The Lunar Interior A Presentation by Kyle Stephens October 2, 2008

Ingredients for Planetary Evolution Basic Questions: 1.Homogeneous or layered interior? 2.Enough heat to cause volcanic activity? 3.How far from the sun? 4.How large is the planetary body?

Planetary Evolution: Key Facts Main elements to planetary differentiation: 1.Segregation into layers of different composition  Usually a rapid process (geologically speaking) 2.Volcanism  A slower process 3. Cataclysmic Bombardment  Separates the crust into different layers through melting

Planetary Evolution: Key Facts a)Body starts as homogeneous interior b)Due to heat, entire body melts c)Layers of crust, mantle, and core begin to form, with more dense materials sinking to the core (Iron, etc.) d)Volcanism begins to affect the composition of the different layers e)Image applies to Earth f)Volcanism causes planet-wide resurfacing

Planetary Evolution: The Moon 1.Homogeneous Interior (4.5 billion years ago) 2.Separation into layers (4.45 billion years ago) 3.Possible formation of metallic core (4.4 billion years) 4.Volcanism ( billion years ago) 5.Major impacts ( billion years ago) 6.Major activity ceases (3.0 billion years ago) 7.Minor impacts continue to form craters

The Moon’s Crust Regolith covers the lunar crust Thickness varies by location:  Lunar maria: 4-5 meters thick  Highlands: ~10 meters thick Regolith is formed by overlapping ejecta blankets from meteor impacts Regolith “grows” by 1.5 mm per million years

The Moon’s Crust Primary composition: Feldspar (rock-forming mineral that crystallizes from magma) Thickness varies with location  Near side (~55 km)  Far side (~100 km) Due to varying crust thickness, the moon’s center of mass is offset

The Moon’s Crust The Moon’s crust represents about 9% of the total mass.

The Moon’s Crust: Lunar Maria About 2.5 to 3 billion years ago, basaltic lava covered 17% of the moon’s surface This lava filled the giant impact basins to form what is known today as the lunar maria Lunar maria is only a few kilometers thick Mascons: Large concentrations of lunar maria that cause a stronger gravitational attraction  Common in younger basins

The Moon’s Crust The moon is a “one plate planet” No tectonic plates like Earth  Moon cooled rapidly Heat is lost by conduction

The Moon’s Mantle The complete structure of the mantle is not known Most data comes from the Apollo missions’ seismometers “Moonquakes” reveal important information about the composition of the interior  Quakes usually originate from tidal effects or meteor impacts

The Moon’s Mantle “Moonquakes” are usually over ten minutes in length Most quakes originate deep within the mantle Figure (a) shows how the seismic wave velocities are used to identify certain elements of the interior

The Moon’s Mantle Over three billion years ago, the mantle was filled with melted basaltic rock Basaltic lava would flood the basins on the surface Through conduction, the interior’s heat was lost Today, the mantle is a lithosphere (unmelted)

The Moon’s Mantle The graph to the left displays the fact that today, the moon’s internal temperature is too low to be molten.

The Moon’s Mantle Two possible models of the lunar interior:

The Moon’s (Possible) Core If the moon does have a core, it would have formed very early on (4.4 billion years ago) Upper limit for the core: km radius Would consist of about 4% of the moon’s volume Current data supports, but does not prove that the moon has a core Better seismic data is needed

Questions? Sources: Planetary Interiors, Surfaces and Interiors of Terrestrial Planets, Encyclopedia of the Solar System, NASA.gov, Wikipedia