As we mentioned at the end of our previous blog post, there are certain chemicals we know Europa would need to have in order for there to be life.
Detection Of Chemicals
At first it may seem strange to be talking about the chemicals of Europa at all. To date, no probes have skimmed the atmosphere of Europa, let alone landed and taken samples for chemical analysis. Yet here you are about to read about the chemicals that are present. This is made possible by of one of the most basic tenets of quantum mechanics, namely the idea that electrons have quantized energy levels. This means that electrons can only exist in certain regions of an atom, with a specific energy inherent to that region. In order to move from one of these energy levels to another an electron must either emit or absorb a photon whose energy is the difference between the energy levels. The energy, and also color of a particular photon is determined by its frequency. Therefore, if you look at the light emitted by a heated substance, you will see distinct bands of colors indicating the electron transitions occurring. What makes this useful for identifying chemicals is that the energy levels that are available to the electrons and the transitions that occur are completely dependent upon the element or compound. Moreover, each unique compound has its own emission and absorption spectra, the features of which can be detected even amongst a whole moon full of chemicals nearly 400 million miles away. This method has discovered the presence of two important chemicals on Europa: oxygen and hydrogen peroxide.
The Importance of Oxygen and Hydrogen Peroxide
The reason that scientists think that finding evidence of hydrogen peroxide and oxygen on Europa is important is because they are among the best oxidation agents known to exist. Oxidation agents are essential to every form of life that we know exists. They are most important because of their role in redox, or oxidation-reduction, reactions. In a redox reaction one compound takes electrons from another. The compound that gains the electrons becomes reduced, and the compound that loses the electrons is oxidized. Examples of redox reactions include everything from the rusting of iron (where iron is oxidized and oxygen is reduced) to the reaction between glucose and oxygen.
Respiration- Both Simple and Complex
Perhaps the most important redox reactions for living organisms are those involved in respiration. Just like all compounds, organic compounds, such as the proteins and carbohydrates that make up organisms, have energy stored in the bonds that hold it together. Taken as a whole, respiration works a lot like combustion, in which a hydrocarbon is oxidized and energy is realized in the form of heat. However, if fires were constantly starting in our mitochondria, we would have some significant problems. Fortunately, the process is broken into many steps, each of which releases a relatively small amount of energy. In complex multicellular life on Earth, this process is done through a series of increasingly powerful oxidizing agents known as the electron transport chain. The electron transport chain transfers electrons from the organic compound being brought down through a series of complex compounds until it ends up being captured by oxygen, the final electron acceptor.
Perhaps the most important substance for life to exist is liquid water. In the first blog post, we talked about how and why scientists suspect that there is liquid water on Europa. Now we are going to talk about why water in particular is so important.
Anyone who knows anything about electronegativity can tell you that water is a polar molecule. The oxygen atom takes on a partial negative charge (denoted 𝛿-) and each of the hydrogen atoms takes on a partial positive charge (denoted 𝛿+). These partial charges allow water to dissolve countless polar and ionic solids, hence why it is referred to as the universal solvent. Most biological reactions will only happen if all of the reactants are in the aqueous state. There is a tendency for polar compounds to dissolve other polar compounds and for non-polar compounds to dissolve other non-polar compounds, and this allows for the easy storage of water. Non-polar molecules like lipids are used to form membranes in cells that are capable of retaining water.
Water’s efficacy as a solvent is part of why it is so vital to life on Earth. All (known) living organisms contain liquid water in their bodies. Water is used to transport nutrients and other vital substances (e.g. glucose) to the areas of our body in need. Other compounds, such as salts are also transported via water. In most animals, blood forms the primary method of active nutrient transfer in the body, and blood’s ability to transport the nutrients depends on water’s solubility.
Because water contains hydrogen atoms bonded to oxygen atoms, it contains in what is called “hydrogen bonds”, the strongest type of intermolecular force. This leads to a series of interesting and important properties of water. For one, each water molecule is strongly attracted to the other water molecules around it, making it hard for water molecules to break away and evaporate into the vapor phase. This gives water a relatively high boiling point (100°C) for such a small molecule. This means that it would take very serious changes in the environment to cause a phase change for large amounts of water. Hydrogen bonding is also responsible for water’s unusual property of having it’s solid phase (ice) be less dense than its liquid phase (water). The optimal bond lengths for the hydrogen bonds are actually greater than the distances water molecules usually are from each other causing them to expand as they freeze. This allows for the phenomenon of underwater oceans to exist because the less dense ice essentially floats on top of the water. The ice basically thermally insulates the rest of the water from the surrounding freezing conditions. In Europa’s case, the ice acts as an insulating blanket, trapping the internal heat generated by Europa’s movement around Jupiter. This insulated layers allow for the possibility of a liquid ocean existing.
Future missions to explore Europa, like NASA’s proposed Europa Clipper aim to analyze the surface of Europa. The mission would launch a satellite to orbit Europa, performing repeated close flybys of the moon’s surface. Various scientific instruments would be used to analyze the surface and trace atmospheric composition of Europa. High-resolution cameras would also enable exquisitely detailed surface imaging of Europa’s icy outer layer. Additionally, there exists the possibility for radar to be included on the satellite, which would allow NASA to determine the depth of Europa’s surface ice.