The Nitrogen Problem: Constraints on the Delivery of Nitrogen to Rocky Planets
Daniel Apai (Space Telescope Science Institute)
Although one of the most abundant elements in the Universe nitrogen is very rare on Earth (ppm levels). In spite of its low concentration in terrestrial environments, nitrogen plays a fundamental role in all ammino acids and proteins and, correspondingly, it is present at highly enriched levels in all living organisms. In this talk I will briefly review the constraints posed by the distributions of nitrogen and its isotopes in primitive Solar System materials on its chemical evolution. I will explore the journey of nitrogen and different possible pathways from the interstellar medium through protoplanetary disks to forming planets. I will also discuss if and how some of the pathways may be affected by the stellar irradiation and what roles JWST and ALMA can play in constraining nitrogen-chemistry in other forming planetary systems.
Observational and Theoretical Constraints on the Water Distribution in Protoplanetary Disks
Edwin Bergin (University of Michigan)
I will outline our current knowledge of the water vapor and ice distribution in protoplanetary disks. In the past few years through both ground and space based efforts we have been able to place important observational constraints on the presence or absence of water vapor both within and beyond the evaporation front or snow-line. In this context, I will discuss the various ways that water vapor and ice can be formed and destroyed depending on the local physical environment. I will place particular emphasis on two new results. First new models suggest that water vapor can form in situ within the snow-line and is capable of protecting itself from molecule-destroying stellar ultraviolet radiation. This has important implications for the presence of water vapor within the planet-formation zone. Second, we will show that the surface of the outer disk appears to be drier than anticipated, which might imply that water ice-coated grains are preferentially found in the disk midplane.
X-ray and UV Radiation During the Era of Planet-Formation
Thomas Bethell (University of Michgan)
The radiative environment inside protoplanetary disks is extremely important for the generation and destruction of a wide range of molecular species; from the simple H_2 molecule to complex organics. High energy stellar photons (UV and X-rays) not only enable a vigorous chemistry through thermodynamic heating/ionization of gas and irradiation of ices, but also provide a destructive role through the direct photodissociation of susceptible molecules (e.g. Bethell & Bergin 2009). In this talk I shall give an overview of phenomena (dust settling and growth, gas dissipation) that control the flow of stellar UV and X-rays; how they depend differentially upon the evolutionary properties of protoplanetary disks, and their implications for the formation and survival of organic species (e.g. Fogel et al 2010). In this perspective we will explore some new and heretofore unrecognized aspects that are critical to our understanding of the environmental conditions in which organics and water must survive. Particular emphasis is given to discussing the form of radiation that approaches the disk midplane during the era of planet formation. References: Bethell, T., & Bergin, E., Formation and Survival of Water Vapor in the Terrestrial Planet-forming Region, 2009, Science, 326, 1675 Fogel, J., Bethell, T., Bergin, E., & Calvet, N., Chemistry of a Protoplanetary Disk with Grain Settling and Ly-a radiation, 2010, ApJ, submitted
Water Vapor and Organics in the Planet-Forming Region of Circumstellar Disks
Geoffrey Blake (Caltech)
With the advent of high dynamic range infrared observations from space (Spitzer IRS) and the ground (principally with Keck-NIRSPEC and VLT-CRIRES), widespread molecular emission from the inner regions of circumstellar disks has recently been observed. The data from Spitzer in particular reveal a mid-infrared molecular line forest dominated by the pure rotational lines of water vapor and the hydroxyl radical along with the bending rovibrational modes of acetylene, hydrogen cyanide, and carbon dioxide. Follow up observations from the ground at high spectral and angular resolution confirm the disk origin of this emission, and together these data sets provide stringent probes of the disk surfaces at distances of a few tenths to a few AU. This talk will present results from an extensive Spitzer IRS survey of the molecular emission from disks around stars of spectral types from A to M, high dispersion ground-based observations of selected species and transitions, and detailed radiative transfer models of the molecular emission from circumstellar disks. As time permits, thoughts about future observations with Herschel, ALMA, and the JWST will be shared.
Spitzer Spectroscopy of Volatiles in the Inner Disks of T Tauri Stars
John Carr (Naval Research Laboratory)
One of the interesting results of the Spitzer mission is the observation that classical T Tauri stars (CTTS) show a rich molecular emission spectrum in the mid-infrared. The spectrum is typically dominated by rotational transitions of H2O, along with vibrational bands of simple organic molecules (HCN, C2H2 and CO2) and rotational transitions of OH. The spectra are well modeled with gas temperatures (300-700 K) and emitting area that are consistent with emission from a warm disk atmosphere at radii less than 2-3 AU, roughly inside the snow line for solar-mass stars and corresponding to the terrestrial planet zone in the Solar system. Furthermore, such emission is widespread among CTTS. These results demonstrate the ability of infrared molecular spectroscopy to probe volatiles in the inner disk and also indicate its potential to contribute to understanding the origin and evolution of water and organic molecules in the planet forming region of disks. While common, molecular emission is not detected among all CTTS; perhaps more interesting, a large disk-to-disk range is observed in the strength of the organic emission to water and in the flux ratios of the different organic bands. The cause of these variations and whether they represent true abundance differences are among the questions being addressed based on the significant body of Spitzer spectra of protoplanetary disks. The Spitzer data will also lay groundwork for spectroscopic studies with future facilities, including the JWST.
The Transport of Water Ice in Protoplanetary Disks
Fred Ciesla (University of Chicago)
Protoplanetary disks are not static objects--rather the gas and solids they contain are constantly in motion, being jostled by gravitational torques and magnetic fields. The net motions are inward, with most of the mass accreting onto the central stars during the final stages of pre-main sequence evolution. As a consequence of this evolution, the disk is heated due to internal dissipation which converts the gravitational and kinetic energy of the gas into thermal energy, and it thins as it loses mas and spreads outward to conserve angular momentum While the evolution above largely describes how the gaseous component of the disk evolves with time, the solid component follows a separate evolutionary path. As solids interact with each other and the surrounding gas, they grow and are transported by diffusion, large-scale flows, and gas drag forces. Small solids are the most mobile, being lofted to high altitudes and subjected to large-scale redistribution as they move along with the gas. As solids grow in size, up until they measure centimeters to meters across, gas drag redistribution dominates the dynamics of the particles, carrying them in the direction of increasing pressure (largely inward). This redistribution continues until particles are finally locked up into planetesimals. The net effect of disk evolution and the dynamics of small solids is that pre-planetary materials are exposed to a wide variety of chemical and physical environments prior to their incorporation into the bodies that accrete to form planets. These different environments can lead to the formation, destruction, or alteration of solids, as the types and rates of chemical reactions that occur will vary with pressure, temperature, radiation flux, and chemical abundances. As such, protoplanetary disks are not just the site of planet formation, but also serve as massive chemical factories where the materials that were present in the parent molecular cloud are processed to various degrees prior to incorporation into the planets. Thus the final properties of the planets that form around a star are intimately linked to the dynamic evolution of dust and aggregates in its evolving protoplanetary disk. In my talk, I will review how these different processes operate together to impact the distribution of water in a protoplanetary disks. Connections will be made to the meteorite record for the formation of our solar system, however I will also discuss implications for the observations of disks around other stars and speculate about the the impact these processes have on the formation of planets elsewhere in the galaxy.
Evidence for a Phyllosilicate-Rich Debris Disk in the Terrestrial Zone Surrounding a 10 Myr-old Star
Thayne Currie (NASA-Goddard)
We describe Spitzer IRS spectroscopic observations of the ~ 10 Myr-old star, EF Cha. Analysis of the EF Cha spectra from 5 microns to 35 microns confirms that it is surrounded by a luminous, terrestrial zone debris disk. Comparing its IR spectrum to sophisticated mineralogical models we find that the EF Cha debris disk is unusually rich in a species or combination of species whose emissivities resemble that of finely-powdered, laboratory-measured phyllosilicate species, which are likely produced by aqueous alteration of primordial anhydrous rocky materials. The large mass volume of grains with sizes comparable to or below the radiation blow-out limit implies that phyllosilicate-rich planetesimals may be colliding at a high rate to yield the emitting dust. These planetesimals either have been dragged from their formation region beyond the ice line into the terrestrial zone earlier, during the protoplanetary disk phase, or they are being dynamically scattered into the terrestrial zone and are colliding with anhydrous rocky planetesimals. In either case, EF Cha's disk is likely undergoing processes thought to be important for Earth's formation and water delivery.
Detecting Water on Super-Earths Using JWST
Drake Deming (NASA's GSFC)
Nearby lower main sequence stars host a class of planets known as Super-Earths, that have no analog in our own solar system. Super-Earths are rocky and/or icy planets with masses up to about 10 Earth masses. They are expected to host atmospheres generated by a number of processes including accretion of chrondritic material. Water vapor should be a common constituent of super-Earth atmospheres, and may be detectable in transiting super-Earths using transmission spectroscopy during primary eclipse, and emission spectroscopy at secondary eclipse. I will discuss the prospects for super-Earth atmospheric measurements using JWST.
Survival of Water in Protoplanetary Disks
Davide Fedele (JHU)
Recent observations have found abundant emission of hot (~1000K) water and hydroxyl radical (OH) vapor in protoplanetary disks around solar type stars. Disks around more massive stars instead do not show any evidence of hot water vapor emission but are rich in OH emission lines. The difference between the two class of stars might be due to the stellar UV field impinging on the disk. I will present the results of a high spectral resolution survey of protoplanetary disks and discuss the impact on the delivery of water in planets.
The Volatile and Organic-rich Carbonaceous Chondrites: Overview and Recent Results
Dante Lauretta (University of Arizona)
I will provide an overview of the CI, CM, and CR carbonaceous chondrite meteorite groups. In addition, I will review recent work linking the CI chondrites with samples returned from comet Wild 2 by the Stardust mission, the alteration of metal in the CM chondrites, and the relationship between the petrology, geochemistry, and organic content of the CR chondrites.
Modeling the Early Dynamical Evolution of the Solar System: Implications for Earth's Volatiles.
Hal Levison (SwRI)
The formation and early dynamical evolution of the Solar System was a violent process that led to the wholesale redistribution of its small bodies. Many of these objects collided with the Earth. Indeed, the Earth currently contains material from every corner of the original proto-planetary disk - including the regions where most of the Earth's volatiles were solids. I will review our current understanding of the dynamical evolution of the Solar System. I will describe the current models of volatile delivery to the Earth via the impact of small objects.
Tracing Organic Chemistry in Primordial Disks Through High-Resolution NIR Spectroscopy
Avi Mandell (NASA GSFC)
Characterizing the gas chemistry in the planet-forming regions of young circumstellar disks is essential to understanding the origins of planetary systems. Near-infrared spectroscopy (1 - 5 µm) covers both low-energy and high-energy ro-vibrational transitions for a large variety of molecules, allowing accurate measurements of the excitation state and kinematics of the warm gas in the inner disk. Detection of new tracers of molecular chemistry can provide new insight into the thermodynamic and kinematic processes occurring in the inner disk. We have developed new reduction techniques to allow very accurate spectroscopic observations of Herbig Ae and T Tauri stars in the 3.0 - 3.7 µm wavelength range (L-band), allowing us to push our sensitivity to unprecedented precision. Our results reveal emission from multiple molecular disk tracers, including OH, H2O, and simple organics. Initial results reveal a lack of H2O features in spectra from higher-mass stars, while water is easily detected in TT stars; constraining the reasons for this disparity may provide important clues to the processes driving chemical balance in disks of different masses. Results from ongoing searches for new molecular tracers in the L-band will also be discussed, and we will describe new modeling techniques to constrain the spatial distribution of different molecular tracers and determine the role of non-LTE radiative excitation in the disk surfaces. Detailed investigations of the formation and excitation of these inner-disk constituents will help to expand our understanding of the chemistry occurring in the warm gas surrounding young protoplanets and how it may affect the final composition of fully-formed planets and planetary atmospheres.
Origin of Terrestrial volatiles : constraints from the isotopic compositions of ET material returned by the Stardust and the Genesis missions
Bernard Marty (CRPG-CNRS)
The origin(s) of volatile elements in the terrestrial atmosphere (including air, sediments and hydrosphere) have been debated for decades. Several sources have been proposed : (i) solar nebula, (ii) solar wind implanted into pre-accretional material, (iii) chondritic (meteoritic) material, (iv) comets, (v) any other matter not any more represented in the solar system. The Genesis mission returned solar wind ions sampled during 27 months by implantation into pure material. The idea of the mission was to document the composition of the solar nebula. Solar wind nitrogen is depleted by 40 % in 15N relative to Earth [1]. Put in another way, all bodies of the solar system except giant planets are enriched in 15N relative to the solar nebula, possibly due to chemical exchange between gaseous N2 and organics under UV irradiation. Furthermore, the Earth, the Martian mantle, and primitive meteorites have comparable nitrogen isotopic compositions, whereas comets are even more enriched in 15N. A similar figure emerges from D/H systematics of solar system reservoirs. Thus major volatiles (N, water, and probably carbon) on Earth (and Mars) appear to be derived from a chondritic source rather than from cometary or solar nebula reservoirs. We have also analyzed noble gases trapped in cometary matter sampled by the Stardust mission [2]. Neon is isotopically comparable to atmospheric Ne and is elementally rich in the analyzed material, making it possible that the late heavy bombardment around 3.8 Ga ago supplied some of the atmospheric noble gases. 1. Marty B. et al. Geochim. Cosmochim. Acta (2010) 74, 340-355. 2. Marty B. et al. Science (2008) 319, 75-78.
Origins of Earth's Water - Key research insight Developedat an Interdisciplinary Workshop Held in Hawaii in 2008, and a Look to the Future.
Karen Meech (Institute for Astronomy)
The origin of Earth's water is one of the fundamental unanswered questions about the early Solar System. The location of the regions within the nascent Solar System, which may have delivered water-rich material during accretion, is under intense debate. Water is formed from two of the three most abundant elements and so is abundant in interstellar space, our Solar System, and on Earth, where it is essential for the existence of life. Water ice acts as a substrate and reactant in interstellar clouds, enabling the formation of organic compounds that are important precursors to life and that eventually became incorporated into comets and asteroids. During collapse of the Solar nebula, icy interstellar grain mantles sublimated and recondensed onto other grains, influencing the transport of energy, mass, and angular momentum in the disk as well as disk opacity and temperature structure. The bulk silicate Earth contains only 500-1100 ppm water, an amount small enough to explain by wet accretion, although most of it probably accumulated with the latter half of Earth's mass from wetter planetary embryos originating beyond 1.5AU. Degassing on impact delivered water to Earth's surface, where it dissolved into a magma ocean. Although most of Earth's water probably came from meteoritic material, Earth's depletion of Xe relative to Kr strongly suggests a role for comets. We will present where the key areas of interdisciplinary research can make headway in addressing the origin of terrestrial water and organics as identified in a workshop held in Hawaii during February 2008.
Bio-essential Element Inventories to Earth (and Earth-like Worlds) in Late Heavy Bombardments
Stephen Mojzsis (University of Colorado)
Our baseline for understanding the delivery of organics and volatiles to Earth and probable Exoearths begins with studies of the first billion years of our solar system. Presently, there exists no direct measure of the influx of extraterrestrial matter to our world from 4.1 – 3.85 Ga (e.g. Anbar et al., 2001) during the purported Late Heavy Bombardment of the solar system (LHB). Calculations suggest that the total mass of impactors to the Moon at the LHB climax ca. 3.9 Ga was ~6 x 10^21 g (e.g. Ryder et al., 2000), and that the mass accumulated in the post-primary accretion epoch of the Hadean subsequent to the Moon-forming event (4.53-4.46 Ga; Halliday, 2008) was roughly equivalent to that which formed ~50 the major (>300 km diameter) lunar basins. The total amount of dust accumulated to the Moon during this time interval could have been 10X this estimate, on the order of 1.2 x 10^22 g. Because evidence for the LHB epoch from terrestrial rocks is scant (Trail et al., 2007), all claims for a substantial role for extraterrestrial matter in the delivery of some key “biogenic elements” (S, C, P) that boosted (or thwarted?) the incipient biosphere on Earth must be treated with caution (Abramov and Mojzsis, 2009). Marty and Yokochi (2006) calculated the total of all extraterrestrial material including dust and other meteoritic debris collected on Earth's surface from 4.5 – 3.8 Ga to have been in the range of 1.8-7.2 x 10^23 g. Dynamical models which explore the timing and source region for the Solar System’s LHB (Gomes et al., 2005; Tsiganis et al., 2005) are in general agreement with these estimates. With the bulk average sulfur, carbon and phosphorus contents of CM and CI meteorites as a guide, this conservatively amounts to an exogenic accumulation of S, C and P to the Hadean Earth integrated over the course of the LHB for both comets and asteroids to be: Sulfur =1.5 x 10^22 g; Carbon = 1.04 x 10^22 g; Phosphorus = 3.6 x 10^20 g. Although these values are apparently significant when compared to the cumulative mass delivered to the Moon, they amount to ≤1% of the total S inventory for all sediments and seawater at the surface zone. Terrestrial S was therefore always dominantly indigenous to the Earth. This is important, because mass-independently fractionated S isotopes documented in the oldest Earth rocks and minerals can with reasonable confidence be assigned an endogenous origin (Mojzsis, 2007). The amounts of extraterrestrial C and P delivered to Earth by the LHB on the other hand are substantial; so much so that they are equivalent to thousands of times the current inventory of the combined surface biosphere+atmosphere+hydrosphere system. If LHB-type epochs are a general rule rather than an odd exception for young solar systems (Gaspar et al., 2009; Brasser et al., 2009), the accompanying bio-essential element augmentation from a relatively late migration of gas giants and perturbation of asteroids and comets may represent a rich bouillon to nascent biomes - especially on marginally habitable worlds - rather than a coup de grace. References: Abramov, O. and Mojzsis, S.J. (2009) Microbial habitability of the Hadean Earth during the late heavy bombardment. Nature 459: 419-422. Anbar, A.D., Zahnle, K.J., Arnold, G. and Mojzsis, S.J. (2001) Extraterrestrial iridium, sediment accumulation and the habitability of the early Earth’s surface: J. Geophys. Res. 106(2); 3219-3236. Brasser, R., Morbidelli, A., Gomes, R., Tsiganis, K. and Levison, H.F. (2009) Constructing the secular architecture of the solar system II: The terrestrial planets. Astron. Astrophys. 507: 1053-1065. Gaspar, A., Rieke, G.H., Sy, K.Y.L., Blaog, Z., Trilling, D., Muzzerole, J., Apai, D. and Kelly, B.C. (2009) The low level of debris disk activity at the time of the late heavy bombardment: A Spitzer study of Praesepe. Astrophys. J. 697: 1578-1596. Gomes, R., Levison, H.F., Tsiganis, K. and Morbidelli, A. (2005) Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial planets. Nature 435: 466-469. Halliday, A. (2008) A young Moon-forming giant impact at 70-110 million years accompanied by late-stage mixing, core formation and degassing of the Earth. Phil. Trans. R. Soc. A. 366: 4163-4181 Marty, B., Yokochi, R., 2006. Water in the early Earth. Rev. Mineral. Geochem. 62: 421–450. Mojzsis, S.J. (2007) Sulphur on the Early Earth. In, Earth’s oldest rocks (Van Kranendonk, M..J., Smithies, R.H. and Bennett, V. Eds.) Developments in Precambrian Geology 15: 923-970. Ryder, G., Koeberl, C., and Mojzsis, S.J. (2000) Heavy bombardment of the Earth at ~3.85 Ga: The search for petrographic and geochemical evidence, In: Origin of the Earth and Moon; edited by K. Righter and R. Canup: University of Arizona Press pp.475-492. Tsiganis, K., Gomes, R., Morbidelli, A. and Levison, H.F. (2005) Origin of the orbital architecture of the giant planets of the Solar System. Nature 435: 459-461. Trail, D., Mojzsis, S.J. and Harrison, T.M. (2007) Thermal events documented in Hadean zircons by ion microprobe depth profiles. Geochim. Cosmochim. Acta 71: 4044-4065.
Water on Earth: its abundance, distribution, and cycling
Michael Mottl (University of Hawaii)
Whereas we often view Earth as the watery planet, it is in fact quite dry in bulk, at least in its silicate portion. Earth’s core, 32% of its mass, may contain as much as 100 oceans worth of hydrogen, or it may contain virtually none. Earth’s interior holds gases of undisputed solar origin, based on isotopes of He, Ne, Ar, and Xe. If this solar gas was gravitationally captured by the growing Earth directly from the solar nebula, as seems likely from its concentration and high ratio of 3He/22Ne, rather than by implantation from the solar wind onto particles that subsequently accreted, then H from such a primary atmosphere would have readily dissolved into an early magma ocean and been segregated into the iron core. If there is H in the core it is most likely trapped there, except for some reaction at the core-mantle boundary with FeO in silicates. The mass of water in the oceans is 1371 x 1018 kg, equivalent to 38 ppm H in the bulk silicate Earth (BSE = crust + mantle = 4.05 x 1024 kg). The rest of the hydrosphere contributes another 7 ppm H to the BSE, mostly from marine pore water, and the rest of the exosphere another 9 ppm, mostly from shales, for a total exospheric concentration of 53 ppm H in the BSE, or 1.4 ocean’s worth of water. The amount of H in the mantle is much less well known. While serpentinization of the lithospheric mantle beneath the oceans is important for transport of water to the deeper mantle via subduction, it represents ≤0.02 ppm H in the BSE. Whereas the water content of the upper mantle is fairly well constrained at ~120 ppm H2O by mid-ocean ridge magmas, representing ~2 ppm H in the BSE, the amounts in the transition zone (410-670 km) and the lower mantle are poorly known. While the transition zone could contain a great deal of water if it were saturated, it almost certainly is not; a reasonable estimate has it contributing 2-26 ppm H to the BSE. The lower mantle is likely quite dry, perhaps 20 ppm H2O, which would contribute 2 ppm H to the BSE, although this value is poorly constrained. The largest uncertainty concerns the mysterious D” layer just above the core-mantle boundary, which may contain 20-500 ppm H2O and could contribute up to 2 ppm H to the BSE. Summing these estimates indicates that the mantle could contribute 5-70 ppm H to the BSE. (In other words, most of Earth’s water could be in the oceans, or in the mantle, or in the core.) The estimated range for the total BSE is therefore ~60-120 ppm H, equivalent to 1.6 to 3.2 ocean’s worth of water. If Earth’s silicate interior were saturated with water, by contrast (which it certainly is not), with ~30,000 ppm H2O in the transition zone and ~2000 ppm H2O everywhere else in the mantle, the BSE would contain 630 ppm H = 0.57 wt% H2O, or ~17 ocean’s worth. While H is clearly a trace element in the BSE, even this small concentration has a drastic effect on tectonic processes, lowering the viscosity of Earth’s mantle enough to allow for the more or less continuous activity of plate tectonics on a billion-year time scale. Plate tectonics, in turn, recycles hydrated oceanic crust and upper mantle back into the deep mantle, thereby keeping the mantle hydrated and “lubricating” it for further motion. Plate tectonics also forms a second type of crust on Earth, that which underlies the continents, and thus produces dry land on our watery planet. Earth’s water thus appears to be responsible for both its oceans and its dry land. Venus, by contrast, is much drier and has neither plate tectonics nor continental crust, presumably because its mantle is too dry and stiff to convect regularly.
Comets as a Possible Source for Volatiles and Organics on Earth
Michael Mumma (NASA's GSFC)
Viewed from a cosmic perspective, Earth is a dry planet yet its oceans are enriched in deuterium by a large factor relative to nebular hydrogen. The question of exogenous delivery of organics and water to Earth and other young planets is of critical importance for understanding planetary systems, and for assessing the prospects for existence of Earth-like exo-planets. Icy bodies today reside in two distinct reservoirs, the OC and the KB region (divided into the classical KB, the scattered disk, and the detached or extended disk populations). Comets injected into the inner planetary system are classified dynamically as isotropic (LPC or HTC) or ecliptic (Centaur-type, Encke-type, or JFC). Ecliptic comets come from the KB reservoir while the isotropic comets come from the Oort cloud [1]. All except Centaur-type comets have the potential of becoming sufficiently bright to obtain sensitive detection of their volatile fraction through high-resolution NIR spectra. Strong gradients in temperature and chemistry in the proto-planetary disk, coupled with dynamical dispersion of an outer disk of icy planetesimals, imply that comets from the formative phase of the Solar System should have diverse composition. The “Nice” model [2,3] predicts that comets formed in (and ejected from) the giant-planets’ feeding zones (5 - 14 AU) probably entered the outer disk [4], whose subsequent disruption contributed some of the mass impacting Earth during the late heavy bombardment. Comets formed in the outer proto-planetary disk (beyond Rh ~ 17 AU) entered both the Kuiper disk and the OC, though likely in different proportions [5,6]. However, recent dynamical studies suggest that many Oort cloud comets may have been captured from sibling stars in the Sun’s birth cluster (7), posing additional questions regarding diversity among comets in that reservoir. While orbital parameters can indicate the cosmic storage reservoir for a given comet, identifying its region of origin depends on quantitative knowledge of its components – dust and ice. The so-called parent volatiles provide the preferred metrics for building taxonomy for cometary ices, but until recently they were difficult to measure. Secondary (or even tertiary) free radicals such as CN, C2, etc., are more easily detected and so form the basis of extensive taxonomies that have provided valuable insights (A’Hearn et al. 1995, Fink 2009). However, these derivative species can have multiple origins and these are sometimes poorly constrained, complicating their interpretation. Taxonomies based on native species (parent volatiles) are now beginning to emerge [5, 10, 11]. I will provide an overview of these aspects. References: [1] Levison, H. F. (1996), in Completing the Inventory of the Solar System, Astr. Soc. Pac. Conf. Proc. 107, 173-191. [2] Gomes, R. et al., Nature 435-7041, 466-469 (2005). [3] Morbidelli, A. et al. (2008), in The Solar System beyond Neptune (in press). [4] Dones, L. et al. (2004), in Comets II, 153-174. [5] Crovisier, J., (2007). arXiv:astro-ph/0703785. [6] Morbidelli, A. (2005), eprint arXiv:astro-ph/0512256. [7] Levison, H. et al. 2010, [8] A’Hearn, M. et al. (1995), Icarus 118, 223, [9] Fink, U. (2009), Icarus 201, 311, [10] Biver, N. et al. (2007), Plan. Sp. Sci. 55, 1058 , [11] DiSanti, M. and Mumma, M. (2008), Sp. Sci. Rev. 138. 127.
The Release of Volatiles and Organics - During the post-main-sequence phase of stellar evolution.
David Neufeld (Johns Hopkins University)
When stars leave the main sequence, having exhausted their supply of hydrogen at the core, their luminosities increase dramatically and the snow line moves outward. Any comets - or dwarf planets - within a Kuiper belt analog will be vaporized, releasing water and other volatiles into the gas phase. This process may provide a distinctive observational signature, and has been suggested to explain the anomalously high abundances of water vapor and formaldehyde observed in the circumstellar outflow from IRC+10216, a carbon-rich AGB star. Recent Herschel observations have led to the detection of water in 6 other carbon-rich AGB stars, with abundances orders of magnitude larger than those expected in thermochemical equilibrium. However, in the particular case of IRC+10216, Herschel observations of multiple water transitions have argued AGAINST the vaporization of comets being the origin of the observed water. I will review these recent results from Herschel, and will discuss the prospects for probing the inventory of orbiting ices around post-main-sequence stars.
Evidence for Volatile Gradients in the Protoplanetary Disk
Keith Noll (STScI)
Kuiper Belt objects are the surviving remnants of the Sun's protoplanetary disk. The large range of colors in these objects can be shown to be primordial for objects larger than D~100 km. Dynamical segregation of objects with differing surface composition can be traced to differences in the origin of these objects. The abundance of red objects that apparently formed at large heliocentric distances argues for a gradient in one or more volatile components (other than water ice) in the protoplanetary disk. Color gradients should be present in the disks around other young stars.
Organic Molecules and Water in Brown Dwarf Disks: Differences and Similarities with Disks around Sun-like Stars
Ilaria Pascucci (STScI)
I will present a comparative Spitzer/IRS study of the gas properties of disks around young Sun-like stars and very low-mass stars/brown dwarfs. I will show that there is a striking difference in the two samples in the presence of simple, gas-phase organic molecules, such as HCN and C2H2, as well as of water. This could originate from the large difference in the UV irradiation around the two types of sources. These results reveal how the chemistry of protoplanetary disks is affected by the central star's properties with possible implications for the organics available during planet formation.
GJ 1214b And The Prospects for Liquid Water on Super Earths
Leslie Rogers (MIT)
GJ 1214b, one of the first discovered transiting super Earths, must have a significant gas component to account for its low average density (M_p=6.55 M_earth, R_p=2.678 R_earth). We use interior structure models to constrain GJ 1214b’s gas envelope mass, and to explore the conditions needed to achieve within the planet pressures and temperatures conducive to liquid water. We consider three possible origins for the gas layer: direct accretion of gas from the protoplanetary nebula, sublimation of ices, and outgassing from rocky material. Despite having an equilibrium temperature below 647 K (the critical temperature of water) GJ 1214b does not have liquid water under most conditions we consider. Even if the outer envelope is predominantly sublimated water ice, in our model a low intrinsic planet luminosity (less than 2 TW) is needed for the water envelope to pass through the liquid phase; at higher interior luminosities the outer envelope transitions from a vapor to a super-fluid then to a plasma at successively greater depths. We generalize our approach to consider the prospects for super Earths with substantial gas envelopes to harbor liquid water.
Water Vapor Absorption and Emission at 5–7.5 Microns Wavelength in Spitzer-IRS Spectra of Protoplanetary Disks
Benjamin Sargent (Space Telescope Science Institute)
We present spectra of T Tauri stars in the Taurus-Auriga star-forming region showing emission and absorption in Spitzer Space Telescope Infrared Spectrograph (IRS) 5–7.5 micron spectra from water vapor and possibly other gases in these stars’ protoplanetary disks. Emission and absorption from water vapor in protoplanetary disks has been seen in spectra at near-infrared wavelengths (< 2.3 microns) and longer mid-infrared (> 10 microns) wavelengths, but finding such spectral signatures in 5–7.5 micron spectra of protoplanetary disks around low-mass stars is unusual. Some of the stars’ spectra show an emission feature at 6.6 microns, likely a blend of many lines, suggesting emission from fairly warm (>500K) water vapor. Other stars have spectra showing an absorption band peaking in strength at 5.6–5.7 microns, which may be due to water vapor and other gases. We present spectral models of these emission and absorption features. The water vapor suggested by these spectra likely originates in the inner regions of these protoplanetary disks and is relevant to studies of the origin of water on planets in the habitable zones of stars.
From Molecular Clouds to Planetary Systems: the Journey of an Interstellar Ice Mantle.
Paule Sonnentrucker (STScI)
Interstellar grains in molecular clouds are coated with icy mantles once the visual extinction exceeds about 3 magnitudes within the cloud. The existence of these mantles is revealed by the presence of various molecular absorption bands that are detected against strong background continuum emission in the near- and mid- infra red wavelength ranges. The bands routinely observed either from the ground or with space-based instruments are due to vibrational modes of water ice (3.1 and 6.0 microns), CO ice (4.7 microns), and CO2 (15.2 microns) ice. Most of what we know about the composition, the formation and the evolution of these ice mantles has come through studies of point-like background sources. Because such background sources are neither numerous nor contiguous, our knowledge of the onset of ice mantle formation and mantle evolution is based on relatively small number of observed sight lines. The Spitzer spectral mapping capability offered the unique opportunity to generate contiguous and extended maps of the distributions of water and CO2 ices that can be directly compared with the distributions of tracers of local physical conditions within star-forming regions. In the following we will discuss: the ice mantle evolution when altered by local physical processes, the use of specific ice constituents to infer the physical conditions pertaining in the probed region, the challenges that we face when investigating the composition of interstellar ice mantles and, their implication when searching for links between interstellar ice mantles and icy bodies in (proto-)planeraty systems.
Hayabusa-2, The Future Sample-Return Mission to the C-type asteroid 1999JU3
Shogo Tachibana (Dept. Earth Planet. Sci., Univ. Tokyo)
The Hayabusa spacecraft returned to the Earth on June 13, 2010, and its sample capsule was safely recovered. It is expected that the capsule contains particles from the asteroid Itokawa although the sampling did not go as originally planned. X-ray fluores-cence spectroscopy of Itokwa showed that the surface chemical composition of Itokawa resembles to that of LL- or L-chondrites (Okada et al., 2006), which indicates that the retuned samples may consist mainly of inorganic minerals. Recent progress in research of extraterrestrial materials has re-vealed that the most pristine materials in the solar system are an interacted mixture of minerals, ice, and organic matter. It is ac-cordingly important to study the interactions between minerals, ice, and organic matter within the pristine materials in the dy-namically active protosolar disk to understand the very early evo-lution of minerals, ice, and organic matter, which would have later evolved to the Earth, ocean, and life, respectively. How-ever, there have been returned samples neither containing ice and volatile organics nor keeping the interactions between inorganics, ice and organics intact. In this talk, we will illustrate the importance of sample-return return missions from undifferentiated asteroids and comets, which preserve pristine minerals, ice, and organics, and introduce a future Japanese asteroidal sample return mission, Hayabusa-2, to the C-type asteroid 1999 JU3.
The Chemical History of Volatiles in Protoplanetary Disks
Ruud Visser (Leiden Observatory)
In order to probe possible pathways for delivering volatiles to the Earth and other rocky planets or exoplanets, it is necessary to understand when and where these volatiles were formed in the first place. We present a model that follows the chemical evolution all the way from pre-stellar cores to protoplanetary disks. It traces gas and dust falling in from the natal cloud core to the star and the disk, and it solves for the abundances of a few hundred gas-phase and grain-surface species as function of time and position. The necessary observational constraints come from data taken with, e.g., the SMA (the PROSAC survey) and Herschel (the WISH and DIGIT key programs). The model predicts that many volatiles are formed already in the infalling envelope, before reaching the disk. They undergo further processing within the disk, thus determining the abundances of complex organic molecules in the building blocks for planets and comets. The predicted abundances of several key species match the abundances observed in comets, but for other species the match is very poor. We discuss what both the good and the poor matches mean for the possible delivery mechanisms for volatiles on Earth and elsewhere.
The Primitive Material Explorer (PriME) Mission: Determining the Role of Comets in Delivering Volatiles Throughout the Solar System
Hal Weaver (JHU Applied Physics Laboratory)
The Primitive Material Explorer (PriME) is a proposed NASA Discovery mission that will launch in 2016 and rendezvous with comet 46P/Wirtanen in 2021. During an intensive yearlong science phase starting in August 2023 and ending two months after perihelion in July 2024, PriME will obtain an unprecedented compositional inventory of the most primitive material in the Solar System, the icy component of a comet nucleus. PriME accomplishes this objective using MASPEX (MAss Spectrometer for Planetary EXploration), which has higher mass resolution and is more sensitive than any mass spectrometer ever flown on a space mission. MASPEX will measure the abundances of noble gases and their isotopes, D/H in H2O and CH4, several isotopes of C-, O-, and N-bearing species, and complex molecular compounds up to masses of 1000 amu. These measurements should be key to resolving the role comets played in delivering water and other biologically important materials throughout the Solar System.
The Survival of Water Within Extrasolar Minor Planets
Siyi Xu (UCLA)
We describe a simple model that shows that extrasolar minor planets can retain much of their internal water during their host star’s red giant stage. These water-rich bodies might be accreted onto helium white dwarfs (DB) and produce an observable amount of hydrogen, which is quite likely in the case of GD 362. One observational test of this possibility is to search for excess oxygen that is likely contributed by common oxides in the atmospheres of polluted white dwarfs. Reference: Jura, M. & Xu, S. 2010, AJ, in press
Processes Affecting the Volatile Budget of the Earth
Reika Yokochi (University of Chicago)
Astrophysical models and observations of extra-solar protoplanetary disks provide information on possible processes occurring during and materials available to planetary accretion. In search of habitable exoplanets, the actual processes that were significant to the redistribution of water in the planetary system need to be identified. As proto-planets grow in size, the impact velocity of accretion increases, which causes volatilization of the colliding objects. The volatiles released at the surface of the accreting planet are lost to space until gravitational energy of the planet exceeds the kinetic energy of gas constituents. If this latter condition is fulfilled before dissipation of the solar nebula, the planet can also capture hydrogen-dominated nebula gas. Hydrostatic stability of the proto-atmosphere could be disturbed by events such as large impacts and intense UV radiation from T-Tauri phase of the young Sun, which results in erosion of the early hydrogen- and water-bearing atmospheres. Numerous physical parameters remain unconstrained in this context, including the feeding zone of each planet, the rate of proto-planet growth, and relative timing of the events. Being the most easily observable planet in the Universe, the volatile budget of the Earth has been keenly studied in order to set further constraints on the generalized picture described above. In particular, noble gases constitute ideal tracers in deciphering the physical processes (e.g. mixing, partial loss and phase separation) affecting volatile elements during planetary evolution owing to their chemical inertness. Noble gases in the terrestrial atmosphere have two important characteristics that helps understanding the processes that affected volatile elements at the surface of the Earth: (i) the general enrichment in heavier isotopes and elements of noble gases compared to the Sun, and (ii) the “missing Xe” problem: Mars and Earth have a deficiency in Xe relative to other noble gases when compared to meteoritic compositions. Furthermore, the distinct non-radiogenic noble gas isotopic compositions between the surface and interior (mantle) of the Earth suggest that there has been a quantitative loss of volatiles accompanying kinetic fractionation that only affected the atmosphere. Timing of these volatile redistribution events can be constrained by isotopic abundances of radiogenic noble gases. Based on the processes responsible for the presently observed noble gas elemental and isotopic compositions in different geochemical reservoirs, possible fate of reactive volatile species and mass balance constraints will be discussed.