Planetary Solids Older than Earth

Abstract

David Stevenson in his Pathways of Discovery Essay “Planetary science: A space odyssey” (11 Feb., p. 997) surveys the role that planetary science has played and is playing in the evolution of the world views held by modern humankind, but he does not include one new window of wide philosophic interest that is so stunning as to contribute to the immense sweep of his canvas. Pieces of small planets fall to the ground (meteorites) containing within them small, solid particles that are themselves older than Earth and older than our entire planetary system. These presolar solids (mostly SiC and graphite) floated amid the interstellar dust and gas that collapsed with the solar cloud in the event that gave birth to our solar system, and they were incorporated intact into the surface debris of the small planets from which meteorites arise. They are recognized by laboratory experiments to contain bulk isotopic compositions of C, Si, Mg, Ti, N, and O and heavy trace elements that are wildly different from the mean composition shared by all of our planetary bodies. Presolar solids came from long ago and far away, and they contain precise new information on the origin of the elements of which our solar system would later be born. Humankind thereby holds in its hands and studies in its laboratories solid samples that substantially predate the birth of our planetary system, but it is, ironically, our planetary system that delivers them to Earth. Stones from a time before there was an Earth, they speak not only of other systems in our universe but of times before our world existed. Their very existence shattered the belief that studying objects of such antiquity on Earth is impossible.