A double ringed basin on Mercury image last week by the Messenger spacecraft during a swing past Mercury. Double and multiple ringed basins, although rare, are also found on Mars, Venus, Earth, and Earth's Moon. Most large multiple ringed basins are caused initially by the impact of an asteroid or comet fragment. Messenger has now completed its last flyby of Mercury but will return and attempt to enter orbit in 2011 March.

Homework #4 is due tomorrow (Tuesday) at 11 pm

Terrestrial planets and many large moon had an extended period where their interiors were “molten”. During this time, denser material sank towards center of planet while less dense material “floated” towards top

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

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

Layering by strength (mantle)

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

Larger planets take longer to cool, and thus: 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

lithospheres of the Terrestrial planets: Geological activity is driven by the 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. lithospheres of the Terrestrial planets:

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

important repercussions for life: This has important repercussions for life: Outgassing produces atmosphere Magnetic fields protect planetary surfaces from high energy particles from a stellar wind. The existence of these fields depends upon molten cores.

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 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

Planetary Surfaces 4 major processes affect planetary surfaces: Impact cratering – from collisions with asteroids and comets Volcanism – eruption of molten rocks Tectonics – disruption of a planet's surface by internal stresses Erosion – wearing down or building up geological feature by wind, water, ice, etc.

Impact Cratering: The most common geological process shaping the surfaces of rigid objects in the solar system (Terrestrial planets, moon, asteroids)

Volcanism Volcanoes help erase impact craters

Volcanic outgassing: source of atmospheres and water

Erosion: the breakdown and transport of rocks and soil by an atmosphere. Wind, rain, rivers, glaciers contribute to erosion. Erosion can build new formations: sand dunes, river deltas, deep valleys). Erosion is significant only on planets with substantial atmospheres.

Tectonics: refers to the action of internal forces and stresses on the lithosphere to create surface features. Tectonics can only occur on planets or moons with convection in the mantle Earth & Venus Jupiter’s moons Europa & Ganymede?

Tectonics… raises mountains creates huge valleys (rifts) and cliffs creates new crust moves large segments of the lithosphere (plate tectonics)

Tectonic plates

divergent plate boundary (plates move away from each other). Atlantic Ocean Great Rift Valley in Africa Valles Marineris (Mars)

Portion of Valles Marineris on Mars – created by tectonic stresses

convergent plate boundary with subduction : plates move towards each other & one slides beneath the other. Nazca plate being subducted under the South American plate to form the Andes Mountain Chain. Island arc system

convergent plate boundary without subduction : plates move towards each other and compress. Formation of Himalayas.

Plates sliding past each other: earthquakes, valleys, mountain building

Half of the world’s volcanoes surround the Pacific plate Tectonic plates