Astronomie - From dust to, possibly, life: New experiments show complex astrochemistry on thin ice covering dust grains


Astronomers from the Max Planck Institute for Astronomy (MPIA) and the University of Jena have obtained a clearer view of nature's tiny deep-space laboratories: tiny dust grains covered with ice. Instead of regular shapes covered thickly in ice, such grains appear to be fluffy networks of dust, with thin ice layers. In particular, that means the dust grains have considerably larger surfaces, which is where most of the chemical reactions take place. Hence, the new structure has fundamental consequences for astronomers' view of organic chemistry in space – and thus for the genesis of prebiotic molecules that could have played an important role for the origin of life on Earth.


Schematic figure showing dust grains (in grey) mixed with ice molecules (in blue), as well as the main external influences that facilitate chemical processing in deep space: heat, bombardment by atoms, ultraviolet radiation, and cosmic particle streams (cosmic rays).

Creating complex molecules in deep space is anything but easy. To the best of current knowledge, the natural laboratories in which the necessary reactions take place are interstellar dust grains with icy surfaces. Now, new experimental results by Alexey Potapov of the MPIA laboratory astrophysics group at Jena University and his colleagues demonstrate that, under realistic conditions, the ice layers may well be so thin that the surface structure of the dust grains themselves plays an important role.

This opens up a new field of study: Those who are interested in the cosmic origins of the organic precursor molecules of life will need to take a closer look at the different properties of the surfaces of cosmic dust grains, their interactions with small amounts of ice, and at the role the resulting complex environments play in helping to synthesize complex organic molecules. 

Cosmic precursors of life

When we think about how life, and how we ourselves, have come to be in this universe, there are several important steps, encompassing physics, chemistry, and biology. As far as we know, the earliest biology of our own origins story took place here on Earth, but the same is not true for either physics or chemistry: Most chemical elements, including carbon and nitrogen, have been created by nuclear fusion inside stars ("We are star stuff", as Carl Sagan famously said).

Molecules, including the organic molecules necessary to form amino acids, or our own DNA, can form in the interstellar medium. On the few occasions that probes have managed to analyze cosmic dust directly, namely the Stardust and Rosetta missions, the analysis found complex molecules, such as the simple amino acid glycine. Over the course of the evolution of a planetary system, organic molecules can be transported to planetary surfaces by meteorites and early comets.

How those molecules can form in the first place, in the nearly empty expanses between stars, is not a simple question at all. In outer space, most of the atoms and molecules are part of an ultrathin gas, with hardly any interaction – let alone the interactions needed to build up more complex organic molecules.

Icy dust grain laboratories

In the 1960s, astronomers interested in interstellar chemistry began to develop the idea that interstellar dust grains could serve as "interstellar laboratories," which would facilitate more complex chemical reactions. Such grains, whether carbon-based or silicate-based, typically form in the outer layers of cool stars or in the aftermath of supernova explosions. In a cloud of gas and dust, different kinds of molecules would stick to the (cold) grain, molecules would accumulate, and eventually, interesting chemical reactions would take place. Specifically, it would take on the order of 100,000 years for a dust grain to accumulate a mantle of ice (mostly water ice, but also some other molecules like carbon monoxide). This icy layer would then serve as a tiny cosmic chemistry lab.

Astronomers interested in this topic soon realized that they needed experiments in order to interpret their observations of interstellar gas clouds. They would need to study ice-covered dust grains and their interaction with molecules in laboratories here on Earth. To this end, they would use vacuum chambers, simulating the emptiness of space, as well as appropriate temperatures. Since the assumption at the time was that what counted was chemistry on the icy surface, it became common practice to use ice layers for such experiments, applied to an ordinary surface such as a potassium bromide (KBr) crystal plate or a metal surface. But that, the new results show, can only be part of the picture, at best.

Artificial dust grains and their ice


Electron microscopy images of the artificial cosmic dust grains, at different resolutions (transmission electron microscopy on the left, scanning electron microscopy on the right). Both show the complex, intricate surface structures of the grains, resulting in large surfaces.
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