The Milky Way is not low fat: grease in space

Article by Michael Burton, Director of the Armagh Observatory and Planetarium

NASA’s Curiosity Mars rover recently discovered organic molecules in 3 billion year old Martian rocks. This material may have been delivered by meteorites and comets to the young planet. Deep space is full of organic matter. Organic matter contains carbon, an element considered essential to life.

There is considerable uncertainty surrounding the abundance of carbon in the cosmos. In the space between stars, only half the carbon that we think should be there can be accounted for as simple molecules, free atoms and ions. The remainder is chemically bound in two main forms, grease-like (aliphatic) and mothball-like (aromatic).

A new study by UNSW Sydney in collaboration with researchers from Ege University (Turkey) and Armagh Observatory and Planetarium (UK), has determined how much interstellar carbon there is in its greasy, aliphatic form. By creating material with the same properties as interstellar dust in the laboratory, the researchers were able to combine magnetic resonance and infrared spectroscopy to determine how strongly the material absorbed light with a certain infrared wavelength.

“We first set out to measure the amount of carbon in grains in interstellar space using the UKIRT infrared telescope in Hawaii”, explained Burcu Gunay, a PhD student from Ege University in Turkey, “by measuring the absorption of starlight at a wavelength of 3.4μm which provides a signature of the presence of organic molecules”.

“Combining laboratory measurements with astronomical observations allows us to measure the amount of aliphatic carbon in interstellar space, between us and the stars” explained Professor Tim Schmidt, from the Australian Research Council Centre of Excellence in Exciton Science in the School of Chemistry at UNSW Sydney.


An aliphatic carbon molecule in interstellar space.  Credit Dave Young

“Carbon is the fourth most abundant element in the Universe, yet it has been unclear how much is locked away within dust particles of interstellar space” explained Professor Michael Burton, Director of the Armagh Observatory and Planetarium in Northern Ireland.

“The evolution of our Galaxy is driven by the cycle of material between the gas and stars. The raw material for star birth has been expelled from previous generations of stars into the interstellar medium. It is then incorporated into new stars as part of a continuous cycle of materials, driven by immense energy flows originating from the stars – a veritable galactic ecosystem.  The element carbon is the key species for probing the life of the gas during this cycle as its dominant form (hydrogen) is invisible for much of it.”

When it is a part of the gas phase, carbon can be studied using high frequency radio telescopes, operating in the millimetre and terahertz bands. A survey for gaseous carbon in our Galaxy is being carried out by Armagh using the Mopra telescope in Australiasee our Astronotes article on Explorers of the Galaxy.

However there is a missing ingredient in this programme: the solid phase of carbon, which is found incorporated in interstellar dust. A considerable amount of carbon is found in the solid phase making dust a significant reservoir for the element.  Much of this “solid” carbon is found in the form of “grease”.

Chemists know these as aliphatic carbonaceous material  (i.e. complex organic molecules that contain –CH2 and –CH3 groups). These are a mixture of aliphatic chains and interconnected polycyclic aromatic hydrocarbons (PAH).  Their presence in space can be measured using infrared telescopes through the absorption they cause to starlight at a wavelength of 3.4μm.  And they can be simulated in the laboratory where their properties be measured, as the researchers did in their Sydney laboratory at UNSW.

The researchers found that there are about 100 greasy carbon atoms for every million hydrogen atoms, accounting for between a quarter and a half of the available carbon. In the Milky Way Galaxy, this amounts to about 1034 tonnes of greasy matter – 10 billion trillion trillion tonnes of fat!

This space grease is not the kind of thing you’d want to spread on a slice of toast, however! It’s dirty, likely toxic and only forms in the environment of interstellar space (and in the laboratory).  It’s also intriguing that organic material of this kind – material that gets incorporated into planetary systems – is so abundant.

The work was published in Monthly Notices of the Royal Astronomical Society on the 18th of June, 2018.

Link to the published article.