Shock Processing of Ice Mixtures in Protoplanetary Disks

Shock Processing of Ice Mixtures in Protoplanetary Disks George Hassel & Wayne Roberge ghassel@mps. ohio-state. edu RPI Physics Department New York Center for Studies on the Origins of Life http: //www. origins. rpi. edu Astro. Surf 2007 Meeting, June 15, 2007 1

Shock Processing of Ices in Protoplanetary Disks n Purpose of Study: Detailed analysis of ice processing by shocks u Implications for the early Earth u n Research Team: Wayne Roberge & Glenn Ciolek u Doug Whittet & Sachin Shenoy u Origins of Life NSCORT Group u 2

Outline of Project Shocks widespread in protoplanetary disks (Desch & Connolly 2002, Boss 2002, Cuzzi & Alexander 2005) v Icy mantles processed in disk shocks v Formation of comets from resulting ice v Delivery to early Earth ? ocean water and biogenic molecules 3

Comet S 4 1999 (LINEAR) n Depleted abundances with respect to comparison comets (H 2 O 100%): u u u CO CH 4 C 2 H 6 C 2 H 2 CH 3 OH HCN LINEAR / Others 0. 45 – < 1. 9 0. 09 – 0. 15 0. 08 – 0. 3 < 0. 14 < 0. 17 0. 10 / / / 1. 8 - 12 0. 7 0. 6 0. 3 1. 7 0. 2 – 0. 4 Mumma et al. 2001 4

Comet S 4 1999 (LINEAR) 5

Astro. Surf Relevance “Chemistry” models dep. heavily on surface processes: n Binding energy n Desorption kinetics n Crystallization rates 6

A Shock is: • A disturbance moving faster than the signal speed in a medium 7

A Shock is: • Turns organized motion into random motion behind the shock front 8

A Shock is: • Frictional Drag Heating between gas and dust 9

Shock Processing Overview n Disk Model – Hydro Inputs n Hydro / RT Simulation u u u n Temperature Density Velocity Profiles Ice Mantle Processing: u Removal u Crystallization u Guest Retention / Exclusion 10

Aikawa Disk & Mantle Removal Optically Thin Shocks, 1 bin size Neufeld & Hollenbach (1994): vrem < 5 km/s for n. H 0 > 1010 cm-3 11

Temperature Profile from Hydrodynamics Sharp increase in n. H & T n. H 0 = 1012 cm-3, vs = 6 km s-1 12

Mantle Processing n Ice Removal / Re-accretion n Pure Water Model: u u n Crystallization of Water Dominant component Water + 1 Guest: u u u Exclusion? Retention? Clathrate Hydrate Formation? 13

Water Crystallization n Water is dominant component of ice n Phase Change: u Amorphous Ice – retains guests u Cubic Crystalline – excludes guests u Clathrate Hydrate – retains select guests 14

Water Crystallization: Crystallizaton and Growth n Removal: Mobility of Reaccreting Surface Molecules u Kouchi et al. 1994 u n Otherwise: Classical Nucleation Theory u Jenniskens & Blake 1996 u 15

Induction Time & Crystallization Timescale 16

Results! 17

H 2 O: CO Mix Crystallization 18

Inclusion of Guest Molecules n CO: u Trapped in amorphous H 2 O (Collings et al. 2003) u u n Alters crystallization kinetics Estimate fractional retention CH 3 OH: u u u Participates in H-bonding w/ H 2 O Formation of Clathrate Hydrate (Blake et al. 1991) Promotes formation of hexagonal ice 19

H 2 O: CH 3 OH Mix – Clathrate Fraction 20

CO Retention / Exclus 21

CH 3 OH Distribution 22

Summary of Results Crystallization efficient if n vs > vremoval n u small range of speeds for vs < vremoval where crystallization possible from nucleation and growth u vremoval decreases with increasing n. H 0 – “easier” to remove & crystallize mantle at higher densities u shocks due to gravitational instabilities: vs may be < VKepler due to oblique incidence (Pickett et al 2003) CO: u u n removed with H 2 O retained (partially) for no/partial mantle removal CH 3 OH: u formation of clathrate hydrate predicted where crystallization is predicted in pure ice – may affect 23 detectability

Future Projects n n Extension of Shock Processing with Multiple Grain Size / Composition Shock-Enhanced Chemistry: u D/H exchange Shock effects on refractory material: u crystallization of silicates Observational Properties 24

Model Comparison 25

Results – IR Optically Thick Shocks, 1 mm Grains, n. H 0= 1010 cm-3 26

Benchmark for Nucleation Theory TPD 0. 6 K/s Warmup rate Based on Speedy et al. 1996 27

Benchmark for Nucleation Theory T = 146 K Annealing Based on Safarik et al. 2003 28

Table 1 – Delsemme 1999 29