Source regions and timescales for the delivery of water to the Earth

Abstract— In the primordial solar system, the most plausible sources of the water accreted by the Earth were in the outer asteroid belt, in the giant planet regions, and in the Kuiper Belt. We investigate the implications on the origin of Earth's water of dynamical models of primordial evolution of solar system bodies and check them with respect to chemical constraints. We find that it is plausible that the Earth accreted water all along its formation, from the early phases when the solar nebula was still present to the late stages of gas‐free sweepup of scattered planetesimals. Asteroids and the comets from the Jupiter‐Saturn region were the first water deliverers, when the Earth was less than half its present mass. The bulk of the water presently on Earth was carried by a few planetary embryos, originally formed in the outer asteroid belt and accreted by the Earth at the final stage of its formation. Finally, a late veneer, accounting for at most 10% of the present water mass, occurred due to comets from the Uranus‐Neptune region and from the Kuiper Belt. The net result of accretion from these several reservoirs is that the water on Earth had essentially the D/H ratio typical of the water condensed in the outer asteroid belt. This is in agreement with the observation that the D/H ratio in the oceans is very close to the mean value of the D/H ratio of the water inclusions in carbonaceous chondrites.     

The origin of water on Mars

This paper considers the origin of water on Mars, in the context of a dynamical model that accounts for most of the Earth's water as a product of collisions between the growing Earth and planet-sized “embryos” from the asteroid belt. Mars' history is found to be different; to explain the present mass of Mars requires that it suffer essentially no giant collisions and the bulk of its growth is through addition of smaller bodies. Asteroids and comets from beyond 2.5 AU provide the source of Mars' water, which totals 6-27% of the Earth's present ocean (1 Earth ocean≡1.5×10²¹ kg), equivalent to 600-2700-m depth on the martian surface. The D/H ratio of this material is 1.2-1.6 times Standard Mean Ocean Water, the smaller value obtaining for the larger amount of water accreted. The upper half of the range of total water accreted, while many times less than that acquired by the Earth, is consistent with geological data on Mars, and the D/H value is that derived for martian magmatic water from SNC meteorites. Both together are consistent with published interpretations of the high D/H in present-day martian atmospheric water in terms of water loss through atmospheric escape.     

Constraints on the Formation Regions of Comets from their D:H Ratios

Studies of the D:H ratio in H₂O within the Solar nebula provide a relationship between the degree of enrichment of deuterium and the distance from the young Sun. In the context of cometary formation, such models suggest that comets which formed in different regions of the Solar nebula should have measurably different D:H ratios. We aim to illustrate how the observed comets can give information about the formation regions of the reservoirs in which they originated. After a discussion of the current understanding of the regions in which comets formed, simple models of plausible formation regions for two different cometary reservoirs (the Edgeworth–Kuiper belt and the Oort Cloud) are convolved with a deuterium-enrichment profile for the pre-solar nebula. This allows us to illustrate how different formation regions for these objects can lead to great variations in the deuterium enrichment distributions that we would observe in comets today. We also provide an illustrative example of how variations in the population within a source region can modify the resulting observational profile. The convolution of a deuterium-enrichment profile with examples of proto-cometary populations gives a feel for how observations could be used to draw conclusions on the formation region of comets which are currently fed into the inner Solar system from at least two reservoirs. Such observations have, to date, been carried out on only three comets, but future work with instruments such as ALMA and Herschel should vastly improve the dataset, leading to a clearer consensus on the formation of the Oort cloud and Edgeworth–Kuiper belt.     

Deuterium enrichment in giant planets

Recently published laboratory measurements of the isotopic exchange rate constant k⁻(T) between CD₄ and H₂ are used to calculate f(z)—the isotopic enrichment factor between CH₄ and H₂—at every level in the outer atmosphere of the giant planets. The variation of f(z) with local vertical velocity, temperature and pressure has been calculated under the assumption that atmospheres are convective and uncertainties have been calculated by error propagation. Considering only the random errors—mainly the uncertainty on k⁻(T)—the f values in the observable upper atmospheres of giant planets (i.e. at z = 0, P = 1 bar) are: f(0) = 1.25 ± 0.05, 1.38 ± 0.06, 1.68 ± 0.09, and 1.61 ± 0.08 for Jupiter, Saturn, Uranus, and Neptune, respectively. Additional systematic errors due to the uncertainty in calculating the vertical velocity in the framework of the mixing length Prandtl theory lead to an overall uncertainty on f(0) of ±0.12, ±0.15, ±0.23, and ±0.21 for each planet, respectively. The D/H ratios in H₂ derived from the measured CH₃D/CH₄ ratios in the upper atmosphere of the four giant planets are then recalculated. Uranus and Neptune seem to be enriched in deuterium with respect to the protosolar nebula but depleted relative to the Standard Mean Oceanic Water on the Earth (SMOW). However calculations based on current interior models of Neptune suggest that ices which formed the core of the planet had a D/H ratio of the order of the SMOW. The deuterium abundance in proto-Uranian ices remains uncertain. The case where water is a major constituent of the fluid envelope of Neptune is discussed. It is shown that the D/H ratio of the planet would then be higher than the value measured in hydrogen. Even in this case, the D/H ratio in proto-Neptunian ices is less than the recently revised value in P/Halley and less than the value measured in water of the Semarkona meteorite. These results suggest that the ices which formed the core of Neptune did not have an interstellar origin. Similarly, the comparison of the most recent determination of the D/H ratio in the atmosphere of Titan with the value of D/H in P/Halley suggests that this atmosphere was not formed by infalling comets but more likely from grains embedded in the sub-nebula of Saturn.     

Embedded star clusters and the formation of the Oort cloud: II. The effect of the primordial solar nebula

This paper deals with Oort cloud formation while the Sun was in an embedded cluster and surrounded by its primordial nebula. This work is a continuation of Brasser et al. [Brasser, R., Duncan, M., Levison, H., 2006. Icarus 184, 59-82], building on the model presented therein, and adding the aerodynamic drag and gravitational potential of the primordial solar nebula. Results are presented of numerical simulations of comets subject to the gravitational influence of the Sun, Jupiter, Saturn, star cluster and primordial solar nebula; some of the simulations included the gravitational influence of Uranus and Neptune as well. The primordial solar nebula was approximated by the minimum-mass Hayashi model [Hayashi, C., Nakozawa, K., Nakagawa, Y., 1985. In: Black, D.C., Matthews, M.S. (Eds.). Protostars and Planets II. Univ. of Arizona Press, Tucson, AZ] whose inner and outer radii have been truncated at various distances from the Sun. A comet size of 1.7 km was used for most of our simulations. In all of our simulations, the density of the primordial solar nebula decayed exponentially with an e-folding time of 2 Myr. It turns out that when the primordial solar nebula extends much beyond Saturn or Neptune, virtually no material will end up in the Oort cloud (OC) during this phase. Instead, the majority of the material will be on circular orbits inside of Jupiter if the inner edge of the disk is well inside Jupiter's orbit. If the disk's inner edge is beyond Jupiter's orbit, most comets end up on orbits in exterior mean-motion resonances with Saturn when Uranus and Neptune are not present. In those cases where the outer edge of the disk is close to Saturn or Neptune, the fraction of material that ends up in the subsequently formed OC is much less than that found in Brasser et al. [Brasser, R., Duncan, M., Levison, H., 2006. Icarus 184, 59-82] for the same cluster densities. This implies that for comets of roughly 2 km in size, the presence of the primordial solar nebula hinders OC formation. A byproduct of some of our simulations are endresults with a substantial fraction of the comets in the Uranus-Neptune scattered disk. A subsequent followup of this material is planned for the near future. In order to determine the effect of the size of the comets on OC formation efficiency, a set of runs with the same initial conditions but different cometary radii have been performed as well, from which it is determined that the threshold comet size to begin producing significant Oort clouds is roughly 20 km. This implies that the presence of the primordial solar nebula acts as a size-sorting mechanism, with large bodies unaffected by the gas drag and ending up in the OC while small bodies remain trapped in the planetary region, in the models studied.     

Prediction of deuterium abundance in comets

Using a chemical scheme based on ion-molecule reactions in cool interstellar clouds, the possibility of a deuterium enrichment of volatile material in comets is discussed. It is assumed that the hydrogen-containing molecules are deuterated before accretion and condensation on the coremantle dust particles from which the cometesimals are formed. The D/H ratio in comets may be enhanced in respect to the average value by a factor of 10². Therefore comets are not promising objects for testing the primordial deuterium abundance.     

Water and related chemistry in the solar system. A guaranteed time key programme for Herschel

“Water and related chemistry in the Solar System” is a Herschel Space Observatory Guaranteed-Time Key Programme. This project, approved by the European Space Agency, aims at determining the distribution, the evolution and the origin of water in Mars, the outer planets, Titan, Enceladus and the comets. It addresses the broad topic of water and its isotopologues in planetary and cometary atmospheres. The nature of cometary activity and the thermodynamics of cometary comae will be investigated by studying water excitation in a sample of comets. The D/H ratio, the key parameter for constraining the origin and evolution of Solar System species, will be measured for the first time in a Jupiter-family comet. A comparison with existing and new measurements of D/H in Oort-cloud comets will constrain the composition of pre-solar cometary grains and possibly the dynamics of the protosolar nebula. New measurements of D/H in giant planets, similarly constraining the composition of proto-planetary ices, will be obtained. The D/H and other isotopic ratios, diagnostic of Mars’ atmosphere evolution, will be accurately measured in H₂O and CO. The role of water vapor in Mars’ atmospheric chemistry will be studied by monitoring vertical profiles of H₂O and HDO and by searching for several other species (and CO and H₂O isotopes). A detailed study of the source of water in the upper atmosphere of the Giant Planets and Titan will be performed. By monitoring the water abundance, vertical profile, and input fluxes in the various objects, and when possible with the help of mapping observations, we will discriminate between the possible sources of water in the outer planets (interplanetary dust particles, cometary impacts, and local sources). In addition to these inter-connected objectives, serendipitous searches will enhance our knowledge of the composition of planetary and cometary atmospheres.     

Theoretical predictions of deuterium abundances in the Jovian planets

Using current concepts for the origin of the Jovian planets and current constraints on their interior structure, we argue that the presence of large amounts of “ice” (H₂O, CH₄, and NH₃) in Uranus and Neptune indicates temperatures low enough to condense these species at the time Uranus and Neptune formed. Yet such low temperatures imply orders-of-magnetude fractionation effects for deuterium into the “ice” component if isotopic equilibration can occur. Our models thus imply that Uranus and Neptune should have a D/H ratio at least four times primordial, contrary to observation for Uranus. We find that the Jovian and Saturnian D/H should be close to primordial regardless of formation scenario. The Uranus anomaly could indicate that there was a strong initial radial gradient in D/H in the primordial solar nebula, or that Uranus is so inactive that no significant mixing of its interior has occurred over the age of the solar system. Observation of Neptune's atmospheric D/H may help to resolve the problem.     

Comets and the formation of planets

A morphological study of the physical and dynamical processes of planet formation is presented, with emphasis on the intermediary role of comet nuclei. Although guided by a particular model of the evolution of the pre-planetary solar nebula, implying the freezing-out of hydrogen in the region of the giant planets, the derivations and conclusions are of wider import, applicable to other cosmogonic models as well as to certain phases of star formation. The items evaluated physically, dynamically, or statistically comprise: (1) the total number mass of comets in Oort's cloud; (2) a re-evaluation of the diameters and masses of cometary nuclei; (3) the processes of nucleation from gravitational and Boltzmann instabilities of gaseous media to agglomerations of particulate matter as conditioned by inbuilt angular momentum; (4) the statistical-dynamical conditions and time scales of orbital interaction of comets with the planets and the consequences of disintegration.A consistent model proposes the formation of comets and planets in pre-planetary rings of the residual solar nebula, with subsequent ejection, chiefly by Jupiter, of the comets to Oort's sphere. Screening by absorbing matter is not only probable, but necessary to protect the comets from dis-integration during the process of ejection.Paper dedicated to Prof. H. C. Urey on the occasion of his 80th birthday on 29 April, 1973.This work has been currently supported by grants from the National Aeronautics and Space Administration, Goddard Space Flight Center, Greenbelt, Maryland.     

New constraints on the delivery of cometary water and nitrogen to Earth from the N/N isotopic ratio

New independent constraints on the amount of water delivered to Earth by comets are derived using the ¹⁵N/¹⁴N isotopic ratio, measured to be roughly twice as high in cometary CN and HCN as in the present Earth. Under reasonable assumptions, we find that no more than a few percent of Earth’s water can be attributed to comets, in agreement with the constraints derived from D/H. Our results also suggest that a significant part of Earth’s atmospheric nitrogen might come from comets. Since the ¹⁵N/¹⁴N isotopic ratio is not different in Oort-cloud and Kuiper-belt comets, our estimates apply to the contribution of both types of objects.     

Accretion in the inner nebula: The relationship between terrestrial planetary compositions and meteorites*

Abstract— The bulk compositions of the terrestrial planets are assessed. Venus and Earth probably have similar bulk compositions, but Mars is enriched in volatile elements. The inner planets are all depleted in volatile elements, as shown by K/U ratios, relative to most meteorites and the CI primordial values. Terrestrial upper mantle Mg/Si ratios are high compared with CI data. If they are representative of the bulk Earth, then the Earth accreted from a segregated suite of planetesimals that had non-chondritic major element abundances. The CI meteorite abundances, despite aqueous alteration, match the solar data and provide the best estimate for the composition of the solar nebula, including the iron abundance. The widespread depletion of volatile elements in the inner solar nebula is most likely caused by heating related to early violent solar activity (e.g., T Tauri and FU Orionis stages) which, for example, drove water out to a “snow line” in the vicinity of Jupiter. The variation in composition among the meteorites and the apparent lack of mixing among the groups indicates accretion from narrow feeding zones. There appears to have been little mixing between meteorite and planetary formation zones, as shown by the oxygen isotope variations, lack of mixing of meteorite groups, and differences in K/U ratios. In summary, it appears that the final accretion of planets did not result in widespread homogenization, and that mixing zones were not more than about 0.3 A.U. wide. Although the composition of the Moon is unique, and its origin due to an essentially random event, its presence reinforces the planetesimal hypothesis and the importance of stochastic processes during planetary accretion in the inner solar system.     

Detection of CO and Ethane in Comet 21P/Giacobini-Zinner: Evidence for Variable Chemistry in the Outer Solar Nebula

Ethane and carbon monoxide were detected in a short-period comet of probable Kuiper Belt origin. Ethane is substantially less abundant compared with Hyakutake and Hale-Bopp, two comets from the giant-planet region of the solar nebula, suggesting a heliocentric gradient in ethane in precometary ices. It is argued that processing by X-rays from the young Sun may be responsible.     

Deuterium fractionation in the presolar nebula: Kinetic limitations on surface catalysis

Models assuming low temperature equilibrium fractionation have previously been employed to explain the range of D/H values observed in the solar system and to make predictions of deuterium enhancement in the outer planets. While the reaction rates of the homogeneous partitioning reactions are prohibitively low at nebular temperatures, it has been suggested that catalysis on grains could shorten equilibrium times sufficiently. This idea is quantitatively tested here. We find that under highly idealized conditions—the full cosmic abundance of nickel available for catalysis in pure 5-μm grains—the equilibrium time constant becomes greater than the lifetime of the nebula at temperatures lower than 560°K. Even this firm lower limit is not cold enough to allow strong fractionation. Speculations are offered on alternative explanations for the distribution of hydrogen isotopes in the solar system.     

The search for other planets: Clues from the solar system

Studies of element abundances and values of D/H in the atmospheres of the outer planets and Titan support a two-step model for the formation of these bodies. This model suggests that the dimensions of Uranus provide a good index for the sensitivity required to detect planets around other stars. The high proportion of N₂ on the surfaces of Pluto and Triton indicates that this gas was the dominant reservoir of nitrogen in the early solar nebula. It should also be abundant on pristine comets. There is evidence that some of these comets may well have brought a large store of volatiles to the inner planets, while others were falling into the sun. In other systems, icy planetesimals falling into stars should reveal themselves through high values of D/H.Paper presented at the Conference on Planetary Systems: Formation, Evolution, and Detection held 7–10 December, 1992 at CalTech, Pasadena, California, U.S.A.     

Cosmic Ray-Induced Polycondensate Hydrocarbons in Giant Molecular Clouds

Astrophysical and cosmochemical data show that many kinds of hydrocarbons are widespread in space, including giant molecular clouds, diffuse interstellar medium, comets, interplanetary dust particles, and carbonaceous meteorites. Here an effort is made to show the close relation between high-molecular weight hydrocarbons observed in space and existing on Earth. Results of astrochemical modelling of dust grains in dense collapsing cores of giant molecular clouds are also presented. They show that about 10% of the total abundance of dust grains may be the result of aliphatic hydrocarbons. This dust serves as initial material for comets, formed in protosolar nebula. The problem of survival of cometary organics during impact onto the Earth is discussed, and it is shown that the so-called soft-landing comet hypothesis may explain the accumulation of complex hydrocarbons on the Earth's surface. We conclude that a significant fraction of terrestrial prebiotic petroleum was delivered by extraterrestrial matter.     

Protosolar nebula and composition of comets

Comets seem to be composed of matter, which is supposed to have the same molecular composition as protosolar nebula. Although there are no unbiased evidence that cometary nuclei retain the molecular composition inherited from the protosolar cloud, the observed properties of comets indicate that there is at least a resemblance between cometary composition and the material properties of dense interstellar clouds. Therefore the origin of comets could be searched in the cold stages of the protosolar nebula and molecular abundances of grain mantles in this nebula may be similar to those in the cometary dust. It is suggested that comets may contain pristine, virtually unaltered protosolar material and their study might be very relevant way to more information about processes in early stages of the solar nebula. Our knowledge about composition of the cometary nucleus is still relatively scarce, but we can partly deduce it from data obtained either by ground-based spectroscopy or by in situ mass spectrometry from space experiments. Most important were the discovery of fluffy CHON particles composed partly or even completely from compounds containing light elements. No consensus concerning the presence of interstellar pristine matter in comet has been reached from various approaches to determine the relationship between comets and interstellar grains. Most of these studies are based on infrared spectroscopy. Another method is the comparison on the chemical models of the protosolar nebula with the volatile compounds of the cometary nuclei. Both gas-phase and grain-surface chemistry are considered and initial gas-phase atomic abundances are assumed to be protosolar. The cometary matter is certainly not identical with the typical material of dense interstellar cool dense clouds, but it is closer to it than any other type of matter in solar system so far accessible to us. The data from comets combined with models of chemical evolution of matter in environment similar as prevailed the early stage of presolar nebula may at least impose constrains on the condition for comet formation. Here presented study is a preliminary contribution to such studies.     

The Origin and Evolution of the Atmospheres of Venus, Earth and Mars

The chemical compositions of the primordial atmospheres of Venus, Earth and Mars have long been a topic of debate between the experts. Some believe that the original atmospheres were a product of outgassed volatiles from the newly accreted terrestrial planets and that these atmospheres consisted primarily of carbon dioxide, nitrogen, water vapor and residual hydrogen and helium (e.g., Lewis and Prinn, <it>Planets and their Atmospheres,</it> Academic Press, Orlando, FL, 1984, pp. 62–63, 81–84, 228–231, 383). Still others think the earliest atmospheres were composed of the gas components of the solar nebula from which the solar system formed (i.e., hydrogen, helium, methane, ammonia and water). I consider the latter to be the correct scenario. Presented herein is a proposed mechanism by which the original atmospheres of Venus, Earth and Mars were transformed to atmospheres rich in carbon dioxide and nitrogen. An explanation is proposed for why water is so common on the surface of Earth and so scarce on the surfaces of Venus and Mars. Also presented are the effects the “great impact” (single cataclysmic event that was responsible for producing the Earth–Moon system) had upon the early atmosphere of Earth. The origin, structure and composition of the impacting object are determined through deductive analyses.     

星云中继假说: 星云中的原始生命和地球生命的起源

提出了关于地球生命起源的新模型---星云中继假说, 它是宇宙胚种论的修改版本. 在这个模型中, 作为宇宙``种子''的原始生命起源于太阳系的前身恒星系统中的生物化学过程, 并且在前身恒星死亡后充满整个原太阳星云. 地球生命的起源可以分为3个阶段: 太阳前身恒星的原始生命起源, 原太阳星云时期和太阳系形成与地球生命时期. 这个模型最主要的推论是原始生命(或其后裔)以及它们的化石存在于太阳系内各种天体之中.     

On the origin of comets

The fact that comets are rich in volatile material shows that they were formed and kept for a long time in the outer, low-temperature regions of the solar system. In this paper we analyse the structure in the outer edge of the solar nebula and show that no formation zone of comets can exist there. Our view is that the comets evolved from the residual planetesimals in the zone between Jupiter and Neptune.     

The D/H Ratio in Methane in Titan: Origin and History

We propose a new interpretation of the D/H ratio in CH₄ observed in the atmosphere of Titan. Using a turbulent evolutionary model of the subnebula of Saturn (O. Mousis et al. 2002, Icarus156, 162-175), we show that in contrast to the current scenario, the deuterium enrichment with respect to the solar value observed in Titan cannot have occurred in the subnebula. Instead, we argue that values of the D/H ratio measured in Titan were obtained in the cooling solar nebula by isotopic thermal exchange of hydrogen with CH₃D originating from interstellar methane D-enriched ices that vaporized in the nebula. The rate of the isotopic exchange decreased with temperature and became fully inhibited around 200 K. Methane was subsequently trapped in crystalline ices around 10 AU in the form of clathrate hydrates formed at 60 K, and incorporated into planetesimals that formed the core of Titan. The nitrogen-methane atmosphere was subsequently outgassed from the decomposition of the hydrates (Mousis et al. 2002). By use of a turbulent evolutionary model of the solar nebula (O. Mousis et al. 2000, Icarus148, 513-525), we have reconstructed the entire story of D/H in CH₄, from its high value in the early solar nebula (acquired in the presolar cloud) down to the value measured in Titan's atmosphere today. Considering the two last determinations of the D/H ratio in Titan—D/H=(7.75±2.25)×10⁻⁵ obtained from ground-based observations (Orton 1992, In: Symposium on Titan, ESA SP-338, pp. 81-85), and D/H=(8.75^+3.25_−2.25)×10⁻⁵, obtained from ISO observations (Coustenis et al. 2002, submitted for publication)—we inferred an upper limit of the D/H ratio in methane in the early outer solar nebula of about 3×10⁻⁴. Our approach is consistent with the scenario advocated by several authors in which the atmospheric methane of Titan is continuously replenished from a reservoir of clathrate hydrates of CH₄ at high pressures, located in the interior of Titan. If this scenario is correct, observations of the satellite to be performed by the radar, the imaging system, and other remote sensing instruments aboard the spacecraft of the Cassini-Huygens mission from 2004 to 2008 should reveal local disruptions of the surface and other signatures of the predicted outgassing.