Comets: Atmospheres, water, and life?

As you travel out from the centre of the Solar System you initially pass the hot, rocky inner planets, with their dense compositions and silicate surfaces. Continuing past 3 AU (AU is a measurement of distance, short for Astronomical Unit), beyond the asteroid belt you notice that giant planets of gaseous water and carbon dioxide clouds have replaced the silicate surfaces of the inner Solar System, and the temperature is becoming increasingly chilly. Venturing further, you find that you need even more layers than at Neptune! Intriguingly, the bodies zipping by don’t have clouds anymore, but solid surfaces. However, these surfaces aren’t rocky like before, they are composed of dusty ices. You are now amongst an enormous cloud of comets! Collectively, the region beyond Neptune, called the transneptunian region (between 30 and 50 AU), and the Oort cloud (an enormous cloud of bodies between 50,000 and 100,000 AU) contain billions of comets. These comets happily orbit the sun in the incubated outer Solar System unless something, such as a passing planet or star, or impact, changes their orbit, sending them on an excursion to the hot inner Solar System. Just as we noticed on our outwards journey, the temperature changes depending upon distance from the sun. As our perturbed comet nears the inner Solar System ices change to gases, and it leaves a spectacular tail of vapourised ice and dust in its wake. The ultimate fate of a perturbed comet is typically destruction when, after multiple close passes of the Sun all the ices have changed to gases, or the comet impacts the Sun or a planet.

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Figure 1, Image of comet Wild 2 taken from Stardust spacecraft (Source: NASA)

Observations of comets containing amino acids (biologically important compounds), high concentrations of water, and other volatile chemical species, have led to suggestions that they may have brought the water that gave rise to the oceans, the atmosphere, and potentially life to Earth. One way to test this is to compare precise measurements of chemicals in the Earth’s atmosphere with comets. I am in the second year of my PhD project which aims to do just that!

My project involves measuring the relative concentrations of isotopes of the noble gases in a sample of comet Wild 2 (Figure 1) returned by NASA’s Stardust mission, this will help us work out how likely it was that comets impacted the primitive Earth, formed an atmosphere, and brought the prebiotic building blocks of life.

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Figure 2, Silica aerogel used by Stardust spacecraft to gently capture comet samples. Those pores and tunnels also make it an excellent insulator! (Source: NASA)

The Stardust spacecraft met comet Wild 2 in 2004, where it flew through the tail of the comet collecting material in trays containing silica aerogel (Figure 2), before returning them to Earth (Figure 3). Aerogel is a very porous silicate ‘sponge’ with the same chemical composition as glass, arranged in a disorderly network of pores and tunnels. It was chosen due to its ability to slow and stop particles gently and with minimal frictional heating, much in the same way a sponge would gently stop a falling marble. However, this porous labyrinth of holes and tunnels also sucked up lots of atmospheric gases, just like a sponge sucks up water. This atmospheric contamination provides a challenge for working out the composition of gases returned by the comet, how do we know whether the gases are indeed from the comet or are just atmospheric contamination? An additional problem is that this contamination may also confuse the measurements of gases released from dust particles collected from the comet.

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Figure 3, Stardust return capsule being dismantled after returning to Earth full of comet goodies! (Source: NASA)

How can this be overcome? Typically, noble gases such as xenon are extracted from extraterrestrial samples by heating, releasing gases from increasingly heat-resistant materials. Applying this method to an aerogel tile returned by Stardust would release gases from the tiny particles collected from comet Wild 2, but also way more gas from the spongy aerogel, making the determination of small differences in composition very hard.

My project uses a Closed System Stepped Etching (or CSSE) apparatus (Figure 4) to avoid this contamination, a series of gold pipes and vessels with acid inside them. Stardust samples will firstly be exposed to gentle acid steps which will etch away the aerogel, releasing the atmospheric contamination and trapped gases from Wild 2. The more acid-resistant particles will then be etched by more vigorous acid steps, releasing gases for analysis.

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Figure 4, Image of CSSE apparatus (Source: Thomas Lawton)

The results of these measurements will help scientists work out how likely it is that comets could have brought the air we breathe, the water we drink, and maybe even life itself!

We’ve already made some preliminary measurements of aerogel, lunar meteorites, and an acid resistant organic residue found in meteorites (“Q”) using this CSSE technique – keep an eye out for my next post where I’ll explain how it works, and update you on the progress of my work!

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