Читать книгу Astrobiology - Charles S. Cockell - Страница 58

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.