Talk of the "International Workshop on Paolo Farinella (1953-2000): the Scientists, the man", Pisa, 14-16 June 2010

1. MAIN BELT COMETS
A 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

2. A new class of objects
Main belt comets are objects orbiting in the Main Belt,
showing cometary activity, with Tj>3
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
Jewitt et al., 2008; 2009
Dynamical transition from outer Solar System is Hsieh et al., 2009
nowadays considered almost impossible, and their
orbits 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).

3. A new class of objects
4 objects known imply
many more currently
active, and a greater
number currently
inactive
It has been suggested
that the observed
activity could have
been triggered by
impacts.
Jewitt, 2008
They could be the third known comet source.
They are potential contributor to Earth oceans.

4. Crater formation rate in the Main Belt
Which is now the crater formation rate in the Main Asteroid
Belt (2. – 3.27 AU)?
The range of interest is 0.1 – 1 km, because MBCs are small
and 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 Main
Belt impact rate and impact velocity.
Impacts are converted into craters using an appropriate scaling law
from hydrocode simulations (Nolan et al .1996).
Then we compute the cumulative distribution of expected crater
diameters.
Marchi et al, 2005; 2009

5. Crater formation rate in the Main Belt
The average time of formation of a crater in the size-range
from 0.1 to 1 km is:
133P/Elst- one crater per 0.9 Myr
Pizarro
P/2005 U1 one crater per 54 Myr
differences in the
Read
time scale are
176P/LINEAR one crater per 0.8 Myr solely due to
P/2008 R1 one crater per 10 Myr differences in the
(Garradd) size of the bodies

6. Thermal modeling
We are assuming that a MBC is a comet-like body: a porous
intimate mixture of water ice and refractary particles.
We are assuming that an impact has recently
happened, triggering a more or less stable
cometary activity.
We run thermal models to simulate this kind of activity
and study possibility and duration of an active phase.

7. Thermal modeling
The nucleus model is composed by a Rome model
porous mixture of ices and a Capria et al., 2000; 2005; 2009
refractory component (spherical De Sanctis et al., 2003;2005;2008.,
grains distributed in different size
classes).
The numerical code is solving heat
and gas diffusion equations, Due to the rising temperature, ices
computings how the heat diffuses start to sublimate, and the nucleus
in the porous cometary material, differentiates giving rise to a
inducing the sublimation- layered structure, in which the
recondensation of water and boundary between different layers is
volatiles. a sublimation front.
The temperature on the surface is When the ices begin to sublimate
obtained by a balance between the the dust particles become free and
solar input and the energy re- can undergo the drag exerted by the
emitted in the infrared, conducted escaping gas, so that they move
in the interior and used to sublimate toward the surface and can be blown
surface ices. off or accumulate to form a crust.

8. A model comet: P/2005 U1 Read
Input parameters P/2005 U1
Read
a 3.165
e 0.253
Diameter (m) 600
Dust/ice 3
Meech and Svoren, 2005
Rotation period (h) 10
Average density 586
(kg/m3)

9. A model comet: P/2005 U1 Read
Exposed ice
Gas flux
equator
85°
30°

10. A model comet: P/2005 U1 Read Exposed ice
Dust flux
Erosion per orbit
at equator is >2
m in 10 years.
Stratigraphy
30°
equator
85°

11. A model comet: P/2005 U1 Read Buried ice
An impact could trigger some activity even without exposing
fresh ice, but simply bringing the heat wave closer to an ice-rich
layer. Ice can sublimate under a porous mantle.
We are assuming that an impact has recently happened and that
an ice-rich layer has been brought closer to the surface. The surface
is still covered by a devolatilized, porous mantle.
We ran models with different mantle thickness and properties, to
determine possibility and duration of an active phase.
Two kind of dust particles (and mantles): silicatic and silicatic/CHON
McDonnell et al.1991

12. A model comet: P/2005 U1 Read Buried ice
Gas flux
Silicatic mantle (0.1 m and 0.5 m)
Silicatic/CHON mantle (0.1 m and 0.5 m)

13. A model comet: P/2005 U1 Read Buried ice
Silicatic mantle 0.1 m thick: the sublimation front recedes 0.15 m in 1000 years
Temperature under the mantle: 162 K
Silicatic mantle 0.5 m thick: no changes in 1000 years
Temperature under the mantle: 155 K
Organic mantle 0.1 m thick: the sublimation front recedes 0.05 m in 1000 years
Temperature under the mantle: 162 K
Organic mantle 0.5 m thick: no changes in 1000 years
Temperature under the mantle: 155 K
The characteristics of the mantle have a strong influence on the activity
of 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 very
reduced.

14. 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 434
(kg/m3)

15. A yet unknown MBC in the outer Main Belt
H2O flux
CO2 flux
CO flux

16. A yet unknown MBC in the outer Main Belt
Mantle forms
CO2 sublimation front
CO sublimation front

17. Conclusions
Small MBCs become quickly inactive due to rapid
degassing 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 be
sustained on "original" bodies, which soon after their
formation/injection into the Main Belt became inactive.
In the MB, an impact don’t need to necessarily expose ice to
activate a MBC: even a very small impact could activate a
MBC, bringing the heat closer to ice-rich layers

18. Conclusions
A number of bodies could exists with a faint gasesous activity
triggered by small impacts. These small impacts could have been
devolatilized the upper layers of MBCs.
A buried snow line must exist, defined by the depth at which
ice can survive for a very long time (T< 145 K). It depends on
heliocentric distance and the physical properties of the mantle.
What about the ice on 24 Themis?
24 Themis is big! A possible replenishment nechanism:
A thin porous insulating layer exists, shielding ice-rich layers
Micrometeoroids impacts erodes the surface, bringing Sun
heat closer to the surface and triggering a faint ice sublimation
No dust flux, gas recondenses on the surface and slowly
sublimates
Micrometeoroids impacts erodes the surface…