Collisional de-weathering ofasteroids?

Paolo Paolicchi (Univ.Pisa), Simone Marchi,Monica Lazzarin, Sara Magrin (Univ.Padua)

Introduction

In a series of recent papers our group hasanalyzed the effects of the so-called spaceweathering on the spectral properties of MainBelt and Near Earth Asteroids. The spectralslope appears to be correlated to the exposure ofthe asteroids to the weathering effects of theSun.

Introduction - 2

The spectral slope is obtained from the observedspectra (and also, with different methods, fromphotometric data).

The exposure is defined in terms of the meandistance from the Sun and of the mean age.

The age of a Main Belt asteroid may beconstrained by its collisional lifetime.

Introduction - 3

Also the Yarkovsky time (i.e. the time requiredfor a significant orbital evolution, maybecausing injection into major resonances) mayplay a role, but only for the smallest bodies (atleast for Main Belt).

From Marchi et al. 2006 AJ 131,1138

Introduction - 4

Given the collisional lifetime, the mean age of aMB asteroid can be estimated with a formulainspired to the classical “optical depth”computations (see AJ paper).

The age can be used to compute the exposureand thus to obtain a slope-exposure plot fordifferent types of asteroids.

From Lazzarin et al. 2006 ApJ 647, 179L

Other effects

A more detailed analysis introduces someadditional effects acting on the spectralproperties:

_ (especially for NEAs) close encounters withother bodies (mainly planets) may alter thesurface, causing a sort of de-weathering.

From Paolicchi et al. 2007, A&A 464,1139

Other effects - 2

_ (Especially for NEAs) Passing very close to theSun introduces heating effects. They may beinvoked to explain other spectral effects, suchas the quasi complete absence of hydrationfeatures on C-type NEAs (Marchi et al., S236Proc. and in preparation).

From Paolicchi et al. 2007, A&A 464,1139

Other effects - 3

_ Another, general effect, is connected tocollisions. We have two reasons for it: inprinciple, between two disrupting events i.e.during the lifetime of an asteroid, we havemany minor collisions, causing cratering, partialshattering and all that. These collisions shouldaffect the surface directly and indirectly.

Other effects - 4

There also is a second argument: while ingeneral our models represent a convincing andcoherent scenario, also in connection with themeteorites and with laboratory experiments,there is a severe discrepancy between thetimescales for which the observational slopeincreases to a partial saturation (also by forlarge bodies) and those suggested by laboratory

Other effects - 4

experiments (of the order of not many My).

The most convincing explanation can beconnected with the de-weathering effect ofminor (also cratering) collisions.

A thorough theoretical analysis is urged. At avery preliminary and qualitative stage a figureexplains the effect.

Collisional de-weathering - 1

Collisional de-weathering-2

_ The interaction between partially de-weatheringcollisions with the general weathering effectmay result into a slower increase of the slope.However, this is not the only possibleexplanation.

_ Maybe the mean age or lifetime to be used inthe exposure plots is not the time between twocatastrophic processes.

Collisional de-weathering - 3

_ The “effective age” might be connected to therejuvenation of a thin surface layer throughinfall of dust and regolith created by lessenergetic collisions; a -by far- more frequentprocess, maybe with a timescale comparable tothe laboratory timescales. Maybe this timescales with the size similarly as the collisionallifetime.

Conclusions

_ The spectral alterations are due not only to thespace weathering but also to other effects (closeencounters, heating, collisions) sometimescausing rejuvenation of the surface.

_ The effect of collisions might be the mostgeneral, but a lot of additional theoretical workand modelling has to be done.