Comets-IV

The Comet Decade

Deep Impact http://deepimpact.jpl.nasa.gov/index.cfm

One aspect of comet research that begged for investigation was: what is inside the nucleus? Every time a comet enters the inner solar system, the sublimation of water ice preferentially removes the smallest dust particles. With time the outer layers will lose the ice to sublime (becoming “devolatilized”) unless they are first smothered by the debris or remaining big particles. Thus, the surfaces may not represent the initial state of the material that went into forming the comet.

How to sample the initial material? Excavation! In Nov. 1999, work began on the Deep Impact spacecraft, destined to sample the nucleus below its surface by ramming an impactor into the nucleus at high speed, and observe the ejected material. In this case, it was a 370 kg “bullet” hitting at 10.3 km/s!

Movies & Animations:

http://www.physics.uc.edu/~sitko/AdvancedAstro/29-Comets-IV/DeepImpact/117657main_Maas_DI_Long_320.240.mov

http://www.physics.uc.edu/~sitko/AdvancedAstro/29-Comets-IV/DeepImpact/121572main_its_approach_inv.mov

http://www.physics.uc.edu/~sitko/AdvancedAstro/29-Comets-IV/DeepImpact/121520main_HRI-Movie.mov

http://www.physics.uc.edu/~sitko/AdvancedAstro/29-Comets-IV/DeepImpact/122129main_Stream.mov

SUMMARY

Impact:

· 370 kg impactor @ 10.3 km/s - 19 GJ

· Water ejected primarily as icy particles

· gravity dominated

· < 65 Pa shear strength (weak, powdery)

· ejecta v≤5 km/s, T_initial~ 1000 K

· no new ‘permanent’ vent created

Excavated:

· Volatiles:

· H₂O, C₂H₆, HCN, CO, CH₃OH, H₂CO, C₂H₂, CH₄

· Refractories:

· amorphous olivine & pyroxenes

· amorphos carbon

· small (sub-mm) crystalline silicates

· Large dust/ice ratio

· comets are icy dirtballs, not dirty snowballs

Structure & Surface:

· nearly uniform albedo (reflectivity) ~4%

· Surface non-uniform - agglomerate?

· layered fluffy material - r_bulk<0.6 g/cm³

· cratering history unlike Wild 2 (Stardust)

Stardust http://stardust.jpl.nasa.gov/home/index.html

Iron sulfide, enstatite (pyroxene), and other material	Material dominated by anhydrous (water-free) chondritic material, but 2 subgrains seem to be typical of more hydrated material.
Grain similar to Calcium Aluminum Inclusion (CAI), the highest-temperature condensate known in the protosolar nebula	Crystalline Forsterite grain!

Comet 73P/Schwassmann-Wachmann 3

Letting nature provide us with a “deeper impact”! Would the various fragments have different properties?

See animations:

http://www.physics.uc.edu/~sitko/AdvancedAstro/29-Comets-IV/73PR.MOV

http://www.physics.uc.edu/~sitko/AdvancedAstro/29-Comets-IV/2006-18-a-high_quicktime.mov

Observations of Component B using the Gemini North telescope.

Observations with the Spitzer Space Telescope

Component B at 24 mm	Component C at 24 mm

Removing the underlying blackbody emission from the spectrum of Component C	Comparison of the normalized spectra for B and C

The spectral characteristics were similar, but not identical.

The strength of the silicate band is an indicator of the size of the grains. Only small (less than a few mm diameter) can produce a strong silicate emission feature. Structure within the band generally indicates more nicely crystalline material. The spectrum of SW3C indicates larger grains than Hale-Bopp and the Deep Impact ejected material from Tempel 1. The degree of crystallinity also seems less than either of those two objects. This may indicate that they were formed in different regions of the solar nebula.

Comet C/2006 P1 McNaught

Most observers in the northern hemisphere did not get a great “deep” view of the comet while it was at its brightest. However, it was possible to actually see it in the daytime! This was the brightest comet since Ikeya-Seki in 1965.

It will be possible to track the trajectories of the grains using synchrones & syndynes. The sporadic ejection from a source that is alternately in sunlight and in shade probably accounts for the structure.

By enhancing the color within the boxed area, it is possible to see strange variations in color. These may provide additional information about the sizes of the grains. This comet was observed by the Spitzer Space Telescope on May 4, 2007.