MAIN BELT COMETSA new class of objects
Maria Teresa Capria, Simone Marchi, Maria
Cristina De Sanctis, Angioletta Coradini and
Eleonora Ammannito
International Workshop on Paolo Farinella, Università di Pisa, June 14-16, 2010
name a (AU) e i Tj q (AU) d (km)
133P/Elst-Pizarro 3.164 0.153 1.38 3.184 2.636 4.6
P/2005 U1 Read 3.165 0.253 1.267 3.153 2.365 <0.6
176P/LINEAR 3.218 0.144 1.40 3.166 2.581 4.8
P/2008 R1 (Garradd) 2.726 0.342 15.9 3.216 1.793 <1.4
Dynamical transition from outer Solar System is
nowadays considered almost impossible, and theirorbits are stable: they formed in in place, or are there since a lot of time . Orbital elements point to Themis family.Infrared spectroscopy shows ice widespread on the surface of 24 Themis (Campins et al. ,2010).
Main belt comets are objects orbiting in the Main Belt, showing cometary activity, with Tj>3
Jewitt et al., 2008; 2009Hsieh et al., 2009
A new class of objects
4 objects known implymany more currentlyactive, and a greaternumber currentlyinactiveIt has been suggestedthat the observedactivity could havebeen triggered byimpacts.
They could be the third known comet source.They are potential contributor to Earth oceans.
Jewitt, 2008
A new class of objects
Which is now the crater formation rate in the Main AsteroidBelt (2. – 3.27 AU)?The range of interest is 0.1 – 1 km, because MBCs are smalland we are not looking for fragmentations.
The method is based on the dynamical model by Bottke et al. (2002; 2005). First the flux of impactors is derived: we consider the average MainBelt impact rate and impact velocity.Impacts are converted into craters using an appropriate scaling lawfrom hydrocode simulations (Nolan et al .1996).Then we compute the cumulative distribution of expected craterdiameters.
Marchi et al, 2005; 2009
Crater formation rate in the Main Belt
The average time of formation of a crater in the size-rangefrom 0.1 to 1 km is:
133P/Elst-Pizarro
one crater per 0.9 Myr
P/2005 U1 Read
one crater per 54 Myr
176P/LINEAR one crater per 0.8 Myr
P/2008 R1 (Garradd)
one crater per 10 Myr
differences in the time scale are solely due todifferences in the size of the bodies
Crater formation rate in the Main Belt
We are assuming that a MBC is a comet-like body: a porousintimate mixture of water ice and refractary particles.
We are assuming that an impact has recentlyhappened, triggering a more or less stablecometary activity.
Thermal modeling
We run thermal models to simulate this kind of activity and study possibility and duration of an active phase.
The nucleus model is composed by aporous mixture of ices and arefractory component (sphericalgrains distributed in different sizeclasses).
The numerical code is solving heatand gas diffusion equations,computings how the heat diffusesin the porous cometary material,inducing the sublimation-recondensation of water andvolatiles.The temperature on the surface isobtained by a balance between thesolar input and the energy re-emitted in the infrared, conductedin the interior and used to sublimatesurface ices.
Due to the rising temperature, icesstart to sublimate, and the nucleusdifferentiates giving rise to alayered structure, in which theboundary between different layers isa sublimation front.When the ices begin to sublimatethe dust particles become free andcan undergo the drag exerted by theescaping gas, so that they movetoward the surface and can be blownoff or accumulate to form a crust.
Rome modelCapria et al., 2000; 2005; 2009De Sanctis et al., 2003;2005;2008.,
Thermal modeling
Input parameters P/2005 U1 Read
a 3.165
e 0.253
Diameter (m) 600
Dust/ice 3
Rotation period (h) 10
Average density (kg/m3)
586
A model comet: P/2005 U1 Read
Meech and Svoren, 2005
A model comet: P/2005 U1 ReadExposed ice
equator
30°
85°
Gas flux
A model comet: P/2005 U1 ReadExposed ice
Dust flux
Stratigraphy
85°
Erosion per orbitat equator is >2 m in 10 years.
equator
30°
An impact could trigger some activity even without exposingfresh ice, but simply bringing the heat wave closer to an ice-richlayer. Ice can sublimate under a porous mantle.
We are assuming that an impact has recently happened and thatan ice-rich layer has been brought closer to the surface. The surfaceis still covered by a devolatilized, porous mantle.
We ran models with different mantle thickness and properties, todetermine possibility and duration of an active phase.
A model comet: P/2005 U1 Read
Two kind of dust particles (and mantles): silicatic and silicatic/CHON
McDonnell et al.1991
Buried ice
A model comet: P/2005 U1 Read Buried ice
Silicatic mantle (0.1 m and 0.5 m)Silicatic/CHON mantle (0.1 m and 0.5 m)
Gas flux
A model comet: P/2005 U1 Read Buried ice
The characteristics of the mantle have a strong influence on the activityof the body.Gas (and dust) fluxes are severely quenched.Under a 2 m thick porous dust layer, ~ 2 x 106 years could be needed to
devolatilize 1 m of an ice-rich layer..In many cases, the mantle tends to grow, because the dust flux is veryreduced.
Silicatic mantle 0.1 m thick: the sublimation front recedes 0.15 m in 1000 yearsTemperature under the mantle: 162 K
Silicatic mantle 0.5 m thick: no changes in 1000 yearsTemperature under the mantle: 155 K
Organic mantle 0.1 m thick: the sublimation front recedes 0.05 m in 1000 yearsTemperature under the mantle: 162 K
Organic mantle 0.5 m thick: no changes in 1000 yearsTemperature under the mantle: 155 K
A yet unknown MBC in the outer Main Belt
Following Levison (2009), the violent dynamical evolution of the giant-planet orbits required by the Nice model leads to the insertion of primitive trans-Neptunian objects into the outer belt.
The captured bodies, composed of organic-rich materials, would have been more susceptible to collisional evolution than typical main-belt asteroids.
These objects should be similar to the resonant Trojans and Hildas: D- or P-type and probably organic-rich.
Input parameters P/ ?
a 4.0
e 0.3
Diameter (m) 1000
Dust/ice 1
Ice H2O, CO2, CO
Average density (kg/m3)
434
A yet unknown MBC in the outer Main Belt
H2O fluxCO2 fluxCO flux
A yet unknown MBC in the outer Main Belt
Mantle forms
CO2 sublimation front
CO sublimation front
Small MBCs become quickly inactive due to rapiddegassing of upper layers. Exposed ice lasts very few time.
Ice buried under a thin porous mantle sublimates slowly, while deep-buried ice can last for a very long time.
This is also an indication that the observed activity cannot besustained on "original" bodies, which soon after theirformation/injection into the Main Belt became inactive.
In the MB, an impact don’t need to necessarily expose ice toactivate a MBC: even a very small impact could activate a MBC, bringing the heat closer to ice-rich layers
Conclusions
A number of bodies could exists with a faint gasesous activitytriggered by small impacts. These small impacts could have beendevolatilized the upper layers of MBCs.
A buried snow line must exist, defined by the depth at whichice can survive for a very long time (T< 145 K). It depends on heliocentric distance and the physical properties of the mantle.
Conclusions
What about the ice on 24 Themis?
24 Themis is big! A possible replenishment nechanism: A thin porous insulating layer exists, shielding ice-rich layersMicrometeoroids impacts erodes the surface, bringing Sun
heat closer to the surface and triggering a faint ice sublimationNo dust flux, gas recondenses on the surface and slowly
sublimatesMicrometeoroids impacts erodes the surface…
