in Figure 3.15. In this state, the molecules are more widely spaced apart than in the liquid in which the molecules are all jumbled together and more closely packed. The consequence is that ice is less dense than liquid water.
The biological consequences of this behavior of water are important because it means that when water freezes, it becomes less dense and thus floats on the surface of a lake. This allows large multicellular organisms to remain active and alive when the outside temperatures have dropped to below a temperature required to freeze water – the floating ice insulates the water below from freezing. Here we can now see how knowledge of the atomic structure of matter allows for an understanding of phase diagrams.
3.13 Other States of Matter
In the previous section, we looked at gases, liquids, and solids and their interrelations. We explored a few examples of the consequences this can have for planetary sciences and life.
In this section, we look at some other states of matter. These states do not have any role in the structure of life, but they do have an important role to play in the structure of the Universe, particularly stars, their characteristics, and the potential environments around them in which planets might exist. They show how biology makes use of just a subset of the different states of matter in the Universe and that contrary to our everyday experiences, we live in a Universe with many diverse and extraordinary states of matter that exist at much greater extremes of temperature and pressure than are associated with life.
3.13.1 Plasma
Plasma is an important phase of matter, which makes up about 99% of “ordinary” matter in the Universe.
Plasma was first discovered by William Crookes (1832–1919) in 1879, but it wasn't called “plasma” until 1928, when Irving Langmuir (1881–1957) coined the term. It is sometimes called the “fourth state of matter.” Unlike gases, solids, or liquids, plasma has a very large component of ions (Figure 3.20). The electrons in the outer orbitals are stripped away at high temperatures, and the result is a collection of ions and electrons. The free charged electrons mean that plasma responds strongly to electromagnetic fields, which partly explains the complex patterns it can adopt when exposed to such fields.
Figure 3.20 The structure of plasma compared to other states of matter.
Hot plasma is typically at a temperature of thousands of Kelvin. An example is gas in the Sun's atmosphere where high temperatures are able to ionize it. Other examples are plasma generated by atmospheric gases exposed to lightning strikes, or the ionization of atmospheric gases by solar particles, forming the northern lights, Aurora Borealis. Plasma can also be generated at relatively low temperatures. Everyday examples of cold plasma include the glow discharge of gas in neon lighting or fluorescent bulbs, plasmas, which are typically produced at temperatures of ∼300–1000 K.
3.13.2 Degenerate Matter
The Universe contains even more extreme forms of matter. Degenerate matter is, in a simplified way, extremely dense matter (Figure 3.21). Degenerate matter was first described for a mixture of ions and electrons in 1926 by Ralph Fowler (1889–1944). In electron degenerate matter, the material is so compressed that electrons are forced to occupy the lowest energy levels, and they become delocalized from their nuclei. However, as we discovered earlier, the Pauli exclusion principle prevents them from occupying identical quantum states, and this generates a pressure, the electron degeneracy pressure, which inhibits the material from being compressed further. The resulting material has a very high density (∼1000 kg cm−3).
Figure 3.21 Electron and neutron degenerate matter. In electron degenerate matter, the electrons become delocalized from the nuclei of the atoms. In neutron degenerate matter, the electrons are forced to combine with protons to form neutrons.
To understand this type of matter, it is instructive to move from the atomic and molecular scale, which has generally attracted our attention so far, to the astronomical scale. It is within astrophysical objects that we find this material. This also gives us an opportunity to explore some astronomy.
Electron degenerate matter can be found in white dwarf stars. White dwarfs are the final state of low mass stars such as our Sun. Inside a white dwarf, indeed any star, there is a tug of war. On the one hand, gravitational forces have the effect of collapsing the star, but on the other hand, the pressure of the matter tends to prevent this gravitational collapse from occurring. In a white dwarf, this balance is such that the pressures inside the star are sufficient to form electron degenerate matter. The electron degeneracy pressure prevents further collapse. However, there is an upper limit to the mass of an electron degenerate object, the Chandrasekhar limit, beyond which electron degeneracy pressure cannot support the object against collapse under its own gravity. The limit is approximately 1.44 times the mass of the Sun (solar masses) for objects with compositions like the Sun. If we have a higher mass than this, then the star will collapse further.
If we continue compressing matter, the pressure increases to the point where it is energetically favorable for electrons to combine with protons to produce neutrons (Figure 3.21), and neutron degenerate matter is formed. The density of this material is even greater than electron degenerate matter (>105 kg cm−3). This is the material from which neutron stars are constructed. A neutron star has a diameter in the order of one-thousandth of a white dwarf. The interior structure of these objects is uncertain, but one model is shown in Figure 3.22.
Figure 3.22 A hypothetical internal structure of a neutron star.
Neutron stars spin very rapidly with their enormous magnetic fields generating beams of radio or light energy that, if pointing in the direction of Earth, can be detected as pulsars and have a frequency between about 5 and 650 seconds.
Amazingly, even neutron stars have not escaped the attentions of astronomers and planetary scientists as abodes for life. The popular article by Frank Drake, “Life on a neutron star,” published in Astronomy in December 1973 has become something of a classic. This was followed by science fiction stories, for example Robert Forward's books Starquake and Dragon's Egg. These are depictions of the “cheela,” a civilization of tiny beings that live on the surface of a neutron star under its intense gravity. They intervene to help some hapless humans in orbit around their star who are suffering a malfunction on their spaceship. These ideas are fascinating and thought-provoking. However, neutron stars are unlikely places for life. As this textbook progresses, you can consider some of the factors that might cause you to agree or disagree with this statement. You might also like to consider the Discussion Point.
Discussion Point: Can Life be Made from Different States of Matter to Terrestrial Life?
The neutron star dwelling cheela raise an important point about whether life can be made of different states of matter, or even exclusively made from one state of matter. We are made up of solids and liquids, and we exchange gases with the environment. Consider a liquid-only life form. How would information be encoded to allow such a life form to reproduce or repair itself? What about a gas cloud intelligence, such as that found in astronomer Fred Hoyle's science fiction novel The Black Cloud? Would gas molecules be too disordered to allow for information storage, movement, and processing, and how would such an entity evolve in the