Jupiter and Saturn are gas giant planets with 318 and 95
Earth masses respectively. A gas giant forms when a solid core of rock and ice
material with ~10 Earth masses starts accreting hydrogen and helium gas from
the protoplanetary disk of material surrounding a young star. The end result is
a massive hydrogen-helium (H-He) envelop surrounding a small rocky core.
Observations of Jupiter and Saturn have revealed two puzzling properties.
Firstly, Saturn seems to have a more massive core than Jupiter even though
Saturn is only one-third Jupiter’s mass. Jupiter’s core is less than 10 Earth
masses while Saturn core is between 15 to 30 Earth masses. Secondly, there is
an enhancement of heavy elements in the H-He envelops of Jupiter and Saturn.
In a previous article, I mentioned how the low mass of
Jupiter’s core can be explained by the planet’s higher internal temperature
which makes rock material within the planet’s core more dissolvable. This
article will cover another mechanism involving the collisions of planetary-mass
objects with gas giants, and how such events can also account for the heftier
core of Saturn and the enhancement of heavy elements in both Jupiter and
Saturn. An impacting planetary object can range from sub-Earth-mass to
super-Earth-mass (~10 Earth masses). When a planetary object collides into a
gas giant, the outcome largely depends on the object’s mass, but also on the
object’s speed and angle of impact.
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Figure 1: Artist’s impression of a super-Earth colliding
into a gas giant planet. Credit: Jamie Murchison
In a head-on collision event, the impacting object needs to
be sufficiently massive to penetrate deep enough to reach the gas giant’s core.
This is because ablative disintegration of the impactor occurs as it plows
through the H-He envelop of the gas giant. As a result, an impactor needs to
have at least a few Earth masses or more to survive ablative disintegration and
reach the gas giant’s core. The dissipation of impact energy into and around
the gas giant’s core can erode the core and mix the core material with the
overlying H-He envelope. Core erosion and ablative disintegration of the
impactor enhances the abundance of heavy elements in the gas giant’s H-He
envelope. An energetic head-on collision involving a massive impactor (~10
Earth masses) probably occurred for Jupiter, and resulted in Jupiter’s low mass
core and enhancement of heavy elements in its atmosphere.
Less massive impacting objects with less than a few Earth
masses are expected to disintegrate completely in a gas giant’s H-He envelop and
not reach the gas giant’s core at all. Some fraction of the debris being
deposited into the gas giant’s H-He envelop by ablative disintegration of the
impactor eventually settles onto the gas giant’s core. As a result, low mass
impactors promote growth of the gas giant’s core. Saturn’s large core may have
grown to its current size in such a manner from the sedimentation of ablated
debris from a number of low mass impactors. Ablation of these low mass
impactors can also account for the observed enhancement of heavy elements in
Saturn’s atmosphere.
Giant impacts involving the collision of sub-Earth-mass to
super-Earth-mass objects into gas giant planets are likely to increase their
luminosities and puff up their diameters. Such impacts can significantly modify
the core-envelope structure and atmospheric composition of gas giants.
Reference:
Shulin Li et al. (2010), “Embryo impacts and gas giant
mergers I: Dichotomy of Jupiter and Saturn’s core mass”, arXiv:1007.4722
[astro-ph.EP]