Image of Mercury taken by NASA's MESSENGER mission. Credit: NASA/JHUAPL/ASU/Carnegie Institution of Washington
Mercury, the closest planet to our Sun, is also one of the least understood in the Solar System. On the one hand, it is similar in composition to Earth and the other rocky planets, consisting of silicate minerals and metals differentiated between a silicate crust and mantle and an iron-nickel core. But unlike the other rocky planets, Mercury’s core makes up a much larger part of its mass fraction. Mercury also has a mysteriously persistent magnetic field that scientists still cannot explain. In this respect, Mercury is also one of the most interesting planets in the Solar System.
But according to new research, Mercury could be much more interesting than previously thought. Based on new simulations of Mercury’s early evolution, a team of Chinese and Belgian geoscientists found evidence that Mercury may have a layer of solid diamond beneath its crust. According to their simulations, this layer is 15 km (9 mi) thick sandwiched between the core and the mantle hundreds of miles beneath the surface. While this makes the diamonds inaccessible (for now, at least), these findings could have implications for theories about the formation and evolution of rocky planets.
In a recent study published in the Proceedings of the National Academy of Sciences, an international team of researchers led by Monash University in Australia have verified the existence of a rare hexagonal structure of diamond called lonsdaleite, within ureilite meteorites from the inside of a dwarf planet that formed approximately 4.5 billion years ago.
Lonsdaleite is named after Dame Kathleen Lonsdale, a famous British pioneering crystallographer responsible for developing several X-ray methods for studying crystal structures, and was the first woman elected as a Fellow to the Royal Society in 1945. This study holds the potential for further unlocking the secrets of the formation of our solar system, and was conducted with collaboration from RMIT University, the Australian Synchrotron and Plymouth University, and CSIRO.
An experiment conducted by an international team of scientists recreated the "diamond rain" believed to exist in the interiors of ice giants like Uranus and Neptune. Credit: Greg Stewart/SLAC National Accelerator Laboratory
For more than three decades, the internal structure and evolution of Uranus and Neptune has been a subject of debate among scientists. Given their distance from Earth and the fact that only a few robotic spacecraft have studied them directly, what goes on inside these ice giants is still something of a mystery. In lieu of direct evidence, scientists have relied on models and experiments to replicate the conditions in their interiors.
For instance, it has been theorized that within Uranus and Neptune, the extreme pressure conditions squeeze hydrogen and carbon into diamonds, which then sink down into the interior. Thanks to an experiment conducted by an international team of scientists, this “diamond rain” was recreated under laboratory conditions for the first time, giving us the first glimpse into what things could be like inside ice giants.
Uranus and Neptune, the Solar System’s ice giant planets. Credit: Wikipedia Commons
For decades, scientists have held that the interiors of planets like Uranus and Neptune consist of solid cores surrounded by a dense concentrations of “ices”. In this case, ice refers to hydrogen molecules connected to lighter elements (i.e. as carbon, oxygen and/or nitrogen) to create compounds like water and ammonia. Under extreme pressure conditions, these compounds become semi-solid, forming “slush”.
And at roughly 10,000 kilometers (6214 mi) beneath the surface of these planets, the compression of hydrocarbons is thought to create diamonds. To recreate these conditions, the international team subjected a sample of polystyrene plastic to two shock waves using an intense optical laser at the Matter in Extreme Conditions (MEC) instrument, which they then paired with x-ray pulses from the SLAC’s Linac Coherent Light Source (LCLS).
“So far, no one has been able to directly observe these sparkling showers in an experimental setting. In our experiment, we exposed a special kind of plastic – polystyrene, which also consists of a mix of carbon and hydrogen – to conditions similar to those inside Neptune or Uranus.”
The plastic in this experiment simulated compounds formed from methane, a molecule that consists of one carbon atom bound to four hydrogen atoms. It is the presence of this compound that gives both Uranus and Neptune their distinct blue coloring. In the intermediate layers of these planets, it also forms hydrocarbon chains that are compressed into diamonds that could be millions of karats in weight.
The MEC hutch of SLAC’s LCLS Far Experiement Hall. Credit: SLAC National Accelerator Laboratory
The optical laser the team employed created two shock waves which accurately simulated the temperature and pressure conditions at the intermediate layers of Uranus and Neptune. The first shock was smaller and slower, and was then overtaken by the stronger second shock. When they overlapped, the pressure peaked and tiny diamonds began to form. At this point, the team probed the reactions with x-ray pulses from the LCLS.
This technique, known as x-ray diffraction, allowed the team to see the small diamonds form in real-time, which was necessary since a reaction of this kind can only last for fractions of a second. As Siegfried Glenzer, a professor of photon science at SLAC and a co-author of the paper, explained:
“For this experiment, we had LCLS, the brightest X-ray source in the world. You need these intense, fast pulses of X-rays to unambiguously see the structure of these diamonds, because they are only formed in the laboratory for such a very short time.”
In the end, the research team found that nearly every carbon atom in the original plastic sample was incorporated into small diamond structures. While they measured just a few nanometers in diameter, the team predicts that on Uranus and Neptune, the diamonds would be much larger. Over time, they speculate that these could sink into the planets’ atmospheres and form a layer of diamond around the core.
The interior structure of Neptune. Credit: Moscow Institute of Physics and Technology
In previous studies, attempts to recreate the conditions in Uranus and Neptune’s interior met with limited success. While they showed results that indicated the formation of graphite and diamonds, the teams conducting them could not capture the measurements in real-time. As noted, the extreme temperatures and pressures that exist within gas/ice giants can only be simulated in a laboratory for very short periods of time.
However, thanks to LCLS – which creates X-ray pulses a billion times brighter than previous instruments and fires them at a rate of about 120 pulses per second (each one lasting just quadrillionths of a second) – the science team was able to directly measure the chemical reaction for the first time. In the end, these results are of particular significance to planetary scientists who specialize in the study of how planets form and evolve.
As Kraus explained, it could cause to rethink the relationship between a planet’s mass and its radius, and lead to new models of planet classification:
“With planets, the relationship between mass and radius can tell scientists quite a bit about the chemistry. And the chemistry that happens in the interior can provide additional information about some of the defining features of the planet… We can’t go inside the planets and look at them, so these laboratory experiments complement satellite and telescope observations.”
This experiment also opens new possibilities for matter compression and the creation of synthetic materials. Nanodiamonds currently have many commercial applications – i.e. medicine, electronics, scientific equipment, etc, – and creating them with lasers would be far more cost-effective and safe than current methods (which involve explosives).
Fusion research, which also relies on creating extreme pressure and temperature conditions to generate abundant energy, could also benefit from this experiment. On top of that, the results of this study offer a tantalizing hint at what the cores of massive planets look like. In addition to being composed of silicate rock and metals, ice giants may also have a diamond layer at their core-mantle boundary.
Assuming we can create probes of sufficiently strong super-materials someday, wouldn’t that be worth looking into?
Artist's conception of a comet exploding in the Earth’s atmosphere above Egypt. Credit: Terry Bakker
Speak about destruction. A comet slammed into Earth’s atmosphere 28 million years ago and basically killed everything with fire below, leaving a huge deposit of yellow silica glass in its wake, a team of astronomers say.
The evidence — a black pebble found by an Egyptian geologist within this vast tract of glass — is believed to be a part of the comet’s nucleus or heart and not just an ordinary meteorite. The team says this could be the first hard evidence, so to speak, of a comet striking Earth.
The temporary “shockwave of fire” hit 2,300 square miles (roughly 6,000 square kilometers) of Egyptian sand, turning the grains into glass. Given the area’s rich archaeological history, it’s probably not too much of a surprise that a small portion of this is visible in a brooch that belonged to ancient boy-king Tutankhamun.
A brooch that belonged to the Egyptian boy-king Tutankhamun, which reportedly contains a silica glass stone that originated from a comet explosion. Credit: University of the Witwatersrand, Johannesburg
“It’s a typical scientific euphoria when you eliminate all other options and come to the realization of what it must be,” said lead author Jan Kramers of the University of Johannesburg in a statement.
Besides silica, the cosmic blast furnace left teeny-tiny diamonds in its wake, forming from carbon. “Normally they form deep in the earth, where the pressure is high, but you can also generate very high pressure with shock. Part of the comet impacted and the shock of the impact produced the diamonds,” said Kramers.
More information on this find should be available soon when the discovery is published in Earth and Planetary Science Letters. The authors first discussed their find in a public lecture Oct. 10. It will be interesting to see what other scientific teams think of this hypothesis, so stay tuned for the reaction.
Source: University of the Witwartersrand, Johannesburg
