantle and the surface and atmosphere, affecting both the life and climate above ground.

Because mankind is incapable of directly probing the lower mantle – the deepest man-made hole is only around 12 km deep – many details of the global material recycling process are poorly understood.

We do know, however, that the main way materials are transferred from the Earth’s surface and atmosphere back into the deep mantle occurs when one tectonic plate slides under another and is pushed down below another into the mantle.

A strong increase in the viscosity leads to a stiff layer which catches sinking slabs Hauke Marquardt

A trap for sinking plates

So far most researchers assumed that these sinking plates either stall at the boundary between the upper and lower mantle at a depth of around 700 km or sink all the way through the lower mantle to the core-mantle boundary 3,000 km down.

But our new research, published in the latest online issue of Nature Geoscience, shows that many of these sinking slabs may in fact be trapped above a previously undiscovered impermeable layer of rock within the lower mantle.

We found that enormous pressures in the lower mantle, which range from 25 GPa (gigapascal) to 135 GPa, can lead to surprising behaviour of matter. To picture just how high this pressure is, balancing the Eiffel Tower in your hand would create pressures on the order of 10 GPa. These pressures lead to the formation of a stiff layer in the Earth’s mantle. Sinking plates may become trapped on top of this layer, which reaches its maximum stiffness at a depth below 1,500km.

Under pressure

We formed this conclusion after performing laboratory experiments on ferropericlase, a magnesium/iron oxide that is thought to be one of the main constituents of the Earth’s lower mantle. We compressed the ferropericlase to pressures of almost 100 GPa in a diamond-anvil cell, a high-pressure device which compresses a tiny sample the size of a human hair between the tips of two minuscule brilliant-cut diamonds.

A diamond-anvil cell compresses a tiny sample under high pressure between two minuscule diamonds. Image via Hauke Marquardt, Author provided

While under compression, the ferropericlase was probed with high-energy x-rays to investigate how it deforms under these high pressures. We found that the ability of the material to resist irreversible deformation increased by over three times under high pressures.

These results were used to model the change of viscosity with depth in Earth’s lower mantle. While previous estimates have indicated only gradual variations of viscosity with depth, we found a dramatic increase of viscosity throughout the upper 900 km of the lower mantle.

Such a strong increase in viscosity can stop the descent of slabs and, in doing so, strongly affect the deep Earth material cycle. These new findings are supported by 3-D imaging observations based on the analysis of seismic wave speeds travelling through the Earth that also indicate that the slabs stop sinking before they reach a depth of 1500 km.

Surface effects

If true, the existence of this stiff layer in the Earth’s mantle has wide-ranging implications for our understanding of the deep Earth material cycle. It could limit material mixing between the upper and lower parts of the lower mantle, meaning mantle regions with previously different geochemical signatures stay isolated in separate patches instead of mixing over geologic time.

What’s more, a stiff mid-mantle layer could also put stress on slabs much closer to the Earth’s surface, potentially acting as a trigger of deep earthquakes.

We are really just at the beginning of a deeper understanding of the inner workings of our planet, many of which ultimately affect our life on its surface.

The Conversation

This article was originally published on The Conversation. Read the original article.

Newly discovered layer in Earth’s mantle can affect surface dwellers too

Deception Island – the Antarctic volcano that just doesn't make any sense

Luca de Siena, University of Aberdeen and David Macdonald, University of Aberdeen

Only two volcanoes in Antarctica are active. There is Mount Erebus, which is roughly due south of New Zealand, and Deception Island, which lies about 850km south east of Cape Horn.

Mt Erebus has been erupting continuously over the last few decades. Yet the rather smaller Deception Island, in the South Shetland archipelago, is responsible for the largest known eruption in the Antarctic area.

This horseshoe-shaped cauldron-like structure, or caldera, was produced more than 10,000 years ago by an explosive eruption that scattered more than 30km³ of molten rock. The result is an enclosed welcoming bay called Port Foster.

Deception Island from above Wikimedia

Deception was officially discovered by the British sealing captain William Smith in 1820 and was subsequently used for purposes such as seal hunting and whaling before finding its modern calling as a site for science and tourism. Maybe because you cannot see most of the volcano above the sea, tourists rarely appreciate its hidden destructive potential.

The big blunder

Claimed in the past by the UK, Chile and Argentina, it provides a unique enclosed environment in which to monitor a “volcano under the ice”. All three of those aforementioned countries financed observatories there in the 1960s (Spain added its own in 2000).

Yet two consecutive volcanic eruptions in 1967 and 1969 went unpredicted – remarkable failures in the history of volcano monitoring. Only the Argentinian and the Spanish observatories still exist.

Google Maps

Mud from down below

The volcanic events at Deception fall into a rare category called subglacial eruptions. The island is situated in a place where there is a glacier on the ocean floor about 100m thick. Scientists would normally expect that if this were hit by lava from below, it would evaporate benignly into steam.

But the lava moving upwards at Deception has several qualities that made things happen differently: it moves slowly and it has high water content. This meant that it turned the glacier into meltwater as well as steam, creating a large overflow of mud to the surface. This was the main cause of the destruction of the UK and Chilean stations.

The reason why this melting was unexpected was because in scientific terms the glacier was “deceptively thin”. The scientists were not expecting it to produce much more than steam. Ironically, the absence of larger glaciers is what made the island the most hospitable location in Antarctica.

We understand these subglacial eruptions much better now than we did in the 1960s. Nowadays there are hazard maps to make visitors aware of the higher-risk spots on the island.

The Deception enigma

Yet from a volcanic point of view, Deception is a great puzzle. Many volcanoes are caused by subduction, which is where two of the Earth’s tectonic plates crash against one another, sending one plate down and pushing the other upwards. A classic example is the Cascade range in the north-western US, whose most famous volcano is Mt St Helens. The ones that scientists have observed happen on land.

Most volcanoes at sea are like Hawaii and the Azores, which we describe as hot spots. Instead of taking place near the points between tectonic plates, these are holes in the ocean floor where there is a direct line to the Earth’s mantle. The same goes for submerged calderas in the middle of the ocean, of which there are some examples near Japan.

For a time, scientists thought that Deception might be an unusual example of subduction happening in the ocean. But a more recent hypothesis is that the South Shetlands may be what we call a rift zone. This would mean that it is on a point where plates meet, but instead of colliding, there are gaps from them moving away from each other, creating new oceanic crust in the process. A good example of a rift zone is the Iceland, as can be seen in the video eruption below.

The hydrocarbon connection

Detailed geophysical surveys have been carried out across Deception since 2000, mainly financed by Spanish projects. UK geological research on the island has also been extensive.

You may be wondering why governments have spent so much on research there. Don’t be fooled into thinking that this is some kind of place of virtue where different nations fund research just to understand how our Earth works.

Rifts fill up with the remains of volcanic explosions and other sediment eroded from the margins of the valley. This process is critical for the production of oil. Located at the western edge of the arc, Deception is the ideal place to observe rift processes because of the natural harbour, which shelters scientists from the harsh Antarctic weather.

Maybe he can shed light on the situation Christopher Michel, CC BY-SA

Rifting is the reason for all the oil in the North Sea. The oil is not deposited where the rift is located, but some distance away. In the same way, there is almost no likelihood of an oil discovery on Deception. But understanding the process of rifting there will be a strong indication that there is oil to the north of the South Shetland Islands. It would also confer an exploration advantage worldwide – so Deception without oil is as valuable as Deception with oil.

So Deception could be the key to unveiling how rifts form and where oil is, in places where resources are unexploited. In an era where the political claims to the Antarctic have long since receded, that should ensure that this frozen corner of the world remains important for some time to come.

The Conversation

Luca de Siena is Lecturer in Geophysics at University of Aberdeen.
David Macdonald is Professor of Petroleum Geology at University of Aberdeen.

This article was originally published on The Conversation. Read the original article.