Posts Tagged ‘June 2016’

Throughout its history, Earth has accreted microscopic dust falling from space. Decelerating from cosmic speeds at the top of the atmosphere, the smallest particles can take weeks to reach the ground, failing a rate of 1 m−2 day−1. Although usually hidden among terrestrial materials, extraterrestrial particles can be collected from select environments and positively identified by their unique properties. Unmelted cosmic dust is often composed of large numbers of smaller silicate, sulfide, and organic components—the preserved materials from the early Solar System. Cosmic dust particles are samples of comets and asteroids and they are important samples of the initial materials that were to build the solid planets.

Collecting cosmic dust is a tricky business! Despite Earth’s surface being showered by thousands of tons of comic dust every year, such dust is quickly lost in a sea of terrestrial particles. Finding the tiny cosmic treasures requires collecting dust from the cleanest environments where the terrestrial particle background is low. The stratosphere can be sampled via high-flying aircraft, whereas sampling cosmic dust from polar regions and the deep sea requires techniques that concentrate the particles. Collection efforts are worth it. Cosmic dust derives from every dust-producing object in the Solar System, including ancient Solar System materials, possibly even interstellar materials, of a type not found in meteorites.

Many cosmic dust particles have escaped the aqueous and thermal processing, the gravitational compaction, and the impact shocks that often overprint the record, in most larger samples, of how Solar System materials formed. The least-altered types of cosmic dust can, therefore, act as probes into the conditions of the solar protoplanetary disk when the first solids formed. Analyses of these “primitive” particles indicate that the protoplanetary disk was well mixed, that it contained submicron grains formed in a diversity of environments, that these grains were aerodynamically transported prior to aggregation, which was likely aided by organic grain coatings, and that some minerals that condensed directly from the disk are not found in other materials. These protoplanetary aggregates are not represented in any type of meteorite or terrestrial rock. They can only be studied from cosmic dust.

Organics are a significant component of most cosmic dust, as revealed from actual samples of extraterrestrial dust in the Earth’s stratosphere, in Antarctic ice and snow, in near-Earth orbit, and in asteroids and comets. Cosmic dust contains a diverse population of organic materials that owe their origins to a variety of chemical processes occurring in many different environments. The presence of isotopic enrichments of D and 15N suggests that many of these organic materials have an interstellar or protosolar heritage. The study of these samples is of considerable importance because they are the best preserved materials of the early Solar System available.

Every year, tens of thousands of tons of cosmic dust accumulate at the Earth’s surface, representing a continuation of the accretion process that started 4.57 billion years ago. The unique geochemical properties of these materials, compared to the Earth’s surface, render them excellent tracers of Solar System, atmospheric, oceanographic, and geologic processes. These processes can be recovered from the records preserved in marine and terrestrial sediments, including snow and ice. We review evidence from these natural archives to illuminate temporal and spatial variations in the flux and composition of extraterrestrial material to Earth, as well as the terrestrial processes that affect the distribution of extraterrestrial tracers in sediments.