My research focusses on the Earth, other planets such as Mars, the Moon, comets and asteroids – basically a wide range of objects in our Solar System. All of these places have one thing in common, apart from all being somewhat rocky, they can be used to understand how the Solar System formed. My research relies on analysing rocky samples of these Solar System objects in a laboratory, and I get samples from these, often far flung, destinations by a variety of means. Luckily we have huge collections of meteorites on Earth that I can use and I have also been lucky enough to work on samples returned to Earth by space missions too.
Here’s a closer look at some of my more recent laboratory work:
Understanding the early Solar System through the analysis of cometary dust
What do I look at?
Some of my postdoctoral research involves analysing samples from comets to understand the history of the earliest times in the Solar System. The samples I use for this work are pretty special because, coming from comets, they are not easy to collect. However, they sample parts of the Solar System that contain very early-formed primitive components not preserved anywhere else. Despite my samples originating from comets, it isn’t always necessary to go into space to collect them.*
Fortunately space dust (this consists of many things such as tiny pieces of comets, asteroids, planets and even spacecraft debris) literally rains down to Earth. Around 40 tonnes of space dust reaches the Earth’s surface everyday but it’s so small (dust grains are around 20-100 microns in size**) that you won’t find it very easily. So, the best way to collect the space dust is to head into the stratosphere. This is something that the NASA Cosmic Dust Laboratory does using high altitude ‘ER2’ aircraft. They install ‘sticky pads’ under the wings of the planes that are opened during flight to trap the dust particles. It’s a relatively simple and cost effective way to sample the Solar System. We don’t require rockets, spacecraft or astronauts. However, flying at 20km altitude is no mean feat in itself.
I have been to the NASA Cosmic dust laboratory in Houston, Texas, to help select and prepare my samples for analysis. The laboratory is ultra-clean because we need to eliminate the contamination of Earth dust in the lab to make sure that we are preparing only the cometary dust. Above you can see the cleansuit I had to wear when I was working in the laboratory (apologies for looking so miserable, I really wasn’t)!
*NASA has been into space to directly collect comet samples only once. This was called the Stardust mission and I have also worked on some of these samples, but it’s not discussed here, I will add some details about this soon.
**a micron is a millionth of a metre. I will mention ‘nanometre’ later and that is a billionth of a metre.
What do I do with the samples?
When I get the samples into the laboratory, back in the UK, I use a number of techniques to analyse them. Firstly I need to look at the dust with a high powered microscope called a ‘Scanning Electron Microscope’, or SEM for short. This allows me to see the particle at a high resolution so that I can understand the appearance and texture of the tiny components they contain. The pictures below show some particles (note the scale bars, these things are tiny!) in pictures taken on the SEM. The right hand picture shows one of these particles pressed onto gold foil. I have to press the particles because I need to produce a flat surface for the final phase of analysis, the NanoSIMS, and this very expensive and sensitive instrument can’t tolerate rough surfaces. More on this in a minute.
The SEM can also provide me with chemical information about the particles and I have to decide whether I think the particles are truly extraterrestrial, because sometimes even Earth created dust can get up into the stratosphere (e.g. volcanic dust or rocket fuel additives). Once I have classified my dust particles I move on to the first stage of analysis, this involves the organic material in the samples.
Measuring organic components in cometary samples
For this procedure I use an instrument called a Raman which is simply a laser system connected to a microscope. It is a non-destructive technique where the laser helps me to find out what type of carbon exists in the sample. Cometary dust can contain up to 40 weight % carbon and it is part of my research to find out where this carbon formed and how it existed in the comet. There is huge debate in the scientific literature about whether comets might have delivered life to Earth and by measuring the organic material present in comet samples we can start to understand the survival of organic material in space. This is an important first step required in characterising the organics.
Destroying the cometary dust!
Unfortunately the final laboratory technique I use is destructive, meaning that the sample is lost (it is physically sputtered away by an ion beam) during analysis. So, once the analysis is completed the particle is gone and I get all the information I need, although it does feel a bit strange destroying a piece of a comet. The technique I use to do this is ‘secondary ionisation mass spectrometry’ (SIMS) and the instrument is called a NanoSIMS meaning that it has a very small (nano-sized) beam that can provide analyses with a really high spatial resolution (i.e. allows us to resolve very small features) and high sensitivity (i.e. we can produce numbers from analysing very little material). The NanoSIMS is built by a company called Cameca and it is a fantastic instrument but can be a little temperamental at times. It weighs about two tonnes and costs in the region of £2 million, but without it I couldn’t do the most exciting stage of my research.
Whilst I destroy my particles I am analysing a whole range of isotopes. An isotope is simply a variant of a chemical element. So if we take oxygen as an example, as this is something I measure and it’s a useful element in the Solar System, then it has three isotopes called 16O, 17O and 18O. They are different because they contain different numbers of neutrons which gives them a different mass, hence the different number. In my comet dust I measure the abundance of the oxygen isotopes which tell me about the silicate portion of the samples (i.e. the rocky parts) and I also measure the abundance of carbon (12C and 13C), nitrogen (14N and 15N) and hydrogen (1H and 2H) isotopes to tell me about the organic material in the comets (and this information is related back to the Raman analyses mentioned above). It’s good to measure the same material in different ways (Raman and NanoSIMS both measure organics) as a comparison, it means you can have even more confidence in your results. Sometimes I can also see presolar grains, these are tiny (100-200 nanometre-sized) pieces of other stars that were hanging around before the Solar System formed, so they are older than 4.5 billion years old and got incorporated into the comets when they were forming. Pretty exciting stuff.
What does all this tell us?
By analysing cometary dust at these really small scales we can start to understand the conditions in the early Solar System. This is because comets were forming when the Solar System was very young (4.5 billion years ago) and as such, they incorporated all the earliest material from the various compositional reservoirs that were hanging about at the time. The important thing about comets, however, is that they have remained in the cold outer reaches of the Solar System since they formed, acting as a deep freeze, preserving all this material. So, whereas the asteroids and planets were chaotically flying all about the inner Solar System close to the Sun, meaning they were heated causing their composition to be altered, comets were not involved in this process.
My research has shown that there is a huge range of different comets in the Solar System and that they all contain material of different compositions. Whereas I found that some of the comets were like the idealised ones I described above, containing only very primitive early Solar System materials, I found others that were more of a mix of compositions containing more processed material like we find more commonly in asteroids. Recent work by other researchers has shown that the distinction between comets and asteroids is not as simple as we once thought, the distinction may instead be simply a continuum of compositions.
Importantly I also find that the silicate (rocky-parts) and organic material in the cometary samples is related. This tells us that the earliest organic and silicate reservoirs were incorporated into the comets at the same time. However, the large range in comet compositions also shows us that comets either formed over a big timescale, or, at different distances from the Sun. In this respect the comet dust samples containing more processed inner Solar System ‘asteroid-like’ components either originate from comets that formed later in Solar System history or they formed a little closer to the Sun. The very primitive looking comet dust samples came from comets that either formed very early on, or, at a much larger distance from the Sun. Unfortunately I can’t currently put exact timescales or distances on these processes but this is something that my future work can focus on answering. In the meantime I am amazed how much information can be gained from such small pieces of dust. So next time you’re doing the cleaning, have a think about the fact that your everyday household dust may also contain pieces of comets!!
You can read more information about this research in my paper published in the scientific journal ‘Geochimica et Cosmochimica Acta’, it’s available online already (you will only be able to view the paper if you have the rights, through a university for example). Here’s some citations to get you started:
Starkey, N. A. and Franchi, I. A. (2013). Insight into the silicate and organic reservoirs of the comet forming region. Geochimica et Cosmochimica Acta 105, 73-91.
Starkey, N. A., Franchi, I. A. and Lee, M. R. (2014). Isotopic diversity in interplanetary dust particles and preservation of extreme 16O-depletion. Geochimica et Cosmochimica Acta 142, 115-131.
Starkey, N. A., Franchi, I. A., and Alexander, C. M. O’D. (2013). A Raman spectroscopic study of organic matter in interplanetary dust particles and meteorites using multiple wavelength laser excitation. Meteoritics and Planetary Science 48 (10), 1800-1822.