Читать книгу Astrobiology - Charles S. Cockell - Страница 37

There is a caveat to the simple picture described above. Not all atoms of the same element have the same number of neutrons. The value of N can vary. Atoms that have the same number of protons but different numbers of neutrons are called isotopes.

As we will see later, isotopes turn out to be very important tools for astrobiologists. Life has a tendency to prefer lighter isotopes since they are ever so slightly more reactive than heavier isotopes of the same element. This means that living things are often enriched in light isotopes of elements. This isotopic fractionation can be used as a signature of life in ancient Earth rocks and it is a means by which one might try to detect life elsewhere.

Later in the textbook, we return to isotopes to consider in more detail how they can be used to search for life. In the meantime, it is worth pointing out that this is another example of how a fundamental understanding of matter is necessary to understand not just biology, but even the technologies one might use to search for life elsewhere.

As carbon is so important in terrestrial biology, it is the first useful example of isotopes. The element has three important isotopes depicted in schematic form in Figure 3.2. There are actually 15 known isotopes of carbon, but these three are of importance in biological processes. Isotopes are often designated by showing the atomic mass number as a superscript on the left-hand side of the element. The three main isotopes of carbon are:

 Carbon 12 (12C): 6 neutrons, 6 protons (a stable isotope, which makes up 98.9% of the carbon on Earth).

 Carbon 13 (13C): 7 neutrons, 6 protons (a stable isotope, which makes up 1.1% of the carbon on Earth).

 Carbon 14 (14C): 8 neutrons, 6 protons (an unstable isotope, accounting for one part per trillion!).

Figure 3.2 Schematic diagram of the three isotopes of carbon. The number of neutrons varies.

Carbon 12 (¹²C) is the most common form of carbon and constitutes more than 98% of carbon in living matter. ¹³C is rarer but, like ¹²C, it is a long-lived stable isotope. By contrast, ¹⁴C, with two additional neutrons, is not stable. It is called a radioactive isotope or radioisotope. One of the neutrons decays into a proton with the release of an electron and the atom becomes nitrogen 14 (¹⁴N). It transforms into a different element because it has now gained a proton which, you'll recall, defines the type of element. This decay has a half-life of 5730 years. In other words, after 5730 years, half of a sample of ¹⁴C will have decayed.

The unstable nature of ¹⁴C means that it is a small proportion of carbon isotopes but, despite this, it turns out to be enormously useful. As it decays with a known rate, it can be used to determine the age of ancient material from living things that contain carbon (“carbon dating”) such as bones. Living things constantly take up new ¹⁴C when they are alive, but once they are dead, they no longer actively take up carbon through metabolism. The ¹⁴C in the now dead organic matter begins to radioactively decay, allowing us to back-track and work out when that decay must have started and thus how old the object is. Later in the book, we see in more detail how radioisotopes can be used to put absolute dates on the fossil and geological record of Earth and other planetary bodies.