Jupiter Formed with More Tar than Ice

Elemental abundances in Jupiter determined from Galileo probe measurements are compared to recently revised solar system abundances. When normalized to the abundance of sulfur, the most abundant refractory rock-forming element reliably determined in Jupiter's atmosphere by the Galileo probe, abundances of argon, krypton, and xenon are 1 times solar, the observed oxygen is depleted by a factor of 4, and carbon is enriched 1.7 times. The fairly uncertain nitrogen abundance ranges from 1 to 3 times solar. The oxygen abundance in Jupiter derived from the observed atmospheric water abundance is only a lower limit to the total planetary oxygen because oxygen is also bound to rock-forming elements such as magnesium or silicon sequestered deep in the planet. The sulfur abundance constrains the amount of rock-forming elements on Jupiter. Considering the amount of oxygen bound to silicate rock, the total oxygen abundance on Jupiter of 0.47 times solar system indicates an overall oxygen depletion by about a factor of 2. The hydrogen and helium abundances in the Jovian atmosphere are depleted (0.48 and 0.39 times solar system, respectively). These relative depletions may indicate the extent of hydrogen and helium partitioning from the molecular envelope into Jupiter's metallic layer. A formation scenario for Jupiter is proposed to explain the relative oxygen depletion and, at the same time, the relative carbon enrichment. In essence, the model assumes that at the time of Jupiter's formation, abundant carbonaceous matter was present near 5.2 AU rather than abundant water ice, increasing the surface mass density of solids in the solar nebula accretion disk. Carbonaceous matter, which has high sticking probabilities, was the agent that sped up accumulation of solid matter of proto-Jupiter. This led to runaway accretion of the planet. Major consequences of this scenario are that the water ice condensation front (the snow line) typically placed near 5.2 AU in solar nebula models must be replaced by a carbonaceous condensation/evaporation front (the "tar line") and that the snow line is located farther out in the solar nebula.