1. On the Larger Picture in Cometary Science
Paul Weissman
Jet Propulsion Laboratory
67P/CG

2. Questions for Rosetta and OSIRIS to Answer
How does insolation interact with the comet nucleus to create the observed activity?
How do the inactive surface areas on the nucleus develop?
What are the parent molecules of observed species in the coma?
What is the internal structure of a cometary nucleus?
What is (are) the source(s) of cometary outbursts and disruption events?
What is the role of amorphous ices in comets?
Are cometary nuclei heterogeneous?

3. Processes and Properties On and In Cometary Nuclei
Rubble pile?
Pieces breaking off the nucleus -->> total disruption
Low bulk density ~ 0.4 g/cm3
Collimated jets
Layers?

4. 1. How does insolation interact with the comet nucleus to create the observed activity?
Deep Impact found ice on the surface of Tempel 1 in only a few small patches. Thermal measurements of Tempel 1 showed no evidence of surface cooling due to sublimation. Thermal measurements suggest very low thermal conductivity on the nucleus surface. How does energy get into the nucleus?

5. 1. How does insolation interact with the comet nucleus to create the observed activity (continued)?
Collimated jets on comet nuclei suggest nozzles through which gas and dust are escaping. Stardust-Next traced jets in the coma to apparently layered terrain on nucleus surface. However, these layered terrains were not observed on other nuclei, though the resolution of earlier flybys was lower.

6. 2. How do the inactive surface areas on the nucleus develop?
Vacuum
Silicate + carbonaceous dust layer or lag deposit: cm thick
Silicate + carbonaceous dust + H2O ice layer: m thick
Silicate + carbonaceous dust + H2O ice + other volatile ices: most pristine material (?) extends down to the center of the nucleus (?) 2
We have highly idealized thermal models of what the near surface layers may look like, but how accurate are they? Are inactive areas on comet nuclei due to lag deposits and/or fluidized beds, or are they a remnant of a cosmic-ray irradiated crust that comets obtained while resident in the Oort cloud or the Kuiper belt, and could be a meter thick or more? In that case, how do comets blow away the irradiated crusts?

7. 3. What are the parent molecules of observed species in the coma?
The volatile composition of cometary comae strongly resembles the volatile composition in dense cloud cores where stars are forming. In studying the composition of cometary nuclei and comae, we are seeking to understand the composition and the conditions in the primordial solar nebula out of which the Sun and planets and everything else formed. What is the composition of this “primordial soup”?

8. 4. What is the internal structure of a cometary nucleus?
Whipple: Icy conglomerate (1950)
Donn & Hughes: Fractal aggregate (1986)
Weissman: Primordial rubble pile (1986)
Belton et al.: Layered model (2006)
What does the structure tell us about accretionary processes in the solar nebula?

9. 5. What is (are) the source(s) of cometary outbursts and disruption events?
Comet 73P/Schwassman-Wachmann 3 (left) showed evidence of splitting as far back as It split into 4 comets in 1995 and then returned again in This HST image shows one of the major fragments, including dozens of cometesimals shedding from the nucleus (SW3 was the original Rosetta mission target). Comet Linear (1999 S4) (right) disintegrated at 0.79 AU, ~10 days after perihelion. About 4% of long-period comets and 1% of short-period comets randomly disrupt per perihelion passage. Possible mechanisms to explain disruption include rotational spin-up and exposure of amorphous ices.

10. 6. What is the role of amorphous ices in comets?
Prialnik & Bar-Nun (1987) showed that amorphous ices in comets would exothermically change to crystalline ices between 110 and 150 K. This changes a layer up to 100’s of meters thick to crystalline ice. The reaction dies out because some of the exothermic energy is forced to go into heating non-volatile components of the cometary material. The change repeats when the crystalline ice layer is sublimated away and amorphous ices are once again near the surface.
Dynamically new comets from the Oort cloud are often discovered in outburst at ~5-7 AU where the amorphous ice transition should occur for these nuclei as they are warmed by the Sun for the first time. Later outbursts may occur as the crystalline ice surface is sublimated away and the amorphous ice interior is again exposed to sunlight.

11. 7. Are cometary nuclei heterogeneous?
Stardust found a refractory component in Wild 2 that likely came from the asteroid belt. When the giant planets were clearing their zones, did icy cometesimals from different solar distances and hence different compositions accrete? Does 103P/Hartley 2 answer that question in the affirmative?

13. Questions for Rosetta and OSIRIS to Answer
How does insolation interact with the comet nucleus to create the observed activity?
How do the inactive surface areas on the nucleus develop?
What are the parent molecules of observed species in the coma?
What is the internal structure of a cometary nucleus?
What is (are) the source(s) of cometary outbursts and disruption events?
What is the role of amorphous ices in comets?
Are cometary nuclei heterogeneous?