When, and from where, do habitable planets acquire their carbon?
Abstract
Like water, carbon is considered an essential ingredient for life. Questions regarding (1) the timing of delivery and (2) the sources of the water in the Earth's oceans have been welladdressed. However, the same questions regarding carbon still remain largely unanswered. Like water, carbon must have arrived in some kind of condensible form; PAHs would have been the most abundant candidate. We are using models for planet formation, for protoplanetary disks, and for nebular chemistry to provide constraints on how and when Earth acquired carbon. Polycyclic aromatic hydrocarbons (PAHs), a refractory and stable family of carbon compounds, are abundant in the interstellar medium. Carbon in meteorites is mostly bound in large PAH- like structures. Models for the chemistry of sooting flames show that typical inner nebula conditions destroy, rather than form, PAHs. Isotopic studies of PAHs in chondritic meteorites agree that these compounds have a presolar heritage. Ordinary chondrites, thought to come from the asteroids located at ∼2.2 AU, contain very little carbon or water. By contrast, the carbonaceous chondrites (which are thought to come from the outer-belt asteroids, ∼3.3 AU) contain ∼10% by weight water of hydration and as much as 5% carbon, much of it in the form of aromatically-bound PAH-like structures. Simulations of the formation of Earth-like planets demonstrate that large amounts of solid material may be transferred from >3 AU to the habitable zone (∼1AU) throughout the planetforming process. The composition of the inner planets is highly dependent not only on the presence of giant planets, but also on their orbital properties. These simulations also show that much of the Earth's water came from what is now the outer asteroid belt (and the source region of carbonaceous chondrites). Stochastic and systematic variations in the accretion of carbonaceous material can alter the carbon and water budgets of terrestrial planets by up to several orders of magnitude, suggesting the possibility of particularly carbon-rich or carbonpoor worlds.
- Publication:
-
37th COSPAR Scientific Assembly
- Pub Date:
- 2008
- Bibcode:
- 2008cosp...37.1616K