3.12.2 Phase Diagrams and Mars
A good way to understand phase diagrams is to apply them to a real environment. Furthermore, this exercise can also illustrate why fundamental knowledge about matter is important in astrobiology and for investigating other planetary bodies. To do this, let's travel to Mars. The image in Figure 3.18 was taken by NASA's Mars Phoenix Lander, which landed on Mars in 2008 in the north polar region (68.22 °N, 234.25 °E). Water ice was exposed by the robotic arm scoop, which removed some of the surface dust. After four days (or four “sol,” a Martian day), the ice began to disappear. It was vaporizing. But no liquid water formed. This tells us the atmospheric pressure on Mars must be at the triple point or lower. In fact, the mean atmospheric pressure on Mars is approximately 600 Pa, near the triple point.
Figure 3.18 Ice exposed by the robotic scoop at the Phoenix landing site in the north polar region of Mars. Water ice exposed on day 20 (“sol.” 20) had begun to sublime (white arrows) by sol 24. The inset images show a close-up of the bottom left corner of the main images. Three large fragments of ice can clearly be seen to have sublimated over the four days.
Source: Reproduced with permission of NASA.
However, Mars hasn't always been like this. There is plenty of evidence that, in the early history of the planet, there was much more liquid water on the surface. Valley networks, dried lakes, and other features suggest that persistent bodies of liquid water could form on the surface over at least the first billion years or so of the planet's history. We examine more of this evidence later in the book. This evidence tells us that early in the history of Mars, the atmospheric pressure must have been higher to have allowed for the liquid state of water to have existed. Thus, Mars has lost much of its atmosphere since its very early history. We could then ask why the atmosphere was lost, a question we explore in more detail later.
This discussion is sufficient to illustrate why phase diagrams are an elegant way of understanding the different states of matter and how these states change with varying pressure and temperature. Phase diagrams have great explanatory power, as they can tell us about how conditions on planetary scales have changed over time. As liquid water is one essential requirement for life, applying the water phase diagram to Mars allows us to understand how the habitability of Mars has tracked pressure and temperature conditions on the planet, and the subsequent consequences for the stability of liquid water. Phase diagrams allow us to relate geological and biological observations to physical principles.
3.12.3 Phase Diagrams and Life
We might also think about the consequences of the phase diagram for life more directly. Have a look at the melting curve in Figure 3.17 – that is the curve between the solid and liquid regions of water above the triple point.
The line has a negative gradient – it goes from right to left as we follow it upwards in pressure from the triple point. This is unusual. Most materials have a positive gradient (Figure 3.19). This is caused by the fact that solid water (ice) is less dense than liquid water. For most substances, if you take a point in the liquid portion of the diagram near to the melting curve and imagine pressurizing the material (i.e. moving up the y axis), it will transition into a solid. In other words, if you pressurize the liquids of most substances, they get denser and solidify.
Figure 3.19 A simple schematic of a phase diagram for a “typical.” substance. It illustrates the positive melting curve gradient.
However, with water, if we pressurize its solid phase, the density increases, and it turns into liquid water. In other words, now take a point on the water phase diagram in Figure 3.17 in the solid region near the melting curve and move up the y axis. It turns into liquid. The reasons for this behavior lie in the hydrogen-bonded networks of water (review Figure 3.15). In liquid water, the molecules all intertwine and move around in their fluid state. However, when frozen, the molecules move into a more regular aligned network, crudely illustrated in just two dimensions 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
