The Philae Lander – Chemistry on a Comet

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You’ve probably heard the great news over the weekend that the European Space Agency’s Philae lander has awoken from its 7 month slumber on Comet 67P, and is once again relaying collected data back to Earth. You might have less of an idea, however, of what this data is, and why it’s important. This graphic looks at some of the chemical information Philae is hoping to collect, and how it will collect it.

Philae is equipped with ten different scientific instruments, accounting for just over a quarter of its weight. Four of these instruments have a particularly chemistry-oriented role. These are APXS, COSAC, Ptolemy, and CIVA. Their names are almost all acronyms, which can make their exact purpose initially obscure, but here we’ll take a look at each in turn, and what they’re hoping to find on Comet 67P.

APXS

APXS stands for ‘Alpha Particle X-ray Spectrometer’. This device is designed to probe the elemental composition of the surface of the comet. Though this is a single instrument, and weighs only 640 grams, it has a variety of functionalities. Firstly, it can carry out alpha back-scattering spectroscopy. It is lowered to within 4 centimetres of the ground, and emits alpha particles from a Curium-244 source.

Alpha particles are essentially a helium nucleus, consisting of two protons and two neutrons, and they can interact with nuclei at the comet’s surface. Most of the alpha particles emitted will not return to the instrument’s detector. However, a small number will collide with the nucleus of atoms in the surface. The nucleus repels the alpha particles, as like them it has an overall positive charge, and the alpha particles are ‘back-scattered’ back towards the emission source, where they can be detected. The energy of the alpha particles that are detected gives information on the nucleus that they rebounded off.

The alpha particles can also induce X-ray emission from atoms in the surface of the comet, as the particles can also knock electrons from inner shells. A higher shell electron will take then take its place, resulting in the emission of an X-ray. The X-ray emitted will have an energy characteristic of the particular element, allowing composition to be determined.

The APXS instrument was functioning in Philae’s first stint of activity on the comet, although only a measurement of its inbuilt calibration target was performed. It’s thought this was because it wasn’t able to make contact with the ground to allow its shutters to open. The team hope that, with the comet nearer to the sun, sufficient energy will be available this time to provide the force required for the shutters to be opened, and for measurements to be performed.

COSAC

COSAC stands for Cometary Sampling And Composition instrument. This instrument is a combined gas chromatograph and mass spectrometer, and its primary purpose is to analyse the chemical composition of soil samples taken from the comet. It could also potentially answer questions about the molecules that may have led to the origin of life.

The samples COSAC will analyse will be taken from the comet surface using a drill, then heated to 600˚C in an onboard oven. Volatile compounds driven off during the heating process can then be passed through the gas chromatograph and mass spectrometer to gain information about their composition and chemical identity. A further goal is to investigate the chirality of the compounds found on the comet.

Chirality is a property of many organic molecules. A molecule is chiral if it cannot be superimposed upon its mirror-image. Our hands are perfect examples of chiral objects, as your left hand and right hand are non-superimposable mirror-images. Amino acids, the building blocks of proteins, show an odd, unexplained preference for their ‘left-handed’ versions on Earth. COSAC will try to detect amino acids on the comet, particularly the amino acid alanine, and ascertain their chirality.

There are a number of potential outcomes – the first, of course, being that COSAC doesn’t detect alanine at all. However, if it does, it might find that the molecules of alanine on the comet are also predominantly left-handed, supporting the hypothesis that comets could have ‘seeded’ life on Earth. Alternatively, it might find an even mix of left and right-handed alanine, suggesting that the preference for left-handed amino acids emerged on Earth. Finally, it may be found to be mostly right-handed, which would provide evidence against comets seeding life on Earth.

Ptolemy

The Ptolemy instrument is fairly similar to the COSAC instrument, in that it, too, is capable of carrying out gas chromatography and mass spectrometry on excavated samples. Its primary objective, however, differs from COSAC, in that it is aiming to probe the ratios of isotopes of different elements.

An isotope of an element is an atom of the same element, with the same number of protons and electrons making up this atom, but a differing number of neutrons. They have the same chemical properties, as these are dictated by the number of electrons an atom possesses. However, due to the different number of neutrons, they do have a different mass. Isotopes occur naturally – for example, 25% of all chlorine atoms have a mass of 37 atomic mass units, whilst 75% of all chlorine atoms have a mass of 35 atomic mass units.

Isotopes of elements tend to have different distributions in different areas as a result of various processes. Similarly, the ratio of isotopes of an element vary in different bodies in the solar system. The ratios of oxygen isotopes contained in rocks on Mars would be noticeably different from those contained in rocks on Earth. Finding out about the isotope ratios on Comet 67P could tell use more about its origin – if they are very similar to those in other solar system bodies, it might imply the comet was formed around the same time, whilst if there are marked differences, they could be either pre-solar system or formed more recently.

CIVA

CIVA stands for Comet nucleus Infrared and Visible Analyser. It actually consists of a set of different instruments. CIVA-P consists of six cameras, designed to take panoramic and stereoscopic images of the comet surface, gaining information on how much solar energy the comet reflects, the topology of the surface, and small-scale changes as the comet approaches the sun.

The part we’re more interested in from a chemistry perspective is CIVA-M. This consists of a miniaturised visual microscope, intended to observe the texture and reflectance of the surface, as well as an infrared spectrometer. The infrared spectrometer will provide more information on the chemical composition of the surface, as different chemical bonds absorb different frequencies of IR radiation. This should allow the molecular composition of the surface to be determined.

What has Philae already discovered?

Of course, Philae has already accrued 57 hours or so of data during its original stint on the comet, before its long hibernation. As such, it’s already made some tentative discoveries. The COSAC instrument has already detected carbon-based molecules, though as yet how simple or complex these molecules are has yet to be discerned. These compounds were detected in the comet’s atmosphere. As yet, the team of scientists behind Philae believe the lander has failed to successfully drill a soil sample, perhaps due to its landing on the comet at an angle.

It’s to be hoped that Philae’s revival allows it to make further observations on Comet 67P which enhance our understanding of the chemical origins of life, as well as the chemical composition of comets in our solar system. Hopefully, this post has provided a little insight into how it may accomplish that.

Thanks go to Dr Mark Lorch for suggesting this as an idea for a graphic.

Edit 03-08-2015: Since I wrote this article, Philae has sent back some data on the compounds it’s detected on Comet 67P. I made a small graphic (below) to show some details on these; you can also learn more about the findings in this Chemistry World article.

Philae's Findings

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