Tag Archives: chemistry

The lunar history

(Alternative title: The search for the origin of the Late Heavy Bombardment)

Hi there! It is a pleasure for me to present you today a multi-disciplinary study, which mixes celestial mechanics with geochemistry. If you want to know the past of a planetary body, you must go backward: you start from the body as you observe it nowadays, and from this you infer the processes which made it evolve from its formation to its present state. In The timeline of the Lunar bombardment – revisited, by A. Morbidelli, D. Nesvorný, V. Laurenz, S. Marchi, D.C. Rubie, L. Elkins-Tanton, M. Wieczorek and S. Jacobson, the authors exploit our observations of the craters and the chemistry of the Moon, and simulations of the motion of asteroids in the early Solar System, to give new constraints on the bombardment of the Moon between 3.9 and 3.7 Gyr (billions of years) ago, which is famous as the Late Heavy Bombardment (LHB). We will see that the results have implications for Mars. This study has recently been accepted for publication in Icarus.

The Lunar basins

Let us start from what we observe: the Lunar surface. This is a heavily cratered surface. Actually, the absence of atmosphere preserves it from erosion, and the small size of the Moon limits its heating, as a consequence the craters neither erode nor relax. Hence, the surface of the Moon is a signature of the activity in the early Solar System.

Let us focus on the largest structures, i.e. the maria and the basins. The maria are lava plains, which result from a volcanic activity of the early Moon. However, the basins are the largest impact craters. You can find below the largest ones, of course many smaller craters exist.

Basin Diameter (km)
South Pole-Aitken 2,600
Imbrium 1,100
Orientale 930
Serenitatis 920
Australe 880
Nectaris 860
Smythii 740
Crisium 740
Tranquillitatis 700
Tsiolkovsky-Stark 700
Fecunditatis 690
Mutus-Vlacq 690
Nubium 690

The early Moon was hot, because of the impact which created it. As a hot body, it stratified into a fluid core, a mantle and a crust, while cooling. The visible impact craters are younger than the crust, i.e. they are younger than 3.9 Gyr, and were created at least 600 Myr after the formation of the Moon… pretty late, hence due to the Late Heavy Bombardment.

Orientale Basin. © NASA
Orientale Basin. © NASA

Origin of the LHB: cataclysm or accretion tail?

Late Heavy Bombardment means that the inner Solar System have been intensively bombarded late after its genesis. But how did that happen? Two scenarios can be found in the literature:

  1. Cataclysm: the very young Solar System was very active, i.e. composed of many small bodies which collided, partly accreting… and became pretty quiet during some hundreds of Myr… before suddenly, a new phase of bombardment occurred.
  2. Accretion tail: the Solar System had a slowly decreasing activity, and the craters on the Moon are just the signature of the last 200 Myrs. The previous impacts were not recorded, since the surface was still molten.

The second scenario could be preferred, as the simplest one. The first one needs a cause which would trigger this second phase of bombardment. Anyway, many numerical simulations of the early Solar System got such an activity, as a dynamical phenomenon destabilizing the orbits of a group of small bodies, which themselves entered the inner Solar System and collided with the planets, accreting on them. The giant planets Jupiter and Saturn have a dominant dynamical influence on the small bodies of the Solar System, and could have triggered such an instability. One of the theories existing in the literature is the E-Belt, for extended belt. It would have been an internal extension of the Main Belt of asteroids, which would have been destabilized by a secular resonance with Saturn, and has finished as the impactors of the LHB. Why not, this is a theory.

When you model phenomena having occurred several billions years ago, you have so many uncertainties that you cannot be certain that your solution is the right one. This is why the literature proposes several scenarios. Further studies test them, and sometimes (this is the case here) give additional constraints, which refine them.

Thanks to the Apollo mission, samples of the Moon have been analyzed on Earth, and geochemistry can tell us many things on the history of a body. For the Moon, focus has been put on siderophile elements.

What siderophile elements tell us

A siderophile element is a chemical element which has affinity with iron. Among these elements are iron, iridium, palladium, platinum, rubidium… When a planetary body is hot, it tends to differentiate, and its heaviest elements, i.e. iron, migrate to the core. This results in a depletion of highly siderophile elements (HSE). Since a very small abundance of these elements has been observed, then we have no problem, thank you…

NO NO NO there is actually a problem, since these siderophile elements should be present in the impactors, which are supposed to have accreted on the Moon AFTER its stratification… yes we have a problem.

But some of the authors have shown recently that on Earth, another phenomenon could remove the HSEs from the crust, well after the formation of the core: the exsolution and segregation of iron sulfide. In other words, the bombardment could have brought more HSEs than currently recorded. And this motivates to revisite the history of the Lunar bombardment.

Simulating the bombardment

So, the observations are: the craters, and the HSEs. The craters are not only the basins, but also the smaller ones, with diameters larger than 1 km. Even smaller craters could be used, but the data are considered to be reliable, i.e. exhaustive, for craters larger than 1 km. From that size to the large basins, we can fit a function of distribution, i.e. number of craters vs. diameter. Since there is an obvious correlation between the size of a crater and the one of the impactor, a population of craters corresponds to a population of impactors.

The authors dispose of statistics of collisions, which could be seen as mass accretion, between the Moon and small bodies during the early ages of the Solar System. These statistics result from numerical simulations conducted by some of them, and they can be fine-tuned to fit the crater distribution, their estimated ages, and the abundance of highly siderophile elements. Fine-tuning the statistics consist in artificially moving the parameters of the simulation, for instance the initial number of small bodies, or the date of the instability provoking the cataclysm, in the cataclysm scenario.

Cataclysm possible, accretion tail preferred

And here is the result: if the HSEs are only due to the mass accretion after the cooling of the Lunar crust, then the observations can only be explained by the cataclysm, i.e. the LHB would be due to a late instability. This instability would have resulted in a mass accretion from comets, and this raises another problem: this accretion seems to lack of primitive, carbonaceous material, while the comets contain some.

However, if the HSEs have been removed after the cooling of the crust, then the accretion tail scenario is possible.

We should accept that for this kind of study, the solution is not unique. A way to tend to the unicity of the solution is to discuss further implications, in examining other clues. And the authors mention the tungsten.

Tungsten is another marker

Tungsten is rather a lithophile than a siderophile element, at least in the presence of iron sulfide. In other words, even if it does not dislike iron, it prefers lithium (I like this way of discussing chemistry). Something puzzling is a significant difference in the ratios of two isotopes of tungsten (182W and 184W) between the Moon and the Earth. This difference could be primordial, as brought by the projectile which is supposed to have splitted the proto-Earth into the Earth and the Moon (nickname of the projectile: Theia), or it could be due to the post-formation mass accumulation. In that case, that would be another constraint on the LHB.

Implications for Mars

The LHB has affected the whole inner Solar System. So, if a scenario is valid for the Moon, it must be valid for Mars.
This is why the authors did the job for Mars as well. A notable difference is that Mars would be less impacted by comets than the Moon, and this would affect the composition of the accreted material. More precisely, a cataclysmic LHB would be a mixture of asteroids and comets, while an accretion tail one would essentially consist of leftover planetesimals. It appears that this last scenario, i.e. the accretion tail one, can match the distribution of craters and the abundance of HSEs. However, the cataclysmic scenario would not bring enough HSEs on Mars.


This study tells us that the accretion tail scenario is possible. The authors show that it would imply that

  1. The quantity of remaining HSEs on the Moon is correlated with the crystallization of the Lunar magma ocean, which itself regulates the age of the earliest Lunar crust.
  2. For Mars, the Noachian era would have started 200 Myr earlier than currently thought, i.e. 4.3 Gyr instead of 4.1 Gyr. That period is characterized by high rates of meteorite and asteroid impacts and the possible presence of abundant surface water. Moreover, the Borealis formation, i.e. the northern hemisphere of Mars, which seems to be a very large impact basin, should have been formed 4.37 Gyr ago.

Further studies, explorations, space missions, lab experiments,… should give us new data, which would challenge these implications and refine these scenarios. So, the wording prediction can seem weird for past phenomena, but the prediction is for new clues.

The study and its authors

And that’s it for today! Please do not forget to comment. You can also subscribe to the RSS feed, and follow me on Twitter and Facebook.

The chemistry of Pluto

Hi there! The famous dwarf planet Pluto is better known to us since the flyby of the spacecraft New Horizons in 2015. Today, I tell you about its chemistry. I present you Solid-phase equilibria on Pluto’s surface, by Sugata P. Tan & Jeffrey S. Kargel, which has recently been accepted for publication in The Monthly Notices of the Royal Astronomical Society.

The atmosphere of Pluto

I do not want here to recall everything about Pluto. This is a dwarf planet, which has been discovered by Clyde Tombaugh in 1930. It orbits most of the time outside the orbit of Neptune, but with such an eccentricity that it is sometimes inside. It was discovered in 1978 that Pluto has a large satellite, Charon, so large that the system Pluto-Charon can be seen as a binary object. This binary has at least 4 small satellites, which were discovered thanks to the Hubble Space Telescope.

Pluto has a tenuous atmosphere. It was discovered from the Earth in 1985 in analyzing a stellar occultation: when a faint, atmosphereless object is aligned between a star and a observer, the observer does not see the star anymore. However, when the object has an atmosphere, the light emitted by the star is deviated, and can even be focused by the atmosphere, resulting in a peak of luminosity.

Several occultations have permitted to constrain the atmosphere. It has been calculated that its pressure is about 15 μbar (the one of the Earth being close to 1 bar, so it is very tenuous), and that it endured seasonal variations. By seasonal I mean the same as for the Earth: because of the variations of the Sun-Pluto distance and the obliquity of Pluto, which induces that every surface area has a time-dependent insolation, thermic effects affect the atmosphere. This can be direct effects, i.e. the Sun heats the atmosphere, but also indirect ones, in which the Sun heats the surface, triggering ice sublimation, which itself feeds the atmosphere. The seasonal cycle, i.e. the plutonian (or hadean) year lasts 248 years.

Observations have shown that this atmosphere is hotter at its top than at the surface, i.e. the temperature goes down from 110 K to about 45 K (very cold anyway). This atmosphere is mainly composed of nitrogen N2, methane CH4, and carbon monoxide CO.

The surface of Pluto

The surface is known to us thanks to New Horizons. Let me particularly focus on two regions:

  • Sputnik Planitia: this is the heart that can be seen on a map of Pluto. It is directed to Charon, and is covered by volatile ice, essentially made of nitrogen N2,
  • Cthulhu Regio: a large, dark reddish macula, on which the volatile ice is absent.
A map of Pluto (mosaic made with New Horizons data). © NASA
A map of Pluto (mosaic made with New Horizons data). © NASA

The reason why I particularly focus on these two regions is that they have two different albedos, i.e. the bright Sputnik Planitia is very efficient to reflect the incident Solar light, while Cthulhu Regio is much less efficient. This also affects the temperature: on Sputnik Planitia, the temperature never rises above 37 K, while it never goes below 42.5 K in Cthulhu Regio. We will see below that it affects the composition of the surface.

An Equation Of State

The three main components, i.e. nitrogen, methane, and carbon monoxide, have different sublimation temperatures at 11μbar, which are 36.9 K, 53 K, and 40.8 K, respectively (sublimation: direct transition from the solid to the gaseous state. No liquid phase.). A mixture of them will then be a coexistence of solid and gaseous phases, which depends on the temperature, the pressure, and the respective abundances of these 3 chemical components. The pressure is set to 11μbar, since it was the pressure measured by New Horizons, but several temperatures should be considered, since it is not homogeneous. The authors considered temperatures between 36.5 K and 41.5 K. Since the atmosphere has seasonal variations, a pressure of 11μbar should be considered as a snapshot at the closest encounter with New Horizons (July 14, 2015), but not as a mean value.

The goal of the authors is to build an Equation Of State giving the phases of a given mixture, under conditions of temperature and pressure relevant for Pluto. The surface is thus seen as a multicomponent solid solution. For that, they develop a model, CRYOCHEM for CRYOgenic CHEMistry, which aims at predicting the phase equilibrium under cryogenic conditions. The paper I present you today is part of this development. Any system is supposed to evolve to a minimum of energy, which is an equilibrium, and the composition of the surface of Pluto is assumed to be in thermodynamic equilibrium with the atmosphere. The energy which should be minimized, i.e. the Helmholtz energy, is related to the interactions between the molecules. A hard-sphere model is considered, i.e. a minimal distance between two adjacent particles should be maintained, and for that the geometry of the crystalline structure is considered. Finally, the results are compared with the observations by New Horizons.

Such a model requires many parameters. Not only the pressure and temperature, but also the relative fraction of the 3 components, and the parameters related to the energies involved. These parameters are deduced from extrapolations of lab experiments.


The predicted coexistence of states predicted by this study is consistent with the observations. Moreover, it shows that the small fraction of carbon monoxide can be neglected, as the behavior of the ternary mixture of N2/CH4/CO is very close to the one of the binary N2/CH4. This results in either a nitrogren-rich solid phase, for the coolest regions (the bright Sputnik Planitia, e.g.), and a methane-rich solid phase for the warmest ones, like Cthulhu Regio.

Developing such a model has broad implications for predicting the composition of bodies’s surfaces, for which we lack of data. The authors give the example of the satellite of Neptune Triton, which size and distance to the Sun present some similarities with Pluto. They also invite the reader to stay tuned, as an application of CRYOCHEM to Titan, which is anyway very different from Pluto, is expected for publication pretty soon.

The study and its authors

And that’s it for today! Please do not forget to comment. You can also subscribe to the RSS feed, and follow me on Twitter and Facebook.

The composition of Himalia, Elara, and Carme

Hi there! Today I tell you on 3 irregular satellites of Jupiter, you know, these small bodies which orbit very far from the planet. Himalia, Elara and Carme have been observed in the Near-InfraRed (NIR), and this gave Composition of Jupiter irregular satellites sheds light on their origin, by M. Bhatt et al., which has been recently accepted for publication in Astronomy and Astrophysics.

The irregular satellites of Jupiter

Jupiter has 69 known satellites, which we can divide into 3 groups:

  1. The 4 Galilean satellites Io, Europa, Ganymede and Callisto. These are large bodies, discovered in 1610 by Galileo Galilei,
  2. The 4 inner satellites Amalthea, Metis, Adrastea, and Thebe. These are small bodies, orbiting inside the orbit of Io,
  3. The irregular satellites, which orbit very far from Jupiter. These are small bodies as well, which are usually thought to have been captured, i.e. they probably not formed in the protojovian nebula.

Contrary to the inner and the Galilean satellites, the irregular satellites have pretty eccentric and inclined orbits. Their eccentricities may exceed 0.4, and most of them are retrograde, i.e. with an inclination larger than 90°. In fact, plotting their inclination vs. their semimajor axes reveals clustering.

Semimajor axes and inclinations of the irregular satellites of Jupiter. The inclinations are given with respect to the ecliptic.
Semimajor axes and inclinations of the irregular satellites of Jupiter. The inclinations are given with respect to the ecliptic.

At least 4 dynamical groups have been defined, all of them being named after the largest of their members:

  1. The Himalia group is made of prograde bodies, with inclinations between 26.6° and 28.3°, eccentricities between 0.11 and 0.25, and semimajor axes between 159 and 176 Jupiter radii (while Callisto orbits at 27 Jupiter radii),
  2. The Ananke group is composed of bodies with inclinations between 145.7° and 154.8°, eccentricities between 0.02 and 0.28, and semimajor axes between 250 and 305 Jupiter radii,
  3. The Pasiphase group is made of bodies with inclinations between 144.5° and 158.3°, eccentricities between 0.25 and 0.43, and semimajor axes between 320 and 350 Jupiter radii,
  4. The Carme group is made of bodies with inclinations between 164.9° and 165.5°, eccentricities between 0.23 and 0.27, and semimajor axes between 329 and 338 Jupiter radii

The clustering among these bodies suggests a common origin, i.e. a group of objects would have a unique progenitor. It is also interesting to notice that some groups are more dispersed than others. In particular, the dispersion of the Carme group is very limited. This could tell us something on the date of the disruption of the progenitor. Another clue regarding a common origin is the composition of these bodies.

Before addressing our 3 objects of interest, i.e. Himalia, Elara (member of the Himalia group), and Carme, I would like to mention Themisto and Carpo, which seem to be pretty isolated, and so would not share a common origin with the other bodies. Their dynamics might be affected by the Kozai-Lidov mechanism, which induces a correlated periodic evolution of their eccentrities and inclinations.

Himalia, Elara, and Carme

These 3 bodies are the ones addressed in this study. You can find below their relevant characteristics.

Semimajor axis Eccentricity Inclination Discovery Radius Albedo
Himalia 163.9 Rj 0.16 27.50° 1904 70-80 km 0.04
Elara 167.9 Rj 0.22 26.63° 1905 43 km 0.04
Carme 334.7 Rj 0.25 164.91° 1938 23 km 0.04

These were among the first known irregular moons of Jupiter. The inclinations are given with respect to the ecliptic, i.e. the orbital plane of the Earth. As a member of the Himalia group, Elara has similar dynamical properties with Himalia. We can also notice the small albedo of these bodies, i.e. of the order of 4%, which means that only 4% of the incident Solar light is reflected by the surface! In other words, these bodies are very dark, which itself suggests a carbonaceous composition. Spectroscopic observations permit to be more accurate.

Spectroscopic observations

These bodies were observed in the near infrared, at wavelengths between 0.8 and 5.5 μm. The observations were made at the IRTF (InfraRed Telescope Facility), located on the Mauna Kea (Hawai’i), with the SpeX spectrograph, during 4 nights, in 2012 and 2013. In measuring the light flux over a specific range of the spectrum, one can infer the presence of some material, which would absorb the light at a given wavelength. For that, we need to be accurate in the measurements, while the atmospheric conditions might alter them. This difficulty is by-passed by the presence of a star in the field, which serves as a reference for the measured light flux.

Detection of minerals

Once a spectrum reflectance vs. wavelength is obtained, it needs to be interpreted. In this study, the authors assumed that the observed spectra were a mixture of the spectra given by different minerals, which have been obtained in laboratories. They disposed of a database of 30 minerals, and fitted mixtures involving 4 of them, to the obtained spectra. This is an optimization algorithm, here named Spectral Mixture Analysis, which fits the relative proportion of the minerals. 4 minerals is actually the best they could obtain, i.e. they failed to produce a significantly better fit in adding a 5th mineral.

In other words, from the absorption spectrum of such a body, you can guess its 4 main components… at least of the surface.

Himalia and Elara are alike, Carme is different

Well, the title contains the conclusion. This is not very surprising, since Himalia and Elara belong to the same group. We can say that the composition confirms the guess that they should have a common origin. Previous studies gave the same conclusions.

In this specific case, Himalia and Elara have a peak of absorption centered around 1.2 μm, and their spectra are similar to C-type, i.e. carbonaceous, asteroids (52) Europa and (24) Themis, of the outer asteroid belt. The best match for Himalia is obtained with a mixture of magnetite and ilmenite, both being iron oxides, with minnesotaite, which is a ferric phyllosilicate. Elara seems to have a similar composition, but the match is not that good. In particular, the spectrum is more dispersed than for Himalia, and a little redder.

Carme has a different spectrum, with a peak of absorption centered around 1.6 μm, and is probably composed of black carbon, minnesotaite, and ilmenite. Another study has proposed that Carme could have a low-level cometary activity, but that would require to observe it at shorter wavelengths. Out of the scope of this study.

The study and the authors

That’s it for today! Please do not forget to comment. You can also subscribe to the RSS feed, and follow me on Twitter and Facebook.

The lowlands of Mars

Hi there! Today I will give you the composition of the subsurface of the lowlands of Mars. This is the opportunity for me to present you The stratigraphy and history of Mars’ northern lowlands through mineralogy of impact craters: A comprehensive survey, by Lu Pan, Bethany L. Ehlmann, John Carter & Carolyn M. Ernst, which has recently been accepted for publication in Journal of Geophysical Research: Planets.

Low- and Highlands

Topography of Mars. We can see lowlands in the North, and highlands in the South. © USGS
Topography of Mars. We can see lowlands in the North, and highlands in the South. © USGS

As you can see on this image, the topography of Mars can be divided into the Northern and the Southern hemispheres, the Northern one (actually about one third of the surface) being essentially constituted of plains, while the Southern one is made of mountains. The difference of elevation between these two hemispheres is between 1 to 3 km. Another difference is the fact that the Southern hemisphere is heavily cratered, even if craters exist in the lowlands. This Martian dichotomy is very difficult to explain, some explanations have been proposed, e.g., the lowlands could result from a single, giant impact, or the difference could be due to internal (tectonic) processes, which would have acted differentially, renewing the Northern hemisphere only… Anyway, whatever the cause, there is a dichotomy in the Martian topography. This study examines the lowlands.

The lowlands are separated into: Acidalia Planitia (for plain), Arcadia Planitia, Amazonis Planitia, Chryse Planitia, Isidis Planitia, Scandia Cavi (the polar region), Utopia Planitia, Vatistas Borealis,…

Plains also exist in the Southern hemisphere, like the Hellas and the Argyre Planitiae, which are probably impact basins. But this region is mostly known for Olympus Mons, which is the highest known mountain is the Solar System (altitude: 22 km), and the Tharsis Montes, which are 3 volcanoes in the Tharsis region.

To know the subsurface of a region, and its chemical composition, the easiest way is to dig… at least on Earth. On Mars, you are not supposed to affect the nature… Fortunately, the nature did the job for us, in bombarding the surface. This bombardment was particularly intense during the Noachian era, which correspond to the Late Heavy Bombardment, between 4.1 to 3.7 Gyr ago. The impacts excavated some material, that you just have to analyze with a spectrometer, provided the crater is preserved enough. This should then give you clues on the past of the region. Some say the lowlands might have supported a global ocean once.

The past ocean hypothesis

Liquid water seems to have existed at the surface of Mars, until some 3.5 Gyr ago. There are evidences of gullies and channels in the lowlands. This would have required the atmosphere of Mars to be much hotter, and probably thicker, than it is now. The hypothesis that the lowlands were entirely covered by an ocean has been proposed in 1987, and been supported by several data and studies since then, even if it is still controversial. Some features seem to be former shorelines, and evidences of two past tsunamis have been published in 2016. These evidences are channels created by former rivers, which flowed from down to the top. These tsunamis would have been the consequences of impacts, one of them being responsible for the crater Lomonosov.

The fate of this ocean is not clear. Part of it would have been evaporated in the atmosphere, and then lost in the space, part of it would have hydrated the subsurface, before freezing… This is how the study of this subsurface may participate in the debate.

The CRISM instrument

To study the chemical composition of the material excavated by the impacts, the authors used CRISM data. CRISM, for Compact Reconnaissance Imaging Spectrometer for Mars, is an instrument of Mars Reconnaissance Orbiter (MRO). MRO is a NASA spacecraft, which orbits Mars since 2006.
CRISM is an imaging spectrometer, which can observe both in the visible and in the infrared ranges, which requires a rigorous cooling of the instrument. These multi-wavelengths observations permit to identify the different chemical elements composing the surface. The CRISM team summarizes its scientific goals by follow the water. Studying the chemical composition would permit to characterize the geology of Mars, and give clues on the past presence of liquid water, on the evolution of the Martian climate,…

In this study, the authors used CRISM data of 1,045 craters larger than 1 km, in the lowlands. They particularly focused on wavelengths between 1 and 2.6μm, which is convenient to identify hydrated minerals.

Hydrated vs. mafic minerals

The authors investigated different parts of the craters: the central peak, which might be constituted of the deepest material, the wall, the floor… The CRISM images should be treated, i.e. denoised before analysis. This requires to perform a photometric, then an atmospheric correction, to remove spikes, to eliminate dead pixels…

And after this treatment, the authors identified two kinds of minerals: mafic and hydrated ones. Mafic minerals are silicate minerals, in particular olivine and pyroxenes, which are rich in magnesium and iron, while hydrated minerals contain water. They in particular found a correlation between the size of the crater and the ratio mafic / hydrated, in the sense that mafic detections are less dependent on crater size. Which means that mafic minerals seem to be ubiquitous, while the larger the crater, the likelier the detection of hydrated minerals. Since larger craters result from more violent impacts, this suggests that hydrated minerals have a deeper origin. Moreover, no hydrated material has been found in the Arcadia Planitia, despite the analysis of 85 craters. They also noticed that less degraded craters have a higher probability of mineral detection, whatever the mineral.

However, the authors did not find evidence of concentrated salt deposits, which would have supported the past ocean hypothesis.

The study and the authors

That’s it for today! Please do not forget to comment. You can also subscribe to the RSS feed, and follow me on Twitter and Facebook.

Our water comes from far away

Hi there! Can you imagine that our water does not originally come from the Earth, but from the outer Solar System? The study I present you today explains us how it came to us. This is Origin of water in the inner Solar System: Planetesimals scattered inward during Jupiter and Saturn’s rapid gas accretion by Sean Raymond and Andre Izidoro, which has recently been published in Icarus.

From the planetary nebula to the Solar System

There are several competing scenarios, which describe a possible path followed by the Solar System from its early state to its current one. But all agree that there was originally a protoplanetary disk, orbiting our Sun. It was constituted of small particles and gas. Some of the small particles accreted to form the giant planets, first as a massive core, then in accreting some gas around. The proto-Jupiter cleared a ring-shaped gap around its orbit in the disk, Saturn formed as well, the planets migrated, in interacting with the gas. How fast did they migrate? Inward? Outward? Both? Scenarios diverge. Anyway, the gas was eventually ejected, and the protoplanetary disk was essentially cleared, except when it is not. There remains the telluric planets, the giant planets, and the asteroids, many of them constituting the Main Belt, which lies between the orbits of Mars and Jupiter.
If you want to elaborate a fully consistent scenario of formation / evolution of the Solar System, you should match the observations as much as possible. This means matching the orbits of the existing objects, but not only. If you can match their chemistry as well, that is better.

No water below this line!

The origin of water is a mystery. You know that we have water on Earth. It seems that this water comes from the so-called C-type asteroids. These are carbonaceous asteroids, which contain a significant proportion of water, usually between 5 and 20%. This is somehow the same water as on Earth. In particular, it is consistent with the ratios D/H and 15N/14N present in our water. D is the deuterium, it is an isotope of hydrogen (H), while 15N and 14N are two isotopes of nitrogen (N).

These asteroids are mostly present close to the outer boundary of the Main Belt, i.e. around 3.5 AU. An important parameter of a planetary system is the snow line: below a given radius, the water cannot condensate into ice. That makes sense: the central star (in our case, the Sun) is pretty hot (usually more than pretty, actually…), and ice cannot survive in a hot environment. So, you have to take some distance. And the snow line of the Solar System is currently lose to 3.5 AU, where we can find these C-type asteroids. Very well, there is no problem…

But there is one: the location of the snow line changes during the formation of the Solar System, since it depends on the dynamical structure of the disk, i.e. eccentricity of the particles constituting it, turbulence in the gas, etc. in addition to the evolution of the central star, of course. To be honest with you, I have gone through some literature and I cannot tell you where the snow line was at a given date, it seems to me that this is still an open question. But the authors of this study, who are world experts of the question, say that the snow line was further than that when these C-types asteroids formed. I trust them.

And this raises an issue: the C-types asteroids, composed of at least 5% of water, have formed further than they are. This study explains us how they migrated inward, from their original location to their present one.

Planet encounter and gas drag populate the Asteroid Belt

The authors ran intensive numerical simulations, in which the asteroids are massless particles, but with a given radius. This seems weird, but this just means that the authors neglected the gravitational action of the asteroids on the giant planets. The reason why they gave them a size in that it influences the way the gas drag (remember: the early Solar System was full of gas) affects their orbits. This size actually proved to be a key parameter. So, these asteroids were affected by the gas and the giant planets, but in the state they were at that time, i.e. initially Jupiter and Saturn were just slowly accreting cores, and when these cores of solid material reached a critical size, then they were coated by a pretty rapid (over a few hundreds of kyr) accretion of gas. The authors considered only Jupiter in their first simulations, then Jupiter and Saturn, and finally the four giant planets. Their different parameters were:

  • the size of the asteroids (planetesimals),
  • the accretion velocity of the gas around Jupiter and Saturn,
  • the evolution scenario of the early Solar System. In particular, the way the giant planets migrated.

Simulating the formation of the planet actually affects the orbital evolution of the planetesimals, since the mass of the planets is increasing. The more massive the planet, the most deviated the asteroid.

And the authors succeed in putting C-type asteroids with this mechanism: when a planetesimal encounters a proto-planet (usually the proto-Jupiter), its eccentricity reaches high numbers, which threatens its orbital stability around the Sun. But the gas drag damps this eccentricity. So, these two effects compete, and when ideally balanced this results in asteroids in the Main-Belt, on low eccentric orbits. And the authors show that this works best for mid-sized asteroids, i.e. of the order of a few hundreds of km. Below, Jupiter ejects them very fast. Beyond, the gas drag is not efficient enough to damp the eccentricity. And this is consistent with the current observations, i.e. there is only one C-type asteroid larger than 1,000 km, this is the well-known Ceres.

However, the scenarios of evolution of the Solar System do not significantly affect this mechanism. So, it does not tell us how the giant planets migrated.

Once the water ice has reached the main asteroid belt, other mechanism (meteorites) carry it to the Earth, where it can survive thanks to our atmosphere.

Making the exoplanets habitable

This study proposes a mechanism of water delivery, which could be adapted to any planetary system. In particular, it tells us a way to make exoplanetary planets habitable. Probably more to come in the future.

To know more…

  • The study, presented by the first author (Sean N. Raymond) on his own blog,
  • The website of Sean N. Raymond,
  • The IAU page of Andre Izidoro.
  • And I would like to mention Pixabay, which provides free images, in particular the one of Cape Canaveral you see today. Is this shuttle going to fetch some water somewhere?

That’s it for today! Please do not forget to comment. You can also subscribe to the RSS feed, and follow me on Twitter and Facebook.