Neptune and Uranus: ice or rock giants?

Area of Science:

• Planetary Science
• Atmospheric Chemistry
• Interior Modeling

Background:

• Uranus and Neptune's physical properties are consistent with both ice- and rock-dominated interior models.
• The bulk rock:ice ratio is a key parameter differentiating these models.
• Atmospheric observations offer crucial constraints for refining these models.

Purpose of the Study:

• To investigate how Neptune's atmospheric temperature and composition data can constrain interior models.
• To evaluate the implications of carbon monoxide (CO) and deuterium-to-hydrogen (D/H) ratios for Neptune's bulk composition.
• To determine whether Neptune is best described as an ice giant or a rock giant.

Main Methods:

• Analysis of existing observational data for Neptune's atmospheric composition (H2, He, D/H, CO, CH4, H2O, CS).
• Comparison of observed CO profiles with predictions from different interior models.
• Reconciliation of CO and D/H measurements under various mixing scenarios.

Main Results:

• Observed CO profiles are controversial, with deep values ranging from 0 to 0.5 ppm.
• High O/H enrichment implied by most CO profiles (>250x solar) conflicts with D/H observations for an equilibrated Neptune.
• Reconciling CO and D/H requires either incomplete interior mixing (ice giant) or an external CO source (rock giant).

Conclusions:

• Both ice- and rock-dominated models remain viable for Neptune and Uranus.
• A rock-dominated (rock giant) model provides a more consistent explanation for available atmospheric observations, particularly CO and D/H ratios.
• The rock giant model aligns better with likely outer solar system ice source compositions.