Building the Planets IV Nebular Capture Nebular capture

Building the Planets. IV. Nebular Capture Nebular capture – growth of icy planetesimals by capturing larger amounts of hydrogen and helium. Led to the formation of the Jovian planets Numerous moons were formed by the same processes that formed the proto-planetary disk Condensation and accretion created “mini-solar systems” around each Jovian planet

Building the Planets. V. Expulsion of remaining gas The Solar wind is a flow of charged particles ejected by the Sun in all directions. It was stronger when the Sun was young. The wind swept out a lot of the remaining gas

Building the Planets. VI. Period of Massive Bombardment

Planetesimals remaining after the clearing of the solar nebula became comets and asteroids Rocky leftovers became asteroids Icy leftovers became comets Many of them impacted on objects within the solar system during first few 100 million years (period of massive bombardment - creation of ubiquitous craters).

Brief Summary

Three outstanding issues: 1. Outer Jovian planets may not have had enough time to formed in their current locations 2. Rocks returned by astronauts from the heavily cratered lunar highlands are ~ 3. 9 million yrs old – younger than solar system 3. There were no icy planetesimals in the inner solar system. Where did the Earth’s water come from?

Solution? Late Heavy Bombardment (LHB) 1. All Jovians planets formed in orbits closer to that of Jupiter 2. Orbital resonances between Jupiter and Saturn caused outer Jovians to move suddenly to larger orbits 3. Uranus and Neptune interacted with Kuiper Belt objects, scattering large numbers of them into the inner solar system 4. This lead to heavy bombardment & delivery of ices to Terrestrial planets.

The oldest biological markers known to scientists date precisely to the end of the LHB.

The Origin of the Earth’s Moon

The Earth-Moon double planet does not fit well into the nebular theory planetesimal accretion predicts both should have the same chemical composition. They don’t - there are subtle but significant differences Moon is composed of less dense material than Earth

The general view is that the Earth’s moon was created as a result of the impact of a large object, perhaps as large as Mars, with the Earth very early in its existence. The moon was formed from the debris of this collision, which included lower density “mantle” material from the Earth.

The Moon stabilizes the tilt of Earth's spin axis, leading to stable seasonal changes. This is not the case for the other terrestrial planets.

WE NOW UNDERSTAND HOW THE LOCATIONS AVAILABLE FOR LIFE WERE FORMED. NOW, WHAT ARE THE REQUIREMENTS FOR LIFE?

Life depends critically on environment. We will examine how life-friendly environments can form in the universe. Fundamentals: Temperature Liquids (particularly H 2 O) Sources of Energy Chemical environment Radiation environment

What determines the environments of terrestrial -like planets? A look at: interiors surfaces atmospheres (much of what follows also applies to Jovian planets & moons)

Terrestrial planets are mostly made of rocky materials (with some metals) that can deform and flow. Likewise, the larger moons of the Jovian planets are made largely of icy materials (with some rocks and metals) that can deform and flow. The ability to deform and flow has many consequences.

Weight of mountain is determined by its mass & the strength of the gravitational acceleration, Fg = mg weight of mountain If this force exceeds the ability of the underlying rock/ice to support it, the mountain will sink into the crust. The ability to deform and flow leads every object with diameters greater than a few hundred km to become spherical under the influence of gravity.

The ability to deform and flow also created structure in the interiors of planets

Early in their existence, the Terrestrial planets and the large moons had an extended period when they were mostly molten. The heating that led to this condition was caused by impacts, where the kinetic energy of the impacting material was converted to thermal energy. Today, the interiors of planets are heated mainly by radioactive decay.

Differentiation – the process by which gravity separates materials according to their densities Denser materials sink, less dense material “float” towards top

DIFFERENTIATION: During the time when interiors were molten, denser material sank towards the center of a planet/moon while less dense material “floated” towards top. This created density layers: core, mantle, crust

Terrestrial planets have metallic cores (which may or may not be molten) & rocky mantles Earth (solid inner, molten outer core) Earth’s interior structure Mercury (solid core)

Differentiated Jovian moons have rocky cores & icy mantles Europa Ganymeade Io Callisto

Interior structure of the Terrestrial planets:

The Lithosphere… Layer of rigid rock (crust plus upper mantle) that floats on softer (mantle) rock below While interior rock is mostly solid, at high pressures stresses can cause rock to deform and flow (think of silly putty) This is why we have spherical planets/moons

The interiors of the terrestrial planets slowly cool as their heat escapes. Interior cooling gradually makes the lithosphere thicker and moves molten rocks deeper. Larger planets take longer to cool, and thus: 1) retain molten cores longer 2) have thinner (weaker) lithospheres

Geological activity is driven by thermal energy of the interior of the planet/moon The stronger (thicker) the lithosphere, the less geological activity the planet exhibits. Planets with cooler interiors have thicker lithospheres!

Earth has lots of geological activity today, as does Venus. Mars, Mercury and the Moon have little to no geological activity (today) This has important repercussions for life: 1) Outgassing: produces atmosphere 2) Magnetic fields (need molten cores): protect planet surface from high energy particles from a stellar wind.

Larger planets stay hot longer. Earth and Venus (larger) have continued to cool over the lifetime of the solar system thin lithosphere, lots of geological activity Mercury, Mars and Moon (smaller) have cooled earlier thicker lithospheres, little to no geological activity

Initially, accretion provided the dominant source of heating. Very early in a terrestrial planet’s life, it is largely molten (differentiation takes place). Today, the high temperatures inside the planets are due to residual heat of formation and radioactive decay heating.

Stresses in the lithosphere lead to “geological activity” (e. g. , volcanoes, mountains, earthquakes, rifts, …) and, through outgassing, leads to the formation and maintenance of atmospheres. Cooling of planetary interiors (energy transported from the planetary interior to the surface) creates these stresses Convection is the main cooling process for planets with warm interiors.

Convection - the transfer of thermal energy in which hot material expands and rises while cooler material contracts and falls (e. g. , boiling water).

Convection is the main cooling process for planets with warm interiors.

Side effect of hot interiors - global planetary magnetic fields Requirements: • Interior region of electrically conducting fluid (e. g. , molten iron, salty water) • Convection in this fluid layer • “rapid” rotation of planet/moon

Earth fits requirements Venus rotates too slowly Mercury, Mars & the Moon lack molten metallic cores Sun has strong field