The Link Between Comets and Biology

For several  months, Comet ISON was the center of attention in the scientific community, and for a good reason. After a journey that lasted almost a million years, ISON put on a spectacular show right up until its final demise on November 28, 2013.

 

And while ISON did not exactly live up to its fame as being the “Comet of the Century”,  as the NASA infographic below demonstrates, it was one of the most closely watched and studied comets in history.

image of NASA assets that studied ISON

courtesy of NASA

This is important, not only for astronomy, but for the study of biology and evolution.  We know  that comets have played an important role in the evolution of life on our planet. So, as one last tribute to ISON, here are a few things that you should know about comets, biology, and the evolution of life.

Comets Played an Important Role in Delivering Water to the Early Earth

The conditions of the early Earth did not include the presence of large quantities of water. The earth was too hot for water to condense. Yet, now our planet is almost 70% covered by liquid water, and it is estimated that there are almost 13,000 cubic kilometers of water in the atmosphere at any given time.  So, where did this water come from? The answer appears to be  the bombardment of the earth by comets.

But how can you trace the origins of water? The answer is by studying the ratios of radioactive isotopes in the hydrogen atoms of water  molecules (need a reminder of isotopes? – check out our animation). For example, one isotope of hydrogen is called deuterium. Unlike normal hydrogen, deuterium possesses a single neutron, which is why it is sometimes called “heavy hydrogen”. By measuring the ratios of deuterium atoms to regular hydrogen atoms, scientists can generate an atomic “fingerprint” of water molecules.  The water in comets shares this deuterium fingerprint with the water on earth – suggesting that at least some, if not the majority, of earth’s water was delivered by comets.

However, it should be noted that not all comets have the same fingerprint, and that comets the originate from the Oort Cloud (which sits well outside of Pluto’s orbit) do not possess the same fingerprint, but those that originate from the Kuiper Belt (located outside the orbit of Neptune) do. Why? No one is really sure, which is why  studies of ISON (an Oort Cloud comet) have been so important.

Comets  Most Likely Delivered Many of the Organic Compounds Needed for Early Chemical Evolution

The basic building blocks of life as we know it are the organic molecules. Organic simply means that they contain carbon-hydrogen bonds. You know these as proteins, carbohydrates, lipids and nucleic acids. Of these, the ones that interest scientists the most, with regards to comets and early life, are the proteins and their building blocks, the amino acids. Why? Because in order for life to begin there  first needed to be a method of assembling molecules from other molecules. This is typically done by organic molecules that have a catalytic ability – such as the enzymes,  and the majority of enzymes are proteins ( although a few are forms of nucleic acids). As this synthesis process occurs, new compounds are formed with new abilities. This is the basis of chemical evolution, an important precursor to the formation of early cells (called proto-cells), and, eventually, biological evolution.

 

Therefore, a lot of attention has focused on the presence of organic materials, such as amino acids, in comets. Two recent studies shed some light on this process:

  • Researchers at UC Berkeley produced dipeptides (which consist of two amino acids) by bombarding a combination of  small hydrocarbons (such as methane), carbon dioxide and ammonia with high energy electrons under conditions that mimic those in deep space (vacuum, intense cold). They were successful in producing glycine-glycine and leucine-alanine dipeptides under these conditions.
  • In another study, researchers at the Imperial College London tested whether the energy from the impact of comets or large meteors would be able to drive the formation of amino acids. The study involved taking simple organic compounds that are know to be found in comets, and bombarding them with steel projectiles to simulate the energy of impact. The team was successful in producing the amino acid alanine and other amino acid precursors.

Organic materials are found elsewhere in the solar system, such as Saturn’s moons Titan and Enceladus. The origins of these molecules is still being questioned, but it does seem that comets played a potential role in delivering these compounds to the early earth.

Comets May be Linked to Some Mass Extinction Events

While comets may have played an important role in the development of life on earth, they have also probably been a key factor in some of the mass extinctions that have occurred during earth’s history.How would this happen? Most scientists agree that if a large enough comet or asteroid struck the planet, a significant amount of debris and dust may be ejected into the atmosphere. This would reduce the amount of sunshine reaching the surface of the planet, which would have the effect of not only dropping the average global temperature, but more importantly, restricting the amount of photosynthesis. The subsequent reduction in plant material, especially if the debris remained in the atmosphere for a considerable amount of time, would reverberate through the food chains, and could potentially cause the extinction of many species.

Has this happened in the past? Here are some of the mass extinction events in which comets are suspect:

  • Creataceous extinction event (66 MYA). This is the most famous of the extinction events, since it led to the demise of the dinosaurs. While often attributed to an asteroid impact, some scientists contend that a large comet may have produced the same results.
  • Permian Triassic Extinction (250 MYA). An asteroid or comet impact is believed to have caused the extinction of almost 90% of all life on the planet. However, no impact crater has been found, and several other geological hypotheses are also being investigated.

How will we know if the points presented here are true? Basically, we need to expand our understanding of comets. Comet ISON generated a considerable amount of attention, but far more ambitious studies are already in the works.  For example, in 2014, the European Space Agency’s Rosetta project will begin a mission to study, and eventually land on, Comet 67P/Churyumov-Gerasimenko. The video below outlines highlights of this project:

What will we find? At the moment, we are not sure, but most scientists would agree that the data obtained from the study of comets, such as Comet ISON and  Comet 67P/Churyumov-Gerasimenko will greatly enhance our understanding of the processes that may have influenced the early processes of chemical evolution on our planet.

 

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