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The search for water on Mars
The search for water on Mars

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Specific heat capacity

Another effect of hydrogen bonding between water molecules is that it takes a lot of energy (supplied as heat) to boil or evaporate water, i.e., to break the bonds between the water molecules. Water is therefore described as having a high specific heat capacity, where the specific heat capacity is a measure of how much energy it takes to raise the temperature of 1 kg of a particular substance by 1 °C. This makes water fairly stable over a wide temperature range and it helps to protect cells, which contain water, even if external temperatures are high.

Water’s specific heat capacity also allows large bodies of water, such as oceans, to absorb a lot of heat. Conversely, landmasses have low specific heat capacity and heat up more than oceans. Oceans can absorb the heat of nearby landmasses, moderating the temperature and - on a planetary scale - the climate. This allows ecosystems to thrive that would otherwise suffer from large temperature fluctuations. Although it takes longer for water to heat up than other substances, when water evaporates, the cooling effect is efficient. This is why forests are cooler in the summer (from the evaporation of water from the leaves) and why we sweat when our bodies need to cool down.

The transition of water between liquid, gas and solid (ice) is not only dependent on temperature, but also pressure – in particular the pressure of the atmosphere. Figure 10 shows which phase of water might exist at any given temperature and pressure.

This is a diagram illustrating the different phases that water can take. It is an x-y graph, with temperature (in degrees Celsius) on the horizontal (x) axis and pressure (in mega pascals) on the vertical (y) axis. The x and y axes cross at approximately -75 degrees Celsius 0 mega pascals. The x axis reads from -75 to 150 degrees Celsius and 0 °C and 100 degrees Celsius are labelled. The y axis has a logarithmic scale and reads from 0 to 1 mega pascal, but only 0.1 mega pascals is labelled, about half way up the axis. The diagram is divided into three areas by three lines. Central to this is a point at about one quarter above the x-axis at 0.06 mega pascals and about one third to the right of the Y-axis at 0 °C. This is labelled the triple point of water. From the origin of the graph to the triple point a concavely curved line divides two areas – one to the right of the line labelled gas (steam) and one above the line labelled solid (ice). The line ends at the triple point. From there, another line extends, also concavely curved through a point marked at 0.1 mega pascal and 100 °C up towards the top of the diagram. This line divides the area labelled gas (steam) from an area labelled liquid (water). A third line extends from the triple point to the top of the diagram, running slightly back towards the y axis. This line divides the areas labelled solid (ice) on the left of the line from the area labelled liquid (water) to the right of the line.
Figure 10 Diagram showing the temperatures and pressures at which different phases of water can exist.
  • Look at Figure 10 and refer back to Table 1 in Section 1. Considering the average temperature and pressure on Mars, do you expect water to be solid, liquid or gas?

  • The average temperature on Mars is -63 °C and the average pressure is 75 mbar. Reading from Figure 10, this would suggest that any water on Mars would be ice (a solid).

The likely presence of ice on Mars is still significant. When water freezes, its molecular structure becomes ‘frozen’ into a uniform, three-dimensional arrangement. Figure 11 shows that this structure has a repeating pattern, characterised by large hexagonal open channels. This structure means the molecules are more spread out than in liquid water, so ice is less dense than liquid water and can float. This is very important for life on Earth because it protects the water below the ice from cold air temperatures. Water under the ice stays liquid, providing life a more clement range of temperatures in which life can survive. As you will see later, ice on Mars may also protect bodies of liquid water that could be significant in the search for life.

Whole molecules are represented as blue spheres (i.e. H and O are not shown separately).
Figure 11 Schematic diagram showing the molecular structure of frozen water. When the water is frozen solid, the molecules fit together in a rigid structure.

The ‘openness’ of the ice structure also has another important function - it can allow other molecules, such as methane, to become trapped. On Earth such systems - called clathrates - have been found underneath ocean floors. Methane is a molecule that can be generated by life (and also by natural processes), so finding these on Mars has been a focus of some exploration missions looking for evidence that life was once present.