Comet Chemistry

Dirty snowballs, snowy dirtballs and the leftovers of solar system formation. These names are commonly used to describe the often visually stunning ice extravaganzas that streak across our skies from time to time - comets. These somewhat simplistic names mask the importance of comets as repositories for material from which the planets formed. Comets contain volatile chemicals - materials that vapourize at relatively low temperatures, such as water, carbon dioxide, ammonia, and methane. This means comets have very limited lifetimes - they get vapourized and lose a lot of their mass each time they pass close to the Sun - so we know that each one we see is relatively new to the inner solar system, giving astronomers a window through which to observe the elemental and isotopic evolution of the early solar system.

The Sun emits light across much of the spectrum, from the blues, greens and reds we can see down into the infrared and up into the ultraviolet. When that light reaches an object, like a comet, some wavelengths of light get absorbed while others are reflected, depending on which materials are present. Similarly, certain molecular interactions also emit light but only at certain wavelengths. This leads to the creation of “spectral lines”. However, even the Sun’s spectrum contains thousands of absorption lines, known as the Frauenhofer lines.

Different elements and chemicals absorb and emit light at different wavelengths, so by analysing these spectral lines we can determine which materials are present. (Public domain image)

Current methods for determining the composition of a comet from the relative comfort of the Earth rely on the analysis of these spectral lines, but this approach is hampered in a number of ways. Firstly, there is the problem of the sheer number of spectral lines detected, and the superposition of these lines in relation to one another. Identifying the distribution of one molecule amongst the thousands of lines that can be detected in one observation is a complex and difficult process. This can also limit the detection of rarer chemicals as they can be masked by more prevalent compounds. Interference from the Earth's atmosphere is also an issue when ground based telescopes are used. The atmosphere can absorb photons at a number of different wavelengths, so some lines of certain species (molecular oxygen and water for example) are not detected at all.

On top of all this, as a fast moving object hurtling towards the Sun, the only volatiles available to study are those located in the comet's coma - the cloud of gas surrounding the solid ‘nucleus’ of a comet. The composition of the coma is assumed to be different from that of the nucleus because the volatile elements vapourize more quickly, so the coma is expected to have more of them and may not reveal some non-volatile elements present in the nucleus. As the relationship between the chemicals observed in the coma and those contained in the nucleus is unknown, abundances in the protoplanetary disk that seeded our solar system cannot be accurately inferred either.

Annotated photograph of Comet Hartley 2, showing the solid nucleus in the centre, the gaseous coma surrounding it, and a faint tail. Comet 103P Hartley was once described as a weird little comet by NASA, due to its high levels of activity.
Photograph ©Nick Howes, used with permission. All rights reserved.

At present, knowledge of a comet's interior is limited mostly to theoretical models. Models suggest that comets are composed of a non-volatile component, usually referred to as dust, and a volatile one consisting of ice^[1]. To date, only two comets have been comparatively well studied; Halley's comet from its 1986 sojourn around the solar system and comet Tempel 1. Comet Tempel-1 was the main focus of the successful NASA Deep Impact mission which provided the first opportunity to look inside a comet.

What is Homochirality?

Since the publication of Charles Darwin’s seminal book On the Origin of Species in 1859, biologists have been looking back through the evolutionary chain to figure out what early life forms all species are descended from. From the mid-20th century, great advances in molecular biology have allowed us to determine what the earliest single-celled organisms were like, but how these life forms developed - the origin of life itself - is still largely a mystery.

Deoxyribonucleic acid (DNA) is a molecule
which serves as the genetic blueprint
for all known living organisms.
Image credit: ynse (Creative Commons)

There are many questions yet to be answered about the origin of life. What were the first self-replicating components - DNA, RNA or something else entirely? What materials and conditions on the early Earth were present to build these components? And why is it that these and the other building blocks of life are all homochiral - that is, why are their molecules all structured in the same way, and not a mixture of the different ways possible?

To understand homochirality, we first need to look at the most basic building blocks that make up all lifeforms. The body of any living creature is made up of molecules – groups of atoms bonded together in a useful way. These range from the very small, such as water made from only three atoms, to the very large, such as the DNA strands that make up our genetic code.

Many of the big molecules in living creatures are made of smaller units joined together: starch, a carbohydrate found in food, is made from the sugar glucose, and proteins are made up of many different amino acids.

Molecules that have the same number and type of atoms but joined together in different ways are called “isomers” of each other. For example, depending on how the atoms are arranged, the C₃H₆O (that is, 3 carbon, 6 hydrogen and one oxygen atom) could be acetone, found in nail varnish remover, or methyl vinyl ether, used to make plastic, or also allyl alcohol used to make glycerol, which is found in many medicines and foodstuffs.

Isomers have the same chemical formula, but different layout of the constituent atoms.