Collisional evolution of asteroids: evolving paradigms

Paolo Paolicchi,University of Pisa, Italy

Pisa, June 2010, Paolo Farinella Workshop

In the year 1982 Paolo Farinella (with me and Enzo Zappala), published a paper:

We were aware of the overwhelming difficulties of the task....however

We tried to define a”general scenario”

The AstI/II scenario

The 1982 paper was representative of the ideas, about the collisional evolution of asteroids, which were dominant in the 15+y period starting from Asteroids I (1979), including Asteroids II (first difficulties...)

The collisions were considered as the fundamental evolutionary process. It became of critical importance to understand the physics of hypervelocity impacts, which was partially known from a few experimental papers.

Later on....

As happens usually in science, the scenario is now partially obsolete. What have been the major changes in data and underlying physics?

New data

­ The completeness limit for Main Belt was estimated to be at 40km; the size distribution was showing the “bump” around 100km, but no significant structure at small sizes. Presently the size distribution can be ­ maybe­ reliably reconstructed for D> 1km. A new bump at 3­4 km can be identified.

New data­ The rotational properties of several very small

asteroids are known. The “V shape” of vs. D, showing a minimum around 100km, and increasing values for larger and smaller objects, was already known. Now we have discovered that the mean rotation period does not change much for small sizes, due also to the “2hr barrier”. Shorter periods, with a 1/D (?) asymptotic trend are present only for D < 100m.

New data

­ The data on the spin vectors remain rather sparse, but the new observations have been enough significant to detect anomalies in a few family asteroids (“Slivan asteroids”), the retrograde dominance in NEAs. These effects support the relevance of Yarkovsky and YORP effects, both theoretically suggested, but now also confirmed observationally.

Note

Paolo has been one of the most relevant scientists involved in the creation of the “old” scenario. Later, he has been one the

discoverers of the importance of Yarkovsky effect for asteroidal evolution. It has been a decisive breakthrough to pass to the “new”

scenario.

New data­ The spectroscopic data are much better and

much more numerous. They allowed a new taxonomical classification, a better connection taxonomy­chemical composition, a more significant comparison asteroids­meteorites. The “space weathering” of asteroids has been introduced, experimentally tested, its effects identified in the observations, especially for S­complex asteroids.

New data

­ New observational techniques have been implemented, such as radar, and, finally, several asteroids have been observed from the space, furnishing first­hand information on cratering, surface properties (regolith), densities, shapes, satellites.

New tools and ideasThe analysis of asteroid families has been thoroughly

refined, both improving the classification and the proper elements (another relevant contribution by Paolo).

The physics of collisional fragmentation is more profoundly understood, both through experiments (now extended to various target sizes, impact properties, structures of the target..) and through hydrodynamical simulations, capable to reproduce most of the experimental results, and usable also to analyze events involving planetary bodies.

New tools and ideasThe naïve “energy scaling” has been progressively

corrected, introducing various theoretical concepts (from the self­gravitational compression to the strain­rate) and comparing to the outcomes of hydrocodes. A QD

* function of the size is defined, showing a minimum around 200m: smaller bodies are stronger, larger ones are more difficult to destroy due to self gravity. In terms of the old jargon, S is low, FKE is very small. Asteroids are easily shattered by a collision, but often reaccumulate.

New tools and ideas

The size distribution of families is dominated ­at all the observable sizes­ by reaccumulation; maybe the presence of preexistent fractures is relevant. All the observable fragments should be rubble piles or gravitational aggregates. The same holds for almost all asteroids, except the very small ones and ­maybe­ the largest ones.

HoweverAs well known, a linear velocity field (such as Hubble law, or a zero­app. ejecta field) has no intrinsic scale: all reaccumulates, a small subcondensation does, and viceversa. You can introduce a multiple reaccumulation breaking the simmetry (as in SEM) but few bodies reaccumulate more than two­three fragments. The simulations entail clusters of fragments reaccumulating together. Why? Pre­existing fractures? Converging motion in the initial velocity field? What else? And, more important, is it real? (I would like to discuss again this points with Paolo, as we did in the past....)

New tools and ideasAccording to the new ideas the transition strength­

gravity takes place for small objects, and the possible transition in the size distribution is marked by a secondary “bump” around 3km.

By the way, the old concept of a “pile of rubble” similar to a fluid is also obsolete. The “new” rubble piles (GA) take into account the interactions among the solid components and may be severely different from figures of equilibrium.

New tools and ideasThe bump in the size distribution around 120 km

corresponds to some changes in physical properties (minimum of the mean rotation rate Ω; minimum of the lightcurve amplitude,a maximum of the relative excess of prograde asteroids, represented in terms of the cumulative excess for bodies larger than a given size in units of the expected statistical deviation).

New tools and ideasIs the ~100km bump connected not to the transition

strength­gravity (old paradigm), but to the transition between a collisionally evolved population and a quasi primordial one?. The larger bodies may retain original properties, or consequences of the primordial evolution. The bodies smaller than 120km might be the only real “outcomes of collisional evolution”, due to the former breakup of large bodies or to mutual collisional events.

The mass of MB, after the first dramatic era, has been not much larger than the present one.

New tools and ideasThe “low mass belt model” entails, differently from

the previous ones, a quasi constant creation of fragments and, through the Yarkovsky driven delivery into resonances, a regular creation of NEAs. The result is in agreement with the analysis of lunar craters. Moreover, the model is consistent with the observed properties of Vesta (its partially intact basaltic crust, with one very big crater).

New tools and ideas: familiesThe families stand as the most relevant outcomes of big

(“catastrophic”)collisional events. Their dynamical structure is not only due to the original properties, but also to the post­impact evolution mainly due to Yarkovsky­Yorp effects . The combined analysis allows also to estimate the age. Note the recent rediscussion of Yorp effects suggesting a possible random­walk evolution of the spin rate.

New tools and ideas: the “collisional weathering”

The solar wind (ion implantation) is the main cause of asteroids space weathering. The timescale of the process is around 106y. However we find a marked slope increase with age for old asteroids. This different (and much larger) timescale can be connected to the partial surface refreshment due to cratering collisions and to the formation of regolith. The slope increase with time may be a witness of the collisional evolution and may help to constrain some parameters of the process (work in progress).

New tools and ideas: general considerations

The collisional evolution is intimately entangled with other physical processes, such as Yarkovsky­YORP and the space weathering. Moreover, direct consequences of the primordial formation and evolution are also presently observable in the high­size tail of the asteroidal belt. The analysis is now more difficult to do!

Work TBD and critical points­ I

Topics which deserve a further analysis:­ Under what conditions the reaccumulation may create a

“democratic” size distribution, avoiding runaway into a few very large reaccumulated bodies or formation of much debris?

­ Does really the 100­120km bump mark the transition between evolved and primordial asteroids?

­ The evolution of the GA concept (texture, shape, rotational properties...) has to be finalized.

Work TBD and critical points­ II­The RP problem: are all bodies larger than 100m

GA? ­ May different data and theoretical models lead to

not consistent age estimates?

Ghosts from the old paradigm? Probably points to be clarified or first indications for the next paradigm.

CONCLUSIONS

Sometimes, the evolution of scientific knowledge is slackened by personal attitudes: we stick to our ideas (especially whenever our

contribution to their assessment has been significant), and are skeptic about new suggestions. On the other hand, we sometimes overestimate the predictive power of “our” new ideas. Paolo was

fundamental to define the first paradigm, to introduce new physical ideas essential to support the second. We miss him, his

“candid” attitude towards science, to enter the scenario to come...