[Paper Review] Modeling the Jovian subnebula: II - Composition of regular satellites ices

O. Mousis, Y. Alibert|ArXiv.org|Oct 28, 2005

Astro and Planetary Science21 citations

TL;DR

This study models the composition of ices in Jupiter's regular icy satellites using a turbulent subnebula evolution model, showing that volatiles like CO2, CH4, NH3, and N2 were trapped in clathrates or hydrates in Jupiter's feeding zone. The results are consistent with surface CO2 on Ganymede and Callisto and subsurface NH3 in their oceans, supporting two formation scenarios: accretion from pre-formed planetesimals or in-situ subnebula formation with minimal chemical processing.

ABSTRACT

We use the evolutionary turbulent model of Jupiter's subnebula described by Alibert et al. (2005a) to constrain the composition of ices incorporated in its regular icy satellites. We consider CO2, CO, CH4, N2, NH3, H2S, Ar, Kr, and Xe as the major volatile species existing in the gas-phase of the solar nebula. All these volatile species, except CO2 which crystallized as a pure condensate, are assumed to be trapped by H2O to form hydrates or clathrate hydrates in the solar nebula. Once condensed, these ices were incorporated into the growing planetesimals produced in the feeding zone of proto-Jupiter. Some of these solids then flowed from the solar nebula to the subnebula, and may have been accreted by the forming Jovian regular satellites. We show that ices embedded in solids entering at early epochs into the Jovian subdisk were all vaporized. This leads us to consider two different scenarios of regular icy satellites formation in order to estimate the composition of the ices they contain. In the first scenario, icy satellites were accreted from planetesimals that have been produced in Jupiter's feeding zone without further vaporization, whereas, in the second scenario, icy satellites were accreted from planetesimals produced in the Jovian subnebula. In this latter case, we study the evolution of carbon and nitrogen gas-phase chemistries in the Jovian subnebula and we show that the conversions of N2 to NH3, of CO to CO2, and of CO to CH4 were all inhibited in the major part of the subdisk. Finally, we assess the mass abundances of the major volatile species with respect to H2O in the interiors of the Jovian regular icy satellites. Our results are then compatible with the detection of CO2 on the surfaces of Callisto and Ganymede and with the presence of NH3 envisaged in subsurface oceans within Ganymede and Callisto.

Motivation & Objective

• To determine the composition of ices in Jupiter’s regular icy satellites using a time-dependent turbulent subnebula model.
• To assess whether volatiles like CO2, CH4, NH3, N2, and noble gases were preserved in clathrates or hydrates during satellite formation.
• To compare two formation scenarios: accretion from planetesimals formed in Jupiter’s feeding zone versus in-situ formation in the Jovian subnebula.
• To evaluate the consistency of predicted volatile abundances with observed surface and subsurface compositions of Ganymede and Callisto.
• To constrain the D:H ratio in water ice based on isotopic exchange in the subnebula, providing a testable prediction for future in-situ measurements.

Proposed method

• Utilizes a 2D time-dependent α-turbulent model of the Jovian subnebula from Alibert et al. (2005a), with initial accretion rates consistent with late-stage Jupiter formation.
• Applies clathrate hydrate and hydrate trapping theory (Lunine & Stevenson 1985) to determine volatile incorporation into solids in Jupiter’s feeding zone under thermodynamically consistent conditions.
• Models the thermal and chemical evolution of the subnebula, tracking the condensation and vaporization of ices during early, hot phases and subsequent cooling.
• Analyzes gas-phase chemistry in the subnebula, particularly the inefficiency of reactions converting CO to CH4, CO to CO2, and N2 to NH3, preserving initial feeding zone ratios.
• Compares two formation scenarios: (1) accretion from pre-vaporized, preserved planetesimals from the feeding zone; (2) accretion from subnebula-produced planetesimals after cooling and re-condensation.
• Estimates mass abundances of volatiles relative to H2O in satellite interiors, using constraints from Jupiter’s atmospheric enrichments (A05b) and observed surface compositions.

Experimental results

Research questions

• RQ1 What is the composition of ices in Jupiter’s regular icy satellites, as determined by volatile incorporation in clathrates and hydrates?
• RQ2 How do the thermodynamical and chemical conditions in the Jovian subnebula affect the preservation of volatile species like CO2, CH4, NH3, and N2?
• RQ3 To what extent are the observed surface detections of CO2 on Ganymede and Callisto consistent with the predicted internal composition of these satellites?
• RQ4 How do the two proposed formation scenarios—accretion from feeding zone planetesimals versus in-situ subnebula formation—affect the predicted volatile abundances and D:H ratios in satellite ices?
• RQ5 Can isotopic exchange between HDO and H2 in the subnebula produce a measurable difference in the D:H ratio in water ice, providing a testable signature for distinguishing formation scenarios?

Key findings

• Volatiles such as CO2, CH4, NH3, N2, Ar, Kr, Xe, and H2S were trapped in clathrates or hydrates in icy solids formed in Jupiter’s feeding zone under thermodynamically consistent conditions.
• Ices entering the Jovian subnebula before ~0.5 Myr were fully vaporized due to high temperatures, necessitating two distinct formation scenarios for satellite formation.
• In both scenarios, the CO2:CO:CH4 and N2:NH3 gas-phase ratios were preserved due to inefficient chemical reactions in the subnebula, supporting initial feeding zone compositions.
• The model predicts that satellite interiors contain CO2 and NH3 in amounts consistent with surface detections of CO2 on Ganymede and Callisto and the presence of subsurface oceans inferred from internal magnetic fields.
• The D:H ratio in water ice is predicted to be ~4–5 times the solar value if satellites formed from feeding zone planetesimals, but lower if formed in the subnebula due to isotopic exchange with H2, offering a testable distinction between formation pathways.
• The results are consistent with the observed volatile enrichments in Jupiter’s atmosphere and support the idea that Ganymede and Callisto’s subsurface oceans may be stabilized by NH3.