Introduction to Astronomy !AST0111-3 (Astronomía)

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Semester 2014B

Prof. Thomas H. Puzia

NEOs Orbits of known Near Earth Asteroids (NEOs).

!There are about 2000 known NEOs

larger than 1 km. ~1400 of these are on potentially

hazardous orbits! Types of Orbits:

Cosmic CollisionsComet Shoemaker-Levy collided with Jupiter in 1994. Before the crash, the tidal forces shredded the comet. !The collision of the 12 pieces had long-term effects observable from Earth.

Cosmic CollisionsComet Shoemaker-Levy collided with Jupiter in 1994. Before the crash, the tidal forces shredded the comet. !The collision of the 12 pieces had long-term effects observable from Earth.

Cosmic Collisions

Effects of collisions in Earth's history Craters (later eroded) Mass extinctions (which give rise to other species)

Arizona, made 50.000 yrs agoQuebec, made 1.4 Myrs ago

Tunguska, 1908 (exploded in air)

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The Tunguska Impact - Simulation

- - - - - - - - -

Each month Each year

Each decade Each century

Each millineum Each 10000 yrs

Each 100000 yrs Each 1 Myrs

Each 10 Myrs ! ! ! ! ! ! ! ! ! ! ! 0.01 1 100 10000 1M

Tunguska

Extinction of the dinosaurs

Impact Frequency

Average current frequency of meteorite impacts of various sizes on the surface of the Earth. Energy depends on both size and speed.

Energy in Megatons (TNT equivalent)

~1 m ~10 m ~100 m ~1 km ~10 km

Rough size equivalent

Hiroshima Tunguska

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Another past impact...

The Eltanin asteroid impact (estimated to be as large as 4km in diameter, or equivalently 10,000 Megatons) is thought to have created a giant tsunami in the southern hemisphere. !In 20 hours the tidal wave would have reached the other end of the ocean !70 m in average height !! !

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Apophis will pass close to us on April 13, 2029: !325m 107 kg Visible! probability of collision in 2068 calculated in 2013, revised down to ~ 1 in 256,000 http://neo.jpl.nasa.gov/risk/

The Next Impact?

• Asteroids: small rocky bodies, 3 main types, various preferred locations/orbits,

• Comets: small ice-rich bodies with cloud + dust and ion tails (strongly affected by Sun), huge variety of (elliptical, non-ecliptic) orbits,

• Dwarf Planets: larger ice-rich bodies, larger variety of (elliptical, non-ecliptic) orbits

• Kuiper Belt and Oort Cloud: reservoirs for comets (and dwarf planets)

• Meteors/Meteorites: meteor showers, cosmic collisions, effect on life !Key Concepts:

What are asteroids? How and where did they (and the asteroid belt) form? Jupiter likely affected/frustrated early growth of planetesimals.

What are comets? How and where did they form? What are dwarf planets? How and where did they form? How do we study and learn the history of these objects? What do they say about the formation and history of the solar system? Why is the solar system (or objects within it) constantly evolving?

Asteroids, Comets, Dwarf Planets, & Beyond

Formation of the Solar SystemThe Solar Nebula Hypothesis

Formed from the collapse of an interstellar gas cloud. !1. Gravitational collapse of large, diffuse, slowly-rotating nebula (largely H + He w/ trace heavy elements) !2. Formation of the protosun w/ jets and a hot protoplanetary disk via conservation of momentum + energy (which explains uniform ecliptic plane and orbital direction) !3. Temperature (and hence radius) dependent condensation of planetesimals (molten rock, ices, etc). Lighter elements in disk pushed out of inner solar system by Sun’s radiation and disk temperature. !4. Accretion of planetesimals to form planet seeds. !5. Formation of Jovian planets through nebular capture of gas. !6. The solar wind of young Sun clears away the remaining gas. !7. Period of heavy bombardment.

200,000 AU 10,000 AU 500 AU

100 AU 100 AU 50 AU

A schematic diagram of the solar nebula as it was still accreting dust to it. Planets have not yet formed. Materials heated near the Sun circulate to the outer Solar System to the cold regions where comets formed

During Protostar/T Tauri Phases

Formation of the Terrestrial Planets During Pre-Main Sequence Phase

Clues from Extrasolar Stars

Subsequent Evolution of Solar SystemAccording to the nebular hypothesis, the outer two planets (and the Kuiper belt) are in the "wrong place". !Ice giants Uranus and Neptune currently reside in a region where the reduced density of the solar nebula and longer orbital times render their formation highly implausible. Moreover, the Kuiper belt should be much denser and closer (15-30AU) to the Sun. !Uranus and Neptune probably formed in orbits near Jupiter and Saturn, where more material was available (Figure a), but migrated outward to their current positions over hundreds of millions of years: in 300M yrs, Jupiter and Saturn could move into a 2:1 resonance, which would have provided enough force to move the icy giants out (Figure b). !The outward migration of Neptune and Uranus over next ~200M yrs would kick most Kuiper belt objects inward either to be captured or eventually ejected by Jupiter to form the Kuiper Belt and Oort Cloud. Jupiter’s orbit would tighten slightly and send asteroids from belt everywhere. The Jovian orbits eventually circularize due to interaction with planetesimals (Figure c). This migration process would have led to many objects entering the inner solar system on elliptical orbits (i.e., Heavy Bombardment epoch)

Subsequent Evolution of Solar SystemThe gradual warming of the Sun over the next 3-4 billion years will also cause the “habitable zone” to migrate outward over time. This will cause Earth’s temperature to rise to the point where water will be vaporized into thick clouds, driving rapid onset of Greenhouse house effect...at which point Earth will become very “Venus”-like. Mars will heat up and many of its icy gases will melt to form oceans and an atmosphere perhaps like Earth’s. !In about 5 billion years, the Sun will exhaust its supply of hydrogen in the core and start burning helium, causing it to go into a puffed up red giant phase where the Sun’s surface extends out to the orbit of Mars. !Eventually, it will shed these outer layers into a planetary nebula and live out its remaining life as a cooling white dwarf.

What is Life? Difficult concept to define: !

• Orderly patterns of cells? DNA/RNA? • Growth and Development? Need nutrients to build from... • Reproduction? • Energy Utilization? Need energy to fuel activities... • Response to Environment? • Requires Liquid (e.g., water)? • Evolutionary Adaptation?

Clearly we should be cautious about limiting what we consider to be “life”. We have already found life in some of the most extreme conditions on Earth.

Could life have migrated to Earth? We see building blocks of life in interstellar material

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WHAT DOES HABITABLE MEAN TO US?

• Right temperature

• Liquid water

• Air to breath

• Light to keep you warm and to see

• Radiation shield

• Asteroid protection

THINGS THAT AFFECT TEMPERATURE• Want temperature so you can have liquid

water on the surface of the planet. What properties define this?

1. Temperature of star 2. Distance from the star

3. Shape of planet’s orbit: circular or elliptical

4. Planet’s atmosphere: greenhouse gases

• These shape the Habitable Zone (HZ) for a star

THE HABITABLE ZONE FOR VARIOUS STELLAR TYPES

The Habitable Zone (HZ) in green is the distance from a star where liquid water is expected to exist on the planets surface. (Kasting, Whitmire and Reynolds, 1993)

A Search for Habitable Planets

The Drake Equation N = R* fp ne fl fi fc fL!

N = The number of communicative civilizations !R* = The rate of formation of suitable stars (stars such as our Sun) !

– ONLY WELL-KNOWN TERM!fp = The fraction of those stars with planets. !

-- evidence mounting that planetary systems may be common for stars like the Sun!

ne = The number of Earth-like worlds per planetary system – not yet known!

fl = The fraction of those Earth-like planets where life actually develops !fi = The fraction of life sites where intelligence develops !

fc = The fraction of communicative planets (those on which electromagnetic communications technology develops) !L = The "lifetime" of communicating civilizations. Jury is still out on us humans…

Are we alone?

Some Basic Definitions for Extrasolar PLANETS• Objects with true masses below the limiting mass for thermonuclear

fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity). Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarfs", no matter how they formed or where they are located. However, because the errors on the mass and metallicity can be rather large, limits as large as 20-25 Jupiter masses are sometimes used.

• The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our solar system.

• Object should orbit star or stellar remnant (no matter how they formed). Free-floating objects in young star clusters with masses below the limiting mass for thermonuclear fusion of deuterium are not "planets", but are "sub-brown dwarfs" (or whatever name is most appropriate).