Interplanetary Dust: A Sneaky Way to Study Comets.

It is estimated that Earth accumulates around 30,000 tonnes of space dust each year. These dust particles are so tiny, that their movement through the solar system can be affected simply by the light from the sun. The light causes some of the particles to spiral towards the inner solar system (The Poynting-Robertson effect), and any crossing Earth’s orbital path will be swept up into the atmosphere. If Earth or other planets don’t intersect them, they will eventually be destroyed by the Sun. This means that the particles have a relatively short lifespan, typically between 1000 to 100,000 thousand years, so we can infer that they are certainly not just floating around as left-over dust from the formation of the solar system. These interplanetary dust particles (IDPs) can be produced during collisions between asteroids, in the same way meteorites are. However, using models that can trace their trajectory (Nesvorny et al. 2010) and studies of their structure and chemical composition, a different source is now favoured for most of the dust, one which everyone is familiar with; the tails of comets.

Comet Schwassmann-Wachmann-3 was imaged by the Hubble Space Telescope as it broke apart into several large fragments and thousands of dust particles. SW3 orbits the sun every 5.4 years, and models have predicted that we may already have dust from this comet in our IDP collections.

Comets originate from the Kuiper belt (30-55 AU) and the Oort cloud (50,000 AU – nearly a light year from the Sun); regions that are extremely cold in comparison to the region of the asteroid belt which is the origin of most meteorites. Studying IDPs and comparing them to meteorites enables us to build up a better picture of what the solar system and its individual bodies formed from, how the initial material was distributed and how it evolved in its first few million years of existence.

In the 1970s, NASA began flying aircraft up to the lower stratosphere equipped with specialised collectors to sample some of the dust that Earth is constantly accreting. Even at such high altitude, the investigating team, led by Don Brownlee (University of Washington), still had to deal with terrestrial contaminants such as volcanic ash and man-made space debris; such as rocket exhaust particles and chips of metal and paint from various space endeavours. After careful examination however, they soon confirmed that a significant proportion of collected dust was extra-terrestrial (E.T.) material from other solar system bodies.

IDP collector with doors closed, ready to open at 65,000 ft. (Credit: NASA)

Since then, several collections have been made and their particles distributed to labs around the world. IDPs have constantly surprised the planetary science community due to their very primitive nature, and they currently remain the most primitive material in our collections. This means that they have been relatively unchanged since their formation around 4.6 billion years ago.

Their typical ‘fluffy’ appearance is unique amongst E.T. material and might be attributed to a porosity left by melted ice, which previously filled the gaps between the mineral grains.

A typical “fluffy” chondritic porous IDP composed of nanometre-sized mineral grains and organic matter. (Credit: N. Spring)

There is now surmounting evidence that ~80% of these particles are of cometary origin. If this prediction is correct, then it makes IDPs only the second collection of cometary material we have besides the ‘Stardust’ collection, from the comet Wild-2. Although space craft missions are extremely important, collection of IDPs from the Earth’s atmosphere clearly makes for a much cheaper cometary mission!

Here at Manchester, we have been lucky enough to obtain around 50 IDPs from the NASA curation facility and in collaboration with L. Nittler (Carnegie Institution of Washington D.C.). We will be studying the structure, mineralogy and isotopic compositions of their components to search for any extremely primitive organic or presolar material, and finally use the world’s most sensitive noble gas spectrometer, RELAX, to detect any Xenon. The latter will help us understand the distribution and evolution of volatile elements in the solar system, and more importantly how they were incorporated into Earth and the other terrestrial planets.

More about comets, IDPs and extra-terrestrial organic matter to come!