Tag Archives: asteroids

The fate of Jupiter’s Trojans

Hi there! Today we discuss about the Trojans of Jupiter. These are bodies which orbit on pretty the same orbit as Jupiter, i.e. at the same distance of the Sun, but 60° before or behind. These asteroids are located at the so-called Lagrange points L4 and L5, where the gravitational actions of the Sun and of Jupiter balance. As a consequence, these locations are pretty stable. I say “pretty” because, on the long term, i.e. millions of years, the bodies eventually leave this place. The study I present today, The dynamical evolution of escaped Jupiter Trojan asteroids, link to other minor body populations, by Romina P. Di Sisto, Ximena S. Ramos and Tabaré Gallardo, addresses the fate of these bodies once they have left the Lagrange points. This study made in Argentina and Uruguay has recently been published in Icarus.

The Trojan asteroids

Jupiter orbits the Sun at a distance of 5.2 AU (astronomical units), in 11.86 years. As the largest (and heaviest) planet in the Solar System, it is usually the main perturber. I mean, planetary objects orbit the Sun, they may be disturbed by other objects, and Jupiter is usually the first candidate for that.

As a result, it creates favored zones for the location of small bodies, in the sense that they are pretty stable. The Lagrange points L4 and L5 are among these zones, and they are indeed reservoirs of populations. At this time, the Minor Planet Center lists 7,039 Trojan asteroids, 4,600 of them at the L4 point (leading), and 2,439 at the L5 trailing point. These objects are named after characters of the Trojan War in the Iliad. L4 is populated by the Greeks, and L5 by the Trojans. There are actually two exceptions: (624) Hektor is in the Greek camp, and (617) Patroclus in the Trojan camp.

Location of the Lagrange points.
Location of the Lagrange points.

These are dark bodies

The best way to know the composition of a planetary body is to get there… which is very expensive and inconvenient for a wide survey. Actually a NASA space mission, Lucy, is scheduled to be launched in 2021 and will fly by the Greek asteroids (3548) Eurybates, (15094) Polymele, (11351) Leucus, and (21900) Orus in 2027 and 2028. So, at the leading Lagrange point L4. After that, it will reach the L5 point to explore the binary (617) Patroclus-Menetius in 2033. Very interesting, but not the most efficient strategy to have a global picture of the Trojan asteroids.

Fortunately, we can analyze the light reflected by these bodies. It consists in observing them from the Earth, and decompose the light following its different wavelengths. And it appears that they are pretty dark bodies, probably carbon-rich. Such compositions suggest that they have been formed in the outer Solar System.

Asymmetric populations

We currently know 4,600 Trojan asteroids at the L4 point, and 2,439 of them at the L5 one. This suggests a significant asymmetry between these two reservoirs. We must anyway be careful, since it could be an observational bias: if it is easier to observe something at the L4 point, then you discover more objects.

The current ratio between these two populations is 4,600/2,439 = 1.89, but correction from observational bias suggests a ratio of 1.4. Still an asymmetry.

Numerical simulations with EVORB

The authors investigated the fate of 2,972 of these Trojan asteroids, 1,975 L4 and 997 L5, in simulating their trajectories over 4.5 Gyr. I already told you about numerical integrations. They consist in constructing the trajectory of a planetary body from its initial conditions, i.e. where it is now, and the equations ruling its motion (here, the gravitational action of the surrounding body). The trajectory is then given at different times, which are separated by a time-step. If you want to know the location at a given time which is not one considered by the numerical integration, then you have to interpolate the trajectory, in using the closest times where your numerical scheme has computed it.

When you make such ambitious numerical integrations, you have to be very careful of the accuracy of your numerical scheme. Otherwise, you propagate and accumulate errors, which result in wrong predictions. For that, they used a dedicated integrator, named EVORB (I guess for something like ORBital EVolution), which switches between two schemes whether you have a close encounter or not.

As I say in previous articles like this one, a close encounter with a planet may dramatically alter the trajectory of a small body. And this is why it should be handled with care. Out of any close encounter, EVORB integrates the trajectory with a second-order leapfrog scheme. This is a symplectic one, i.e. optimized for preserving the whole energy of the system. This is critical in such a case, where no dissipative effect is considered. However, when a planet is encountered, the scheme uses a Bulirsch-Stoer one, which is much more accurate… but slower. Because you also have to combine efficiency with accuracy.

In all of these simulations, the authors considered the gravitational actions of the Sun and the planets from Venus to Neptune. Venus being the body with the smallest orbital period in this system, it rules the integration step. They authors fixed it to 7.3 days, which is 1/30 of the orbital period of Venus.

And these numerical simulations tell you the dynamical fate of these Trojans. Let us see the results!

The Greek are more stable than the Trojans

It appears that, when you are in the Greek camp (L4), you are less likely to escape than if you are in the Trojan one (L5). The rate of escape is 1.1 times greater at L5 than at L4. But, remember the asymmetry in the populations: L4 is much more populated than L5. The rates of escape combined with the overall populations make than there are more escapes from the Greek camp (18 per Myr) than from the Trojan one (14 per Myr).

Where are they now?

What do they become when they escape? They usually (90% of them) go in the outer Solar System, first they become Centaurs (asteroids inner to Neptune), and only fugitives from L4 may become Trans-Neptunian Objects. And then they become a small part of these populations, i.e. you cannot consider the Lagrange points of Jupiter to be reservoirs for the Centaurs and the TNOs. However, there are a little more important among the Jupiter-Family Comets and the Encke-type comets (in the inner Solar System). But once more, they cannot be considered as reservoirs for these populations. They just join them. And as pointed out a recent study, small bodies usually jumped from a dynamical family to another.

The study and its authors

You can find the study here. The authors made it freely available on arXiv, many thanks to them for sharing!

And now, the 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, Facebook, Instagram, and Pinterest.

The Solar System is a mess: Using Big Data to clear it up

Hi there! When you look at our Solar System, you wonder how it came to be that way. I mean, it formed from a nebula, in which grain accreted to create the Sun, the planets,… and all these small bodies. Most of them have disappeared since the origin, they were either ejected, or accreted on a planet, or on the Sun… anyway still many of them remain. You have the Asteroid Main Belt, the Centaurs, the Kuiper Belt, the Oort cloud, the comets… Several studies have tried to determine connections between them, i.e. where does this comet come from? Was it originally a Centaur, a Kuiper Belt Object, something else? And how did it change its orbit? A close approach with a giant planet, maybe?
And to address this question, you simulate the trajectories… which is not straightforward to do. It is pretty classical to simulate a trajectory from given initial conditions, but to answer such a question, you need more.
You need more because you do not know how reliable are your initial conditions. Your comet was there that day… very well. How sure are you of that? You observe a position and a velocity, fine, but you have uncertainties on your measurements, don’t you?
Yes, you have. So, you simulate the trajectories of many comets, which initial conditions are consistent with your observations. That’s better. And you hope that the outcome of the trajectories (trajectories simulated backward, if you want to know the origin) will be pretty much the same, since the initial conditions are very close to each other…
But they are not! This is what we call sensitivity to the initial conditions. This often means chaos, but I do not want to detail this specific notion. But basically, when a comet swings by a giant planet, its trajectory is dramatically deviated. And the deviation is highly sensitive to the location of the comet. So sensitive that at some point, you lose the information given by your initial conditions. C’est la vie.
As a result, there are in the literature many studies presenting their simulations, and which conclusions are sometimes inconsistent with each other.
The study we discuss today, It’s Complicated: A Big Data Approach to Exploring Planetesimal Evolution in the Presence of Jovian Planets, by Kevin R. Grazier, Julie C. Castillo-Rogez, and Jonathan Horner, suggests another approach to clear up this mess. It considers that all of these possible trajectories constitute a reservoir of Big Data. This study has recently been published in The Astronomical Journal.

Architecture of the Solar System

You know the 8 planets of our Solar System, from the closest to the outermost one:

  • Mercury,
  • Venus,
  • Earth,
  • Mars,
  • Jupiter,
  • Saturn,
  • Uranus,
  • Neptune.

And these planets are accompanied by many small bodies, which constitute

  • the Near-Earth Asteroids, which orbit among the 4 terrestrial planets (from Mercury to Mars),
  • the Main Belt Asteroids, which orbit between Mars and Jupiter,
  • the Centaurs, which orbit between Jupiter and Neptune,
  • the Kuiper Belt, which extends between 30 and 50 AU (astronomical units) from the Sun. So, its inner limit is the orbit of Neptune,
  • the scattered disc, which extends to 150 AU from the Sun. These objects are highly inclined. Eris is the largest known of them.
  • the detached objects, like Sedna. This population is very poorly known, and we do not even know if it is truly a population, or just some objects,
  • the hypothetical Oort cloud, which could be as far as one light-year, or 50,000 AU.

Of course, this list is not exhaustive. For instance, I did not mention the comets, which could originate from any of those populations of small objects.

In this study, the authors limit themselves to the orbit of Neptune. They consider 3 populations of objects between the orbits of Jupiter and Saturn, between Saturn and Uranus, and between Uranus and Neptune. And the question is: how do these populations evolve, to the current state? For that, planetary encounters appear to be of crucial importance.

Planetary encounters

Imagine a small body flying by Jupiter. It approaches Jupiter so closely that it enters its sphere of influence, in which the gravity of Jupter dominates the one of the Sun. Virtually, the object orbits Jupiter, but usually this orbit cannot be stable, since the approach is too fast. Locally, its orbit around Jupiter is hyperbolic, and the object does not stay there. Jupiter ejects it, and you do not know where, because the direction of the ejection is highly sensitive to the velocity of the object during its approach. It also depends on the mass of Jupiter, but this mass is very well known. Sometimes, the action of Jupiter is so strong that it fragments the object, as it did for the comet Shoemaker-Levy 9, in July 1994. And you can have this kind of phenomenon for any of the giant planets of the Solar System.

This is how planetary encounters could move, disperse, eject,… entire populations in the Solar System.

The Big Data approach

With so many objects (the authors considered 3 ensembles of 10,000 test particles, the ensembles being the 3 zones between two consecutive giant planets) and so many potential planetary encounters (the trajectories were simulated over 100 Myr), you generate a database of planetary encounters… how to deal with that? This is where the Big Data approach enters the game.

The authors performed it into two stages. The first one consisted to determine close encounter statistics and correlations, for instance with changes of semimajor axis, i.e. how a planetary encounter displaces an object in the Solar System. And the second stage aimed at reconstituting the path of the particles.

And now, the results.

Random walk from one belt to another

It appears that the particles could easily move from one belt to another. Eventually, they can be ejected. As the authors say, the classification of a particle into a population or another is ephemeral. It depends on when you observe it. In other words, a small object you observe in the Solar System could have been formed almost anywhere else. Even in situ. Now let us talk about specific examples.

The origin of Ceres

For instance, Ceres. You know, this is the largest of the Main Belt Asteroids, and the first to have been discovered, in 1801. It has recently been the target of the mission Dawn, which completed in October 2018.

Ceres seen by Dawn. © NASA
Ceres seen by Dawn. © NASA

Ceres is rich in volatiles like ammonia and carbon dioxides, as are other asteroids like Hygeia. Hygeia is itself a large Main Belt Asteroid. Knowing the origin of Ceres could give you the origin of these volatiles… but they could have been partly accreted after the migration… You see, it is difficult to be 100% sure.

Ceres could have formed in situ, i.e. between Mars and Jupiter, but this study shows that it could have originated from much further in the Solar System, and migrated inward.

The origin of trapped satellites

Most of the main satellites of the giant planets are thought to have been formed with the planet, in the protoplanetary nebula.
But in some cases, you have satellites, which orbit far from the parent planet, on an irregular orbit, i.e. a significantly inclined and eccentric one. In such a case, the body has probably not been formed in situ, but has been trapped by the planet. Among them are Saturn’s Phoebe and Neptune’s Triton, which are large satellite. I have discussed the case of Triton here. The trapping of Triton probably ejected mid-sized satellites of Neptune, which are now lost.

Phoebe seen by <i>Cassini</i> in August 2017 © NASA/ESA/JPL/SSI
Phoebe seen by Cassini in August 2017 © NASA/ESA/JPL/SSI
Mosaic of Triton taken by Voyager 2 in 1989. © NASA
Mosaic of Triton taken by Voyager 2 in 1989. © NASA

Phoebe and Triton entered the sphere of influence of their parent planet, but did not leave it. And where did they come from?

It seems probable that Triton was a Trans-Neptunian Object (TNO) before. In that part of the Solar System, the velocities are pretty low, which facilitate the captures. However, several scenarios are possible for Phoebe. The study show that it could have originated from an inner or from an outer orbit, and have jumped to Saturn from close encounters with Jupiter / Uranus / Neptune.

Something frustrating with such a study, which goes back to the origins, is that you lose some information. As a consequence, you can only conclude by “it is possible that”, but you cannot be certain. You have to admit it.

A way to secure some probabilities is to cross the dynamics with the physical properties, i.e. if you see that element on that body, and if that element is thought to have formed there, then you can infer something on the body, and the authors discuss these possibilities as well. But once more, you cannot be 100% sure. How do you know that this element has been formed there? Well, from the dynamics… which is chaotic… And when you see an element at the surface of a planetary body, does it mean that it is rich in it, or just coated by it, which means it could have accreted after the migration?

You see, you cannot be certain…

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, Facebook, Instagram, and Pinterest.

An active asteroid

Hi there! Today we will detail a recent study by Jessica Agarwal and Michael Mommert, entitled Nucleus of active asteroid 358P/Pan-STARRS (P/2012 T1). This study has recently been accepted for publication in Astronomy and Astrophysics, and consists in increasing our knowledge of a recently discovered object, i.e. P/2012 T1. This object proved to have some activity, like a comet. The authors realized several observations to try to characterize its activity, and infer some physical properties like its size and its rotation.

Comet vs. active asteroid

First of all, I would like to make clear what is a comet, and what is an active asteroid. I am very ambitious here, since these two notions actually overlap. For instance, our object is both an active asteroid, and a main-belt comet.

Let us say that a comet is an active asteroid, while an active asteroid is not necessarily a comet. The difference is in the nature of the activity.

A comet is a dirty snowball, i.e. you have water ice, and some silicates. Its orbit around the Sun is usually pretty eccentric, so that you have large variations of the distance Sun-object. The location of the orbit, at which the distance is the smallest, is called pericentre. When the comet approaches the pericentre, it approaches the Sun, heats, and part of its water ice sublimates. This results in a dusty tail (actually there are two tails, one being composed of ionized particles).

But when you see dust around a small body, i.e. when you see activity, this is not necessarily ice sublimation. There could be for instance rock excavated by an impact, or material expelled by fast rotation. In that case, you still have an active asteroid, but not a comet. One of the goals of this study is to address the cause and nature of P/2012 T1’s activity.

The asteroid P/2012 T1

P/2012 T1, now named 358P, has been discovered in October 2012 by the Pan-STARRS-1 survey. Pan-STARRS stands for Panoramic Survey Telescope and Rapid Response System, it uses dedicated facilities at Haleakala Observatory, Hawaii, USA.

Discovery of P/2012 T1. © Pan-STARRS
Discovery of P/2012 T1. © Pan-STARRS

Its provisional name, P/2012 T1, contains information on the nature of the object, and its discovery. P stands for periodic comet, 2012 is the year of the discovery, and T means that it has been discovered during the first half of October.

Interestingly, this object appeared on images taken in December 2001 at Palomar Observatory in California, while acquiring data for the survey NEAT (Near-Earth Asteroid Tracking).

You can find below its orbital elements, from the Minor Planet Center:

Semi-major axis 3.1504519 AU
Eccentricity 0.2375768
Inclination 11.05645°
Period 5.59 y

From its orbital dynamics, it is a Main-Belt object. As a comet, it is a Main-Belt Comet.

New observations

Once an object is known and we know where it is, it is much easier to reobserve it. The authors conducted observations of 358P from the Southern Astrophysical Research (SOAR) telescope, and the Very Large Telescope.

The SOAR telescope is based on Cerro Pachón, Chile. This is a 4.1-m aperture facility, located at an altitude of 2,700 m. The authors took images with the Goodman High Throughput Spectrograph during one night, from July 27 to July 28, 2017. They wanted to analyze the reflected light by the asteroid at different wavelengths, unfortunately the observational constraints, i.e. cloud coverage, permitted only two hours of observations. Only the observations made with the VR filter, centered at 610 nm, were useful.

These data were supplemented by 77 images taken during 10 hours from August 17 to August 18, 2017, at the Very Large Telescope. This instrument depends on the European Southern Observatory (ESO), and is located on Cerro Paranal, once more in Chile, at an altitude of 2,635 m. The authors used the FOcal Reducer and low dispersion Spectrograph 2 (FORS2), which central wavelength is 655 nm.

The observations give raw images. The authors treated them to get reliable photometric and astrometric measurements of 358P, i.e. they corrected from the variations of the luminosity of the sky, in using reference stars, and from the possible instrumental problems. For that, they recorded the response of the instrument to a surface of uniform brightness, and used the outcome to correct their images.

Let us now address the results.

Measuring its rotation

Such a small (sub-kilometric) body is not spherical. This results in variations of luminosity, which depend on the surface element which is actually facing your telescope. If you acquire data during several spin periods of the asteroid, then you should see some periodicity in the recorded lightcurve.

The best way to extract the periods is to make a Fourier transform. Your input is the time-dependent lightcurve you have recorded, and your output is a frequency-dependent curve, which should emphasize the periods, which are present in the recorded lightcurve. If the signal is truly periodic, then it should exhibit a maximum at its period and its harmonics (i.e. twice the period, thrice the period, etc.), and almost 0 outside (not exactly 0 since you always have some noise).

In the case of 358P, the authors did not identify any clear period. A maximum is present for a rotation period of 8 hours, but the result is too noisy to be conclusive. A possible explanation could be that we have a polar view of the asteroid. Another possibility is that the albedo of the asteroid (the fraction of reflected light) is almost uniform.

Dust emission

The authors tried to detect debris around the nucleus of the comet, in widening the aperture over which the photometry was performed. They got no real detection, which tends to rule out the possibility of non-cometary activity.

A 530m-large body

Finally, the magnitude of the asteroid is the one of a sphere of 530 meters in diameter, with an albedo of 6%. This means that a higher albedo would give a smaller size, and conversely. The albedo depends on the composition of the asteroid, which is unknown, and can be only inferred from other asteroids. The authors assumed it to be a carbonaceous asteroid (C-type), as 75% of the asteroids. If it were an S-type (silicateous) body, then it would be brighter. A wide band spectrum of the reflected light would give us this information.

The study and its authors

  • You can find the study here, on Astronomy and Astrophysics’ website. Moreover, the authors uploaded a free version on arXiv, thanks to them for sharing!
  • Here is the webpage of the first author, Jessica Agarwal,
  • and here the website of Michael Mommert.

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, Facebook, Instagram, and Pinterest.

Saturn sends us meteorites

Hi there! First I would like to thank you for following me on Facebook. The Planetary Mechanics page has reached 1,000 followers!

OK, now back to business. Did you know that our Earth is intensively bombarded from space? You have recently heard of this Chinese space station, Tiangong-1… in that case, it was man-made stuff. But we are intensively bombarded by natural space material. Most of it is so small that it is destroyed when entering the atmosphere, but sometimes it arrives to us as stones… And in extreme cases, the impactor is so large that its impact may generate an extinction event. The Chicxulub crater, in Mexico, is thought to result from the impact, which aftermath provoked the extinction of the dinosaurs, some 66 Myr ago.

The meteorites I speak about today are the ones, which fall on the Earth every year. This is the opportunity to discuss about Identification of meteorite source regions in the Solar System, which has recently been accepted for publication in Icarus. In that study, the authors determine the origin of 25 meteorites, from their observed trajectories just before they hit us.

Meteorites bombard the Earth

We estimate that currently 60 tons of cosmic material fall on the Earth every day. This seems huge, but actually most of it arrives to us as dust, since the original object does not survive its entry into the atmosphere. In fact, the larger the meteorite, the less frequent it falls on us. 4-m objects arrive every ~16 months, 10-m ones every ~10 years, and 100-m ones every ~5,200 years. And they arrive somewhere on Earth… do not forget that most of the surface of our planet is water. So, don’t worry.

The contact of such a small object with the atmosphere may generate an airburst, which itself could be detected, in many frequencies. I mean, you may hear it, you may see it (make a wish), it can also disturb the radio emissions. This motivated the existence of several observation programs, dedicated to the detection of meteors.

Observation networks

Programs of observation exist at least since 1959, originally under the impulse of Ondřejov Observatory (Czech Republic). These are usually national programs, e.g.

and there are probably more. These are networks of camera, which systematically record the sky, accumulating data which are then automatically treated to detect meteors. The detection of a meteors from different location permit to determine its trajectory.

Detection of a fireball by FRIPON, in September 2016. © FRIPON
Detection of a fireball by FRIPON, in September 2016. © FRIPON

Identifying the source

As I said, multiple detections, at different locations, of a fireball, permit to derive its trajectory. This trajectory gives in particular the radiant, which is the direction from which the meteorite, or the impactor, seems to come. The authors are also interested in the velocity of the object.

The velocity and the radiant are determined with respect to the Earth. Once they are determined, the authors translated them into heliocentric elements, i.e. they determined the pre-impact trajectory of the object with respect to the Sun. And this makes sense, since Solar System objects orbit the Sun! This trajectory is made of orbital elements, i.e. semimajor axis, eccentricity, inclination, and the uncertainties associated. Don’t forget that the observations have an accuracy, which you must consider when you use the data. The magnitude of the fireball tells us something on the size of the impactor as well.

From these data, the authors wondered from where the object should come from.

7 candidates as reservoirs of meteorites

The authors identified 7 possible sources for these impactors. These regions are the densest parts of the Main Asteroid Belt.
These are:

  1. the Hungaria family. These asteroids have a semimajor axis between 1.78 and 2 astronomical units, and an inclination between 16° and 34° with respect to the ecliptic, i.e. the orbit of the Earth,
  2. the ν6 resonance: these are bodies, which eccentricity raise because excited by Saturn. They orbit at a location, where they are sensitive to the precessional motion of the pericentre of Saturn. The raise of their eccentricity make these bodies unstable, and good candidates for Earth-crossers. Their semimajor axis is slightly smaller than 2 AU.
  3. the Phocaea family: this is a collisional family of stony asteroids. Their semimajor axes lie between 2.25 and 2.5 AU, their eccentricities are larger than 0.1, and their inclinations are between 18° and 32°. They are known to be a source of Mars-crossers.
  4. the 3:1 MMR (mean-motion resonance with Jupiter): these bodies perform exactly 3 orbits around the Sun while Jupiter makes one. They lie at 2.5 AU. The perturbation by Jupiter tends to empty this zone, which is called a Kirkwood gap.
  5. the 5:2 MMR, at 2.82 AU. This is another Kirkwood gap.
  6. the 2:1 MMR, at 3.27 AU, also known as Hecuba gap,
  7. the Jupiter Family Comets. These are comets, which orbital periods around the Sun are shorter than 20 years, and which inclinations are smaller than 30° with respect to the ecliptic. They are likely to be significantly perturbed by Jupiter.

For each of the 25 referenced meteorites, the authors computed the probability of each of these regions to be the source, in considering the orbital elements (semimajor axis, eccentricity, and inclination) and the magnitude of the object. Indeed, the magnitude is correlated with the size, which is itself correlated with the material constituting it. The reason is that these Earth-crossers orbit the Sun on eccentric orbits, and at their pericentre, i.e. the closest approach to the Sun, they experience tides, which threaten their very existence. In other words, they might be disrupted. Particularly, a large body made of weak material cannot survive.

And now, the results!

Saturn send meteorites to the Earth!

The authors find that the most probable source for the meteorites is the ν6 secular resonance, i.e. with Saturn. In other words, Saturn sends meteorites to the Earth! Beside this, the Hungaria family and the 3:1 mean-motion resonance with Jupiter are probable sources as well. On the contrary, you can forget the Phocaea family and the 2:1 MMR as possible sources.
It appears that the inner belt is more likely to be the source of meteorites than the outer one. Actually, the outer belt mostly contains carbonaceous asteroids, which produce weak meteoroids.

The authors honestly recall that previous studies found similar results. Theirs also contains an analysis of the influence of the uncertainty on the trajectories, and of the impact velocity with the Earth. This influence appears to be pretty marginal.

Anyway, the future will benefit from more data, i.e. more detections and trajectory recoveries. So, additional results are to be expected, just be patient!

The study and its authors

  • You can find the study here, on the website of Icarus. This study is in open access, which means that the authors paid extra fees to make the study available to us. Many thanks to them!
  • You can visit here the website of Mikael Granvik, the first author of the study,
  • and the one of the second author, Peter Brown.

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, Facebook, Instagram, and Pinterest.

Analyzing a crater of Ceres

Hi there! The space mission Dawn has recently visited the small planets Ceres and Vesta, and the use of its different instruments permits to characterize their composition and constrain their formation. Today we focus on the crater Haulani on Ceres, which proves to be pretty young. This is the opportunity for me to present you Mineralogy and temperature of crater Haulani on Ceres by Federico Tosi et al. This paper has recently been published in Meteoritics and Planetary Science.

Ceres’s facts

Ceres is the largest asteroid of the Solar System, and the smallest dwarf planet. A dwarf planet is a planetary body that is large enough, to have been shaped by the hydrostatic equilibrium. In other words, this is a rocky body which is kind of spherical. You can anyway expect some polar flattening, due to its rotation. However, many asteroids look pretty much like potatoes. But a dwarf planet should also be small enough to not clear its vicinity. This means that if a small body orbits not too far from Ceres, it should anyway not be ejected.

Ceres, or (1)Ceres, has been discovered in 1801 by the Italian astronomer Giuseppe Piazzi, and is visited by the spacecraft Dawn since March 2015. The composition of Ceres is close to the one of C-Type (carbonaceous) asteroids, but with hydrated material as well. This reveals the presence of water ice, and maybe a subsurface ocean. You can find below its main characteristics.

Discovery 1801
Semimajor axis 2.7675 AU
Eccentricity 0.075
Inclination 10.6°
Orbital period 4.60 yr
Spin period 9h 4m 27s
Dimensions 965.2 × 961.2 × 891.2 km
Mean density 2.161 g/cm3

The orbital motion is very well known thanks to Earth-based astrometric observations. However, we know the physical characteristics with such accuracy thanks to Dawn. We can see in particular that the equatorial section is pretty circular, and that the density is 2.161 g/cm3, which we should compare to 1 for the water and to 3.3 for dry silicates. This another proof that Ceres is hydrated. For comparison, the other target of Dawn, i.e. Vesta, has a mean density of 3.4 g/cm3.

It appears that Ceres is highly craterized, as shown on the following map. Today, we focus on Haulani.

Topographic map of Ceres, due to Dawn. Click to enlarge. © NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
Topographic map of Ceres, due to Dawn. Click to enlarge. © NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

The crater Haulani

The 5 largest craters found on Ceres are named Kerwan, Yalode, Urvara, Duginavi, and Vinotonus. Their diameters range from 280 to 140 km, and you can find them pretty easily on the map above. However, our crater of interest, Haulani, is only 34 km wide. You can find it at 5.8°N, 10.77°E, or on the image below.

The crater Haulani, seen by <i>Dawn</i>. © NASA / JPL-Caltech / UCLA / Max Planck Institute for Solar System Studies / German Aerospace Center / IDA / Planetary Science Institute
The crater Haulani, seen by Dawn. © NASA / JPL-Caltech / UCLA / Max Planck Institute for Solar System Studies / German Aerospace Center / IDA / Planetary Science Institute

The reason why it is interesting is that it is supposed to be one of the youngest, i.e. the impact creating it occurred less than 6 Myr ago. This can give clues on the response of the material to the impact, and hence on the composition of the subsurface.
Nothing would have been possible without Dawn. Let us talk about it!

Dawn at Ceres

The NASA mission Dawn has been launched from Cape Canaveral in September 2007. Since then, it made a fly-by of Mars in February 2009, it orbited the minor planet (4)Vesta between July 2011 and September 2012, and orbits Ceres since March 2015.

This orbit consists of several phases, aiming at observing Ceres at different altitudes, i.e. at different resolutions:

  1. RC3 (Rotation Characterization 3) phase between April 23, 2015 and May 9, 2015, at the altitude of 13,500 km (resolution: 1.3 km/pixel),
  2. Survey phase between June 6 and June 30, 2015, at the altitude of 4,400 km (resolution: 410 m /pixel),
  3. HAMO (High Altitude Mapping Orbit) phase between August 17 and October 23, 2015, at the altitude of 1,450 km (resolution: 140 m /pixel),
  4. LAMO (Low Altitude Mapping Orbit) / XMO1 phase between December 16, 2015 and September 2, 2016, at the altitude of 375 km (resolution: 35 m /pixel),
  5. XMO2 phase between October 5 and November 4, 2016, at the altitude of 1,480 km (resolution: 140 m / pixel),
  6. XMO3 phase between December 5, 2016 and February 22, 2017, at the altitude varying between 7,520 and 9,350 km, the resolution varying as well, between
  7. and is in the XMO4 phase since April 24, 2017, with a much higher altitude, i.e. between 13,830 and 52,800 km.

The XMOs phases are extensions of the nominal mission. Dawn is now on a stable orbit, to avoid contamination of Ceres even after the completion of the mission. The mission will end when Dawn will run out of fuel, which should happen this year.

The interest of having these different phases is to observe Ceres at different resolutions. The HAMO phase is suitable for a global view of the region of Haulani, however the LAMO phase is more appropriate for the study of specific structures. Before looking into the data, let us review the indicators used by the team to understand the composition of Haulani.

Different indicators

The authors used both topographic and spectral data, i.e. the light reflected by the surface at different wavelengths, to get numbers for the following indicators:

  1. color composite maps,
  2. reflectance at specific wavelengths,
  3. spectral slopes,
  4. band centers,
  5. band depths.

Color maps are used for instance to determine the geometry of the crater, and the location of the ejecta, i.e. excavated material. The reflectance is the effectiveness of the material to reflect radiant energy. The spectral slope is a linear interpolation of a spectral profile by two given wavelengths, and band centers and band depths are characteristics of the spectrum of material, which are compared to the ones obtained in lab experiments. With all this, you can infer the composition of the material.

This requires a proper treatment of the data, since the observations are affected by the geometry of the observation and of the insolation, which is known as the phase effect. The light reflection will depend on where is the Sun, and from where you observe the surface (the phase). The treatment requires to model the light reflection with respect to the phase. The authors use the popular Hapke’s law. This is an empirical model, developed by Bruce Hapke for the regolith of atmosphereless bodies.

VIR and FC data

The authors used data from two Dawn instruments: the Visible and InfraRed spectrometer (VIR), and the Framing Camera (FC). VIR makes the spectral analysis in the range 0.5 µm to 5 µm (remember: the visible spectrum is between 0.39 and 0.71 μm, higher wavelengths are in the infrared spectrum), and FC makes the topographical maps.
The combination of these two datasets allows to correlate the values given by the indicators given above, from the spectrum, with the surface features.

A young and bright region

And here are the conclusions: yes, Haulani is a young crater. One of the clues is that the thermal signature shows a locally slower response to the instantaneous variations of the insolation, with respect to other regions of Ceres. This shows that the material is pretty bright, i.e. it has been less polluted and so has been excavated recently. Moreover, the spectral slopes are bluish, this should be understood as a jargony just meaning that on a global map of Ceres, which is colored according to the spectral reflectance, Haulani appears pretty blue. Thus is due to spectral slopes that are more negative than anywhere else on Ceres, and once more this reveals bright material.
Moreover, the bright material reveals hydrothermal processes, which are consequences of the heating due to the impact. For them to be recent, the impact must be recent. Morever, this region appears to be calcium-rich instead of magnesium-rich like anywhere else, which reveals a recent heating. The paper gives many more details and explanations.

Possible thanks to lab experiments

I would like to conclude this post by pointing out the miracle of such a study. We know the composition of the surface without actually touching it! This is possible thanks to lab experiments. In a lab, you know which material you work on, and you record its spectral properties. And after that, you compare with the spectrum you observe in space.
And this is not an easy task, because you need to make a proper treatment of the observations, and once you have done it you see that the match is not perfect. This requires you to find a best fit, in which you adjust the relative abundances of the elements and the photometric properties of the material, you have to consider the uncertainties of the observations… well, definitely not an easy task.

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, Facebook, Instagram, and Pinterest.