Who can time the solar system? It's hidden in the stone →

In Shizi, written by Shi Jiao, one of the Hundred Schools of Thought in the Pre-Qin Dynasty, it is mentioned that "the four directions and the up and down are called the universe, and the past and the present are called the universe". The universe is a space-time full of curiosity and wonder. In the boundless sea of ​​stars, there is a type of star called Cepheid variable stars, whose brightness changes periodically over time. By studying the relationship between the luminosity and period of Cepheid variable stars, astronomers can reveal the distances between galaxies and star clusters. These magical Cepheid variable stars are known as the "cosmic ruler".

However, simply measuring distances cannot fully tell the story of the universe. To gain a deeper understanding of the origin and evolution of the universe, we need to focus on humanity’s home, the solar system. Astrochemists have a unique “magic weapon” that uses radioactive isotopes to determine the age of rocks and minerals. Different radioactive isotope systems are like natural “timers” placed in the solar system, recording the magma activity on celestial bodies, collisions between celestial bodies, and chemical reactions that occur, allowing astrochemists to decipher the formation and evolution history of the solar system from the dimension of time.

▲Figure 1 Pseudo-color image of zircon from Jack Hill, Australia. This zircon represents the oldest sample on Earth (4.4 billion years old). Image source: Internet

Long-lived isotopes: the solar system's "clock"

Astrochemists have a variety of timers at their disposal, the most commonly used of which is uranium-lead (U-Pb) dating (there are also 87Rb-87Sr and 40K-40Ar, etc.). Taking the uranium-rich mineral zircon as an example, the dating principle is as follows: when zircon is formed, it captures a certain proportion of the uranium element. Natural uranium contains two radioactive isotopes, 235U and 238U, which decay into 207Pb and 206Pb respectively over time at their respective rates (a certain half-life, that is, the time it takes for half of the radioactive nuclei to decay). The two isotope systems 235U-207Pb and 238U-206Pb are actually two independent timers, and by definition, they should give the same age. By measuring the isotope ratios in minerals with a mass spectrometer, it is possible to calculate the formation time of the mineral or rock.

▲Figure 2 Backscattered electron image of baddeleyite in the Antarctic Martian meteorite GRV 020090. This baddeleyite confirms that the youngest volcanic activity on Mars lasted until 200 million years ago. Image source: Purple Mountain Observatory

Zircons (including zircon-containing minerals) in meteorites are small in size, usually only a few microns to tens of microns (for comparison, the diameter of an adult hair is about 70 microns). Zircons have very strong resistance to erosion and are not easily disturbed by later impacts. At the same time, their structure does not easily accommodate interfering daughter elements (such as lead), making them very suitable for uranium-lead dating and can faithfully record their crystallization time in magma.

Zircons, as tiny time capsules, have been widely used in planetary science. For example, the oldest sample on Earth to date comes from zircons found in Jack Hills, Australia, which were crystallized 4.4 billion years ago. Despite the vicissitudes of life, this zircon still gives scientists the opportunity to gain insight into the geological history of the Earth's early formation. Its youngest volcanic activity lasted until 200 million years ago, which was known from baddeleyite in Martian meteorites. The research team led by Academician Li Xianhua of the Institute of Geology and Geophysics, Chinese Academy of Sciences, used the ultra-high spatial resolution technology independently developed to analyze zircon-containing minerals as small as 3 microns (about 1/25 of the diameter of an adult hair) in the samples returned by Chang'e 5, confirming that the youngest volcanic activity on the moon to date occurred 2 billion years ago.

▲Figure 3 Zircon-containing minerals (badgezircon and titano-zircon-thorite) in the lunar basalt of my country's Chang'e 5, confirming that the youngest volcanic activity on the moon so far occurred 2 billion years ago. Image source: adapted from reference 3

Short-lived isotopes: The solar system's stopwatch

The half-lives of the above radioactive isotopes are very long (the half-lives of 238U and 235U are about 4.5 billion years and 700 million years respectively), which can accurately date events that occurred in the solar system over billions of years. However, for events that occurred in the first few million to tens of millions of years of the solar system, the accuracy of those long-lived isotopes is not enough. At this time, nature "thoughtfully" placed another type of timer - short-lived radioactive isotopes. These nuclides decay very quickly and are now extinct, so they are also called extinct nuclides. Although they have long decayed, we can infer that they existed in the early solar system based on their isotope daughters after decay.

If long-lived isotopes are likened to the "clock" of the solar system, then short-lived isotopes can be likened to the "stopwatch" of the solar system. The "stopwatches" widely used by astrochemists include the 26Al-26Mg system (with a half-life of only 700,000 years), 53Mn-53Cr (with a half-life of about 3.7 million years), and 182Hf-182W (with a half-life of about 9 million years). These short-lived radioactive isotopes can accurately date events that occurred in the early solar system, with an accuracy of hundreds of thousands of years.

Scientists used the 26Al-26Mg and 53Mn-53Cr systems to discover the oldest andesite in the solar system (a relatively silicon-rich, sodium-rich, and potassium-rich magmatic rock). Its age is consistent with the earliest high-temperature condensed solid material refractory inclusions in the solar system within the error range, indicating that within one or two million years after the formation of the solar system, planetesimals were able to melt and differentiate into core-mantle-crust and quickly evolve into andesite. It is worth mentioning that short-lived radioactive isotopes can only give relative ages, so they must be combined with long-lived radioactive isotopes to obtain the absolute age of rocks.

▲Figure 4 Schematic diagram showing that within the first one or two million years of the solar system, planetesimals quickly melted and differentiated to form andesite. The achondrite Erg Chech 002 represents the oldest igneous rock in the solar system. Image source: adapted from the Internet

Cosmic ray exposure age of meteorites

When a meteorite is knocked out of its parent body, it roams in space and is constantly bombarded by cosmic rays. Cosmic rays interact with atoms on the surface of meteorites to generate some cosmogenic nuclides, such as 3He, 10Be, 14C, 21Ne, 26Al and 26Cl. By measuring the content and generation rate of these cosmogenic nuclides, it is possible to estimate the time the meteorite has been exposed to cosmic rays, that is, the time the meteorite has been roaming in space. Please note that the 26Al detected in the meteorite at this time is no longer an extinct nuclide, because as long as the meteorite is exposed to cosmic rays, its surface is constantly bombarded by cosmic rays, and 26Al is continuously generated. It only begins to decay and decrease after entering the Earth's atmosphere, and the decay will be completed after about 5 million years. The meteorites collected so far generally stay on Earth for less than 5 million years (those that exceed 5 million years are estimated to have decomposed), so relative to the meteorites formed in the early solar system, the 26Al generated by cosmic rays here is no longer an extinct nuclide.

▲Figure 5 The cosmic ray exposure age records the time from when the meteorite was released from its parent body and wandered in space until it entered the Earth's atmosphere. Source: Internet

The age of meteorites

When a meteorite falls to the surface, it no longer interacts with cosmic rays due to the shielding of the Earth's atmosphere, so cosmogenic nuclides no longer increase. At this time, those cosmogenic radionuclides (such as 14C, 10Be and 36Cl, etc.) begin to decay. By measuring the concentration of these nuclides, the time when the meteorite fell on the Earth can be estimated. The residence age of desert meteorites (meteorites found in the desert) is usually greater than 50,000 years, and some can even reach 250,000 years. The residence age of Antarctic meteorites (meteorites found in Antarctica) can be as long as 2 million years, which shows that the dry and cold Antarctica is like a natural large freezer that can preserve meteorites for a longer time.

▲Figure 6 Desert meteorites (above) and Antarctic meteorites (below). The settlement age records the time when the meteorites fell on Earth. Image source: Internet

Conclusion

Radioactive isotopes are like time travelers in the universe, revealing the history of the universe from ancient galaxies to the formation and evolution of the solar system. These solar system timepieces carry secrets from billions of years ago, allowing astrochemists to interpret the chapters of the universe from the long river of time.

References:

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2. Jiang Y. and Hsu WB 2012. M&PS. 47 (9): 1419–1435.

3. Li QL et al. 2021. Nature. 600: 54–58.

4. Anand A. et al. 2022. M&PS. 57 (11): 2003–2016.

5. Jiang Y. et al. 2023. GCA. 345: 1–15.

6. Kong Ping et al. 2007. Science China: Earth Sciences. 37 (8): 1020–1023.

7. Jull AJT 2006. Meteorites and the Early Solar System II. 889–905.

Source: Purple Mountain Observatory, Chinese Academy of Sciences

Science Communication Research Center of Chinese Academy of Sciences

Editor | Li Sijin

Proofreading | Cao Ruiyue Li Chun

Audit | He Yong