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convection

Scientists Just Narrowed Down The Age of Earth’s Inner Core

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At some point in Earth’s 4.5-billion-year history, its entirely liquid iron core cooled enough to form a solid ball in the centre. Today, our planet’s core consists of a solid iron inner core surrounded by a molten iron outer core, but pinning down exactly when this change occurred has proven quite difficult.

 

Estimates range from 4.5 billion years ago – the age of Earth itself – to 565 million years ago; now, a new study has finally narrowed it down. According to data obtained in laboratory experiments that create conditions approaching those in the planetary core, the age of the inner core should be somewhere between 1 billion and 1.3 billion years.

In turn, this helps us to narrow down the age of the geodynamo, which powers the magnetic field around Earth. This magnetic field contributes to conditions hospitable to life as we know it by protecting the planet’s atmosphere from being blown away by the solar wind.

Therefore, it will come as no surprise that scientists are deeply interested in how it came to exist, and how it is maintained.

“People are really curious and excited about knowing about the origin of the geodynamo, the strength of the magnetic field, because they all contribute to a planet’s habitability,” said geoscientist Jung-Fu Lin of The University of Texas at Austin.

The geodynamo is created by the circulation of conducting iron in the outer core, driven by convection that’s powered by two mechanisms.

 

Firstly, there’s thermal convection, generated by temperature fluctuations; this can occur in a fully liquid core. Secondly, there’s compositional convection, in which lighter elements released at the inner core boundary rise through the liquid outer core, creating movement.

In both cases, this conducting liquid creates electric currents which charge the core, essentially turning it into a giant electromagnet. Et voila! A magnetic field. Currently, both types of convection are present in Earth’s core, equally contributing to the geodynamo.

But before the solid core crystallised, only thermal convection was possible in Earth’s core. This is capable of generating the geodynamo, but in order to maintain it over billions of years, as is required for the younger estimates of the inner core’s age, the iron would have needed to be extremely hot – unrealistically so.

To conduct and maintain such temperatures, the thermal conductivity of iron – as in, the ability to conduct heat efficiently – needs to be high. So, the team decided to look into the thermal conductivity of iron under pressures and at temperatures approaching those in the core.

 

To do this, they took an iron sample, blasted it with lasers to heat it up, and squished it in a diamond anvil. It took much longer to do than it does to describe: many attempts over two years. Finally, however, the team managed to measure the electrical and thermal conductivity of the sample under 170 gigapascals of pressure (that’s 170 million times the atmospheric pressure at sea level), and temperatures of 3,000 Kelvin.

The pressures in the outer core range from 135 to 330 gigapascals from the outer boundary to the boundary of the inner core, while temperatures range from 4,000 to 5,000 Kelvin. The inner core is thought to reach above 6,000 Kelvin (but the iron solidifies under the intense pressure).

When the team measured the conductivity in the sample, they found it 30 to 50 percent lower than what would be required for the 565-million-year age estimate for the inner core. Hence, the researchers could place an upper limit on the thermal conductivity of liquid iron under core conditions – which, in turn, places an upper limit on how much heat could be conducted and retained.

With all this, they could finally estimate the age of Earth’s inner core.

“Once you actually know how much of that heat flux from the outer core to the lower mantle, you can actually think about when did the Earth cool sufficiently to the point that the inner core starts to crystallise,” Lin said.

The team’s timeline, interestingly, falls neatly with a change in Earth’s magnetic field. The arrangement of magnetic materials in rocks dating back 1 to 1.5 billion years ago shows that there was an increase in magnetic field strength at around this time – as would be expected for the time at which the inner core crystallised.

However, a similar increase was also seen at 565 million years ago. If the inner core crystallised earlier, that means whatever Earth did 565 million years ago is still a mystery.

“Further interrogations between mineral physics, geodynamics, and paleomagnetism are needed to resolve this discrepancy,” the researchers wrote.

The research has been published in Physical Review Letters.

 



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An Ancient Meteorite Is The First Chemical Evidence of Volcanic Convection on Mars

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For many years, we thought Mars was dead. A dusty, dry, barren planet, where nothing moves but the howling wind. Recently, however, pieces of evidence have started to emerge, hinting that Mars is both volcanically and geologically active.

 

Well, the idea of a volcanically active Mars just got a little more real. A meteorite that formed deep within the belly of Mars has just provided the first solid chemical proof of magma convection within the Martian mantle, scientists say. 

Crystals of olivine in the Tissint meteorite that fell to Earth in 2011 could only have formed in changing temperatures as it was rapidly swirled about in magma convection currents – showing that the planet was volcanically active when the crystals formed around 574 to 582 million years ago – and it could still be intermittently so today.

“There was no previous evidence of convection on Mars, but the question ‘Is Mars a still volcanically active planet?’ was previously investigated using different methods,” explained planetary geologist Nicola Mari of the University of Glasgow to ScienceAlert.

“However, this is the first study that proves activity in the Mars interior from a purely chemical point of view, on real Martian samples.”

Olivine, a magnesium iron silicate, isn’t rare. It crystallises from cooling magma, and it’s very common in Earth’s mantle; in fact, the olivine group dominates Earth’s mantle, usually as part of a rock mass. On Earth’s surface, it’s found in igneous rock.

 

It’s fairly common in meteorites. And olivine is also fairly common on Mars. In fact, the presence of olivine on the surface of Mars has previously been taken as evidence of the planet’s dryness, since the mineral weathers rapidly in the presence of water.

But when Mari and his team started studying the olivine crystals in the Tissint meteorite to try to understand the magma chamber where it formed, they noticed something strange. The crystals had irregularly spaced phosphorus-rich bands.

We know of this phenomenon on Earth – it’s a process called solute trapping. But it was a surprise to find it on Mars.

magma olivine(Mari et al., Meteoritics & Planetary Science, 2020)

“This occurs when the rate of crystal growth exceeds the rate at which phosphorus can diffuse through the melt, thus the phosphorus is obliged to enter the crystal structure instead of ‘swimming’ in the liquid magma,” Mari said.

“In the magma chamber that generated the lava that I studied, the convection was so vigorous that the olivines were moved from the bottom of the chamber (hotter) to the top (cooler) very rapidly – to be precise, this likely generated cooling rates of 15-30 degrees Celsius per hour for the olivines.”

 

The larger of the olivine crystals were also revealing. Traces of nickel and cobalt are in agreement with previous findings that they originated from deep under the Martian crust, a depth of 40 to 80 kilometres (25 to 50 miles).

This supplied the pressure at which they formed; along with the equilibration temperature of olivine, the team could now perform thermodynamic calculations to discover the temperature in the mantle at which the crystals formed.

They found that the Martian mantle probably had a temperature of around 1,560 degrees Celsius in the Martian Late Amazonian period when the olivine formed. This is very close to the ambient mantle temperature of Earth of 1,650 degrees Celsius during the Archean Eon, 4 to 2.5 billion years ago.

That doesn’t mean Mars is just like an early Earth. But it does mean that Mars could have retained quite a bit of heat under its mantle; it’s thought that, because it lacks the plate tectonics that help to dissipate heat on Earth, Mars may cool more slowly.

“I really think that Mars could be a still volcanically active world today, and these new results point toward this,” Mari told ScienceAlert.

“We may not see a volcanic eruption on Mars for the next 5 million years, but this doesn’t mean that the planet is inactive. It could just mean that the timing between eruptions between Mars and Earth is different, and instead of seeing one or more eruptions per day (as on Earth) we could see a Martian eruption every n-millions of years.”

We’ll need more research to confidently say this hypothesis checks out. But these results also mean that previous interpretations of the planet’s dryness based on surface olivine may need to be revisited. (Although let us be clear, Mars is still extremely dry.)

The ongoing NASA InSight mission that recently found evidence of Marsquakes, measures – among other things – the heat flux from the Martian crust. If Mars is still volcanically active, we may know more about it really soon.

The research has been published in Meteoritics & Planetary Science.

 



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Breathtaking High-Res Images of Jupiter Reveal The Secrets of Its Wild Storms

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Jupiter is not a serene place. The giant planet is wracked with tempestuous storms, wide bands of roiling cloud that encircle the entire globe, extending to depths many times thicker than the atmospheric distance between Earth and space.

 

The gas giant’s wild weather is so different from what happens on Earth that astronomers have struggled to understand it. But we just got another piece of the puzzle – in the form of breathtaking, near-infrared and optical images, taken using the powerful Gemini Observatory and Hubble Space Telescope.

The Gemini near-infrared images capture thermal radiation glowing through the clouds from Jupiter’s interior. When combined with Hubble’s optical images taken within hours of the Gemini ones, scientists can piece together the internal and external activity.

The high-resolution images reveal that regions of cloud that appear darker in optical images actually glow the most brightly in infrared, indicating those regions have little to no cloud compared to the lighter bands.

jupiter full(International Gemini Observatory/NOIRLab/NSF/AURA M.H. Wong & team/Mahdi Zamani)

“It’s kind of like a jack-o-lantern,” said astronomer Michael Wong of the University of California, Berkeley. “You see bright infrared light coming from cloud-free areas, but where there are clouds, it’s really dark in the infrared.”

This included a line curving around the edge of the Great Red Spot, a permanent storm currently a little larger than an entire Earth. Similar features had been seen in the storm before, but it was unclear what was causing them.

 

“Visible-light observation couldn’t distinguish between darker cloud material, and thinner cloud cover over Jupiter’s warm interior, so their nature remained a mystery,” said planetary scientist Glenn Orton of NASA’s Jet Propulsion Laboratory.

The new imagery cleared that question up rather neatly. When the two images were compared, a glowing infrared arc neatly matched up to an optical shadow, showing that the colouration marked a deep crack in the storm’s swirling clouds.

jupiter spot(NASA, ESA & M.H. Wong/UC Berkeley & team)

That’s really cool. But things got even more interesting when data from NASA’s Jupiter orbiter Juno was thrown into the mix. As Juno orbits and makes close flybys of Jupiter’s poles, it has been detecting atmospheric radio signals, called sferics and whistlers, from powerful lightning strikes.

In its first eight flybys, Juno’s Microwave Radiometer Instrument detected 377 lightning discharges, clustered around the planet’s polar regions. This is basically the opposite of Earth, where lightning storms are more common around the equator.

Planetary scientists believe that this has to do with how the Sun warms both planets. On both, the equator is warmed by the Sun. On Earth, this generates convection currents that drive tropical thunderstorms.

 

On Jupiter, which is much farther away from the Sun, equatorial warming is gentler, stabilising the upper atmosphere; but, scientists have theorised, this stabilising warmth doesn’t reach the poles, so they’re rather more tempestuous.

Combining this Juno data with the Gemini and Hubble images sheds more light on these wild storms, revealing the cloud structures around where lightning forms. “The data from Hubble and Gemini can tell us how thick the clouds are and how deep we are seeing into the clouds,” explained planetary scientist Amy Simon of NASA.

The team found that the lightning strikes are generated in regions with large, convective towers of moist air over deep clouds of water, both frozen and liquid. Clear regions around these storms are probably caused by a downwelling of drier air outside the convection cells.

jupiter lightning(NASA, ESA, M.H. Wong/UC Berkeley, A. James & M.W. Carruthers/STScI)

These coincide with what are known as folded filamentary regions, because the clouds are stretched and folded by Jupiter’s insane winds. This new information suggests that they’re teeming with convective activity, “the turbulent mixing process that transports Jupiter’s internal heat up to the visible cloud tops”, according to Wong.

“These cyclonic vortices could be internal energy smokestacks, helping release internal energy through convection. It doesn’t happen everywhere, but something about these cyclones seems to facilitate convection,” he added.

The Juno mission is ongoing, scheduled to end in July of next year. These findings will inform how to probe the data it is still collecting, as well as future ground- and space-based observations. And we are finally getting a handle on Jupiter’s savage weather.

“Because we now routinely have these high-resolution views from a couple of different observatories and wavelengths, we are learning so much more about Jupiter’s weather,” Simon said.

“This is our equivalent of a weather satellite. We can finally start looking at weather cycles.”

The research has been published in The Astrophysical Journal Supplement Series.

 



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Betelgeuse Just Keeps Getting Dimmer, And We Have No Idea Why

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Betelgeuse keeps getting dimmer and everyone is wondering what exactly that means. The star will go supernova at the end of its life, but that’s not projected to happen for tens of thousands of years or so. So what’s causing the dimming?

 

Villanova University astronomers Edward Guinan and Richard Wasatonic were the first to report Betelgeuse’s recent dimming. In a new post on The Astronomer’s Telegram, the pair of astronomers report a further dimming of Betelgeuse. They also point out that although the star is still dimming, its rate of dimming is slowing.

Betelgeuse is a red supergiant star in the constellation Orion. It left the main sequence about one million years ago and has been a red supergiant for about 40,000 years.

It’s a core-collapse SN II progenitor, which means that eventually, Betelgeuse will burn enough of its hydrogen that its core will collapse, and it will explode as a supernova.

It’s known as a semi-regular variable star, which means its brightness is variable. One of its cycles is about 420 days long, and another is about five or six years. A third cycle is shorter; about 100 to 180 days. Though most of its fluctuations are predictable and follow these cycles, some of them are not, like the current dimming.

Betegeuse's varying magnitude.Betelgeuse’s magnitude in visible light, August 2018 to January 2020. (AAVSO – AAVSO Light Curve Generator 2

Astronomers have been monitoring Betelgeuse for a long time. Visual estimates of the star go back about 180 years, and since the 1920s, the American Association of Variable Star Observers (AAVSO) have taken more systematic measurements.

About 40 years ago astronomers at Villanova University began taking systematic photometric measurements of Betelgeuse’s brightness. The photometry data from the last 25 years is the most thorough, and according to that data the star is as dim as it’s ever been.

 

According to Guinan and Wasatonic’s post on Astronomer’s Telegram, Betelgeuse’s temperature has dropped by 100 Kelvin since September 2019, and its luminosity has dropped by nearly 25 percent in the same time frame.

According to all of those measurements, the star’s radius has grown by about 9 percent. This swelling is expected as Betelgeuse ages.

In a way, we’re lucky to have Betelgeuse so close by, in astronomical terms at least. It’s only about 650 light-years away, and that makes it a great teacher. It’s the only star other than our Sun on which we can see surface details. That helps astrophysicists understand what’s happening there, and on other similar stars.

Like all stars, Betelgeuse generates heat in its core through fusion. The heat is transferred to its surface via convection. The currents that carry the heat are called convection cells, which can be seen on the surface as dark patches.

As the star rotates, these cells rotate in and out of view, which contributes to Betelgeuse’s observed variability. Convection cells can be massive, even more so on the surface of a huge star like Betelgeuse.

 

In 2013 scientists reported evidence of convection cells on the Sun that lasted for months. It wasn’t conclusive, but could something like that be happening on Betelgeuse, contributing to the dimming?

This dimming episode may not be the star itself, but rather a cloud of gas and dust obscuring the light. As time goes on, and Betelgeuse burns more of its fuel, it loses mass.

As it loses mass, its gravitational hold on its outer edges is weakened, and clouds of gas escape the star into the surrounding regions. This could cause the current dimming episode.

Or could it be something else? We know a lot about stars, but we don’t know everything. We’ve also never been able to observe any other red super-giants the way we can with Betelgeuse.

Astronomers know what’ll happen, they just don’t know when

Whatever the cause, we know what the eventual end for Betelgeuse looks like: a supernova explosion.

Whether this dimming is directly related to the approaching cataclysmic death of this unstable star is unknown at this point. As Guinan and Wasatonic say on Astronomer’s Telegram, “The unusual behavior of Betelgeuse should be closely watched.”

 

When Betelgeuse does eventually go supernova, it will be the most fascinating act of nature witnessed by any human ever. Other supernovae like SN 185 and SN 1604 were much farther away than Betelgeuse.

When Betelgeuse goes supernova, it will the third brightest object in the sky, after the Sun and the full Moon. Some estimates say it’ll be even brighter than the Moon.

That brightness will last months, and it’ll cast shadows on Earth even at night. Betelgeuse will light up the sky like no other supernovae, and will last for months, visible in daytime, and casting shadows at night. Then in about three years, it will fade to its current brightness.

Then in about six years after it goes supernova, Betelgeuse won’t even be visible in the night sky. Orion the Hunter will be no more.

When exactly all this will happen, nobody knows. And though this recent dimming likely isn’t directly connected to Betelgeuse’s eventual supernova explosion, astronomers don’t know that for sure either.

This article was originally published by Universe Today. Read the original article.

 



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