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.

The six-tailed comet, and other mysteries

Comets are one of the spectacles of the solar system and some only pass by in view of the Earth every few thousand years (Comet Hale-Bopp is only in view of Earth every 2,500 years). At the end of 2013, astronomers observed the marvels of both Comet ISON and a “pseudo” comet with six tails! On Monday, ESA's Rosetta mission will wake from hibernation to continue its mission to orbit and land on a comet. This week, TWDK's physics editor Cait has interviewed Nick Howes, the Pro-Am Programme Manager for the Faulkes Telescopes in Hawaii and Australia. Nick is also an active amateur astronomer, with a particular focus on comets and other solar system bodies.

The main tail of a comet that you see in the sky is caused by the ice of the comet subliming (turning straight from solid to gas) as the comet approaches the Sun. This sublimation of ice also lifts dust of the surface of the comet (this nucleus is generally only a few kilometres in size) which then streams away to form a dust tail that is millions to hundreds of millions of kilometres in length. Looking closer, astronomers also observe another ‘ion’ tail - a tail of ionised gas. It is thinner and has a slightly different colour. The dust tail often appears curved as it is a trail of dust left behind in the comet’s path whereas the ion tail is straight as the charged ions are pushed outwards in the solar wind. Both tails always face away from the Sun, so the tails can appear to proceed the comet when it is travelling out of the solar system.

With Comet Lulin here in 2009, we sometimes also see an “Anti-tail” which looks like it's facing towards the Sun, but this is an optical illusion caused by line-of-sight effects with the comet.  Then in some rare comets, we may also "see" a third main tail, which is a sodium tail. "See" being specialist filters on large telescopes. This was notable in Comet Hale-Bopp in the late 1990s. Image credit: Wikimedia commons

The majority of asteroids in our solar system orbit the Sun in a belt between the orbits of Mars and Jupiter known as the “main belt” and were once thought to be the leftover ingredients for a planet that failed to form, but have a combined mass much smaller than would be necessary for this to be the case. Exactly what asteroids are made of and how many are out there, are questions that scientists are still working on - but they’re mainly rocky or metallic bodies or piles of rubble, sometimes with an icy coating. Comets, on the other hand, are generally thought of as ‘dirty snowballs’, and are more ice than rock. Regular comets come from the outer solar system, in the Kuiper Belt/Trans Neptunian zone or from the Oort cloud, and in the latter case in long, looping orbits that take extremely long periods of time - hundreds or thousands of years, or even longer.

Juno - halfway there and home again

Artist's impression of the Juno spacecraft near the planet Jupiter. Image credit: NASA/JPL

On August 12 2013, NASA's Juno Spacecraft reached the halfway point on it's journey to Jupiter. Since launching back in 2011, it has travelled over ten times the distance from the Earth to the Sun, and has performed a series of planetary flybys and deep space manoeuvres. Tomorrow on October 9, Juno will come within 350 miles of Earth's surface. This is known as a gravity assist, or a gravitational slingshot. After Juno says its farewells to planet Earth for the final time, it will race towards the Jovian system before its slated arrival time of 22:29 EST on July 4, 2016, give or take a few minutes!

But what has Juno done since launch? Well, the Jovian explorer has been sent out past the orbit of Mars, and performed crucial Deep Space Maneuvers to set itself up for tomorrow's flyby of Earth.

Juno Mission Project Manager Rick Nybakken explains further;

"On Oct. 9, Juno will come within 347 miles (559 kilometers) of Earth. The Earth flyby will give Juno a kick in the pants, boosting its velocity by 16,330 mph (about 7.3 kilometers per second). From there, it's next stop Jupiter... Almost like a second launch for free!"

Juno's path to Jupiter. Image credit: NASA

Confused? Well, think of it this way: if a golfer putts a ball towards the edge of the hole, and the ball does not fall into the cup, instead hitting the very edge of the hole and "lipping out", the ball will shoot off in another direction at a faster speed than before. You got the hang of this? Right, let's move on.

However, it is also worth noting that as well as this extremely predictable increase in speed, the spacecraft also experiences a tiny tiny change in velocity due to something else. But just why do spacecraft that flyby Earth receive this tiny change in acceleration? Well, to put it simply, no one is really quite sure! This is known as the Flyby Anomaly, and it's something that science doesn't really have an answer for at the moment.

What Causes the Flyby Anomaly? 

Scientists have theorised (and later dismissed) this unexpected source of energy as being caused by atmosphere, tides, magnetism or radiation. The possible remaining solutions for this problem include that there might be a halo of dark matter around the Earth, as well the theory that it is caused by the rotation of the Earth itself.

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.