Comet Hale-Bopp. Photo: Courtesy NASA/JPL

Comets, the icy wanderers that occasionally grace the inner Solar System, have long been considered the oldest and most pristine components of the primordial material out of which the planets and their cortege of satellites were spawned. Now, new research has cast fresh doubts on this long-cherished impression. In a paper published in the April 27 edition of Nature, the American planetary scientist Scott Messenger, at Washington University, has unearthed new and provocative evidence suggesting that a class of interplanetary dust grains (IDPs) might well have pre-dated the planetary system.

IDPs are a diverse assemblage of dusty material, with typical sizes between 0.1 and 1.0 microns (a micron is one millionth of a metre), that have retained much of their volatile inventory in the form of water ice and other ices. And while this material continually rains down on the earth, it is only in the upper atmosphere, particularly in the stratosphere, that unaltered IDPs are routinely retrieved.

Messenger chemically analysed a variety of IDP specimens, including isolated dust grains and cluster IDPs, comprising of many, loosely-bound grains. After analysing 40 individual IDPs and 28 cluster IDPs, Messenger showed that the latter were considerably more enriched in deuterium - a 'heavy' species of atomic hydrogen, containing an extra neutron in its atomic nucleus - relative to isolated individual IDPs.

Deuterium is a sensitive indicator of the physical environment in which the grains formed. In particular, because its formation and concentration inside dust grains is favoured in very cold environs, deuterium tends to become enriched in the coldest parts of space, particularly cool molecular clouds - the cocoons out of which new stars emerge - where temperatures can be as low as 10 K.

But the biggest surprise came when Messenger compared the relative abundance of deuterium in comets, primitive meteorites and cluster IDPs studied to date. Though IDPs exhibited a larger overall range in deuterium enrichments, they were often up to ten times more enriched in deuterium relative to cometary material. Indeed, the enrichments are consistent with the idea that cluster IDPs may have pre-dated the Solar System and may even have been derived from a variety of cool molecular clouds that brushed by this neck of the Galaxy before the Sun was even a distant idea.

More work will undoubtedly serve to establish the precise age of these tiny and ancient interstellar caravels.