What scientific mysteries are hidden behind the “rare treasure helium element” on Earth?

Produced by: Science Popularization China

Author: Li Zhongping (Oil and Gas Resources Research Center, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Gas Geochemistry Committee, Chinese Society of Mineralogy, Petrology and Geochemistry)

Producer: China Science Expo

In the vast universe, various elements are constantly born and die. Among them, there is a type of element that has attracted much attention since its discovery, and that is rare gases. When it comes to rare gases, the first thing that comes to mind is: the Group ⅧA elements in the periodic table, namely: helium (He), neon (Ne), argon (Ar), krypton, and xenon; they are very rare in nature and do not react with other substances, so they are also called inert gases.

But have you ever wondered why some rare gases are very rare on Earth but very common in the universe? Do you know how scientists use rare gas isotopes to reveal the history of Earth's evolution? How is the measurement of rare gas isotope ratios achieved? What are the application areas of rare gases? How do humans obtain these precious gases?

In fact, each noble gas hides a magical mystery. I believe you are also curious! Let us enter the mysterious world of noble gas geochemistry. In this article, we will explore the mysteries of helium isotope geochemistry!

From Argon to Helium: William Ramsay's Journey into the Gases

Did you know that helium was discovered by humans more than 100 years ago?

It was the evening of April 19, 1894, and William Ramsay attended a lecture given by Sir Rayleigh (John William Strat), who had previously discovered that a compound called ammonium nitrite could produce nitrogen with a different density from that in the air. Rayleigh and Ramsay discussed and decided to jointly explore the cause of this phenomenon. They immediately began research in their respective laboratories and kept in touch almost every day to inform each other of the progress of their work.

In August of the same year, Ramsay and Rayleigh announced the discovery of a new element, argon. In 1895, Ramsay separated helium from yttrium uranium ore (an impure radioactive uranium-containing uranium ore), proving that this element also exists on Earth. In the following years, Ramsay discovered neon, krypton and xenon. Because Ramsay discovered the noble gas elements in the air and determined their position in the periodic table, he was awarded the Nobel Prize in Chemistry in 1904.

Ramsey was an outstanding scientist. His discovery not only filled the gap in the periodic table, but also made significant contributions to human understanding of nature and the advancement of science. His achievements have inspired later generations to continue to pursue scientific truth and promote the development of science and technology. We should remember his name and his great contribution to the development of science, and thank him for his unremitting efforts for the progress of human civilization.

Figure 1. William Ramsay (1852-1916) was a Scottish chemist who discovered the noble gases and, with his collaborator John William Strathairn (Rayleigh), won the Nobel Prize in Chemistry in 1904. After the pair discovered argon, Ramsay studied other atmospheric gases, making pioneering contributions with his work in separating argon, helium, neon, krypton, and xenon, leading to the development of the periodic table.

(Photo source: Nobel Prize Committee official website)

Helium: What kind of cosmic secrets are hidden behind this “rare treasure” on Earth?

The origin of helium can be traced back to the Big Bang about 13.8 billion years ago. At this time, there were only two elements in the universe, hydrogen and a small amount of helium. These elements were held together by gravity in space to form stars and galaxies. When stars burn hydrogen in their cores, they create an environment of high temperature and pressure, which causes hydrogen atoms to fuse into helium atoms. When stars die and experience explosions, they release large amounts of helium and other elements into space.

Looking at the entire universe, helium accounts for 23% by mass, but in the Earth's atmosphere, the concentration of helium is very low, only 5.2 parts per million by volume (5.2 ppmv). This is because helium has a relatively small atomic weight and its molecular speed is faster than other gases in the atmosphere, which makes it easy for it to escape from the Earth's atmosphere.

In addition, helium is also a very stable gas. It is not easily adsorbed or converted by chemical reactions of other gases in the Earth's atmosphere, which also causes it not to accumulate in the atmosphere. In contrast, although hydrogen is lighter, it can form compounds, such as combining with oxygen to form water and combining with carbon to form various hydrocarbons, so the content of hydrogen on Earth is relatively high.

Figure 2. Dry air is a mixture of nitrogen, oxygen, argon, and other gases in small quantities.

Composition of the Earth's atmosphere by number of molecules, excluding water vapor

The graph below represents trace gases, which together make up about 0.0434% of the atmosphere (the concentration in August 2021 was 0.0442%). The numbers are mainly from 2000, with CO2 and CH4 from 2019, and do not represent any single source.

(Image source: engineeringtoolbox-Air Composition and Molecular Weight)

Helium has 8 isotopes, of which only two are stable, namely helium-3 and helium-4. The main component of helium in the Earth's atmosphere is helium-4, which accounts for about 99.99986% of helium. Helium-3 is rare on Earth, accounting for only about 0.00014%, and a small amount of helium-3 is produced by nuclear weapons testing. As one of the earliest elements formed in the universe, the main cause of helium-3 on Earth is the Big Bang and nuclear fusion reactions inside stars.

The other is helium-4 of radioactive origin, which is an alpha particle released when radioactive elements (such as thorium and uranium) decay inside the earth. Helium-4 mainly exists in the earth's crust, especially in sedimentary basins, and can be released to the surface through fractures or geological fluids.

Helium isotope ratios, exploring the secrets of the deep Earth

Since helium-3 and helium-4 have different sources on Earth, there are significant differences in the 3He/4He ratio of helium from different sources. Therefore, by analyzing the 3He/4He ratio in helium that rises from deep Earth to the surface, scientists can understand the composition and movement of materials deep in the Earth, such as crustal movement, thermal and chemical history of rocks, and the evolutionary history of the Earth.

Primitive helium, such as that from solar system material, usually has a high 3He/4He ratio because the isotopic ratio of helium was fixed when the universe was formed, so it can be used to judge the origin and evolution of solar system material.

Helium of radioactive origin, such as the helium in the earth's crust, usually has a low 3He/4He ratio, about 10^-8 to 10^-7. This is because the helium produced inside the earth's crust mainly comes from 4He from radioactive decay, and the decay of radioactive isotopes will cause the ratio of helium isotopes to change.

How to use helium isotope ratios to study the evolution and circulation of matter inside the Earth?

Scientists can measure the helium isotope ratios in samples from the mantle in different regions to study the origin and evolution history of the mantle material. If the helium isotope ratio in a region is higher than the average, it may mean that the test samples in the region come from more primitive mantle materials; conversely, if the ratio is lower, it may mean that the specific region represented by the sample has experienced more material mixing and recycling.

For example, on the island of Hawaii, the helium isotope ratio in volcanic rocks is as high as 40 times or even higher than the atmospheric level, indicating that the volcanic magma of the island of Hawaii may come from the deep, primitive or unique mantle material of the earth, and form unique volcanic rocks (scientists call it "hotspot"). On the Atlantic Ocean Ridge, the helium isotope ratio in the seafloor basalt is only 8 times the atmospheric level, which may indicate that the Atlantic Ocean Ridge comes from a relatively shallow layer, accompanied by mixing, or a common mantle source.

Figure 3. Using 3He/4He and 40Ar/36Ar ratios to demonstrate the migration and evolution of mantle materials.

(Image source: Hirochika-Sumino 2010)

The application of helium isotopes, along with other noble gases (neon, argon, krypton, xenon), in geochemical research has made a lot of progress, but there are still many unknown areas for us to explore. For example, scientists still do not know the helium isotope composition and distribution of certain regions in the Earth's interior, and how the Earth's evolution affects them. In addition, helium isotopes can also be used to study extraterrestrial bodies, such as planets and meteorites in the solar system, to better understand the formation and evolution of the universe.

Helium isotope technology is currently widely used in research in the fields of earth science, space science, and nuclear science. For example, the ratio of noble gas isotopes in rocks, groundwater, or meteorites can be measured to infer their formation age, source, and evolution process. Helium from different sources has different isotope ratios because they have undergone different evolution processes inside the earth. By measuring the ratio of noble gas isotopes, we can explore the movement and evolution of the earth's interior, determine the origin and distribution of mineral deposits, and even predict the possibility of earthquakes and volcanic eruptions.

Helium isotope geochemistry also faces some possible challenges:

First, how to improve the accuracy and sensitivity of helium isotope analysis to meet the needs of measuring extremely low abundance samples has always been a concern of scientists; secondly, how to expand the scope and depth of application of helium isotopes in different media (such as rocks, fluids, gases, etc.) in the future to reveal more earth science problems is also an important research direction; in addition, how to explore the coupling relationship between helium isotopes and other rare gas isotopes (such as neon, argon, krypton, xenon, etc.) to enhance the understanding of the complex processes of the Earth system is also a hot topic of academic concern.

Noble Gas Mass Spectrometer: An Important Tool for Revealing the History of the Earth

A noble gas mass spectrometer is an instrument specifically used to analyze the isotope ratio of noble gases. It uses mass spectrometry to separate gas samples into different isotope ions, and then determines the isotope ratio by detecting the mass ratio of these ions. This technology is widely used in geology, chemistry, physics, astronomy and other fields, and can be used to study substances and processes in nature, as well as the evolutionary history of the earth and the universe.

In order to achieve higher measurement accuracy, the rare gas mass spectrometer uses static vacuum technology. The so-called static vacuum means that there is no gas flow in the system during the measurement process, and the pressure in each part is the same and remains unchanged for a long time, which can reduce gas interference and improve measurement accuracy.

Modern noble gas mass spectrometers use high-resolution magnetic sector fields and multi-collector techniques to measure the isotope ratios of noble gases in trace samples. This instrument is essential for research in fields such as earth science, astronomy, and chemistry.

Figure 4. Static vacuum mass spectrometer for noble gas isotope measurement.

(Image source: Website of Gas Isotope Laboratory, Oil and Gas Resources Research Center, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences)

Mining lunar helium-3: a challenge that is currently almost impossible for humans to accomplish

Helium-3 also has a potential energy source. It can undergo nuclear fusion reactions with hydrogen isotopes. Unlike general nuclear fusion reactions, helium-3 does not produce neutrons during the fusion process, so it has low radioactivity, and the reaction process is environmentally friendly, safe and easy to control. However, the reserves of helium-3 on Earth are very limited, with a total amount of less than a few hundred kilograms, which cannot meet human needs.

Scientists have discovered that the moon has a very rich reserve of helium-3. The reason why the lunar surface is rich in helium-3 is that it is an important component of the solar wind, and the lunar surface is exposed to the radiation of the solar wind. When the solar wind flows through the lunar surface, it interacts with the surface material, causing helium-3 to attach to the lunar surface. Because the moon has no atmosphere and magnetic field, helium-3 can be captured and accumulated on the lunar surface without interference.

Figure 5. The fusion of two helium-3 atoms produces a large amount of energy.

(Image source: Quantum Science)

Scientists are also studying how to collect helium-3 from the moon as a future energy resource. But developing helium-3 from the moon is a challenging task, mainly due to the following aspects:

1. The technical difficulty of obtaining samples is high: To collect helium-3 on the lunar surface, it is necessary to build mining facilities on the lunar surface and use sophisticated robots and equipment to collect samples. This technology requires highly autonomous robotics and precision control technology, which is still a huge challenge for the current technical level.

2. The cost of collecting and transporting helium-3 is high: Even if helium-3 can be successfully collected, the cost of bringing it back to Earth is very high. This requires the use of expensive transportation technology and equipment, such as space vehicles and return capsules.

3. Technical difficulty and cost increase the risk of helium-3 mining: The process of helium-3 mining also needs to solve many technical problems and safety hazards, such as radiation, temperature changes, dust and electromagnetic interference in the lunar environment.

In summary, mining lunar helium-3 is a technically difficult, costly, and risky challenge that requires us to overcome many problems before we can successfully achieve it.

Helium: A versatile element for aerospace, nuclear physics, and more

Helium plays an important role in the aerospace industry. It is widely used in gas buoyancy devices such as balloons, airships and satellites. Since helium has an extremely low density and is about 7 times lighter than air, it can provide strong buoyancy, allowing these devices to float in the air or enter space outside the atmosphere. In addition, helium can also be used in the gas supply system of spacecraft to ensure the normal operation of the spacecraft.

Secondly, helium is also an important material for nuclear physics research. Helium can be used as a refrigerant in low-temperature physics experiments, helping researchers study superconductivity, quantum mechanics, and physics of various substances. At the same time, helium can also be used as a coolant in nuclear magnetic resonance imaging equipment to ensure the normal operation of the equipment and high-precision imaging effects. In addition, helium has many other applications. For example, helium can be used as a coolant for helium-neon lasers to improve the efficiency and life of the lasers.

Helium has many other special properties. For example, when helium-3 and helium-4 are mixed together, they can lower the temperature to close to absolute zero. When the temperature is below 2.6 milliK, liquid helium-3 will show a "superfluidity" phenomenon, that is, it has no viscosity and can "crawl" out of the cup that holds it. Superfluidity is a very strange phenomenon that can not only help us better understand quantum phenomena in nature, but also let us see the broad development space and application prospects in this field.

Figure 6. Liquid helium in the superfluid phase will slowly climb up the inside of the cup, climb over the cup mouth, then slowly slide down the outside of the cup, gather together to form a drop of liquid helium, and finally drip into the liquid helium below. In this way, the liquid helium will drip drop by drop until the cup is completely empty.

(Image source: Wikipedia)

In addition, helium can also be used in high-voltage experiments, welding, and other high-tech fields such as semiconductor manufacturing and optical manufacturing. It should be noted that despite the wide range of applications of helium, the reserves of helium are very limited. At present, the main sources of helium are a few countries such as the United States, Russia, and Australia. Therefore, we should use helium with caution and reduce waste as much as possible to ensure the sustainable use of helium.

The magic of helium: the science and risks of pitch-shifting voices

Helium is also a very interesting gas because its speed of sound is three times faster than that of air, through which human voices are usually transmitted. When the composition of air changes from 78% nitrogen and 21% oxygen to 80% helium and 20% oxygen, the density becomes one-third of normal air. This change in composition causes the speed of sound to propagate nearly three times faster, so the voice of a person who inhales helium becomes higher frequency.

This is because the pitch of the sound is related to the vibration frequency of the vocal cords, and helium changes the resonance state of the human vocal cords, making them emit a higher frequency, which also has its unique value for entertainment and performance. Although this phenomenon may be fun, inhaling helium is risky because excessive inhalation of helium may cause hypoxia, which is harmful to health. Don't try it on your own.

Exploring the mystery of helium: From natural gas to uranium ore, four ways to obtain it will make you shine

Helium is an extremely important element with a wide range of applications in industry, medicine and science. So how do we obtain this precious gas?

Currently, there are four main ways to obtain helium.

The first is the natural gas separation method. This method uses natural gas containing helium as raw material, and finally obtains pure helium through steps such as liquefaction fractionation and activated carbon adsorption purification;

The second is the synthetic ammonia method. In the industrial production process of synthetic ammonia, the tail gas contains helium, which can be obtained by separation and purification;

The third method is air fractionation. This method is to extract helium from the neon-helium mixture by fractional distillation from liquid air;

Finally, there is the uranium ore method, which involves roasting uranium ore containing helium to separate the gas, and then removing impurities through chemical methods to finally obtain pure helium.

Although these methods are different, they can all effectively obtain helium. With the continuous advancement and development of science and technology, we believe that there will be more and more efficient ways to obtain helium, so as to better meet human needs.

Protect helium resources and be the helium guardian around you!

In the medical field, nuclear magnetic resonance (NMR) technology requires very strong magnetic fields to achieve high-precision experiments, which usually requires the use of superconducting magnets. Superconducting magnets need to work at very low temperatures and usually require the use of liquid helium for cooling. Liquid helium has a very low boiling point of about -269°C, which can cool the magnet to near absolute zero, which can reduce the resistance of the superconducting magnet to zero, thereby achieving higher magnetic field strength and stability.

At the same time, liquid helium also has good thermal conductivity, which can quickly dissipate the heat generated by the magnet, thereby maintaining the stability of the magnet. The use of liquid helium also brings certain challenges and costs. Liquid helium is a rare resource, with a high price, and requires special equipment and technology for storage and transportation. Therefore, NMR instruments using liquid helium refrigeration are usually high-end scientific instruments used for high-precision scientific research.

However, due to the shortage of helium, Harvard University physicists Amir Yakobi and Philip Kim shut down about half of their laboratory projects in the summer of 2022 due to the shortage of helium supply. At the same time, the University of California, Davis also reported that one of its helium suppliers cut its quota by half, including for medical use. Some hospitals have to stop using MRI machines for examinations, which will be a very unfortunate thing for patients.

In scientific research, helium is often used as a refrigerant to keep the experimental environment low. In the aerospace and military fields, helium is also widely used to manufacture rocket and missile thrusters. Helium also plays an important role in scientific research, aerospace and military fields. my country is a helium-poor country. At present, most of the helium is imported. Therefore, it is imperative to promote and apply technologies for saving and recycling helium, strengthen supervision and management of helium resource consumption, and increase public awareness and participation in the protection of helium resources.

As ordinary people, we can also do some small things in our daily lives to protect helium resources. For example, we can buy fewer or no helium-filled balloons, because once these balloons float into the air, they will release precious helium into the atmosphere and cannot be recycled. The next time we go out with our children, we can try some more interesting activities to avoid wasting precious helium resources.

After all, helium is not only used to blow up balloons, it can also make aircraft take off, make nuclear magnetic resonance imaging more accurate, and make radio telescope observations clearer. Let us work together to protect helium resources and allow it to play a greater role in scientific and technological development, medical diagnosis and space exploration!

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