is held together, we now look at different types of matter, their interrelationships, and some of the important consequences for life.
Matter can be found in different forms. The major forms are solid, liquid, gas, and plasma. Which one of these states of matter exists in any given environment is determined by the pressure and temperature conditions. The relationship that describes the state of matter under any given conditions is called the equation of state.
3.12.1 Phase Diagrams
One very common way to show the equation of state is to focus on just the pressure and temperature (although we could also look at relationships between pressure and volume or temperature and volume, but these tend to be less interesting and give us less information). This depiction of the state of matter as a function of pressure and temperature is called a phase diagram. You can see a phase diagram for water in Figure 3.17.
Figure 3.17 A phase diagram for water. The axes are not drawn to a fixed scaling, but they are drawn to exaggerate values of important features of the diagram.
Let's examine some of the main features of this diagram. On the x axis is temperature and on the y axis pressure. Follow the horizontal line shown on the figure from the left to the right. This line shows the change in the form of water at a pressure of 1 atm (our usual experience).
At atmospheric pressure and low temperatures below 0 °C water is a solid. This is consistent with our experience of snow and ice on a cold winter's day. Or more trivially, the reason why we use a freezer to make ice. If we warm the ice, then at 0 °C it undergoes a phase change when it meets the melting curve. The melting curve defines when a substance transforms from a solid into a liquid (or vice versa) for any given combination of pressure and temperature. This is also an experience we are all familiar with when we see ice melting in a drink or on the roads. As we heat it up further, the water undergoes another phase change at 100 °C as it turns into gas when it reaches the vaporization curve. The vaporization curve defines when a substance transforms from a liquid into a gas (or vice versa) for any given combination of pressure and temperature. At this point, water boils. If we continue to heat it to very high temperatures (several thousand K), it would turn into a plasma as electrons are driven off the nuclei (this phase is not shown on the diagram).
There are two features of the diagram to point out. At high temperatures and pressures, there is a point called the critical point, at which gas and liquid become indistinguishable. Matter in this part of the phase diagram is called a supercritical fluid. This state of matter is not used in biological systems, although it has general importance for understanding the behavior of matter. Some exoplanets with high surface pressures and temperatures have been suggested to have atmospheres and surfaces with supercritical water (Chapter 20).
You will also notice that at pressures lower than atmospheric pressure, water boils at lower temperatures than 100 °C. This is consistent with the experience of mountaineers. The higher they go, the lower the temperature at which water boils (making it more difficult to cook vegetables). At the summit of Mount Everest (a height of 8848 m), the boiling point of water is 71 °C. If we continue to reduce the pressure, we hit a point on the graph called the triple point. The triple point is the point at which all three phases of matter can co-exist. You will see that at this point and at lower pressures, if we heat ice it turns directly into gas – it undergoes sublimation. The triple point of pure water is at 0.01 °C (273.16 K) and 611.2 Pa. There is no liquid phase in this region of the phase diagram.
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,