Jupiter and Uranus
Same start, about 4.5 billion years ago. One core won the gas. The other arrived after the gas was gone.
From the pair notes by Addison Dean and Emma Terrill.
Jupiter
- Where
- Closer to the Sun
- Air
- Mostly hydrogen and helium
- Clouds
- Ammonia ice on top
- Weather
- Driven by internal heat
- Impact
- ~10 Earths, possibly head-on
Uranus
- Gas
- A thin layer only
- Bulk
- Water, ammonia, methane ices
- Clouds
- Methane ice
- Weather
- Driven by sunlight
- Tilt
- 98°, from a graze
Question 1
Formation pathway
About 4.5 billion years ago, Jupiter and Uranus began forming around the same time. Jupiter formed closer to the Sun and had a denser core. That core grew much faster, so Jupiter became a gas giant and captured mostly hydrogen and helium. Uranus’s core reached the size for runaway gas accretion only after solar wind had blown away most of the hydrogen and helium. Uranus captured a thin layer of gas, and mostly water, ammonia, and methane ices.
Move the clock. The short answer above does not depend on it.
About 4.5 billion years ago
Both begin around the same time. Jupiter is already the one closer in, with the denser core.
Jupiter’s core wins the gas
The core grows much faster. Jupiter pulls in hydrogen and helium and becomes a gas giant while that gas is still there.
Uranus misses the disk
By the time Uranus’s core is large enough for runaway accretion, solar wind has cleared most of the hydrogen and helium. Only a thin gas layer is captured. The mass that remains is water, ammonia, and methane ice.
Jupiter, outside in
Clouds, then gaseous hydrogen, liquid hydrogen, and metallic hydrogen.
Uranus, outside in
Upper cloud layer. Then an atmosphere of hydrogen, helium, and methane. Then a mantle of water, ammonia, and methane ices. Then a core of silicate and iron-nickel rock.
Evidence. The timing contrast is the one in the notes: same start, faster Jovian core, Uranus after the gas is blown off. The layer lists are the labels on their interior diagrams.
Question 2
Collisions and captures
Both events were about 4.5 billion years ago. Both involved protoplanets of significant size. Both changed the interior and redistributed heavy elements. Jupiter’s impactor is theorized at about 10 Earth masses, and possibly head-on. Uranus was only grazed, and the graze tilted it 98 degrees. That impact also left a massive debris disk, which formed Uranus’s current moons.
Jupiter
Theorized impactor of about 10 Earth masses. Possibly head-on. The notes do not give Jupiter a new tilt from this hit. They do say the interior and the heavy elements were rearranged.
Uranus
The protoplanet only grazed Uranus. The tilt is 98 degrees. The debris disk from that impact formed the moons Uranus has now.
Evidence. Shared age, about 4.5 billion years. Jupiter’s ~10 Earth-mass impactor is stated as a theory. Uranus’s 98° tilt is stated as the result of the graze.
Still open. “Theorized” and “possibly head-on” are the notes’ own limits on the Jupiter impact. The graze and the 98° tilt are stated more directly.
Question 3
Volatiles and air
Jupiter’s atmosphere is mostly hydrogen and helium. Uranus is mostly heavy volatiles: the water, ammonia, and methane ices captured when the gas disk was already thin. Jupiter’s dominant upper cloud is ammonia ice. Uranus’s is methane ice. Jupiter’s weather is driven by internal heat. Uranus’s is driven by solar radiation.
Composition
Jupiter: hydrogen and helium. Uranus: heavy volatiles under a thin gas layer.
Upper cloud
Jupiter: ammonia ice. Uranus: methane ice.
What drives it
Jupiter: internal heat flux. Uranus: sunlight.
Evidence. Those three contrasts are stated in that order in the notes. The interior labels add methane to Uranus’s gas layer and metallic hydrogen to Jupiter’s deep interior.
Question 4
A constrained future
When the Sun becomes a red giant, both planets change, and neither is described as swallowed. Jupiter survives and goes through extreme atmospheric heating, about 1000 K. Its moons thaw briefly. Uranus is far enough to survive engulfment, but the extra sunlight strips the upper atmosphere and melts outer volatiles. Later, in the white-dwarf phase, the Sun sheds about half its mass. Weaker gravity lets both drift outward. The notes then say the orbits may become chaotic enough for a collision or an ejection.
Red giant
Jupiter: extreme heating, about 1000 K, and a brief thaw of the moons. Uranus: too distant to be engulfed. Increased radiation strips the upper atmosphere and melts outer volatiles.
White dwarf
The Sun sheds half its mass. Jupiter and Uranus drift into wider orbits. They may enter chaotic orbits that could make them collide, or eject them farther out.
Evidence. About 1000 K for Jupiter’s red-giant heating. Half the Sun’s mass lost in the white-dwarf stage. Uranus survives engulfment because of distance, and still loses upper air.
Still open. The chaotic orbits, the collision, and the ejection are “may” in the notes. The heating and the outward drift are stated as the direction of change.