Tag Archives: chemistry

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.

On the interior of Mimas, aka the Death Star

Hi there! Today I will tell you on the interior of Mimas. You know, Mimas, this pretty small, actually the smallest of the mid-sized, satellite of Saturn, which has a big crater, like Star Wars’ Death Star. Despite an inactive appearance, it presents confusing orbital quantities, which could suggest interesting characteristics. This is the topic of the study I present you today, by Marc Neveu and Alyssa Rhoden, entitled The origin and evolution of a differentiated Mimas, which has recently been published in Icarus.

Mimas’ facts

The system of Saturn is composed of different groups of satellites. You have

  • Very small satellites embedded into the rings,
  • Mid-sized satellites orbiting between the rings and the orbit of Titan
  • The well-known Titan, which is very large,
  • Small irregular satellites, which orbit very far from Saturn and are probably former asteroids, which had been trapped by Saturn,
  • Others (to make sure I do not forget anybody, including the coorbital satellites of Tethys and Dione, Hyperion, the Alkyonides, Phoebe…).

Discovered in 1789 by William Herschel, Mimas is the innermost of the mid-sized satellites of Saturn. It orbits it in less than one day, and has strong interactions with the rings.

Semimajor axis 185,520 km
Eccentricity 0.0196
Inclination 1.57°
Diameter 396.4 km
Orbital period 22 h 36 min

As we can see, Mimas has a significant eccentricity and a significant inclination. This inclination could be explained by a mean-motion resonance with Tethys (see here). However, we see no obvious cause for its present eccentricity. It could be due to a past gravitational excitation by another satellite.

Mimas, seen by Cassini. We can the crater Herschel, which makes Mimas look like Star Wars' Death Star. Credit: NASA
Mimas, seen by Cassini. We can the crater Herschel, which makes Mimas look like Star Wars’ Death Star. Credit: NASA

The literature is not unanimous on the formation of Mimas. It was long thought that the satellites of Saturn formed simultaneously with the planet and the rings, in the proto-Saturn nebula. The Cassini space mission changed our view of this system, and other scenarios were proposed. For instance, the mid-sized satellites of Saturn could form from the collisions between 4 big progenitors, Titan being the last survivor of them. The most popular explanation seems to be that a very large body impacted Saturn, its debris coalesced into the rings, and then particles in the rings accreted, forming satellites which then migrated outward… these satellites being the mid-sized satellites, i.e. Rhea, Dione, Tethys, Enceladus, and Mimas. This scenario would mean that Mimas would be the youngest of them, and that it formed differentiated, i.e. that the proto-Mimas was made of pretty heavy elements, on which lighter elements accreted. Combining observations of Mimas with theoretical studies of its long-term evolution could help to determine which of these scenarios is the right one… if there is a right one. Such studies can of course involve other satellites, but this one is essentially on Mimas, with a discussion on Enceladus at the end.

The rotation of Mimas

As most of the natural satellites of the giant planets, Mimas is synchronous, i.e. it shows the same face to Saturn, its rotational (spin) period being on average equal to its orbital one. “On average” means that there are some variations. These are actually a sum of periodic oscillations, which are due to the variations of the distance Mimas-Saturn. And from the amplitude and phase of these variations, you can deduce something on the interior, i.e. how the mass is distributed. This could for instance reveal an internal ocean, or something else…

This rotation has been measured in 2014 (see this press release). The mean rotation is indeed synchronous, and here are its oscillations:

Period Measured
amplitude (arcmin)
Theoretical
amplitude (arcmin)
70.56 y 2,616.6 2,631.6±3.0
23.52 y 43.26 44.5±1.1
22.4 h 26.07 50.3±1.0
225.04 d 7.82 7.5±0.8
227.02 d 3.65 2.9±0.9
223.09 d 3.53 3.3±0.8

The most striking discrepancy is at the period 22.4 h, which is the orbital period of Mimas. These oscillations are named diurnal librations, and their amplitude is very sensitive to the interior. Moreover, the amplitude associated is twice the predicted one. This means that the interior, which was hypothesized for the theoretical study, is not a right one, and this detection of an error is a scientific information. It means that Mimas is not exactly how we believed it is.

The authors of the 2014 study, led by Radwan Tajeddine, investigated 5 interior models, which could explain this high amplitude. One of these models considered the influence of the large impact crater Herschel. In all of these models, only 2 could explain this high amplitude: either an internal ocean, or an elongated core of pretty heavy elements. Herschel is not responsible for anything in this amplitude.

The presence of an elongated core would support the formation from the rings. However, the internal ocean would need a source of heating to survive.

Heating Mimas

There are at least three main to heat a planetary body:

  1. hit it to heat it, i.e. an impact could partly melt Mimas, but that would be a very intense and short heating, which would have renewed the surface…nope
  2. decay of radiogenic elements. This would require Mimas to be young enough
  3. tides: i.e. internal friction due to the differential attraction of Saturn. This would be enforced by the variations of the distance Saturn-Mimas, i.e. the eccentricity.

And this is how we arrive to the study: the authors simulated the evolution of the composition of Mimas under radiogenic and tidal heating, in also considering the variations of the orbital elements. Because when a satellite heats, its eccentricity diminishes. Its semimajor axis varies as well, balanced between the dissipation in the satellite and the one in Saturn.

The problems

For a study to be trusted by the scientific community, it should reproduce the observations. This means that the resulting Mimas should be the Mimas we observe. The authors gave themselves 3 observational constraints, i.e. Mimas must

  1. have the right orbital eccentricity,
  2. have the right amplitude of diurnal librations,
  3. keep a cold surface.

and they modeled the time evolution of the structure and the orbital elements using a numerical code, IcyDwarf, which simulates the evolution of the differentiation, i.e. separation between rock and water, porosity, heating, freezing of the ocean if it exists…

Results

The authors show that in any case, the ocean cannot survive. If there would be a source of heating sustaining it, then the eccentricity of Mimas would have damped. In other words, you cannot have the ocean and the eccentricity simultaneously. Depending on the past (unknown) eccentricity of Mimas and the dissipation in Saturn, which is barely known, an ocean could have existed, but not anymore.
As a consequence, Mimas must have an elongated core, coated by an icy shell. The eccentricity could be sustained by the interaction with Saturn. This elongated core could have two origins: either a very early formation of Mimas, which would have given enough time for the differentiation, or a formation from the rings, which would have formed Mimas differentiated.

Finally the authors say that there model does not explain the internal ocean of Enceladus, but Marc Neveu announces on his blog that they have found another explanation, which should be published pretty soon. Stay tuned!

Another mystery

The 2014 study measured a phase shift of 6° in the diurnal librations. This is barely mentioned in the literature, probably because it bothers many people… This is huge, and could be more easily, or less hardly, explained with an internal ocean. I do not mean that Mimas has an internal ocean, because the doubts regarding its survival persist. So, this does not put the conclusions of the authors into question. Anyway, if one day an explanation would be given for this phase lag, that would be warmly welcome!

To know more…

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.

Predicting the chemical composition of (4)Vesta

Hi there! Today I present you a study entitled Chlorine and hydrogen degassing in Vesta’s magma ocean, by Adam R. Sarafian, Timm John, Julia Roszjár and Martin J. Whitehouse. This study has recently been published in Earth and Planetary Science Letters. The goal here is, from the chemical analysis of meteorites which are supposed to come from Vesta, understand the evolution of its chemical evolution. In particular, how the degassing of its magma ocean impacts its chemical evolution.

(4)Vesta

I have presented the small planet (4)Vesta in that post. Basically, it is one of the largest Main-Belt asteroids, with a mean radius of some 500 km. The craters at its surface and the dynamical models of the early Solar System show that Vesta has been intensively bombarded. The largest of these impacts were energetic enough to melt Vesta and trigger its differentiation between a pretty dense core, a shallow magma ocean and a thin crust.

Despite having been visited by the spacecraft Dawn, the magma ocean has not been detected. Its presence is actually confirmed by the analyses of meteorites which fell on Earth.

The HED meteorites

Every day, about 6 tons of material hit the surface of the Earth, after having survived the atmospheric entry. Mineralogists split these meteorites into several groups. 5% of these meteorites are HEDs, for Howardite-Eucrite-Diogenites. These are achondritic basaltic meteorites, which are supposed to present similarities with Vesta. This hypothesis has been proposed in 1970 after comparison of the spectrum of Vesta and the one of these meteorites, and enforced since by the observations and theoretical works. So, it is now accepted that these meteorites come from Vesta or bodies similar to it, and studying them is a way to study the chemical composition of Vesta.
In this study, only the Eucrites will be addressed. They represent most of the HEDs, and contain 2 phosphates: the merrillite and the apatite. Moreover, they are systematically depleted in volatile elements, compared to carbonaceous chondrites and the Earth.

Chemical analysis

The authors have analyzed the chemical composition of 7 samples of eucrites, which were found on Earth. They present a variety of thermal alteration. Comparing them would be like watching a movie of the process of evolution of the material during the degassing in the magma ocean. The analyses were conducted on two sites: the Natural History Museum Vienna, in Austria, and the Woods Hole Oceanographic Institution (MA, USA). The involved technology is the scanning electron microscopy, which consists in obtaining images from the interaction of the sample with a focused bean of electrons, supplemented with an energy-dispersive X-ray spectrometer. This spectrometer gives the spectral signature of the interactions of the electrons with the rock sample, and so reveals the elements which constitute it.

The authors were particularly interested in measuring the concentrations of halogen (fluorine, chlorine, bromine and iodine), of stable isotopes of the chlorine, isotopes of hydrogen, and water. Comparing the relative concentration of these elements in the seven samples would give information on their volatilization during the outgassing process of the magma ocean, in conditions that do not exist on Earth.

Conclusions

The samples show different compositions in volatile elements (H2, H20, and metal chlorides), which show that there is some outgassing in Vesta’s magma ocean. The authors show in particular a large variability in the ratio [Cl]/[K], i.e. chlorite with respect to potassium. This means that not only the thermal evolution tends to reject volatile elements, but also that they are effectively ejected. This might be a concern since the ocean cannot be seen at the surface of Vesta. Anyway, this does not preclude outgassing, either through the crust, which is supposed to be thin, and/or with the assistance of giant impacts, which created craters deep enough to reach the ocean.

This way, we have a signature of the history of a planetary body in material found on the Earth. These results might have implications beyond Vesta, i.e. could be extended to other dwarf planets, and so give us information on the chemical evolution of the Solar System.

I hope you enjoyed this article. As usual, I am interested in your feed-back. So please, leave me some comments, share it, and happy new year!

To know more…

  • The study, which can also be found on ResearchGate, thanks to the authors for sharing!
  • The webpage of Adam Robert Sarafian, grad student at the Woods Hole Oceanographic Institution (USA)
  • The webpage of Timm John, Freie Universität Berlin, Germany
  • The webpage of Julia Roszjár, Natural History Museum, Vienna, Austria
  • The webpage of Martin Whitehouse, Swedish Museum of Natural History, Stockholm, Sweden