Cometary delivery of Biogenic Elements to Europa
E. Pierazzo, C.F. Chyba

Icarus 157 (2002) 120-127

31st LPSC (2000) Abstract #1656
Jupiter: Planet, Satellites & Magnetosphere Conference (25-30 June 2001) Boulder, CO

For more information contact betty@psi.edu


INTRODUCTION

Strong evidence that a liquid water ocean exists beneath the surface of Jupiter's moon Europa fuels speculations about the possibility of life on that world and resulting plans for future spacecraft exploration. However, the inventory of elements needed for life (the "biogenic" elements C, H, N, O, P, S, etc.) in Europa's ocean is nearly entirely unknown. A bulk carbonaceous chondrite composition is sometimes assumed, but Europa's formation conditions within the circum-jovian nebula are poorly known, so the accuracy of these models is correspondingly uncertain. We calculate lower limits on the exogenous contribution to Europa's inventory of biogenic elements over geologic time by carrying out high resolution hydrocode simulations of cometary impacts on Europa. Jupiter-family comets constitute more than 90% of the impactor population striking Europa with impact velocities centered around 26 km/s. By recording the velocity evolution of the impactor material we determine the fraction of cometary material accreted by Europa. At typical impact velocities, most impactor material escapes Europa's gravity, but a significant fraction (at the 10% level) of lower velocity comets striking within 30 degrees of the vertical is retained. Integrated over solar system history, this mechanism provides Europa with a substantial inventory of biogenic elements (for example, few times more carbon than is present in Earth's upper-ocean biomass). Regardless of its formation conditions, Europa should have the raw materials needed to support a biosphere.


Impact Simulations:

Target: Ice layer, 5 km thick, ocean below (although projectile does not go through)
      Surface temperature: 110 K; Ocean temperature: about 270 K (used to calculate thermal gradient in target)

Projectile: Comet (Jupiter-family about 99% of impacts on Galilean satellites) with density ranging between 0.6 and 1.1 g/cc, about 0.5 and 0.25 km in radius, impacting at 26.5 km/s (median impacts velocity for Europa; Zahnle et al., Icarus 136 (1998) 202-222).
Simulation cover about 2.6 sec (1.3 for 0.25 km radius). During this time about 70% of the projectile is accelerated to a velocity larger than escape for Europa (about 2 km/s)

Fractions of amino acids (in % of initial amount in projectile) surviving the impact of a fully dense comet (1.1 gr/cc) and remaining on Europa (in parenthesis are surviving amino acids that escape Europa's gravitational field), for various impact velocities.

D=1 km - v=21.5 km/s D=1 km - v=26.5 km/s D=1 km - v=30.5 km/s D=0.5 km - v=26.5 km/s
Volume lost (%) 44.87 69.43 81.06 68.72
Aspartic Acid 0.611   (0.841) 0.314   (1.120) 0.129   (1.070) 3.170   (8.140)
Glutamic Acid 0.282   (0.781) 0.021   (0.265) 1.71E-3   (0.107) 0.847   (3.563)
Glycine 0.020   (0.122) 5.E-4   (1.9E-3) 1.32E-5   (5.97E-3) 0.132   (0.874)
Asparagine 11.947   (9.547) 5.260   (14.02) 2.625   (14.403) 12.93   (20.886)
Phenylalanine 3.147   (9.918) 0.682   (3.150) 0.165   (2.060) 4.729   (13.266)

Mass of cometary material and relative amounts of biogenic elements accreted on Europa over 4.4 Gyr

Comet Density (g cm-3)

  

Total Accreted
 

Mass (g)

Carbon (g)

Nitrogen (g)

Sulfur (g)

Phosphorous (g)

0.6

5 ´ 1015

9 ´ 1014

2 ´ 1014

2 ´ 1014

2 ´ 1013

0.8

1 ´ 1016

2 ´ 1015

6 ´ 1014

4 ´ 1014

6 ´ 1013

1.1

6 ´ 1016

1 ´ 1016

3 ´ 1015

2 ´ 1015

3 ´ 1014

COLOR FIGURES (To download GIF files click on the figures)

Temperature map of (a) peak shock temperature and (b) postshock (after 2.5 seconds) temperature inside the projectile for a Jupiter-family comet 1 km in diameter, impacting at 26.5 km/s on Europa.


Survival map for aspartic acid, subsequent to the impact characterized by the peak and post shock temperatures above.
Left: map of the surviving fraction of the projectile after the impact (due to the axial symmetry of the simulations, the right half of the projectile is a mirror image of the left half).
Right: surviving fraction deposited to teh surface of Europa (the blackened region represents the part of the projectile that reaches escape velocity). Asterisks indicate the initial position of Lagrangian tracers.

Fraction of projectile material that reaches Europa's escape velocity in the impact simulations as a funtion of projectile bulk density (x-axis) and impact velocity (various lines/symbols) for a comet 1 km in diameter.