Neptune and raining of diamonds

 Deep within Neptune and Uranus, it rains diamonds

Deep inside the planet Neptune and Uranus, the pressure is so extreme that carbon atoms are crushed into actual diamonds, which then rain downward in solid showers through the planet's interior, and recent laboratory experiments have successfully recreated the conditions in which this happens, producing tiny synthetic diamonds in the process. The interiors of the two ice giants of the outer solar system, Uranus, at about 2.9 billion km's from the Sun, and Neptune, at about 4.5 billion, are difficult to study by any direct means. Astronomers and physicists have suspected for nearly 40 years about raining of diamonds. The outer planets of our Solar System are hard to study, however. Only a single space mission, Voyager 2, has flown by to reveal some of their secrets, so diamond rain has remained only a hypothesis. Beyond the lingering mystery of the diamond rain, there’s a big loss in our failure to study Uranus and Neptune inside and out. It limits our understanding of the Solar System and the galaxy, because planets of this size have turned out to be extremely common in the Milky Way. The number of planets similar in size to Uranus and Neptune which have been found in the galaxy is roughly nine times greater than the number of much larger planets similar in size to Jupiter and Saturn. The outermost planets also seem to bear scars which could tell us a lot about the formation of our own Solar System. So there’s a growing sense of urgency to explore Neptune and Uranus, both to better understand where and how planetary systems form and also to refine our ideas about where to look for planets which can sustain life.

The interiors of the two ice giants of the outer solar system, Uranus and Neptune, are difficult to study. The only spacecraft ever to visit either planet was Voyager 2, which flew past Uranus in January 1986 and Neptune in August 1989, observed both worlds for a few hours each, and continued on into interstellar space. No probe has entered either planet’s atmosphere, and none is planned within this decade. What scientists know about the interiors of Neptune and Uranus comes almost entirely from theoretical models, indirect observations of magnetic fields and gravitational moments, and laboratory experiments designed to recreate the extreme pressures and temperatures believed to exist at depth. Although we’ve been limited by spacecraft and ground-based telescopes regarding how much we can learn about the exteriors of Uranus and Neptune, advances in laboratory simulations are enabling remarkable new insights about what’s happening in their interiors, including what gives rise to diamond rain. Discoveries such as these reveal the complexity of the chemical processes involved in the evolution of these planets. Our simulations give clues to the internal nature of worlds far beyond the Solar System, even worlds which we may never see directly from the outside.

One of the more striking predictions of these models is that, somewhere between 5,000 and 10,000 km's below the planets’ visible cloud tops, the atmospheric pressure becomes so extreme that the methane molecules abundant in the atmosphere are torn apart. The methane (CH₄) decomposes into its constituent carbon and hydrogen atoms; the carbon atoms then arrange themselves into the diamond crystal lattice and crystallise out as solid diamond particles. The particles, being denser than the surrounding fluid, sink toward the planetary core under the pull of gravity. According to the 2017 SLAC National Accelerator Laboratory announcement of the first direct laboratory observation of this process, the phenomenon has been theorised for nearly four decades but had never been directly observed in any experimental setup. Neptune and Uranus are called the “ice giants“ of our Solar System because their outer two layers consist of compounds which include hydrogen and helium. In astronomy slang, ice refers to all compounds of light elements which contain hydrogen, so the planets’ water (H2O), ammonia (NH3) and methane (CH4) make them “icy.” The beautiful bluish hue of both planets is the result of methane traces in their atmospheres.

The 2017 results have been refined in subsequent experiments. According to a 2017 Eos article published by the American Geophysical Union, the simulated conditions corresponded to a depth of approximately 10,000 km's below Neptune’s surface, and produced nanodiamonds of unambiguous crystalline structure within the femtosecond observation window. Subsequent experiments at SLAC in 2020 and 2022 used variations of the technique, including replacing the polystyrene with plastics containing oxygen (more closely approximating the actual chemical mixture inside Neptune, which includes water and ammonia along with methane), and confirmed that the diamond formation persisted under these more realistic conditions. The most recent refinement, published in Nature Astronomy in January 2024, contained a notable surprise. According to a SLAC announcement accompanying the 2024 paper, the diamond rain forms at substantially lower pressures and temperatures than the earlier experiments had suggested. The implication is that diamond formation occurs over a much larger region of Neptune’s and Uranus’s interiors than had been previously assumed, extending from much shallower depths than the originally calculated ~10,000 km's. The 2024 paper also linked the diamond rain to the unusual magnetic fields of Neptune and Uranus, which differ from those of Earth and Jupiter in being substantially offset from the planets’ rotational axes, possibly because the diamond rain disrupts the convective flows in the planets’ fluid interiors in ways which distort the magnetic field geometry.

However, it is the “ice” in the deep middle layers which really shapes their properties. On Neptune, for example, beneath a hydrogen-helium atmosphere there is 3,000 km's thick lies an ice layer that is 17,500 km's thick. Simulations suggest that gravity compresses the “ices” in this middle layer to high densities, and the internal heat raises the internal temperatures to several thousand kelvins. Despite the high temperature, pressures more than one million times greater than the atmospheric pressure on Earth compress the so-called ices into a hot, dense fluid. Under such heat and pressures, ammonia and methane are chemically reactive. Scientists have modeled exotic processes, including diamond formation, taking place between the compounds deep within the ice layers. Marvin Ross of Lawrence Livermore National Laboratory first introduced the diamond-rain idea in a 1981 article in Nature titled, “The Ice Layer of Uranus and Neptune, Diamonds in the Sky?” He suggested that the carbon and hydrogen atoms of hydrocarbons such as methane separate at the high pressures and high temperatures inside the ice giant planets. Clusters of isolated carbon atoms would then be squeezed into a diamond structure, which is the most stable form of carbon under such conditions.

The technical challenge of recreating Neptune’s deep interior is substantial. The pressures involved are on the order of 1.5 to 3 million times Earth’s atmospheric pressure. The temperatures are several thousand kelvin. Sustaining these conditions in any kind of bulk sample is impossible with current technology available, no container could survive the pressures, and no heating system could maintain the temperatures without destroying everything around it. The trick the SLAC team developed is to recreate the conditions for only an extremely brief instant, just long enough for a single chemical reaction to occur and be observed. The technique works as follows. According to a Lawrence Livermore National Laboratory technical summary of the experiment, a thin sample of polystyrene plastic, a substance whose chemical composition (carbon and hydrogen in long molecular chains) approximates that of the hydrocarbon compounds in Neptune’s atmosphere, is placed in a target chamber at SLAC’s Matter in Extreme Conditions instrument. The sample is then struck simultaneously by two pulses of an extremely high-powered optical laser, which together create two compression shock waves which travel through the plastic. The shock waves briefly compress the sample to roughly Neptune-deep-interior conditions of 1.5 to 2 million atmospheres of pressure and 5,000 to 6,000 K of temperature, sustaining these conditions for a few quadrillionths of a second.

During this femtosecond-long window, the SLAC team uses X-ray diffraction or X-ray scattering, fired through the same sample by the laboratory’s Linac Coherent Light Source X-ray free-electron laser, to monitor in real time what is happening to the carbon and hydrogen atoms inside the plastic. The diffraction patterns produced by the X-rays reveal the atomic arrangement of any crystalline material present. When the SLAC team performed the experiment in 2017, the X-ray diffraction patterns showed unambiguous evidence of diamond, the carbon atoms had separated from the hydrogen, organised themselves into the characteristic cubic crystal structure of diamond, and condensed into nanometre-sized crystalline grains, all in the brief moment of peak shock pressure. Diamond is denser than the methane, ammonia and water left in the ice layer, so the carbon crystal would start to sink toward the planet’s core. It would accumulate new layers as it falls when it touches other isolated carbon atoms or diamonds, allowing individual diamond blocks to reach a size meters in diameter. We think that, as a result, a thick layer of carbon surrounds the rocky cores of Uranus and Neptune. This carbon layer may consist of blocks of solid diamond, or, if the temperature is extremely high (as some planet models suggest), it might transform into liquid carbon, or a mix of solid carbon and liquid carbon.

If the layer is a mix of solid and liquid carbon, the solid carbon would be of lower density than the liquid, with the result that large “diamond bergs” would float on top of an ocean of liquid carbon. Each possible composition of the carbon layer, solid, liquid, or mixed, would affect the core of the planet differently. Solid diamond, for example, is electrically insulating and has a stiff crystal lattice, whereas liquid carbon is a metallic conductor and flexible. Determining the properties of the carbon layer could reveal whether or not Neptune and Uranus formed from a rocky protoplanet core billions of years ago. The diamonds produced in the SLAC experiments are nanoscale, a few nanometres across, because the experimental shock lasts only femtoseconds. On Neptune itself, the same chemical process would operate over millions of years and produce diamonds of much larger size. Theoretical models suggest that individual diamond crystals could grow to weights of millions of carats before sinking toward the planet’s core. The total mass of diamond produced over the planet’s history could amount to a substantial fraction of the total mantle material, with some estimates suggesting that Neptune may have a layer of solid diamond hundreds of km's thick surrounding its rocky core.

The mass of diamond inside Neptune and Uranus, if these estimates are even approximately correct, would dwarf the total terrestrial diamond resource by many orders of magnitude. Diamond on Earth occurs in trace quantities in rocks called kimberlites and lamproites, with the global annual production of mined diamond on the order of 100 million carats/year, or about 20 tonnes. Neptune’s hypothesised diamond layer, if it exists at the scale current models suggest, would contain hundreds of millions of years of Earth’s diamond production in any given cubic kilometre. The diamonds would be inaccessible, Neptune is gaseous in its outer layers and the diamond rain region sits beneath thousands of kilometres of hydrogen and helium, but the resource itself, if any future technology could ever reach it, would be approximately the most abundant supply of pure crystalline carbon anywhere in the solar system. Although Ross’s idea was certainly fascinating, it was mainly hypothetical at the time and needed to be verified by observations. It is impossible with any imaginable technology to design and build a probe which could penetrate deep into Neptune or Uranus and directly observe the formation of diamonds. Scientists instead have tried to recreate the extreme conditions of planetary interiors in their laboratories. Even this more limited goal is extremely challenging, since we need to reliably generate and measure pressures of several million atmospheres and temperatures of several thousand kelvins to simulate their effects on the elements found inside the ice giants. In essence, we need to build a piece of a planet in the lab.

Facilities around the world are tackling the problem by compressing a sample material, such as methane, between two diamond anvils with very small tips that press on the sample. The same effect of pressure enhancement can be seen on a different scale by placing something underneath the heel of a high-heeled shoe. Even though diamond anvils can generate pressures of several megabars (comparable to the pressure that would be produced by placing several thousand African elephants on top of that high-heeled shoe), the sample also needs to be heated by electrical currents or lasers in order to mimic hot planetary interiors. Using such a setup, some experiments have indeed formed diamond. However, in these setups the materials representing the planetary ice layers, methane, ammonia, or water, start to react with the diamond anvils and the gaskets. Those reactions can strongly alter and contaminate the results. Another way to generate the extreme pressure and temperature conditions found inside the ice giant planets is to create shock compression using strong explosives, high-velocity gun projectile impacts, or pulsed high-energy lasers. Although this process both compresses and heats the sample at the same time, the samples remain in the interesting state for only a tiny fraction of a second. Particularly for the high-energy lasers, which can achieve gigabar pressures and temperatures of millions of kelvins (comparable to the temperature at the center of the Sun), the conditions usually last a few nanoseconds or less. That is a very limited time in which to obtain precise and direct measurements of structural changes of the sample.

The diamond-rain experiments have implications beyond Neptune and Uranus. Hydrocarbon-rich atmospheres are common in the universe. Many of the exoplanets discovered by the Kepler and TESS missions in the past 15 years are believed to be Neptune-like ice giants or larger “mini-Neptunes,” with atmospheres dominated by hydrogen, helium and varying mixtures of methane, water, and ammonia. The same physics which produces diamond rain in Neptune should, in principle, operate in many of these exoplanetary atmospheres. If even a small fraction of the known ice giants in the galaxy have functioning diamond-rain processes in their interiors, the total mass of crystalline carbon distributed across exoplanetary mantles in the Milky Way may be enormous. The next step in the experimental programme, according to the SLAC team, is to recreate the conditions found in still-deeper layers of these planets, pressures of 5 to 10 million atmospheres, temperatures approaching 10,000 K, and observe what other exotic chemistry occurs. The team has speculated that, at sufficient pressure, additional unusual phenomena may include the formation of metallic hydrogen, exotic compounds of helium and other noble gases, and possibly forms of carbon chemistry that have no Earth analogues at all. The diamond rain, in this view, is just the most accessible of a range of strange chemical processes which operate inside planets very different from Earth, and the SLAC experiments are the first direct experimental window into a kind of geology which exists nowhere on the visible surface of our own planet but is plausibly the dominant kind in much of the rest of the universe.

This situation changed in 2009 with the completion of the world’s first x-ray free-electron laser: the Linac Coherent Light Source at Stanford University. Combining this machine with a powerful pulsed-laser system allows us to study chemical reactions at conditions comparable to those in the deep interiors of giant planets in real time. Plastics, which are mainly made out of carbon and hydrogen, are useful substances to mimic the material mix in the ice layers of Neptune and Uranus. Understanding the inner processes of the ice giants gives clues to the features of these planets. For example, diamond precipitation releases gravitational energy, which is converted to heat by friction between the diamonds and the surrounding material as they descend. This effect could explain why Neptune is emitting more energy than it receives from the Sun. Such an internal energy source may help to account for the origin of the surprisingly violent storms which are observed on the planet’s surface. Diamond formation may also explain why the ice giant planets’ magnetic fields are so exotic. Unlike Earth’s magnetic field, the fields around Uranus and Neptune are not symmetrical, and they don’t extend from each pole. These properties suggest that ice giant fields probably originate not in the core but in a thin, rather variable layer of conducting material, such as metallic hydrogen formed as a by-product of making diamonds. Other exotic processes inside the planets may also contribute to their magnetic fields. We will continue to study these phenomena in the lab, but a new space probe mission to Neptune or Uranus (or both) could add a wealth of information about the planets’ internal processes and about how such planets have formed in our Solar System and others. NASA is currently considering such a mission. In 2030, the planets of our Solar System will be favorably aligned for a spacecraft to launch and reach Uranus or Neptune by 2040. Another fortuitous alignment of the planets won’t come for another two generations, so now is the time to start thinking about exploring the ice giants up close and learning more about the Solar System’s intriguing diamond worlds in our universe.

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East Asia’s tallest tree

 "The Heaven Sword" new tallest tree with whopping 81 meters

Determined researchers have found East Asia’s tallest known tree in a secluded valley near one of Taiwan’s longest rivers. The team named the massive fir tree “Heaven Sword of the Da’an River”, a nod to the legendary weapon from Jin Yong’s martial arts novels. Researchers estimate the Heaven Sword,  towering 84.1 meters (276 feet) tall, is about 1,000 years old. The quest to identify the Taiwania fir spanned a decade. The Indigenous Rukai people who inhabit the island’s southern mountains refer to the evergreen species, formally known as Taiwania cryptomerioides, as “the tree that hits the moon.” After nearly 10 years of forest treks and scientific sleuthing, researchers have finally pinpointed East Asia’s tallest tree: “The Heaven Sword.” Measuring 84.1 meters (nearly 276 feet), you might expect it to be relatively easy to find such a huge object, but the task was anything but simple. “When we confirmed the tallest tree, we were so relieved because it was really a long journey to achieve that result,” Dr Rebecca Chia-Chun Hsu, study author from the Taiwan Forestry Research Institute, said.

Researchers used LiDAR and field surveys to confirm an 84.1-meter Taiwania fir after a decade-long search and a map of 941 giant trees. Researchers identified East Asia's tallest tree, 'The Heaven Sword,' in a secluded valley in Taiwan, according to a study published in the journal Frontiers in Forests and Global Change. A group nicknamed the Taiwan Tree Seekers spent nearly 10 years surveying vegetation, using LiDAR technology and crowdsourced volunteers to analyze 57,065 candidate images to find the tallest specimen. It was discovered in Taiwan, an island that’s known to harbor exceptionally tall trees amid its rugged mountain terrain. Chief among them is a species known as the Taiwania fir (Taiwania cryptomerioides), a giant that the local Indigenous Rukai people would call “The tree that hits the moon.” The team managed to track them down, with the tallest measuring 69.3 meters (227 feet) and a trunk diameter of nearly 3 meters (nearly 10 feet), certainly impressive, but the researchers suspected even bigger firs were out there. “Taiwan is an island across the Tropic of Cancer. So the island holds many tree species from both temperate and tropical regions. Thanks to the massive area of steep mountains, many old-growth forests remain untouched by past logging,” added Chia-Chun Hsu. The search was launched in August 2014, when researchers from the Taiwan Forestry Research Institute set out in pursuit of a legendary grove known as the "Chilan Three Sisters." 

Located in the Chilan conservation area, rumors about this giant trio had circulated for years, yet they had never been scientifically documented. For reference, the world’s tallest known living tree is Hyperion, a coast redwood in California’s Redwood National Park, which currently measures 116 meters (381.3 feet) tall. Dramatically mountainous, Taiwan is home to a rich diversity of plant life. Forests cover about 60% of the island, which has an estimated 950 million trees, according to an earlier study. The colossal trees which grow in many of Taiwan’s forests made searching for the Heaven Sword a formidable task. Undaunted, a group of Taiwan tree seekers, including professional tree climbers, ecologists, geologists and remote-sensing specialists, has been working since 2014 to document the island’s tallest trees, including Taiwania firs such as the Heaven Sword. They next turned their attention to a more remote region near Mount Benya and Great Ghost Lake, where locals suggested the largest population of Taiwania firs might be found. The expedition was grueling, requiring four days of heavy hiking through dense forest. They managed to climb a 71.7-meter (235-foot) tree during the trip, but still suspected that more scientific methods might reveal something taller. Lead study author Dr. Rebecca Chia-Chun Hsu from the Taiwan Forestry Research Institute scaled the massive tree, using tape-drop measurement which confirmed the record height of 84.1 meters. Taiwan's steep, mountainous terrain and stable oceanic climate create a rare environment sustaining giant trees, though the Heaven Sword remains shorter than the 116-meter Hyperion coast redwood in California. 

To become more rigorous with their approach, the team used LiDAR (Light Detection and Ranging), a technology that sweeps across vast swaths of landscape by firing laser pulses from an aircraft. But there was a small hiccup. Taiwan's unique geography can be deceiving to these airborne, technological methods. On rugged, uneven terrain, trees perched beside steep cliffs can appear far taller than they actually are, for instance. To overcome this hurdle, they simply used hundreds of people to double-check the LiDAR findings, as human eyes are much better at recognizing these subtleties. It turned out that the algorithm had misjudged the height of around 93% of the trees in the survey. “Taiwan’s giant forests [are] some of the most carbon-dense environments in the entire world, comparable to the most famous old-growth forests on Earth. These ‘trees that hit the moon’ are not just natural wonders; they are essential guardians of the environment,” they added. The group undertook years of aerial scanning surveys, created the Taiwan Giant Tree Map and sought input from citizen scientists. Ultimately, the team identified the Heaven Sword the old-fashioned way, by scaling the massive tree and dropping a tape measure from the top, according to a study. The careful and creative mixture of methods employed by the team could be used to identify large trees on broader scales, a crucial task for conserving Earth’s biodiversity and safeguarding against the climate crisis, experts say. 

As a result, the team published the "Taiwan Giant Tree Map" by the end of 2022, flaunting 941 individual trees which stood over 65 meters (213 feet) in height. In January 2023, during the Lunar New Year holiday, they launched a fresh expedition to find the tallest tree on the map. When they believed they had found it, climbers scaled the trunk and dropped a measuring tape from the very top to confirm it. It was official: the “Heaven Sword of the Da’an River” was recorded as the tallest specimen in Taiwan and East Asia, measuring 84.1 meters (nearly 276 feet). The tree atlas also led the scientists to an extraordinary "temple of giants" near Mount Benya, featuring a grove of 11 trees each exceeding 65 meters (213 feet). They also discovered a new "pure forest" of around 30 giant Taiwania firs near Great Ghost Lake, growing in a dense, ancient cluster. These pockets of gigantic Taiwania firs are vital not only to the species' health, but to the health of the planet itself. As immense stores of living wood, these trees draw down huge quantities of carbon from the atmosphere, locking it away in their trunks and root systems. Without them, Earth would be in an even sadder state. “These trees are vital for the planet’s health,” the researchers said after their discoveries. “Taiwan’s giant forests [are] some of the most carbon-dense environments in the entire world, comparable to the most famous old-growth forests on Earth. These ‘trees that hit the moon’ are not just natural wonders; they are essential guardians of the environment,” they added. Abundant yearly rainfall and a steady climate have made Taiwan one of the rare environments on Earth capable of sustaining for hundreds and thousands of years the continuous growth of old, giant trees, said lead study author Dr. Rebecca Chia-Chun Hsu. Industrial logging between 1912 and 1991 depleted some of Taiwan’s ancient forests, but the island’s incredibly steep terrain kept old trees out of the reach of loggers, she added. Now, many of the trees grow in protected areas.

The Taiwan tree seekers first came together 12 years ago to measure and document “The Three Sisters,” a trio of giant Taiwania firs long known to locals in the expansive Cilan conservation area across northern and northwestern Taiwan. The group galvanized into action when Hsu encountered experts in lidar, or light detection and ranging, at a conference. They described the difficulties in searching for giant trees while relying solely on raw remote-sensing data. Professional tree climbers and members of Indigenous communities joined the group as it embarked on increasingly challenging expeditions, sometimes involving days of hiking to reach one site. The team realized that identifying the tallest trees from the ground was a nearly impossible task. The group had climbers who could scale towering trees, but the members needed a bird’s-eye view of the multitiered canopy to gain a broader perspective of Taiwan’s dense forests. Partnering with remote-sensing experts from Taiwan’s National Cheng Kung University, the team used lidar to transmit pulses of light from aircraft, measuring how long it took for the light to bounce back to them. The technique enabled the team to create a detailed 3D map which highlighted the height of the trees. Human eyes and thought processing also played vital roles. 

Taiwan’s uneven terrain can trick remote sensing, which will measure trees as taller than they are depending on nearby features, such as steep cliffs. Reaching the Heaven Sword involved over 20 km's (12 miles) of swimming and rock climbing upstream, followed by two days of an arduous, uphill hike. Then, it was time to ascend the tree. “On-site, we use drones to investigate the target tree before climbing,” Hsu wrote in an email. “However, the most accurate way to determine a giant tree’s height is tape-drop measurement.” When the tape measure read 84.1 meters, Hsu recalled feeling immense relief. The grueling hike in the middle of nowhere had been worth it. Forester Michael Taylor, codiscoverer of the Hyperion tree and lidar specialist for the Columbia Land Trust in California, applauded the efforts of the team for putting in the hard work of manual measurements for confirmation. Taylor was not involved in the study. “Very few other groups actually do that, instead over-relying on these inflated figures using automated height generation software alone without consideration of hillside leaners,” Taylor said. Hillside-leaning trees often grow in a way in which they are trying to seek additional sunlight or compensate for shifting soil. Hsu believes every expedition has been worth the effort, even when navigating unpredictable weather and unfavorable conditions, because the group has had to forge their own paths through stunningly beautiful but isolated locales that they might not otherwise see.

Giant trees play an ecologically important role in forests, absorbing CO2 and acting like protective guardians of their ecosystems. Taiwan’s forests of giant trees may be some of the most carbon-dense environments in the world. “Beyond their carbon value, large trees contribute to the structural complexity and functional diversity of forests and provide critical habitat for other organisms,” said Lalasia Bialic-Murphy, head of the forest diversity and biodemography group at the Swiss Federal Research Institute WSL. “At the same time, forests are increasingly threatened, with recent estimates suggesting that over 30% of the world’s tree species face elevated extinction risk.”. Tall trees are some of the most vulnerable to climate change, according to the new study. They face greater sensitivity to drought and extreme weather events, such as the strong typhoons and heavy rainfall-driven landslides which plague Taiwan.

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