Organic material in planetary systems : nature or nurture ?

Organisateurs : Cristina Dalle Ore (Ames Research
Center, NASA) & Marcello Fulchignoni (LESIA)

The importance of organic molecular material in astrophysics and Solar System science has become clear in recent years. From the Solar System to the distant galaxies, organics are the principal carrier of carbon, and the study of this material spans the full range of astrochemistry from the formation of stars and planets to the origin of life. Many basic questions remain on the processes that contribute to the formation, evolution, and distribution of organics that are found in the interstellar medium, in circumstellar debris disks, and on
planetary surfaces.

Organics are found on carbonaceous meteorites, comets, surfaces of icy objects from the Saturn satellites to Kuiper Belt Objects, as well as in Titan’s atmosphere, the interstellar medium (ISM) in our Galaxy and others and circumstellar debris disks.

The importance of organics in Solar System chemistry has been accepted over the last few years as the surface component responsible of the color of many outer Solar System bodies. There are other views, however, and an open and frank discussion of all the possibilities would be beneficial in achieving a clearer understanding of the wide-spread phenomenon of coloration. The origin of organics, particularly the macromolecular material, is still uncertain and the focus of several studies.

While organics in comets and meteorites originated, at least in part, in the ISM, a current view for the origin of organics on the surface of icy bodies is that these materials develop as a result of weathering on the surface of solid objects. However, the importance of nano-size particles of metallic iron as contaminants producing the coloring of the surface materials is currently being explored. Tholins produced by
irradiation of mixtures of simple molecules containing C, H, and N have been studied for many years. Laboratory
work shows that irradiation of simple hydrocarbons as CH4 and CH3OH result in the formation of red complex organics. The variation in amount of redness and therefore organic material has been attributed to a number of counteracting mechanisms that “refresh” the surface of the objects.

A new interpretation of the observations has been offered by Grundy (2009) where the varying color of TNOs is attributed to the different amount of organics mixed at the molecular level into the ice (water and other) that makes up part of the composition of these bodies. Depending on the concentration of organics the color varies as shown by radiative transfer modeling of icy objects. Assuming that the amount of organics is constant and its origin is pre-solar, the relative amount of ice present on the surface drives the concentration and therefore the color of the object : when the concentration of organics is small ice prevails and little or none red slope is present, as ice sublimates away the concentration of impurities increases along with the red coloring, when all ice is gone only the dark organics are left. Variations on temperature and/or size of the object end up impacting the stability of the icy component and indirectly its color.

As both theories are plausible it is possible that an inter-play of the two is at work to produce the observed colors. If this is the case are there any bodies that are pristine and show their original make-up ? If they are a function of their size where the biggest are subject to weathering and therefore “production” of organics as a result of irradiation, where is the size transition between pristine and evolved ?

Other planetary systems at various stages of evolution have recently been the object of compositional studies. Debes et al (2008) find the presence of complex organic materials in the debris disk of HR4796A manifesting as a red slope between 0.5 and 1.6µm in the dust spectrum. The fast advancement in space and ground-based observing techniques of circumstellar disks brings us closer to other planetary systems. The problem of the origin of organics can be “exported” to these systems with several advantages. A large sample of circumstellar disks around different spectral types can elucidate where, when, and how organics are created on the surfaces of planetesimals.
However, both observations and modeling of these disks can greatly benefit from the detailed knowledge found within our own Solar System.

A few questions come to mind :
• Can we trace this behavior in exoplanetary systems of different ages ?

• What observations are already available and what should we obtain to study this
scenario in circumstellar disks ?

And even more generally :
• How do pre-solar organics in the ISM relate to the compositions of circumstellar disks ?

• Can we reproduce in the lab the processes relevant to planetary surfaces with ices with
varying concentration of organic impurities ?

These are some of the preliminary questions to be discussed dring the three-day workshop.