Monthly Archives: October 2016

Asteroids: Main Belt

Interesting polar craters on Vesta

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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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The first release of Gaia astrometric data

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Hi there! Today is a little bit different, since I will tell you about positions of stars in the sky. WTF??? No Solar System today? Well, actually this is very useful for studying the Solar System. This deals with astrometry, which tells you where your object is.
Another difference with the usual business is that I do not present you a paper, but a series of paper. I have counted 6 papers related to this first release of Gaia data, i.e. the Gaia DR1, for Gaia Data Release 1. They will be published soon in Astronomy and Astrophysics, and some of them are freely available on arXiv. The Gaia Data Release 1 was made available online on Sept, 14th.

Why astrometry?

When you want to study Solar System objects, you need to know where they are, especially if you study their orbital motion, but not only. For that, you use stars as fixed enough reference points, with respect to which you will locate your planetary object of interest. Actually, the stars have some motion with respect to the observer. They have their proper motion, since our galaxy is moving, and a parallax effect, which is a consequence of the motion of the Earth. If you observe something that does not move while you are moving, you will see an apparent motion. This motion will be all the more significant that the object is closer. These problems motivate the use of even further objects, the quasars, with respect to which the stars will be located. These quasars, for quasi-stellar radio-sources, are actually galaxies with an active nucleus. As galaxies, they are further from us than the observed stars, which belong to our galaxy. Moreover, they are brighter, which make them ideal reference points for defining reference frames, in which the stars will be positioned.

One of the goals of the Gaia mission is to elaborate the most accurate and exhaustive catalog giving the positions of stars.

The first space experiment devoted to high precision astrometry was Hipparcos, for High precision parallax collecting satellite. It was made by the European Space Agency (ESA), launched in 1989, and has operated until 1993. It could detect light sources until the magnitude 12.5. It resulted in 3 catalogs: Hipparcos, Tycho-1 and Tycho-2.

The Hipparcos catalog was constituted of 118,218 entries, giving astrometric and photometric data for almost all of them. The astrometric data were composed of 6 elements: right ascension and declination, which locate the object on the sky, the parallax, which is related to its distance, the proper motion in right ascension and declination, and its radial velocity, i.e. the time variation of its distance.

A more extensive analysis of the stars detected by Hipparcos resulted in 2 more comprehensive catalogs, Tycho-1 and Tycho-2, constituted of respectively 1,058,332 and 2,539,913 entries. Tycho-2 was the most accurate catalog we disposed on until this first release of Gaia data. It gives astrometric data at the mean date J1991.25.

Gaia is an astrometric satellite made by ESA and launched in December 2013. It orbits close to the Lagrange point L2 of the Sun-Earth system. This means that it lies between the Sun and the Earth, at a distance of 1.5 millions km from the Earth, and that its orbit is very stable, since the gravitational attraction of the Earth balances the one of the Sun, at that place. This pretty limited distance from the Earth allows a high rate of data transmission (40 Gbyte / day). From that place, Gaia makes systematic scans of the sky during its 5-years operational phase, which has started on July 25th 2014. It is composed of 2 telescopes with a very stable angle between them, and the whole sky shall be observed 70 times during the 5-years nominal mission.

Gaia can detect light sources up to the magnitude 20. This will permit the discovery of unknown Solar System objects, like asteroids or comets, but also of exoplanets. The discovery of a supernova, named Gaia14aaa, has been announced in September 2014. Moreover, the accurate determination of the proper motion of the stars shall give us an accurate picture of the motion of our galaxy, and permit a better knowledge of the position of the stars in the past and in the future. This shall help to redetermine astrometric position of Solar System objects on old astrometric planets, and so refine their orbital ephemerides, as proposed by the NAROO (New Astrometric Reduction of Old Observations) project.

The Data Release 1

The Data Release 1 has been released on Sep, 14th 2016. It contains positions of more than one billion of stars brighter than magnitude 20.7, and proper motion and parallaxes of about 2 millions of stars, which are the Tycho-2 objects. These numbers are given at the date J2015.0. The data are based on the first 14 months of the operational phase, and they should be seen as very preliminary results.

This release is of high importance, since it represents a major improvement with respect to the catalog Tycho-2, and shows the efficiency of Gaia. We could thus be very confident in the accuracy of the future releases.

In the future

This Data Release 1 is just the first release. Others will come, in which the astrometric data will be accompanied by photometric data. The Data Release 2 is planned for summer 2017, the releases 3 and 4 for 2018 and 2019 respectively, while the final one should come in 2022. This final release shall also include discoveries of Jupiter-like planets out of our Solar System.

At the end, Gaia shall have an astrometric accuracy of 25 micro-arcseconds at the magnitude 15, while Hipparcos reached 1 milli-arcsecond. Reaching such an accuracy is a challenge. For that, the timing must be extremely precise, and second-order relativistic effect of the deviation of the light by the Earth and other object must be considered.

Regarding the parallaxes, i.e. the distance: Hipparcos has given us the parallaxes of 60,000 objects with an accuracy of 20%, while the Gaia Data Release 1 gives us the same information, with the same accuracy, for 1 million objects. The Final Release shall give us 10 millions of parallaxes with an accuracy of 1%, 150 millions of them with an accuracy of 10%, 280 millions of them with an accuracy of 20%. Knowing the distances of stars with such a precision will permit major improvements in the understanding of star clusters and in the structure of the Milky Way.

 

Some links

 

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