Category Archives: Asteroids: Main Belt

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.

The system of (107) Camilla

Hi there! I will present you today a fascinating paper. It aims at a comprehensive understanding of the system composed of an asteroid, (107) Camilla, and its two satellites. For that, the authors acquired, processed and used 5 different types of observations, from all over the world. A consequence is that this paper has many authors, i.e. 27. Its title is Physical, spectral, and dynamical properties of asteroid (107) Camilla and its satellites, by Myriam Pajuelo and 26 colleagues, and it has recently been published in Icarus. This paper gives us the shape of Camilla and its main satellites, their orbits, the mass of Camilla, its composition, its spin period,… I give you these results below.

The system of Camilla

The asteroid (107) Camilla has been discovered in 1868 by Norman Pogson at Madras Observatory, India. It is located in the
outer Main-Belt, and more precisely it is a member of the Cybele group. This is a group of asteroids, named after the largest of them (65) Cybele, which is thought to have a common origin. They probably originate from the disruption of a single progenitor. I show you below some Camilla’s facts, taken from the JPL Small-Body Database Browser:

Discovery 1868
Semimajor axis 3.49 AU
Eccentricity 0.066
Perihelion 3.26 AU
Inclination 10.0°
Orbital period 6.52 yr

We have of course other data, which have been improved by the present study. Please by a little patient.

In 2001 the Hubble Space Telescope revealed a satellite of Camilla, S1, while the second satellite, S2, and has been discovered in 2016 from images acquired by the Very Large Telescope of Cerro Paranal, Chile. This makes (107) Camilla a ternary system. Interesting fact, there is at least another ternary system in the Cybele group: the one formed by (87) Sylvia, and its two satellites Romulus and Remus.

Since their discoveries, these bodies have been re-observed when possible. This resulted in a accumulation of different data, all of them having been used in this study.

5 different types of data

The authors acquired and used:

  • optical lightcurves,
  • high-angular-resolution images,
  • high-angular-resolution spectrum,
  • stellar occultations,
  • near-infrared spectroscopy.

You record optical lightcurves in measuring the variations of the solar flux, which is reflected by the object. This results in a curve exhibiting periodic variations. You can link their period to the spin period of the asteroid, and their amplitudes to its shape. I show you an example of lightcurve here.

High-angular-resolution imaging requires high-performance facilities. The authors used data from the Hubble Space Telescope (HST), and of 3 ground-based telescopes, equipped with adaptive optics: Gemini North, European Southern Observatory Very Large Telescope (VLT), and Keck. Adaptive optics permits to correct the images from atmospheric distortion, while the HST, as a space telescope, is not hampered by our atmosphere. In other words, our atmosphere bothers the accurate observations of such small objects.

A spectrum is the amplitude of the reflected Solar light, with respect to its wavelength. This permits to infer the composition of the surface of the body. The high-angular-resolution spectrum were made at the VLT, the resulting data also permitting astrometry of the smallest of the satellites, S2. These spectrum were supplemented by near-infrared spectroscopy, made with a dedicated facility, i.e. the SpeX spectrograph of the NASA InfraRed Telescope Facility (IRTF), based on Mauna Kea, Hawaii. Infrared is very sensitive to the temperature, this is why their observations require dedicated instruments, which need a dedicated cooling system.

Finally, stellar occultations consist to record the light of a star, which as some point is occulted by the asteroid you study. This is particularly interesting for a faint body, which you cannot directly observe. Such observations can be made by volunteers, who use their own telescopes. You can deduce clues on the shape, and sometimes on the presence of a satellite, from the duration of the occultation. In comparing the durations of the same occultation, recorded at different locations, you may even reconstruct the shape (actually a 2-D shape, which is projected on the celestial sphere). See here.

And from all this, you can infer the orbits of the satellites, and the composition of the primary (Camilla) and its main satellite (S1), and the spin and shape of Camilla.

The orbits of the satellites

All of these observations permit astrometry, i.e. they give you the relative location of the satellites with respect to Camilla, at given dates. From all of these observations, you fit orbits, i.e. you numerically determine the orbits, which have the smallest distances (residuals), with the data.

This is a very tough task, given the uncertainty of the recorded positions. For that, the authors used their own genetic-based algorithm, Genoid, for GENetic Orbit IDentification, which relies on a metaheuristic method to minimize the residuals. Many trajectories are challenged in this iterative approach, and only the best ones are kept. These remaining trajectories, designed as parents, are used to generate new trajectories which improve the residuals. This algorithm has proven its efficiency for other systems, like the binary asteroid (22) Kalliope-Linus. In such cases, the observations lack of accuracy and many parameters are involved.

You can find the results below.

S/2001 (107) 1
Semimajor axis 1247.8±3.8 km
Eccentricity <0.013
Inclination (16.0±2.3)°
Orbital period 3.71234±0.00004 d
S/2016 (107) 2
Semimajor axis 643.8±3.9 km
Eccentricity ~0.18 (<0.23)
Inclination (27.7±21.8)°
Orbital period 1.376±0.016 d

You can deduce the mass of (107) Camilla from these numbers, i.e. (1.12±0.01)x1019 kg. The ratio of two orbital periods probably rule out any significant mean-motion resonance between these two satellites.

Spin and shape

The authors used their homemade algorithm KOALA (Knitted Occultation, Adaptive-optics, and Lightcurve Analysis) to determine the best-fit solution (once more, minimization of the residuals) for spin period, orientation of the rotation pole, and 3-D shape model, from lightcurves, adaptive optics images, and stellar occultations. And you can find the solution below:

Camilla
Diameter 254±36 km
a 340±36 km
b 249±36 km
c 197±36 km
Spin period 4.843927±0.00004 h

This table gives two solutions for the shape: a spherical one, and an ellipsoid. In this last solution, a, b, and c are the three diameters. We can see in particular that Camilla is highly elongated. Actually a comparison between the data and this ellipsoid, named the reference ellipsoid, revealed two deep and circular basins at the surface of Camilla.

Moreover, a comparison of the relative magnitudes of Camilla and its two satellites, and the use of the diameter of Camilla as a reference, give an estimation of the diameters of the two satellites. These are 12.7±3.5 km for S1 and 4.0±1.2 km for S2. These numbers assume that S1 and S2 have the same albedo. This assumption is supported for S1 by the comparison of its spectrum from the one of Camilla.

The composition of these objects

In combining the shape of Camilla with its mass, the authors deduce its density, which is 1,280±130 kg/m3. This is slightly larger than water, while silicates should dominate the composition. As the authors point out, there might be some water ice in Camilla, but this pretty small density is probably due to the porosity of the asteroid.

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.

Some comets are not randomly distributed

Hi there! Everybody knows the comets, which may show us impressive tails, when they approach our Sun. This is due to what we call cometary activity. You can find comets in many places in the Solar System. Today we will focus on the 9 ones, which are located in the Main Asteroid Belt, i.e. between the orbits of Mars and Jupiter. This is the opportunity to present you Orbital alignment of Main-belt comets, by Yoonyoung Kim, Yougmin JeongAhn, and Henry H. Hsieh. This study has recently been published in The Astronomical Journal.

Comets in the Solar System

A comet is a small body, which presents some activity. This activity manifests as 2 tails, which are a gas tail and a dust tail. These two tails have different directions because the dust is heavier than the gas, and so is differently affected by the Sun. The Sun is actually responsible for at least part of this activity: if the body has water ice at its surface, the proximity of the Sun heats it enough to sublimate it.

We distinguish different classes of comets, from their orbital motion. The short-period comets have a period below 200 years, i.e. they make a close approach to the Sun periodically, with less than 200 years between two approaches. This is for instance the case of the famous Halley comet, or 1P/Halley, which period is 75 years. The comets with a period smaller than 20 years are called Jupiter-family comets, their orbits are strongly affected by the gravitational perturbation of Jupiter.
And we also have long-period comets, with periods larger than 200 years, up to several thousands of years, or even more… The extreme case is the one of the parabolic and hyperbolic comets, which eccentricities are close to or larger than 1. In such a case, we just see the comet once.

The Jupiter-family comets should originate from the Kuiper Belt, and have been so strongly perturbed by Jupiter that their semimajor axes became much smaller, reducing their orbital periods. However, we attribute the origin of the longer periods comets to the Oort cloud, which is thought to lie between 50,000 and 200,000 astronomical units (remember: the Sun-Earth distance is 1 AU). The comets we are interested in today are much closer, in the Main-Belt of asteroids.

The Main-Belt Comets

Main Belt Comets (MBCs) are comets, which are located in the asteroid belt. As such, they present some cometary activity. It appears that there is no general agreement on the way to identify them. Some asteroids present an activity, which is mainly driven by dust, and not by sublimation of water ice, so it could be relevant to call them active asteroids instead of comets. But they may have some sublimation driven activity as well.

The first identified MBC is 133P/Elst-Pizarro, which has been discovered in 1979 and is since then identified as an asteroid… and also as a comet since 1996. I mean, this is officially both a comet and an asteroid. The authors considered 9 MBC, there could be a little more of them, since classifying them is not that easy.

The comet Elst-Pizarro seen at La Silla Observatory. © ESO
The comet Elst-Pizarro seen at La Silla Observatory. © ESO

The MBC should originate from the Main-Belt. In this study, we are interested in the orbital dynamics. Let us talk about orbital elements.

Proper, forced, and osculating elements

As I have already told you in a previous post, we usually describe the orbit of a planetary body with 6 orbital elements, which characterize the ellipse drawn by the trajectory.

These orbital elements are

  • the semi-major axes (which would be the distance to the Sun, if the orbit were circular… this remains almost true for slightly eccentric orbits),
  • the mean longitude,
  • the eccentricity of the trajectory (0 means circular, the eccentricity must be smaller than 1 for the orbit to be elliptic),
  • the pericentre, i.e. location of the point of the trajectory, where the distance to the Sun is minimal,
  • the inclination, with respect to a given reference plane,
  • the ascending node, which locates the intersection between the reference plane and the orbit.

We call them osculating elements. These are the elements that the orbit would have at a given time, if it were exactly an ellipse. The real trajectory is very close to an ellipse, actually.

We will just keep in mind the two couples (eccentricity, pericentre), and (inclination, ascending node). Because these variables are coupled: without eccentricity, the pericentre is irrelevant, since the distance Sun-body is constant. And without inclination, the ascending node is irrelevant, since the whole trajectory is in the reference plane.

And these variables are the sums of a proper and a forced component. Imagine you are a MBC. You want to have your own motion around the Sun. This gives you the proper (or free) component, which is actually ruled by your initial conditions, and the interaction with the Sun (what we call the 2-body, or Kepler, problem). Unfortunately for you, there is this big guy perturbing your motion (Jupiter is his name). He is heavy enough to force your motion to follow his. This gives you a forced motion, and the actual motion is the sum of the proper and the forced ones. The forced motion tends to align your pericentre and your ascending node with the ones of Jupiter. The authors studied these motions.

The Main-Belt Comets are clustered

And their conclusions is that the MBC are clustered, in particular the pericentres. They tend to be aligned with the one of Jupiter. This could have been anticipated, but the authors found something more: the MBC are more clustered than the others asteroids, which lie in that region of the outer main-belt.

For quantifying this more clustered, they ran several statistical tests, which I do not want to detail (the Kolmogorov-Smirnov test, the F-test, and the Watson’s U2 test). These tests show that this excess of clustering happened very unlikely by chance. In other words, there is something. And this is more obvious for comets, for which the sublimation activity is overwhelming. This permits the authors to make a link between this activity, i.e. the presence of water ice, and this clustering. And to suggest favorable conditions for the detection of cometary activity for Main-Belt objects.

Where are the other MBCs?

Based on the result that the eccentricities of MBCs are secularly excited by Jupiter, the authors suggest to look for them in the fall night sky, when Jupiter’s perihelion is at opposition.
We would not necessarily be looking for new bodies, but also for cometary activity of already known bodies. Because of the variations of the distance with the Sun, the sublimation of water ice is not a permanent phenomenon. Remember that Elst-Pizarro has been classified as a comet 17 years after its discovery.

Clustering of TNOs suggests the existence of the Planet Nine

I would like to finish with a reminder that the Planet Nine was hinted that way, in 2016. A clustering among the orbits of Trans-Neptunian Objects was statistically proven. Since then, the Planet Nine has not been detected (yet), but other clues have suggested its presence, like the obliquity of the Sun.

More generally, I would say that big objects strongly affect the orbits of small ones, and in observing the small ones, then you can deduce something on the big ones!

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.

Breaking an asteroid

Hi there! Asteroids, these small bodies in the Solar System, are fascinating by the diversity of their shapes. This is a consequence of their small sizes. Another consequence is their weakness, which itself helps to split some of them into different parts, sometimes creating binary objects, asteroids families… The study I present you today, Internal gravity, self-energy, and disruption of comets and asteroids, by Anthony R. Dobrovolskis and Donald G. Korycansky, proposes an accurate computation of the required energy to provoke this break-up, at any place of the asteroid, i.e. you are more efficient when you hit at a given location. This study has recently been accepted for publication in Icarus.

Shapes of asteroids

Please allow me, in this context, to call asteroid a comet, a comet being a small body, i.e. like an asteroid, but with a cometary activity. The important thing is that the involved bodies are small enough.

Beyond a given size, i.e. a diameter of ~400 km, a planetary body is roughly spheroidal, i.e. it is an ellipsoid with it two equatorial axes almost equal and the polar one smaller, because of its rotation. For a tidally despun body, like the Moon, or a satellite of a giant planet, the shape is more triaxial, since the tidal (gravitational) action of the parent planet tends to elongate the equatorial plane. The same phenomenon affects Mercury.

However, for smaller bodies, the self-gravitation is not strong enough to make the body look more or less like a sphere. As a consequence, you can have almost any shape, some bodies are bilobate, some are contact binaries, i.e. two bodies which permanently touch together, some others are rubble piles, i.e. are weak aggregates of rocks, with many voids.

These configurations make these bodies likely to undergo or have undergone break-up. This can be quantified by the required energy to extract some material from the asteroid.

The energies involved

For that, an energy budget must be performed. The relevant energies to consider are:

  • The impact disruption energy: the minimum kinetic energy of an impactor, to shatter the asteroid and remove at least half of its mass,
  • The shattering energy: the minimum energy needed to shatter the asteroid into many small pieces. It is part of the impact disruption energy. This energy is roughly proportional to the mass of the asteroid. It represents the cohesion between the adjacent pieces.
  • The binding energy: this energy binds the pieces constituting the asteroid. In other words, once you have broken an asteroid (don’t try this at home!), you have to make sure the pieces will not re-aggregate… because of the binding energy. For that, you have to bring enough energy to disperse the fragments.
  • The self-gravitational energy: due to the mutual gravitational interaction between the blocks constituting the asteroids. Bodies smaller than 1 km are strength-dominated, i.e. they exist thanks to the cohesion between the blocks, which is the shatter energy. However, larger bodies are gravity-dominated.
  • The kinetic energy of rotation: the spin of these bodies tends to enlarge the equatorial section. In that sense, it assists the break-up process.

This study addresses bodies, which are far enough from the Sun. This is the reason why I do not mention its influences, i.e. the tides and the thermic effects, which could be relevant for Near-Earth Objects. In particular, the YORP effect is responsible for the fission of some of them. I do not mention the orbital kinetic energy of the asteroid either. Actually the orbital motion is part of the input energy brought by an impact, since the relative velocity of the impactor with respect to the target is relevant in this calculation.

I now focus on the two cases studied by the authors to illustrate their theory: the asteroid Kleopatra and the comet 67P/Churyumov-Gerasimenko.

2 peculiar cases: Kleopatra and Churyumov-Gerasimenko

216 Kleopatra is a Main-Belt asteroid. Adaptive optics observations have shown that is is constituted of two masses bound by material, giving a ham-bone shaped. As such, it can be considered as a contact binary. It is probably a rubble pile. Interestingly, observations have also shown that Kleopatra has 2 small satellites, Alexhelios and Cleoselene, which were discovered in 2008.

Reconstruction of the shape of Kleopatra. © NASA
Reconstruction of the shape of Kleopatra. © NASA

However, 67P Churyumov-Gerasimenko is a Jupiter-family comet, i.e. its aphelion is close to the orbit of Jupiter, while its perihelion is close to the one of the Earth. It has an orbital period of 6.45 years, and was the target of the Rosetta mission, which consisted of an orbiter and a lander, Philae. Rosetta orbited Churyumov-Gerasimenko between 2014 and 2016. The shape of this comet is sometimes described as rubber ducky, with two dominant masses, a torso and a head, bound together by some material, i.e. a neck.

Churyumov-Gerasimenko seen by Rosetta. © ESA
Churyumov-Gerasimenko seen by Rosetta. © ESA
216 Kleopatra 67P/Churyumov-Gerasimenko
Semimajor axis 2.794 AU 3.465 AU
Eccentricity 0.251 0.641
Inclination 13.11° 7.04°
Spin period 5.385 h 12.761 h
Mean radius 62 km 2.2 km
Magnitude 7.30 11.30
Discovery 1880 1969

The irregular shapes of these two bodies make them interesting targets for a study addressing the gravitation of any object. Let us see now how the authors addressed the problem.

Numerical modeling

Several models exist in the literature to address the gravity field of planetary bodies. The first approximation is to consider them as spheres, then you can refine in seeing them as triaxial ellipsoids. For highly irregular bodies you can try to model them as cuboids, and then as polyhedrons. Another way is to see them as duplexes, this allows to consider the inhomogeneities dues to the two masses constituting bilobate objects. The existence of previous studies allow a validation of the model proposed by the authors.

And their model is a finite-element numerical modeling. The idea is to split the surface of the asteroid into small triangular planar facets, which should be very close to the actual surface. The model is all the more accurate with many small facets, but this has the drawback of a longer computation time. The facets delimit the volume over which the equations are integrated, these equations giving the local self-gravitational and the impact disruption energies. The authors also introduce the energy rebate, which is a residual energy, due to the fact that you can remove material without removing half of it. This means that the impact disruption energy, as it is defined in the literature, is probably a too strong condition to have extrusion of material.
The useful physical quantities, which are the gravitational potential, the attraction, and the surface slope, are propagated all along the body thanks to a numerical scheme, which accuracy is characterized by an order. This order quantifies the numerical approximation which is made at each integration step. A higher order is more accurate, but is computationally more expensive.

Once the code has been run on test cases, the authors applied it on Kleopatra and Churyumov-Gerasimenko, for which the shape is pretty well known. They used meshes of 4,094 and 5,786 faces, respectively.

Results

The validation phase is successful. The authors show that with a 3rd order numerical scheme, they recover the results present in the literature for the test cases with an accuracy of ~0.1%, which is much better than the accuracy of the shape models for the real asteroids. Regarding Kleopatra and Churyumov-Gerasimenko, they get the gravity field at any location, showing in particular excesses of gravity at the two lobes.

Such a study is particularly interesting for further missions, which would determine the gravity field of asteroids, which would then be compared with the theoretical determination by this code. Other applications are envisaged, the authors mentioning asteroid mining.

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.

And Merry Christmas!

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.