Adam C. SimonPh.D., University of Maryland, 2003Research AssociateDepartment of GeologyUniversity of MarylandCollege Park, MD 20742p: 301 405 0235f: 301 314 9661e-mail: asimon@geol.umd.edu

Starting from ScratchWith the assistance of observational and theoretical astronomy, and by studying meteorites, geologists have developed hypotheses about the origin of our solar system and the Earth.Since these ideas are hard to test (because we have to rely on things very far away) planetary geology remains an actively evolving field. NGC 4414

How to Make a Solar System, pt.1begin with a region of space that has a high proportion of gas and dust (this is not necessarily common: space is dominated by vast areas containing little matter) -- these are celestial leftovers from earlier cosmic eventsdisturb this dusty region with something like a shock wave, perhaps from a nearby supernova: something to get the gas and dust to compress

Eagle NebulaThe Hubble Space Telescope has shown us some striking examples of nebulas in neighboring space--regions of active star formation.

How to Make a Solar System, pt.2gravity then takes over, with most of the matter in the region (which we call the presolar nebula) being compressed into the center of a rotating disk

How to Make a Solar System, pt.3dust in the disk starts to clump together, forming larger masses; meanwhile the central part of the nebula, where most of the mass is, starts to heat up

the internal pressure in the center of the nebula eventually is great enough to initiate nuclear fusion: in this process light elements merge to form heavier elements, and a huge amount of energy is released (the familiar form to us is sunlight)

fusion is how all of the heavy elements in nature are formed, although the most important reaction is where hydrogen atoms combine to form helium atoms

Galactic CatastrophismWe estimate that the entire process of nebula formation and star birth takes < 1 Myr.

...compare this to the ages ofthe Earth (~4.5 Byr)and the universe (~15 Byr) M = million

as the sun forms, the outer parts of the nebula have organized into the rotating protoplanetary diskgas and dust in the disk rapidly clump together (mainly through a combination of gravitational attraction and static forces) into planetesimalsthe clumps get bigger and bigger as each protoplanet sweeps material from its part of the diskas planetesimals grow in size, collisions can be destructive, but eventually planets grow largeBased on meteorite ages, we think that building planets this way would take < 100 Myr to accomplish.Making Planets

Looking for Planets Outside our Solar System One reason why we think the nebular hypothesis of solar system formation is accurate is that we see evidence for planet formation around young stars elsewhere in the universe.

The Terrestrial (inner) Planets The inner planets of the solar system have some common features. They are broadly similar in size and considerably richer in elements like iron (Fe), silicon (Si), oxygen (O), aluminum (Al) and magnesium (Mg) [the elements that make up rocks] than the outer planets of the solar system.MercuryVenusEarthMars

Earth is the Odd One (fortunately for us) Despite the similarities, the Earth enjoys significant differences from the other inner planets:mostly iron;too close to Sunacidic atmosphere at high pressurethe only one with decent atmosphere (nitrogen+oxygen) and abundant waterthin atmosphere (CO2), but had abundant water at some point

Active Volcanism in the Solar SystemJupiters moon Io. Images taken 5 months apart, showing effects of the eruption of the volcano Pillan (the grey spot), adjacent to massive volcano Pele (orange circle). The deposit from the Pillan eruption is 400 km in diameter.beforeafter

Extinct Planetary Volcanismold volcanoes on Venusthe Tharsis Montes on Mars

Olympus Monslack of chemical weathering weaker gravitational field (1/3)larger flux of lava + sluggish surface tectonicsis the largest known volcano in the solar system.

How can Martian volcanoes have been so much bigger (10-100 times) than those on Earth?

Plate Tectonics on Other Planets?To determine if plate tectonics were active on other planets, what would we look for?

Volcanic activity alone does not require plate tectonics, but it is a sign of geological activity. Extinct volcanoes abound on both Mars and Venus.

Comparative Densities of the Planets Although the compositions of the inner planets are broadly similar, in the outer part of the solar system most of the volatile elements condensed, and thus these planets have very low densities.

The Outer Planets: Gas Giants The compositions of Jupiter, Saturn, Uranus and Neptune are all dominated by hydrogen, helium, methane -- gases and ices.

They are 4-10 times larger than the Earth, but have much lower densities.

Why is the solar system chemically segregated the way it is?

Differentiation is the process of taking something that is well-mixed and separating its components to some ordered arrangement. It can refer to mega-scale processes, like the solar system or the Earth, or to smaller scales, like a single lava flow.

differentiate ~ separate ~ order ~ concentrateDifferentiation

Refractory and Volatile ElementsChemical differentiation of the solar system occurred because of the different freezing points of the elements:

Elements with high freezing (and melting) points are called refractory (e.g., iron). Refractory elements condense from a gaseous state at very high temperature.

Elements and compounds with low freezing points are called volatile (e.g., hydrogen, water). Volatile elements remain gases at low temperatures before solidifying.

Differentiation of the Solar SystemThere is a strong thermal gradient across the solar system, meaning that it is much hotter close to the Sun than far away (the vacuum of space does not conduct heat well).

In the early solar nebula, objects close to the young Sun were heated to the point that all but their refractory elements were largely boiled off.

Movement of this mass of volatile-element-rich gas away from the Sun results in chemical differentiation across the disk: the inner planets are more enriched in refractory elements, and generally poor in volatiles; the outer planets very volatile rich and refractory-poor.

The inner planets in the early solar system started out as mixed-up masses of accreted dust. Planetary Leftoverswhich made it a dangerous place. Hence we see the cratered surfaces of the planets.At this stage, there was still a lot of left-over material floating around the solar system,

Early Planets Heat UpImpacts yield energy, which serves to heat material up.

The early planets were also heated up due to:--decay of radioactive elements--gravitational compressionasteroid Eros (33 km long)Eratosthenes crater on the Earths Moon

Planetary DifferentiationAs the early planets heated up, they underwent internal chemical differentiation.

In this process, dense refractory elements tend to sink toward the center, leaving material enriched in the less dense elements to concentrate in the shallower regions.

As a result, all of the inner planets have generally similar internal structures, which include:--iron-rich core--silicon-oxygen-magnesium-rich mantleWith further differentiation, the Earth also developed a chemically distinct surface skin, called the crust.

The Bulk Composition of the EarthFig. 2.08

Internal Structure of the Earth: Coremainly iron (Fe) and nickel (Ni) with some sulfur (S) (also relatively rich in metals like platinum [Pt] and gold [Au])initially mostly liquid: has crystallized over time (cores of other inner planets are essentially solid now)solid-liquid interaction may account for magnetic fieldoverall diameter ~ 6350 km

Internal Structure of the Earth: Mantle>95% silicon (Si), oxygen (O), magnesium (Mg), iron (Fe)minor amounts of aluminum (Al) and calcium (Ca)largest volume layersolid (not molten) but able to flowoverall diameter ~ 6350 km

Internal Structure of the Earth: Crustthin, rigid layer of scum on Earths surfacemainly silicon (Si), oxygen (O), aluminum (Al)significant amount of iron (Fe), calcium (Ca), sodium (Na) and potassium (K), magnesium (Mg)where much of Earths volatile elements are concentratedoverall diameter ~ 6350 km

Water!Water is a weird substance (compared to other liquids) and is critical in the evolution of a planet, and the formation of a biosphere.Some of the odd properties of water:expansion on freezingunusually high melting and boiling point, extremely high heat capacity and heat of vaporizationvery high surface tensionabsorbs much radiation in the UV and IR spectra, but less in the visibleexcellent solvent for ionic substances

Origin of the Earths MoonThe leading theory of lunar origin involves the early Earth being struck in a glancing blow by a planetesimal (or series of planetesimals). The hit was not dead-on, or else the Earth probably would have broken up.

Origin of the Earths MoonIn the hit, a great deal of Earth material was blown off. This ejected debris combined with material from the planetesimal and was gravitationally captured by the Earth.

One important question is when this collision occurred...

Simulating the Collision4.2 minutes8.4 minutes12.5 minutesfrom Kipp and Melosh (86), Tonks and Melosh (93)From this model, what parts of the Earth and the impactor combine to make up the proto-Moon?These figures are from a numerical simulation of an oblique collision between early Earth and a Mars-size impactor.material that becomes the Moon

Lunar Origin and Composition of Earth v. MoonLunar rocks and Earth rocks are very similar in composition except that lunar rocks have much less Fe (and essentially no water).

If the impact that made the Moon happened before the Earth differentiated (when the core formed), we would expect the two bodies to have essentially identical bulk compositions.

The only way to make a Moon that has much less Fe than the Earth by this method is to have the collision occur after the Earths core had formed.

Timing of Lunar OriginSequence:

Earth accretes (accretion means sticking together)core forms, removing much of the Fe from the mantlecollision occurs, Moon coalesces from remains of asteroid and Earth mantle debrisLunar core segregates: Lunar mantle, already Fe-depleted, loses even more of its FeThus, Moon rocks that form after the Moons core formed will be conspicuously low in Fe (but very similar in most other elements) compared to equivalent rocks on Earth.

The ages of the oldest lunar rocks and the estimate for Earth core formation are consistent with this model.

Early Solar System TimelineYou dont need to commit this to memory, but having a general idea of the scales involved is important (i.e., billions v. millions of years, which came first, etc.). 4.56 Byr... solar system assembled4.50... accretion of Earth complete4.45... Earths core formed4.44... age of oldest lunar rocks4.30... Earths crust stabilized4.20... Earths hydrosphere in place4.00... rapid decline in meteorite bombardment4.00... end of activity on Mercury4.00... volcanic activity on Moon3.00... end of activity on Moon2.00... end of activity on Mars1 Byr = 1000 Myr; 0.01 Byr = 10 Myr

Meteorites: Critical Links to the Early Solar System and the Deep Earth planet Marscomet Halleyasteroid MathildeOur understanding of the history of the solar system and of the internal workings of our planet would be grossly incomplete if not for meteorites that fall to Earth.

Meteorites: Not Just Boring Rocks on Museum Shelves Yes, they may be nothing to look at, even compared to Earth rocks, but meteorites are critical to our understanding of not only the origin and composition of the solar system, but of the Earth itself.stony meteorite (chondrite)the big 4 Martian meteorites

Meteorites Are Important -- best estimates of age of the solar system -- homogeneous meteorites called chondrites give us the best estimate of the bulk composition of the Earth -- iron meteorites match a lot of our remote estimates for composition of the core metal crystals in an iron meteoriteiron meteorite (304 kg)

Meteorites Are Important -- prior to the Mars Pathfinder mission, they gave us the only (and still the best) estimate of the composition (and age) of planet Mars -- stony-iron meteorites [photo below] may represent material from the core-mantle interfaceFe-Ni metalMg-silicate (olivine)

Meteorites of Gold!?Yes, iron meteorites are relatively enriched in precious metals like gold (Au), butwere still talking about concentrations in the parts per BILLION.asteroid Ida and satellite Dactyl

Meteorites Can Kill!Every year ~60 MEGATONS of space dust and larger cosmic debris reaches the Earths surface. Our planet is a big target. A ~12 kg meteorite (a chondrite) ended the life of this classic Chevy Malibu in Peekskill, NY, in 1992.

CreditsSome images in this presentation come from: NASAJPLLPIwww.nyrockman.comEarth: Portrait of a Planet (1st ed.) by S. Marshak Univ. Tennesee, Knoxville (ASTR 161)