Lesson in Characteristics of Science Origin of Moon Lesson in Characteristics of Science

Today’s Goals Overall Goal: Use Supporting Evidence Decipher history of moon from features Moon rocks to support formation theory Facts and Synthesis: Moon properties, compared to Earth Moon formation theory – link to Earth, Solar System

Today’s Plan Moon Features Formation Theories of the Moon Failed Theories Fission Capture Co-Accretion Small impacts Current Best Theory Giant Impactor

Earth’s Moon How did our Moon form? http://photojournal.jpl.nasa.gov/catalog/PIA00094

A few data to ponder …. Lower density – “lighter” - relative to planets Less iron than whole Earth, more aluminum and titanium Moon’s chemical signature ~ Earth’s mantle http://photojournal.jpl.nasa.gov/catalog/PIA00405 The moon is enriched in Titanium, Aluminum, and related elements and depleted in iron and volatile elements (and compounds - DRY!) The moon (~3.3) has a low density compared to earth (~5.5) Earth/moon has anomalously large amount of angular momentum compared to other planets - conservation of momentum. Angular momentum is the measure of rotation in a system. The combined momentum of these two closely related bodies must stay the same over time. If moon closer earlier, then earth must have been rotating faster (shorter days). Momentum = mass x velocity Angular momentum = mass x velocity in a circle Earth’s high because earth + moon + impact = higher momentum Old highlands (cratered), young basalt “seas”. 4.5 billion years ago. Formation of the Moons's geosphere (fig p179 McSween) The moon too developed a magma ocean during its formation due to the same sources of heat, impact, internal compression, radiogenic decay and gravitational tides. We know this because when the first moon rocks were brought back to Earth during the Apollo program they were composed almost entirely of one mineral -- Feldspar. The only way to form such a rock (called Anorthosite) is to precipitate crystals from a molten lava and then separate the cystals according to differences in density -- a process known as fractional crystallization. This would happen if feldspar crystals intially crystallized with other minerals but then floated to the surface of a large magma ocean. The denser minerals would sink and the less dense minerals would float. Eventually, when the magma ocean cooled and solidified the crustal surface would be dominated by feldspar and the rock would be Anorthosite. This separating out of minerals of different density is known to occur in magma chambers within the continetal crust on Earth, but the planetary scale of this process on the moon is mind-boggling. (fig p178 Mc Sween) Estimates of the thickness of the Anothosite crustal layer on the moon indicate that it is ~20 km thick. To separate out a 20 km thick layer of feldspar from a molten magma requires a magma ocean that was at least 1000 km thick. So far measurement of radiometric ages of the Anorthosites indicates that the lunar crust solidified 4400 Ma ago, only 200 Ma after the formation of the solar system.

How Did the Moon Form? A few more data to ponder Does not orbit in equatorial plane of Earth, or ecliptic Earth/Moon - high angular momentum http://photojournal.jpl.nasa.gov/catalog/PIA00405 How Did the Moon Form?

Moon Facts Before Moon landings we knew: Shape of Moon, Mass, Diameter, Density, Distance Heavily Cratered, Other Surface Features – Approx. Age Orbital Information, Angular Momentum of System

Age from Cratering Cratering history, number of impacts as function of time Assume rate of impacts during past few billion years decreasing at known rate, estimate how long since last ‘erasure’ activity Get age About 4-5 billion years Crater Clavius, photograph from Celestron.com, courtesy Arpad Kovacsy

Models/Theories Make Predictions Let’s look at the most popular theories for the formation of the Moon PRIOR to our visiting the Moon. Each makes predictions about what we should observe. Three biggest theories: daughter, wife, sister

Theory: Fission Young Earth spun so fast that a bulge occurred at the equator. Over time a chunk from Earth's mantle was flung into orbit. Predict: Amount of angular momentum needed to fling piece off and can compare to current angular momentum of system Current E-M system doesn’t have enough angular momentum. http://www.youtube.com/watch?v=qiYKonNEc-c&feature=related

Fission Predict: Moon should orbit the Earth at its equator BUT: It doesn’t http://www.timezone.com/library/tmachine/tmachine0005

Fission Predict: Moon rocks same composition as Earth mantle rocks Probably not the method of formation: angular momentum not sufficient and orbital orientation is incorrect.

Theory: Capture An “asteroid” gets captured by the Earth in the early solar system Problem: How does it slow down and get into orbit? Predict: Expect different composition

Theory: Co-Accretion/Condensation Moon and Earth form at same time from same material Predict: Expect same density Predict: Same overall composition of Earth and Moon (i.e. iron, water, gases) BUT: We already know not same density

Theory: Colliding Planetesimals Moon forms from bits of Earth blasted off by small impacts Problem: Small impacts don’t send enough material into space. Most recaptured by Earth’s gravitational field before can form Moon. Predict: Same overall composition of Earth’s surface/mantle and Moon

After Lunar Landing: Composition of Moon Rocks No gases Evidence of heating Oxygen isotopes same as Earth, different from other parts solar system Density of surface rocks like average density of whole moon Little iron

After Landing Find: Moon’s surface is lots like Earth’s mantle, even the ratio of oxygen isotopes Supports fission Negates capture Negates co-formation Supports small impacts

Formation of Moon Theory best able to explain based on moon rock evidence Moon rocks almost as old as Earth Moon rocks made up of nearly identical material as crust and mantle Moon rocks lack all volatiles Moon rocks show evidence of heating

Fission? Or Small Impacts? So many other problems with fission that it seems unlikely Small impacts just don’t have the energy to get fragments into Earth’s orbit.

New Theory: Giant Impactor Mars sized body (½ diameter, (1/10 mass) Impact causes mantle material to be ejected Re-coalesce to form Moon Alastair Cameron, Harvard University

Giant Impactor Predictions: Depending on when impact occurred - composition could be very similar to mantle but not to core Oxygen ratios can be same if impactor big enough to modify overall for system Extreme heating of rocks means all volatiles gone Giant impactor can have enough energy to get particles to orbit earth Giant impactor means Moon doesn’t have to orbit at equator

Likely? Is it likely that a huge Mars-sized chunk was around to whack into Earth to make this happen? Where is the Moon’s iron? Let’s go back to age of Moon… What was going on in the solar system? What was Earth like?

Timing We know Moon about 4 Billion years old Lots of impacts then So - Mars sized impactor not out of the question

Earth Formation Very hot initially: heat of formation, internal radioactive decay, collisions Molten at first, then cools & solidifies No atmosphere

Iron on Molten Earth? Know structure of Earth by: Iron Core NOW - how? Overall density vs. surface density Seismic waves Iron Core NOW - how? Differentiation: denser materials sink to center Can only happen if Earth is molten or liquid So - for Moon to have no iron - impact would have had to happen AFTER differentiation Allows us to know when Earth’s iron core formed.

Confirmation: Particle Simulation Canup & Asphaug 2001, SwRI

Summary: Impactor Theory Evidence No volatiles in moon rocks - heating, from impact Moon rocks like Earth mantle rocks – impact occurs after most of iron differentiated Oxygen isotopes (asteroids not like Earth, but Moon is like Earth) Earth tilt, Moon orbit geometry

Characteristics of Science Summary Moon features tell us about Earth’s formation history Evidence suggests Moon formed by Mars sized impactor

Assumed Prior Knowledge Theories must be falsifiable, testable, make predictions that can be tested Formation of the Solar System (nebular hypothesis, planets in a plane, oddities to explain with impacts) Formation of the Earth - Geologic history of the Earth, including higher incidence of impacts in the past, settling of iron core in molten earth, erosion processes, fossil record Radioactive Dating, half-lives, log plots Density, Volume calculations Energy conservation, especially with impacts

Test Questions Describe some of the evidence gleaned from moon rocks that disproves the capture theory When was the last time people were on the moon? Did the Moon have volcanic activity? How do we know? Make a graph (with appropriate labels) estimating the number of impacts as a function of time in the history of the solar system.

More test questions Did the giant impact that formed the moon happen before or after the differentiation of the Earth? What evidence supports that? (be sure you describe the what differentiation is in your answer) Were large impacts more or less likely in the past? Describe the evidence. Calculate the average density of the moon. If avg density of iron is 8000 kg/m^3 and surface rocks on earth have avg density of 2500, which are moon rocks more like? Explain how this fits in to the impactor theory of the formation of the moon.

Other Moon Resources http://www.lpi.usra.edu/mymoon/ Look for Moon Formation and Processes Powerpoint: http://www.lpi.usra.edu/education/resources/s_system/moon.shtml

Moon Origin http://www.psi.edu/projects/moon/moon.html http://www.swri.org/3pubs/ttoday/spring99/moon.htm http://www.swri.org/3pubs/ttoday/fall01/Moon.htm

Moon Facts 0.27 Earth 0.012 Earth 1/50 Earth 0.61 Earth orbit: 384,400 km diameter: 3476 km mass: 7.35 x 10^22 kg volume: density:3340 kg/m3 0.27 Earth 0.012 Earth 1/50 Earth 0.61 Earth

Earth Moon 2160 miles (3476 km) diameter 7 degree tilt (~no seasons) Surface temps - 107 C to –153 C (224 F to –243 F) No atmosphere No liquid water … Ice at poles in shadows? 7930 miles (12,756.3 km) diameter 23 degree axis tilt (seasons!) Surface temps –73 to 48 C (-100 to 120F) Thick atmosphere, mild greenhouse effect Liquid water – lots! - at surface antwrp.gsfc.nasa.gov

Earth Moon No Magnetic Field Small Moon Quakes Small, Offset Core http://www.windows.ucar.edu/tour/link=/earth/moon/earthmoon_interior_gif_image.html No Magnetic Field Small Moon Quakes Small, Offset Core

Gory Details: Moon Rock Composition Moon relatively depleted in volatile elements and enriched in refractory elements. Highly depleted in siderophile elements… b/c scavenged by earlier differentiation..

Formation of the Earth Formed in protoplanetary disk All planets in single plane Think hula hoop, or Saturn's rings Formed via accretion Planetesimals to protoplanets Lots of heat

Structure of Earth Today

Molten Earth? Molten early Earth explains why oldest rocks on Earth are not as old as oldest meteorites or as old as Sun Molten early Earth also explains why no atmosphere present initially - boils atmosphere away Although, didn't form with many volatiles, including water

Theory of Moon Formation Large impact with Mars size object Explains heating Explains lack of volatiles After Earth differentiated Explains why moon similar to crust and mantle Before 4.4 million years ago When impacts were much more frequent Crater counts provide another estimate of age of moon

Lunar Interior Structure A cross section view of the Moon’s internal structure. The outer, rocky crust averages about 60 km thick. Most of the Moon is the mantle, which is also rocky but of somewhat different composition than the crust (more magnesium, less iron and aluminum). There is probably also a small, iron rich core, a few hundred km in size at the center of the Moon. Copyrighted, LPI