Session 4

Water and life in the Solar System

10/11 – Tuesday

Afternoon

15:00 - 15:30 Why is the Earth so dry? (Alessandro Morbidelli)

Authors
  1. Alessandro Morbidelli (CNRS/Observatoire de la Cote d’Azur, Nice, France)
Abstract

A simple calculation of the temperature in the proto-planetary disk as a function of the disk’s accretion rate shows that the snowline should have moved inside 1 AU before that the disk disappeared. Thus, all objects down to 1 AU or even less should be water-rich. However, the Earth, with its budget of only about 0.1% of an Earth mass in water, is water poor. Ordinary chondrites and, particularly, enstatite chondrites are also water poor, despite they assembled about 3 My after the beginning of the Solar System (CAI formation time), a time at which the proto-planetary disk should have been very cold.

Just a few partial solutions of this problem have been proposed in the literature so far. Here we show that even if the disk becomes cold, there cannot be direct condensation of water. This is because the snowline moves towards the Sun more slowly than the radial motion of the gas. Thus the gas in the vicinity of the snowline always comes from farther out, it should have already condensed there and therefore it should be dry. The appearance of ice in a portion of the disk swept by the snowline can only be due to the radial drift of icy particles from the outer disk. However, if a planet with a mass larger than 20 Earth mass is present, the radial drift of particles is interrupted, because such a planet gives the disk a super-Keplerian rotation just outside of its own orbit. From this result, we propose that the precursor of Jupiter achieved this threshold mass when the snowline was still around 3 AU, probably in the first My of Solar System existence. This effectively fossilized the snowline at that location. In fact, even if the disk cooled later, the disk inside of Jupiter’s orbit remained ice depleted because the flow of icy particles from the outer system was intercepted by the planet.

According to this scenario, the water-rich planetesimals should have formed behind Jupiter’s orbit. This may appear inconsistent with the existence of primitive and icy asteroids in the outer asteroid belt. However, the Grand Tack scenario solves this apparent inconsistency. In fact in this scenario the radial migration of Jupiter, first inwards, then outwards, implanted in the asteroid belt a fraction of the planetesimals originally beyond Jupiter’s orbit. The small amount of water on Earth was also delivered by icy trans-Jovian planetesimals scattered inwards during the outward migration of the giant planet.

15:30 - 16:00 Terrestrial planet formation at home and abroad (Sean Raymond)

Authors
  1. Sean Raymond (CNRS/Laboratoire d’Astrophysique de Bordeaux)
  2. Alessandro Morbidelli (CNRS/Observatoire de la Cote d’Azur, Nice, France)
  3. Andre Izidoro (CNRS/Laboratoire d’Astrophysique de Bordeaux)
Abstract

Our vision of planet formation — long tied to studies attempting to reproduce the orbital architecture of the Solar System — has been irrevocably changed by the discovery of thousands of extra-solar planets. The distribution of exoplanets shows a much broader diversity and includes categories of planets that are not represented in the Solar System. The main goal of planet formation is to build a model that naturally encompasses and explains this diversity. The known extra-solar systems can be divided into just a few categories: i) Solar System-like systems with Jupiter-like planets on wide-separation, near-circular orbits; ii) systems with “angry” gas giant planets on eccentric or very short-period orbits; and iii) systems containing only low-mass planets, often referred to as “hot super-Earths”.  The systems in each category have a rich dynamical history that plays the central role in shaping the formation (and in some cases destruction) of rocky worlds. It also controls the delivery (or lack thereof) of water-rich material from the cold icy regions of planet-forming disks into the terrestrial planet zone. Indeed, Earth’s water is thought to have been delivered by the impacts of countless water-rich bodies that condensed in the asteroid belt or perhaps beyond Jupiter and Saturn.

Although both terrestrial planets and gas giants form from the same building blocks — so-called “planetary embryos” — gas giants are constrained to grow very quickly and thus they influence the final assembly of terrestrial planets. In systems without giant planets, interactions with the protoplanetary disk causes embryos to migrate inward and become hot super-Earths, presumably with very large water contents. However, if an embryo grows large enough to capture gas from the disk and become a giant planet, it may act as a barrier to the inward migration of other super-Earths; this process can explain the lack of super-Earths in the Solar System. Systems with gas giants are shaped by two key processes: orbital migration and scattering. Orbital migration of gas giants is typically inward (a possible origin of hot Jupiters) but can in some configurations be directed outward (as in the Grand Tack model for the Solar System). In either case, migrating giants snowplow rocky material and drastically alter the growth and feeding zones of terrestrial planets. In fact, giant planet migration causes such strong radial mixing that it enhances water delivery to terrestrial planets. Finally, their eccentric orbits suggest that at least 75% of the known systems of giant exoplanets are thought to have undergone violent instabilities resulting gravitational scattering between gas giants and the destruction of one or more giant planets. These instabilities systematically destroy terrestrial planets (or their building blocks), typically by driving them into the central star.

To conclude, there are specific branching points between different dynamical outcomes: for instance, the growth of a gas giant planet and an instability in a system of gas giants. We propose that these branching points are central in explaining the diversity of planetary systems and of terrestrial planets.

16:00 - 16:30 Holding back the invaders: Gas giant planets as dynamical barriers to inward-migrating super-Earths (Andre Izidoro)

Authors
  1. Andre Izidoro (CNRS/Laboratoire d’Astrophysique de Bordeaux)
  2. Sean Raymond (CNRS/Laboratoire d’Astrophysique de Bordeaux)
  3. Alessandro Morbidelli (CNRS/Observatoire de la Cote d’Azur, Nice, France)
Abstract

Our current understanding of planet formation has been challenged by the discovery of exotic planetary systems. One of the most intriguing findings is that, unlike our solar system, around 30-50% of the nearby Sun-like stars host planets with orbital period shorter than 100 days. These planets — or their building blocks — may have formed on wider orbits, in water-rich regions of the protoplanetary disk, and migrated inward due to gravitational interactions with the gaseous natal disk. If hot Super-Earths are so abundant the key question then becomes: why are there no hot super-Earths in our Solar System? Here, we address this question by using N-body dynamical simulations. We show that gas giant planets may shape the fate of planetary systems. When a planetary core becomes a gas giant planet it acts as a barrier to the inward migration of planetary cores (or super-Earths) initially placed on more distant orbits. This nicely places the Solar System in the context of extrasolar planetary systems. If we assume that Jupiter was the innermost core and first to grow to a gas giant, it follows that Jupiter held back an invasion of inward-migrating bodies. Thus, Jupiter’s early formation may have prevented Uranus and Neptune (and perhaps Saturn’s core) from becoming hot super-Earths. Importantly, we also show that the passage of multi-Earth-mass ice planets by the terrestrial zone (region inside 2-3 AU) represents a catastrophic event for the formation of terrestrial planets, except if the ice-planets migration speed is sufficiently fast. Inward-migrating super-Earths may sweep away the rocky building blocks of terrestrial planets. The lack of hot-super Earth in the solar system may explain why the Earth does not contain drastically more water. Finally, our model predicts that the populations of hot super-Earth systems and Jupiter-like planets are anti-correlated: gas giants (especially if they form early) should be rare in systems with many hot super-Earths. Testing this prediction will constitute a crucial assessment of the validity of the migration hypothesis for the origin of close-in super-Earths.

17:30 - 17:50 Water and volatile abundances on Earth-like planets in low-mass disks (Maria Paula Ronco)

Authors
  1. Maria Paula Ronco (Astrophysical Institute – University of La Plata – CONICET)
  2. Amaury Thiabaud (Physics Institute and Center for Space and Habitability, University of Bern)
  3. Ulysse Marboeuf (Physics Institute and Center for Space and Habitability, University of Bern)
  4. Yann Alibert (Physics Institute and Center for Space and Habitability, University of Bern – Observatoire de Besancon, France)
  5. Gonzalo Carlos de Elía (Astrophysical Institute – University of La Plata – CONICET)
  6. Octavio Miguel Guilera (Astrophysical Institute – University of La Plata – CONICET)
Abstract

The study of the chemical composition, especially the amounts of water of extrasolar planets, is a topic of current interest since it is partly responsible for their potential habitability. It is particularly interesting to study the chemical composition of Earth-like planets formed within the habitable zone of low-mass protoplanetary disks, which are incapable of forming giant planets. According to observational and theoretical studies, planetary systems composed by only rocky planets seem to be the most common in the universe and owing to that they result to be targets of significative interest.

In this work, we present results of numerical simulations aimed to analyzing planetary formation processes and distribution of volatile elements around a solar-type star and assuming low-mass protoplanetary disks. In particular, we study protoplanetary disks with and without irradiation. For each of such scenarios, we calculate the condensation sequences of the different volatile elements (H2O, CO, CO2, CH4, H2S, N2, NH3 and CH3OH) and then, we incorporate them in a semi-analytical model, which calculates the evolution of planetary embryos and planetesimals during the gaseous phase. Once the gas is dissipated, the distributions of embryos and planetesimals obtained with the semi-analytical model are used as initial conditions to develope N-body simulations, which describe the main dynamical processes associated to the planetary formation.

The main goal of the present work is to analyze the final chemical composition of the planets that remain in the habitable zone for both the irradiated and the non-irradiated disk. In particular, we are interested in studying the differences between both scenarios of work. Moreover, we focus on the discrepancies obtained between the chemical compositions of the planets resulting of our simulations and those associated to the planets of our Solar System. This investigation will allow us to strengthen our understanding about the formation and evolution of a planetary system around solar-type stars.

17:50 - 18:10 Compound model for the origin of earth’s water revised under new theories (Karla Torres)

Authors
  1. Karla Torres (CEFET – MG)
  2. Othon Winter (UNESP – FEG)
Abstract

One of the most important subjects of debate in the formation of Solar System is the origin of Earth’s Water. Izidoro et al. (2013) [ApJ 767, 54] have proposed a compound model that consider local adsorption of water onto dust grains in the primordial nebula, and water delivery through planetesimals and planetary embryos from beyond the snow line. Comets are also considered in the final analysis. Incorporating both endogenous and exogenous theories, and using D/H ratios of different water sources, it was found possible relative contributions from each source, focusing on planets formed in the habitable zone (HZ). Here we analyze those results considering new planetary formation models and evidences, as well as constraints for life. Although the local water adsorption theory still face critics regarding to empirical evidences of meteorites and asteroids (indicating that it was too hot around 1 AU for the adsorption to be efficient), the positions of asteroids today may not be the same as their formation location, something that has been suggested by a variety of dynamical models. If so, the distribution of asteroids compositions today cannot be used to provide strong constraints on the thermal structure on the solar nebula. Despite of that, it has been suggested that not only water but also organics could have been delivered to early Earth via direct adsorption mechanisms, thereby providing an endogenous source also for planetary organics. Asteroids are still considered the main source of Earth’s water not only because of their water’s D/H ratio range close to the terrestrial water value (SMOW), but also because of a new model of inward-then-outward migration of Jupiter and Saturn, known as the ‘Gran Tack’ model. This model is successful in justifying a truncation of the disk of planetary embryos that could form Mars-like planets, and the delivery of water-rich planetesimals and planetary embryos to the terrestrial zone as a natural consequence of the migration processes. However, a truncated disk leads to formation timescales more rapid than suggested by radiometric chronometers. Although comets have been ruled out as a main source of Earth’s water because of previous dynamical models and their D/H ratio values (normally over twice the terrestrial value), it can be noted that two Jupiter-family comets were recently found to have D/H ratio close to SMOW. With both endogenous and exogenous sources considered, planets in the HZ were made in the simulations with 1 to 40 terrestrial oceans. Nonetheless, it has been calculated that the appropriate range of the amount of seawater on a rocky planet for the emergence and evolution of life is quite narrow, ranging from 0.007 to 0.7% of the Earth mass. On planets richer in water, continents would be globally and permanently covered with oceans and, in those circunstances, weathering never operates to supply nutrients continuously from continents, so that life (as we know it) would be hard to emerge and evolve.

18:10 - 18:30 Origin of Terrestrial Water Inferred from Hydrogen Isotopes (Sho Sasaki)

Authors
  1. Sho Sasaki (Osaka University)
Abstract

The Earth is frequently called as a water planet. But there have been debates on the origin of terrestrial water, mostly residing as the ocean. Significant amount of water would be trapped in the interior of the Earth: the present ocean mass 1.4 x 1021 kg could be the minimum terrestrial water mass. The mean deuterium-to-hydrogen ratio (D/H) of the ocean (1.558 x 10-4) is considered to represent the mean value of the Earth.
For a long time, similarity of terrestrial D/H and meteorite D/H had been considered as evidence of which degassing from accreting solid silicate materials should consist of terrestrial water. The D/H values of two typical meteorite types, carbonaceous and ordinary chondrites, varies suggesting heterogeneity of solar nebula, but the peak values of their D/H match the Earth’s value.
On the other hand, previously D/H values observed in long-period comets (e.g. Halley, Hale Bopp, Hyakutake) were twice higher than terrestrial value. These long-period comets are considered to come from the Oort cloud. The difference between meteorite (asteroid) D/H and cometary D/H could be explained by the decrease of initial high D/H (e.g., 0.01 of organic molecules in cold molecular clouds) by kinetic isotopic exchange in the protosolar nebula between D-rich water and molecular hydrogen. Then the meteorite D/H and cometary D/H would represent materials of the inner and outer solar system, respectively.
Recent measured D/H of Jupiter-group comet Hartley by Herschel Space Observatory is similar to the terrestrial value [1], which strongly suggested the cometary (from Kuiper belt zone) origin of terrestrial water. Jupiter group comets are considered to have come from Kuiper belt zone (closer to the sun than the Oort cloud). The present observation data suggests rather uniform D/H of water ice from Kuiper belt zone to asteroid zone (and to the terrestrial zone). One plausible mechanism to homogenize D/H is the so-called “snow line” (frost line), i.e., the boundary between the ice-stable outer zone and ice-unstable inner zone.
There would be other sources of hydrogen. Under the accretion after dissipation of the protoplanetary gas disk, accreting material should have hydrogen from the proto-solar wind with low D/H. On the other hand, under the accretion within the protoplanetary gas disk, the Earth should have the primary atmosphere from nebular gas. Due to the blanketing effect of the atmosphere, the surface of the accreting Earth was molten and interaction of atmospheric hydrogen with silicate produced significant amount of water vapor [2, 3], with D/H of the order of 10^{-5}. However if the initial amount of water from the oxidation of nebular gas is much larger than the present amount, D/H would have been raised through the escape process of the atmosphere.

References
[1] Hartogh., P. et al. (2011) Nature 478, 218. [2] Sasaki, S. (1990) in Origin of the Earth. Oxford Univ. Press. pp. 195– 209., [3] Ikoma, M. and Genda, H. (2006) Astrophys. J. 648, 696.

Abstract

The Earth is frequently called as a water planet. But there have been debates on the origin of terrestrial water, mostly residing as the ocean. Significant amount of water would be trapped in the interior of the Earth: the present ocean mass 1.4 x 1021 kg could be the minimum terrestrial water mass. The mean deuterium-to-hydrogen ratio (D/H) of the ocean (1.558 x 10-4) is considered to represent the mean value of the Earth.
For a long time, similarity of terrestrial D/H and meteorite D/H had been considered as evidence of which degassing from accreting solid silicate materials should consist of terrestrial water. The D/H values of two typical meteorite types, carbonaceous and ordinary chondrites, varies suggesting heterogeneity of the solar nebula, but the peak values of their D/H match the Earth’s value.
On the other hand, previously D/H values observed in long-period comets (e.g. Halley, Hale Bopp, Hyakutake) were twice higher than the terrestrial value. These long-period comets are considered to come from the Oort cloud. The difference between meteorite (asteroid) D/H and cometary D/H could be explained by the decrease of initial high D/H (e.g., 0.01 of organic molecules in cold molecular clouds) by kinetic isotopic exchange in the protosolar nebula between D-rich water and molecular hydrogen. Then the meteorite D/H and cometary D/H would represent materials of the inner and outer solar system, respectively.
Recent measured D/H of Jupiter-group comet Hartley by Herschel Space Observatory is similar to the terrestrial value [1], which strongly suggested the cometary (from Kuiper belt zone) origin of terrestrial water. Jupiter group comets are considered to have come from Kuiper belt zone (closer to the sun than the Oort cloud). The present observation data suggests rather uniform D/H of water ice from Kuiper belt zone to asteroid zone (and to the terrestrial zone). One plausible mechanism to homogenize D/H is the so-called “snow line” (frost line), i.e., the boundary between the ice-stable outer zone and ice-unstable inner zone.
There would be other sources of hydrogen. Under the accretion after dissipation of the protoplanetary gas disk, accreting material should have hydrogen from the proto-solar wind with low D/H. On the other hand, under the accretion within the protoplanetary gas disk, the Earth should have the primary atmosphere from nebular gas. Due to the blanketing effect of the atmosphere, the surface of the accreting Earth was molten and interaction of atmospheric hydrogen with silicate produced significant amount of water vapor [2, 3], with D/H of the order of 10^{-5}. However if the initial amount of water from the oxidation of nebular gas is much larger than the present amount, D/H would have been raised through the escape process of the atmosphere.

References
[1] Hartogh., P. et al. (2011) Nature 478, 218. [2] Sasaki, S. (1990) in Origin of the Earth. Oxford Univ. Press. pp. 195– 209., [3] Ikoma, M. and Genda, H. (2006) Astrophys. J. 648, 696.

18:30 - 18:50 On the origin(s) of water and organics on the terrestrial planets (Bernard Marty)

Authors
  1. Bernard Marty (Université de Lorraine, CRPG-CNRS)
Abstract

Space missions have permitted exceptional in-situ measurements and investigations on samples returned to Earth. In particular, the Stardust (NASA) mission that returned cometary grains, the Genesis (NASA) mission that returned solar wind ions and the Rosetta (ESA) mission which is currently measuring the composition of volatiles released by Comet 67P/CG give insight into the elemental and isotopic compositions of light elements and noble gases in the protosolar nebula and in the outer solar system. These measurements, together with those obtained from the analysis of meteorites and of lunar samples, show large, somewhat outstanding, variations of the stable isotope ratios of hydrogen, nitrogen and oxygen. Compared to the protosolar nebula composition, all objects and reservoirs (but the giant planet atmospheres) are rich in the heavy and rare isotopes 2H, 15N, 17,18O by several tens to hundreds of percents. The origin(s) of these enrichments, presently debated, could be interactions between the nebular gas and UV photons from the young Sun, and possibly, with growing dust grains. These isotope tracers allow us to investigate rthe elationships between meteorites, asteroids, inner planets, and comets. The picture that emerges from the above measurements is that terrestrial and inner planetary volatiles were primarily sourced by the protosolar nebula gas and (then) by bodies akin to present-day volatile-rich asteroids known as carbonaceous chondrites. The latest measurements made by the ROSINA instrument on-board of the Rosetta spacecraft confirm that the oceans and terrestrial organics can hardly derive from a 67P/CG-like reservoir. However, they also leave room for significant contributions of comets to the noble gas inventory of the inner planets.