Thursday, 22 April 2021

Earth Day and Isotopes

Apollo 8's Earthrise Image (NASA)


Today is Earth Day - one of the highest profile of the special named days that occur throughout the year.  High profile enough, at least, to prompt newspapers to run articles on helping to save the planet.

This is a nuclear physics blog, and I rarely write about the kind of issues Earth Day highlights.  The role of nuclear power as a low-carbon electricity source is probably the most obvious intersection of the nuclear and green worlds, and one that has people more expert than me in energy generation campaigning one way or the other on it.

One lesser-known way that nuclear physics makes an impact on environmental issues is in the use of different nuclear isotopes to help understand natural processes going on on Earth now, and back into the distant past.  

For each element in the periodic table, there are many possible different isotopes.  Each element is characterised by the number, Z, of protons in the nucleus: Z=1 for hydrogen, Z=2 for helium, Z=3 for lithium etc.  But for each element there can also be a different number of neutrons in the nucleus.  Hydrogen, for example, comes in 2 different stable isotopes 1H with 1 proton and 0 neutrons, and 2H with 1 proton and 1 neutron.  

The chemical behaviour is largely the same for each different isotope of an element, because the chemistry is mainly determined by the number of electrons in an atom of the element, and that number matches the number of protons in the nucleus.  But there can be slight differences between isotopes in chemical and other processes.  Isotopes with more neutrons are heavier, and they have shorter bond lengths when making molecules than their lighter counterparts.  These differences are enough to make some physical processes - such as the evaporation of water molecules, or the way nitrogen-rich nutrients are metabolised - happen at slightly different rates depending on the isotopes present.

In the case of water molecules, the rates of evaporation and precipitation of different kinds of heavy water (e.g. with heavy hydrogen and/or heavy oxygen isotopes) happens differently depending on the temperature, and by studying ice cores in Greenland's ice sheets, geophysicists have been able to reconstruct the Earth's temperature back long before scientific instruments began recording temperature.   The nuclear isotope ratios can be used to measure back to around 100,000 years ago - vital information in understanding the development of the Earth's climate to understand where we are today.

In the case of nitrogen isotopes, by looking at the reaction rates of different nitrogen molecues (ammonium, without oxygen, and nitrates, with oxygen), biogeophysicists can see evidence in the fossil record of when the Great Oxidation Event (GOE) took place, in which oxygen was released into the atmosphere and life forms began to develop which made use of it.  This is looking back a bit more than 2 billion years, so back to half the lifetime of the Earth ago.

The existence of isotopes was posited in 1913 by radiochemist Fredrick Soddy, with a range of new isotopes being discovered by mass spectrography in the following years.  Finally in 1932 the existence of the neutron was confirmed and the reason for the existence of different isotpes was understood.   Those involved at the time had no idea, of course, that the work would help us piece together the geohistory of our planet, help us understand our historic climate, and hence help us model how it will develop in future.  Such cases of applications of intitially blue-skies research are the way it goes (so the moral is: fund blue-skies research!)

For more details of the oxygen isotope ratios in ice cores see Frozen Annals by W. Dansgaard.  For the nitrogen isotope ratios going back to far prehistory, see Nitrogen Isotopes in Deep Time by Colin Mettam and Aubrey L. Zerkle

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