Category Archives: Asteroids: Main Belt

An asteroid pair

Hi there! Today I present you the study of an asteroid pair. Not a binary, a pair. A binary asteroid is a couple of asteroids which are gravitationally bound, while in pair, the asteroids are just neighbors, they do not live together… but have. The study is entitled Detailed analysis of the asteroid pair (6070) Rheinland and (54287) 2001 NQ8, by Vokrouhlický et al., and it has recently been published in The Astronomical Journal.

Asteroid pairs

I have presented asteroid families in a previous post. These are groups of asteroids which present common dynamical and physical properties. They can be in particular identified from the clustering of their proper elements, i.e. you express their orbital elements (semimajor axis, eccentricity, inclination, pericentre, …), you treat them properly so as to get rid off the gravitational disturbance of the planets, and you see that some of these bodies tend to group. This suggests that they constitute a collisional family, i.e. they were a unique body in the past, which has been destroyed by collisions.
An asteroid pair is something slightly different, since these are two bodies which present dynamical similarities in their osculating elements, i.e. before denoising them from the gravitational attraction of the planets. Of course, they would present similarities in their proper elements as well, but the fact that similarities can be detected in the osculating elements means that they are even closer than a family, i.e. the separation occurred later. Families younger than 1 Myr (1 million of years) are considered to be very young; the pair I present you today is much younger than that. How much? You have to read me before.
A pair suggests that only two bodies are involved. This suggests a non-collisional origin, more particularly an asteroid fission.

Asteroid fission

Imagine an asteroid with a very fast rotation. A rotation so fast that it would split the asteroid. We would then have two components, which would be gravitationally bound, and evolving… Depending on the energy involved, it could remain a stable binary asteroid, a secondary fission might occur, the two or three components may migrate away from each other… and in that case we would pair asteroid with very close elements of their heliocentric orbits.
It is thought that the YORP (Yarkovsky – O’Keefe – Radzievskii – Paddack) could trigger this rotational fission. This is a thermic effect which alter the rotation, and in some cases, in particular when the satellite has an irregular shape, it could accelerate it. Until fission.
Thermic effects are particularly efficient when the Sun is close, which means that NEA (Near Earth Asteroids) are more likely to be destroyed by this process than Main Belt asteroids. Here, we deal with Main Belt asteroids.

The pair 6070-54827 (Rheinland – 2001 NQ8)

The following table present properties of Rheinland and 2001 NQ8. The orbital elements are at Epoch 2458000.5, i.e. September 4th 2017. They come from the JPL Small-Body Database Browser.

(6070) Rheinland (54827) 2001 NQ8
Semimajor axis (AU) 2.3874015732216 2.387149297807496
Eccentricity 0.2114524962733347 0.211262507795103
Inclination 3.129675305535938° 3.128927421642917°
Node 83.94746016534368° 83.97704257098502°
Pericentre 292.7043398319871° 292.4915004062336°
Orbital period 1347.369277588708 d (3.69 y) 1347.155719572348 d (3.69 y)
Magnitude 13.8 15.5
Discovery 1991 2001

Beside their magnitudes, i.e. Rheinland is much brighter than 2001 NQ8, this is why it was discovered 10 years earlier, we can see that all the slow orbital elements (i.e. all of them, except the longitude) are very close, which strongly suggests they shared the same orbit. Not only their orbits have the same shape, but they also have the same orientation.

Shapes and rotations from lightcurves

A useful tool for determining the rotation and shape of an asteroid is the lightcurve. The object reflects the incident Solar light, and the way it reflects it will tell us something on its location, its shape, and its orientation. You can imagine that the surfaces of these bodies are not exclusively composed of smooth terrain, and irregularities (impact basins, mountains,…) will result in a different Solar flux, which also depends on the phase, i.e. the angle between the normale of the surface and the asteroid – Sun direction… i.e. depends whether you see the Sun at the zenith or close to the horizon. This is why recording the light from the asteroid at different dates tell us something. You can see below an example of lightcurve for 2001 NQ8.

Example of lightcurve for 2001 NQ8, observed by Vokrouhlický et al.

Recording such a lightcurve is not an easy task, since the photometric measurements should be denoised, otherwise you cannot compare them and interpret the lightcurve. You have to compensate for the variations of the luminosity of the sky during the observation (how far is the Moon?), of the thickness of the atmosphere (are we close to the horizon?), of the heterogeneity of the CCD sensors (you can compensate that in measuring the response of a uniform surface). And the weather should be good enough.

Once you have done that, you get a lightcurve alike the one above. We can see 3 maxima and 2 minima. Then the whole set of lightcurves is put into a computational machinery which will give you the parameters that best match the observations, i.e. periods of rotation, orientation of the spin pole at a given date, and shape… or at least a diameter. In this study, the authors already had the informations for Rheinland but confirmed them with new observations, and produced the diameter and rotation parameters for 2001 NQ8. And here are the results:

(6070) Rheinland (54827) 2001 NQ8
Diameter (km) 4.4 ± 0.6 2.2 ± 0.3
Spin period (h) 4.2737137 ± 0.0000005 5.877186 ± 0.000002
Spin pole (124°,-87°) (72°,-49°) or (242°,-46°)

We can see rapid rotation periods, as it is often the case for asteroids. The locations of the poles mean that their rotations
are retrograde, with respect to their orbital motions. Moreover, two solutions best match the pole of 2001 NQ8.

Dating the fission

The other aspect of this study is a numerical simulation of the orbital motion of these two objects, backward in time, to date their separation. Actually, the authors considered 5,000 clones of each of the two objects, to make their results statistically relevant.
They not only considered the gravitational interactions with other objects of the Solar System, but also the Yarkovsky effect, i.e. a thermal pull due to the Sun, which depends on the reflectivity of the asteroids, and favors their separation. For that, they propose new equations implementing this effect. They also simulated the variations of the spin pole orientation, since it affects the thermal acceleration.

And here is the result: the fission probably occurred 16,340 ± 40 years ago.

Perspectives

Why doing that? Because what we see is the outcome of an asteroid fission, which occurred recently. The authors honestly admit that this result could be refined in the future, depending on

  • Possible future measurements of the Yarkovsky acceleration of one or two of these bodies,
  • The consideration of the mutual interactions between Rheinland and 2001 NQ8,
  • Refinements of the presented measurements,
  • Discovery of a third member?

To date the fission, they dated a close approach between these two bodies. They also investigated the possibility that that
close approach, some 16,000 years from now, could have not been the right one, and that the fission could have been much older. For that, they ran long-term simulations, which suggest that older close approaches should have been less close: if the pair were older, Yarkovsky would have separated it more.

To know more

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.

Chaotic dynamics of asteroids

Hi there! Today’s post deals with the fate of an asteroid family. You remember Datura? Now you have Hungaria! Datura is a very young family (< 500 kyr), now you have a very old one, i.e. probably more than 1 Gyr, and you will see that such a long time leaves room for many uncertainties... The paper I present is entitled Planetary chaos and the (In)stability of Hungaria asteroids, by Matija Ćuk and David Nesvorný, it has recently been accepted for publication in Icarus.

The Hungaria asteroids

Usually an asteroid family is a cluster of asteroids in the space of the orbital elements (semimajor axis, eccentricity, inclination), which share, or a supposed to share, a common origin. This suggests that they would originate from the same large body, which would have been destroyed by a collision, its fragments then constituting an asteroid family. Identifying an asteroid family is not an easy task, because once you have identified a cluster, then you must make sure that the asteroids share common physical properties, i.e. composition. You can get this information from spectroscopy, i.e. in comparing their magnitudes in different wavelengths.

The following plot gives the semimajor axis / eccentricity repartition of the asteroids in the inner Solar System, with a magnitude smaller than 15.5. We can clearly see gaps and clusters. Remember that the Earth is at 1 UA, Mars at 1.5 UA, and Jupiter at 5.2. The group of asteroids sharing the orbit of Jupiter constitute the Trojan population. Hungaria is the one on the left, between 1.8 and 2 AU, named after the asteroid 434 Hungaria. The gap at its right corresponds to the 4:1 mean-motion resonance with Jupiter.

Distribution of the asteroids in the inner Solar System, with absolute magnitude < 15.5. Reproduced from the data of The Asteroidal Elements Database. Copyright: planetary-mechanics.com

If we look closer at the orbital elements of this Hungaria population, we also see a clustering on the eccentricity / inclination plot (just below).

Eccentricity / Inclination of the asteroids present in the Hungaria zone. Copyright: planetary-mechanics.com

This prompted Anne Lemaître (University of Namur, Belgium) to suggest in 1994 that Hungaria constituted an asteroid family. At that time, only 26 of these bodies were identified. We now know more than 4,000 of them.

The origin of this family can be questioned. The point is that these asteroids have different compositions, which would mean that they do not all come from the same body. In other words, only some of them constitute a family. Several dynamics studies, including the one I present today, have been conducted, which suggest that these bodies are very old (> 1 Gyr), and that their orbits might be pretty unstable over Gyrs… which suggests that it is currently emptying.

This raises two questions:

  1. What is the origin of the original Hungaria population?
  2. What is the fate of these bodies?

Beside the possible collisional origin, which is not satisfying for all of these bodies since they do not share the same composition, it has been proposed that they are the remnants of the E-Belt, which in some models of formation of the Solar System was a large population of asteroid, which have essentially been destabilized. Another possibility could be that asteroids might pass by and eventually be trapped in this zone, feeding the population.

Regarding the fate, the leaving asteroids could hit other bodies, or become Trojan of Jupiter, or… who knows? Many options seem possible.

The difficulty of giving a simple answer to these questions comes partly from the fact that these bodies have a chaotic dynamics… but what does that mean?

Chaos, predictability, hyperbolicity, frequency diffusion, stability,… in celestial dynamics

Chaos is a pretty complicated mathematical and physical notion, which has several definitions. A popular one is made by the American mathematician Robert L. Devaney, who said that a system is chaotic if it has sensitive dependence on initial conditions, it is topologically transitive (for any two open sets, some points from one set will eventually hit the other set), and its periodic orbits form a dense set.

Let us make things a little simpler: in celestial mechanics, you assume to have chaos when you are sensitive to the initial conditions, i.e. if you try to simulate the motion of an object with a given uncertainty on its initial conditions, the uncertainties on its future will grow exponentially, making predictions impossible beyond a certain time, which is related to the Lyapunov time. But to be rigorous, this is the definition of hyperbolicity, not of chaos… but never mind.

A chaotic orbit is often thought to be unstable. This is sometimes true, especially if the eccentricity of your object becomes large… but this is not always the same. Contrarily, you can have stable chaos, in which you know that your object is not lost, it is in a given bounded zone… but you cannot be more accurate than that.

Chaos can also be related to the KAM theory (for Kolmogorov-Arnold-Moser), which says that when you are chaotic, you have no tores in the dynamics, i.e. periodic orbits. When your orbit is periodic, its orbital frequency is constant. If this frequency varies, then you can suspect chaos… but this is actually frequency diffusion.

And now, since I have confused you enough with the theory, comes another question: what is responsible for chaos? The gravitational action of the other bodies, of course! But this is not a satisfying answer, since a gravitational system is not always chaotic. There are actually many configurations in which a gravitational system could be chaotic. An obvious one is when you have a close encounter with a massive object. An other one is when your object is under the influence of several overlapping mean-motion resonances (Chirikov criterion).

This study is related to the chaos induced by the gravitational action of Mars.

The orbit of Mars

Mars orbits the Sun in 687 days (1.88 year), with an inclination of 1.85° with respect to the ecliptic (the orbit of the Earth), and an eccentricity of 0.0934. This is a pretty large number, which means that the distance Mars – Sun experiences some high amplitude variations. All this is valid for now.

But since the Hungaria asteroids are thought to be present for more than 1 Gyr, a study of their dynamics should consider the variations of the orbit of Mars over such a very long time-span. And this is actually a problem, since the chaos in the inner Solar System prevents you from being accurate enough over such a duration. Recent backward numerical simulations of the orbits of the planets of the Solar System by J. Laskar (Paris Observatory), in which many close initial conditions were considered, led to a statistical description of the past eccentricity of Mars. Some 500 Myr ago, the eccentricity of Mars was most probably close to the current one, but it could also have been close to 0, or close to 0.15… actually it could have taken any number between 0 and 0.15.

The uncertainty on the past eccentricity of Mars leads uncertainty on the past orbital behavior of Solar System objects, including the stability of asteroids. At least two destabilizing processes should be considered: possible close encounters with Mars, and resonances.

Among the resonances likely to destabilize the asteroids over the long term are the gi (i between 1 and 10) and the fj modes. These are secular resonances, i.e. involving the pericentres (g-modes) and the nodes (f-modes) of the planets, the g-modes being doped by the eccentricities, and the f-modes by the inclinations. These modes were originally derived by Brouwer and van Woerkom in 1950, from a secular theory of the eight planets of the Solar System, Pluto having been neglected at that time.

The eccentricity of Mars particularly affects the g4 mode.

This paper

This paper consists of numerical integrations of clones of known asteroids in the Hungaria region. By clones I mean that the motion of each asteroid is simulated several times (21 in this study), with slightly different initial conditions, over 1 Gyr. The authors wanted in particular to test the effect of the uncertainty on the past eccentricity of Mars. For that, they considered two cases: HIGH and LOW.

And the conclusion is this: in the HIGH case, i.e. past high eccentricity of Mars (up to 0.142), less asteroids survive, but only if they experienced close encounters with Mars. In other words, no effect of the secular resonance was detected. This somehow contradicts previous studies, which concluded that the Hungaria population is currently decaying. An explanation for that is that in such phenomena, you often have a remaining tail of stable objects. And it seems make sense to suppose that the currently present objects are this tail, so they are the most stable objects of the original population.

Anyway, this study adds conclusions to previous ones, without unveiling the origin of the Hungaria population. It is pretty frustrating to have no definitive conclusion, but we must keep in mind that we cannot be accurate over 1 Gyr, and that there are several competing models of the evolution of the primordial Solar System, which do not affect the asteroid population in the same way. So, we must admit that we will not know everything.

To know more

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

Identifying an asteroid family

Hi there! Today’s post deals with an asteroid family, more precisely the Datura family. The related study is New members of Datura family, by A. Rosaev and E. Plávalová, it has recently been accepted for publication in Planetary and Space Science. The Datura family is a pretty recent one, with only 7 known members when that study started. The authors suggest that 3 other bodies are also members of this family.

Some elements of the dynamics of asteroids

Detailing the dynamics of asteroids would require more than a classical post, here I just aim at giving a few hints.
Asteroids can be found at almost any location in the Solar System, but the combination of the gravitational effects of the planets, of thermal effects, and of the formation of the Solar System, result in preferred locations. Most of the asteroids are in the Main Belt, which lies between the orbits of Mars and Jupiter. And most of these bodies have semimajor axes between 2.1 and 3.2 astronomical units (AU), i.e. between 315 and 480 millions of km. Among these bodies can be found interesting dynamical phenomena, such as:

  • Mean motion resonances (MMR) with planets, especially Jupiter. These resonances can excite the eccentricities of the asteroids until ejecting them, creating gaps known as Kirkwood gaps. At these locations, there are much less asteroids than nearby.
  • Stable chaos. Basically, a chaotic dynamics means that you cannot predict the orbit at a given accuracy over more than a given timespan, because the orbit is too sensitive to uncertainties on its initial conditions, i.e. initial location and velocity of the asteroid. Sometimes chaos is associated with instability, and the asteroid is ejected. But not always. Stable chaos means that the asteroid is confined in a given zone. You cannot know accurately where the asteroid will be at a given time, but you know that it will be in this zone. Such a phenomenon can be due to the overlap of two mean-motion resonances (Chirikov’s criterion).

Anyway, when an asteroid will or will not be under the influence of such an effect, it will strongly be under the influence of the planets, especially the largest ones. This is why it is more significant to describe their dynamics with proper elements.

Proper elements

Usually, an elliptical orbit is described with orbital elements, which are the semimajor axis a, the eccentricity e, the ascending node Ω, the pericentre ω, the inclination I, and the mean longitude λ. Other quantities can be used, like the mean motion n, which is the orbital frequency.

Because of the large influence of the major planets, these elements present quasiperiodic variations, i.e. sums of periodic (sinusoidal) oscillations. Since it is more significant to give one number, the oscillations which are due to the gravitational perturbers are removed, yielding mean elements, called proper elements. These proper elements are convenient to characterize the dynamics of asteroids.

Asteroid families

Most of the asteroids are thought to result from the disruption (for instance because of a collision) of a pretty large body. The ejecta resulting from this disruption form a family, they share common properties, regarding their orbital dynamics and their composition. A way to guess the membership of an asteroid to a family is to compare its proper elements with others’. This guess can then be enforced by numerical simulations of the orbital motion of these bodies over the ages.

Usually a family is named from its largest member. In 2015, 122 confirmed families and 19 candidates were identified (source: Nesvorný et al. in Asteroids IV, The University of Arizona Press, 2015). Many of these families are very old, i.e. more than 1 Gyr, which complicates their identification in the sense that their orbital elements are more likely to have scattered.
The Datura family is thought to be very young, i.e. some 500 kyr old.

A funny memory: in 2005 David Nesvorný received the Urey Prize of the Division of Planetary Sciences of the American Astronomical Society. This prize was given to him at the annual meeting of the Division, that year in Cambridge, UK. He then gave a lecture on the asteroid families, and presented the “Nesvorný family”, i.e. his father, his wife, and so on.

Datura’s facts

The asteroid (1270) Datura has been discovered in 1930. It orbits the Sun in 3.34 years, and has a semimajor axis of 2.23 AU. As such, it is a member of the inner Main Belt. Its orbit is highly elongated, between 1.77 and 2.70 AU, with an orbital eccentricity of 0.209. It rotates very fast, i.e. in 3.4 hours. Its diameter is about 8.2 km.

It is an S-type asteroid, i.e. it is mainly composed of iron- and magnesium-silicates.

This study

After having identified 10 potential family members from their proper elements, the authors ran backward numerical simulations of them, cloning each asteroid 10 times to account for the uncertainties on their locations. The simulations were ran over 800 kyr, the family being supposed to be younger than that. The simulations first included the 8 planets of the Solar System, and Pluto. The numerical tool is a famous code, Mercury, by John Chambers.

The 10 asteroids identified by the authors include the 7 already known ones, and 3 new ones: (338309) 2002 VR17, 2002 RH291, and 2014 OE206. These are all sub-kilometric bodies. The authors point out that these bodies share a linear correlation between their node and their pericentre.

This study also shows that 2014 OE206 has a chaotic resonant orbit, because of the proximity of the 9:16 MMR with Mars. This resonance also affects 2001 VN36, but this was known before (Nesvorný et al., 2006). The authors also find that this chaotic dynamics can be significantly enhanced by the gravitational perturbations of Ceres and Vesta. Finally, they say that close encounters might happen between (1270) Datura and two of its members: 2003 SQ168 and 2001 VN36.

Another study

Now, to be honest, I must mention another study, The young Datura asteroid family: Spins, shapes, and population estimate, by David Vokrouhlický et al., which was published in Astronomy and Astrophysics in February 2017. That study goes further, in considering the 3 new family members found by Rosaev and Plávalová, and in including other ones, updating the Datura family to 17 members.

This seems to be a kind of anachronism: how could a study be followed by another one, which is published before? In fact, Rosaev and Plávalová announced their results during a conference in 2015, this is why they could be cited by Vokrouhlický et al. Of course, their study should have been published earlier. Those things happen. I do not know the specific case of this study, but sometimes this can be due to a delayed reviewing process, another possibility could be that the authors did not manage to finish the paper earlier… Something that can be noticed is that the study by Vokrouhlický is signed by a team of 13 authors, which is expected to be more efficient than a team of two. But the very truth is that I do not know why they published before. This is anyway awkward.

A perspective

I notice something which could reveal a rich dynamics: the authors show (their Figure 7) a periodic variation of the distance between (1270) Datura and 2003 SQ168, from almost zero to about twice the semimajor axis… This suggests me a horseshoe orbit, i.e. a 1:1 mean-motion resonance, the two bodies sharing the same orbit, but with large variations of their distance. If you look at the orbit of the smallest of these two bodies (here 2003 SQ168) in a reference frame which moves with (1270) Datura, you would see a horseshoe-shaped trajectory. To the best of my knowledge, such a configuration has been detected in the satellites of Saturn between Janus and Epimetheus, suggested for exoplanetary systems, maybe detected between a planet and an asteroid, but never between two asteroids…

By the way, 2003 SQ168 is the asteroid, which has the closest semimajor axis to the one of (1270) Datura, in Rosaev and Plávalová’s paper. Now, when I look at Vokrouhlický et al.’s paper, I see that 2013 ST71 has an even closer semimajor axis. I am then tempted to speculate that these two very small bodies are coorbital to (1270) Datura. Maybe a young family favors such a configuration, which would become unstable over millions of years… Speculation, not fact.

Update

This is actually not an horsehoe orbit. The large variation of the distance is due to the fact that 2003 SQ168 is on a orbit, which is close to the one of (1270) Datura, with a slightly different orbital frequency. Regarding 2013 ST71, a numerical simulation by myself suggests the possibility of a temporary (i.e. unstable) capture in a 1:1 MMR.

To know more…

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

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

Interesting polar craters on Vesta

Hi there! Today’s post is on the paper On the possibility of viscoelastic deformation of the large south polar craters and true polar wander on the asteroid Vesta, by Saman Karimi and Andrew J. Dombard, both at the University of Illinois at Chicago during the study; Saman Karimi is now at Johns Hopkins University. This study has recently been accepted for publication in Journal of Geophysical Research: Planets. It is a study of 2 craters of the small planet Vesta, Rheasilvia and Veneneia, which present two unusual features:

  1. they are located close to the South Pole,
  2. they are shallow with a central peak.

The authors have tried to explain these two properties.

The small planet Vesta

Vesta, or more precisely (4) Vesta, is the second largest object of the Main Asteroid Belt. It has a triaxial shape, i.e. (572.6 × 557.2 × 446.4) km, and is large enough to have a differentiated structure. It orbits at a distance of 2.36 AU from the Sun, i.e. 354 millions km, which implies an orbital period of 3.63 years. However, it rotates much more rapidly, in 5.3 hours. This rapid rotation is responsible for the high polar flattening, i.e. you can see from its shape that one of its axes is much smaller than the other ones. This axis is actually the rotation axis. This rotation around one axis permits to define easily the North and the South Poles, close to which are the 2 craters of interest.

(4) Vesta has been recently the target of the space mission Dawn. Dawn has been launched from Cape Canaveral in September 2007. It has orbited Vesta between July 2011 and September 2012, and is orbiting Ceres since March 2015. Dawn permitted invaluable progress on our knowledge of Vesta. It gave us an accurate cartography of the surface, which resulted in a count of the craters, measurements of its shape, of its gravity field, of its rotation… All of these data permit to constrain the interior. Many papers on Vesta followed, the paper I am presenting you is one of these.

Impacts in the Solar System

The Solar System bodies are impacted since the beginning of their formation. During the early ages of the Solar System, the impacts were more frequent than now, because of the presence of a protoplanetary disk composed of small objects before they accrete into larger ones. For instance, the Late Heavy Bombardment (LHB) is known as an episode of intense bombardment which occurred approximately 4 billion years ago. Some models consider that it could have been triggered by a gravitational interaction between giant planets and a former asteroid belt, which has destabilized it. For instance a previous version of the Nice model stated that the LHB could have been the consequence of a former 2:1 mean-motion resonance between Jupiter and Saturn during their migration. That resonance would have raised the orbital oscillations of these planets, which would have favored the destabilization of the asteroid belt and the bombardment of the terrestrial planets.

Meteorites are signatures of impacts on the Earth. Actually, many small objects are destroyed when they enter our atmosphere, this is why we get these small meteorites on the surface. Atmosphereless bodies usually present signatures of bombardment, for instance the Moon is known for its craters. When such a body does not present evidence of craters, it could mean that its surface has been recently renewed by some internal processes, due to tectonic or volcanic activity. So, counting the impacts is a way to age the surfaces.

When large enough, impacts can be responsible for dramatic events such as: the creation of the Moon, which has probably been split from the Earth by an impact, the creation of the rings of Saturn, which could be made of a large impactor, the destruction of the outer envelope of the proto-Mercury, or the extinction of the dinosaurs.

The study I present here deals with two impact basins at the South Pole of Vesta: Rheasilvia and Veneneia, with diameters of 505 and 395 km, respectively. You can compare these numbers with the dimensions of Vesta, and you understand how significant the impacts creating these craters should have been in the history of Vesta.

A viscoelastic rheology

The issue is: how does the surface respond to a large impact? It depends on its structure, of course. Basically, when you hit the surface, you create a crater, ejecta being expelled. After that, the surface of the asteroid tends to relax, i.e. the deformation due to the impact is kind of damped, but the final aspect will not be the initial one, since some material has been displaced, some other ejected, and the heating due to the impact tends to molten the surface. During the process of relaxation, the material tends to converge to the center of the basin, while it was pushed to the edges when the impact occurred, this can result in a central peak. Measuring the topography of the crater, i.e. its width, its depth, and the height of its central peak, can give constraints on the way the surface responds. This response characterizes the rheology of the surface, which is basically viscoelastic. Elastic would mean that the surface would recover its initial shape without any energy loss, and viscous means that you have actually some energy loss, which results in a permanent deformation once the surface is relaxed.

This study

The study first points out the two peculiarities of the two craters, and test the hypothesis that the impacts occurred close to the equator As a consequence Vesta would have been reoriented, this would explain why the impacts are now located close to the South Pole. This would mean that the surface is molten enough to result in the current topography of the craters and in the present polar flattening of Vesta.

To try to understand these facts, the authors assumed that the impactors hit Vesta close to its equator, and ran numerical simulations to check whether Vesta was able to reach its current state, which implies reshaping and reorientation. The numerical simulations consist to propagate the response to the impact not only in time, but also on the surface of Vesta. For that, the surface is discretized on a mesh, and finite elements modeling is used. This is a classical way to integrate Partial Derivative Equations (PDE). A key parameter is the temperature: if the impact is energetic enough, then Vesta heats enough to be molten enough to create the central peak, relax the crater, and reshape according to its new orientation state.

The reader should be aware that such simulations require high computation facilities, and take a long time. This is the reason why the authors ran only 8 of them, with different assumptions to cover most of the physically acceptable properties for the lithosphere of Vesta. These properties are in this study ruled by 6 parameters: the crustal thickness, the temperatures of the surface and of the mantle, the crustal thermal conductivity, the background heat flux, and the isostatic compensation. This last parameter characterizes the capacity of the surface to recover its gravity after the shock of the impact, which displaced the internal masses. This particularly affects the height of the central peak.

None of these 8 simulations result in a Vesta which is close enough to the observed one, since it does not heat enough. This means that the shape of Vesta is not a direct consequence of these two impacts, which probably occurred close to the South Pole, even if impacts at this latitude have a low probability.

A question for the authors

I am no specialist of impacts, but I wonder: if we have two tangent impacts instead of perpendicular ones, I guess they would have resulted in craters with a limited depth, but a strong reorientation of Vesta. The authors do not mention this possibility in the paper, and I would be interested in their opinion on this issue.

Some links

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