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Astro 101 Fall 2013 Lecture 4 T. Howard. The Solar System. Planets Their Moons, Rings Comets Asteroids Meteoroids The Sun A lot of nearly empty space. Ingredients?. Solar System Perspective. Orbits of Planets. All orbit in same direction . Most orbit in same plane.

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slide1

Astro101

Fall 2013

Lecture 4

T. Howard

slide2

The Solar System

  • Planets
  • Their Moons, Rings
  • Comets
  • Asteroids
  • Meteoroids
  • The Sun
  • A lot of nearly empty space

Ingredients?

slide4

Orbits of Planets

All orbit in same direction.

Most orbit in same plane.

Elliptical orbits, but low eccentricity for most, so nearly circular.

slide5

Exceptions:

MercuryPluto

(no longer a planet)

orbital tilt 7o orbital tilt 17.2o

eccentricity 0.21 eccentricity 0.25

slide6

Sun, Planets and Moon to scale

(Jupiter’s faint rings not shown)

slide7

Two Kinds of Planets

"Terrestrial"

Mercury, Venus,

Earth, Mars

"Jovian"

Jupiter, Saturn, Uranus, Neptune

Far from the Sun

Large

Close to the Sun

Small

Mostly Rocky

High Density (3.9 -5.3 g/cm3)

Mostly Gaseous

Low Density (0.7 -1.6 g/cm3)

Slow Rotation (1 - 243 days)

Fast Rotation (0.41 - 0.72 days)

Few Moons

No Rings

Main Elements Fe, Si, C, O, N

Many Moons

Rings

Main Elements H, He

slide8

Asteroids --

  • most between Mars and Jupiter
    • asteroid “belt”
  • some groups of asteroids with unusual locations or orbits
    • “Trojan” asteroids
    • Other “families” -- “Apollo”, etc.
        • Earth-crossing orbits --> perihelia closer to Sun than Earth’s orbit
slide9

Asteroids

Rocky fragments ranging from 940 km across (Ceres) to < 0.1 km. 100,000 known.

Most in Asteroid Belt, at about 2-3 AU, between Mars and Jupiter. The Trojan asteroids orbit 60 oahead of and behind Jupiter. Some asteroids cross Earth's orbit. Their orbits were probably disrupted by Jupiter's gravity.

slide11

39.4

30.1

5.2

9.5

19.2

1

1.5

.07

4.0

5.3

2.5

.58

1.2

THE OUTER SOLAR SYSTEM

Orbital radius (AU)

Jupiter (5.2 AU)

zodiacal

cloud

Light time from earth (hrs)

34.8 min.

Kuiper Belt

Oort Cloud

Asteroid Belt

Earth (1 AU)

Solar wind

Mars (1.5 AU)

4.2 minutes

slide12

CATALOGUED ASTEROIDS (C. 12/98)

(VIEW ALONG ECLIPTIC PLANE)

ecliptic

slide13

CATALOGUED ASTEROIDS (C. 12/98)

(VIEW FROM ABOVE ECLIPTIC)

Jupiter

Trojan groups

Main belt

Source: Guide 7.0

slide15

asteroid Mathilde (253)

Asteroid Eros (closeup)

(from the NEAR mission)

slide16

Ida and Dactyl

Gaspra

Total mass of Asteroid Belt only 0.0008 MEarthor 0.07 Mmoon.

So it is not debris of a planet.

Probably a planet was trying to form there, but almost all of the planetesimals were ejected from Solar System due to encounters with Jupiter. Giant planets may be effective vacuum cleaners for Solar Systems.

slide17

Comets

  • Generally have highly elliptical orbits
  • Perihelion distance close to Sun
  • Aphelion distance in the outer Solar System
  • “Tails” -- two components
  • Dust tail
  • Ion (ionized gas) tail
  • Both directed away from Sun by Solar wind
  • Fuzzy appearance in camera/telescope images
  • Nucleus (solid body)
  • Coma (gaseous cloud surrounding nucleus)
  • Comets aren’t just “rocks”
  • Have volatile chemicals in the form of ices
slide18

Solar System Debris

Comets

Comet Hale-Bopp (1997)

Comet Halley (1986)

Long Period Comets

Short Period Comets

50-200 year orbits

Orbits prograde, close to plane of Solar System

Originate in Kuiper Belt

Few times 105 or 106 year orbits

Orbits have random orientations and ellipticities

Originate in Oort Cloud

slide19

Comet Structure

Coma and tail due to gas and dust removed from nucleus by Solar radiation and wind.

Far from Sun, comet is a nucleus only.

Nucleus: ~10 km ball of ice, dust

Coma: cloud of gas and dust around nucleus (~106 km across)

Tail: can have both gas (blue) and dust tails (~108 km long). Always points away from Sun.

slide20

Comets

Comet Halley

(c. 1986)

Comet Hale-Bopp

(c. 1997)

slide21

Comets

Comet Halley

(c. 1986)

Comet Hale-Bopp

(c. 1997)

Tail

Coma

Nucleus

slide22

Comet Hale-Bopp (1997)

dust tail = white, ion tail = blue

slide23

Closeup --

nucleus of comet Halley seen by Giotto

spacecraft

slide24

Meteor Showers

Comets slowly break up when near Sun, due to Solar radiation, wind and tidal force.

e.g. Halley loses 10 tons/sec when near Sun. Will be destroyed in 40,000 years.

Fragmentation ofComet LINEAR

 Debris spreads out along comet orbit.

IF Earth's orbit crosses comet orbit, get meteor shower, as fragments burn up in atmosphere.

slide25

Other stuff ...

  • Kuiper Belt Objects
  • Group of icy, small (asteroid-size) objects beyond orbit of Neptune
  • Pluto now officially considered a Kuiper Belt Object
  • Origin of Short-period comets
  • Oort Cloud
  • “reservoir” of icy, inactive comets in far outer system
  • most don’t orbit in or near the ecliptic
  • origin of Long-period comets
slide26

Kuiper Belt Objects (KBOs)

  • From Neptune’s orbit (30 AU)
  • to about 50 AU
  • Discovered 1992
  • Small bodies
  • Mostly frozen “volatiles”
  • water, CO2, CH4, etc.
  • About 200 x mass of main
    • asteroid belt
    • Known objects: ~1000
    • Estimated: ~ 70,000
  • Several larger objects known

Green dots = KBOs 

Scale is in AU

slide27

Oort Cloud is a postulated huge, roughly spherical reservoir of comets surrounding the Solar System. ~108 objects? Ejected planetesimals.

A passing star may dislodge Oort cloud objects, plunging them into Solar System, where they become long-period comets.

If a Kuiper Belt object's orbit takes it close to, e.g., Neptune, its orbit may be changed and it may plunge towards the inner Solar System and become a short-period comet.

slide28

Meteors

  • Meteor = streak of light
  • Meteoroid = body (in space) that causes it
  • Meteorite = fragment that makes it to the ground
  • Solar system debris
  • Some meteor showers associated with comets
    • Swarm of debris results in repeated meteor shower
  • Dust grains and very small solids
  • Larger ones are probably from asteroids
  • (possibly debris from broken-up asteroids / collisions)
  • Meteoroid types: rocky or metallic (iron-nickel)
slide29

Meteor Showers

Comets slowly break up when near Sun, due to Solar radiation, wind and tidal force.

e.g. Halley loses 10 tons/sec when near Sun. Will be destroyed in 40,000 years.

Fragmentation ofComet LINEAR

 Debris spreads out along comet orbit.

IF Earth's orbit crosses comet orbit, get meteor shower, as fragments burn up in atmosphere.

slide30

Russian Meteor 2/15/13

  • How big? About 50 – 55 ft diameter, preliminary est.
    • How massive? Using density of known meteorites found on the ground (stony meteorites)
    • mass would have been ~ 10000 tons
  • How fast? Estimated based on images from spacecraft,
  • and consistent with speed it must have had to be in
  • heliocentric, but not Earth orbit: ~ 40,000 MPH
  • Kinetic energy = ½ mass x velocity2
  • = about 1015 Joules = 300+ ktons/TNT
  • = equivalent to ~ 20 – 30 “Fat Man” atomic bombs
slide33

Impact Craters

Chicxulub crater

Yucatan

slide34

Chicxulub crater

Yucatan

Image: NASA

slide35

Comet Shoemaker-Levy, 1994

Image: NASA,

HST, May 1994

slide37

Crater chain on Europa

(from a similar event)

slide39

Artist’s conception:

one fragment (G) if it had hit Earth

Image: John Spencer's Astronomical Visualizations.

slide40

Astroblemes – some known impact sites

Image: Steven Dutch, U. Wisconsin – Green Bay

slide41

Panspermia

Did life originate in space?

slide42

Asteroid Mining ? -- maybe

Finding

Prospecting

Images: Planetary Resources

http://www.planetaryresources.com

Mining

slide43

How did the Solar System Form?

We weren't there. We need a good theory. Check it against other forming solar systems. What must it explain?

- Solar system is very flat.

- Almost all moons and planets orbit and spin in the same direction. Orbits nearly circular.

- Planets are isolated in space.

- Terrestrial - Jovian planet distinction.

- Leftover junk (comets and asteroids).

Not the details and oddities – such as Venus’ and Uranus’ retrograde spin.

slide44

Early Ideas

René Descartes (1596 -1650) nebular theory:

Solar system formed out of a "whirlpool" in a "universal fluid". Planets formed out of eddies in the fluid.

Sun formed at center.

Planets in cooler regions.

Cloud called "Solar Nebula".

This is pre-Newton and modern science. But basic idea correct, and the theory evolved as science advanced, as we'll see.

slide45

A cloud of interstellar gas

a few light-years,

or about 1000

times bigger than

Solar System

The associated dust blocks starlight. Composition mostly H, He.

Too cold for optical emission but some radio spectral lines from molecules. Doppler shifts of lines indicate clouds rotate at a few km/s.

Some clumps within clouds collapse under their own weight to form stars or clusters of stars. Clumps spin at about 1 km/s.

slide46

But why is Solar System flat, and why do planets orbit faster than 1 km/s?

Pierre Laplace (1749 - 1827): an important factor is "conservation of angular momentum":

When a rotating object contracts, it speeds up.

mass x velocity

mass x velocity x "size"

"momentum"

"angular momentum" (a property of a spinning or orbiting object)

Well demonstrated by ice skaters . . .

slide47

So, as nebula contracted it rotated faster.

Could not remain spherical! Faster rotation tended to fling matter outwards, so it could only collapse along rotation axis => it became a flattened disk, like a pizza crust.

Hubble is seeing these now!

slide48

Now to make the planets . . .

Solar Nebula: 98% of mass was gas (H, He)

2% in dust grains (Fe, C, Si . . .)

Condensation theory:

1) Dust grains act as "condensation nuclei": gas atoms stick to them => growth of first clumps of matter.

2) Accretion: Clumps collide and stick => larger clumps. Eventually, small-moon sized objects: "planetesimals".

3) Gravity-enhanced accretion: objects now have significant gravity. Mutual attraction accelerates accretion. Bigger objects grow faster => a few planet-sized objects.

slide49

initial gas and dust nebula

dust grains grow by accreting gas, colliding and sticking

continued growth of clumps of matter, producing planetesimals

planetesimals collide and stick, enhanced by their gravity

Hubble observation of disk around young star with ring structure. Unseen planet sweeping out gap?

a few large planets

result

slide50

Terrestrial - Jovian Distinction

Outer parts of disk cooler: ices form (but still much gas), also ice "mantles" on dust grains => much solid material for accretion => larger planetesimals => more gravity => even more growth.

Jovian solid cores ~ 10-15 MEarth . Strong gravity => swept up and retained large gas envelopes of mostly H, He.

Inner parts hotter (due to forming Sun): mostly gas. Accretion of gas atoms onto dust grains relatively inefficient.

Composition of Terrestrial planets reflects that of initial dust –

not representative of Solar System, or Milky Way, or Universe.

slide51

Asteroid Belt

Perhaps a planet was going to form there. But Jupiter's strong gravity disrupted the planetesimals' orbits, ejecting them out of Solar System. The Belt is the few left behind.

And Finally . . .

Remaining gas swept out by Solar Wind.

slide52

Result from computer simulation of planet growth

Shows growth of terrestrial planets. If Jupiter's gravity not included, fifth terrestrial planet forms in Asteroid Belt. If Jupiter included, orbits of planetesimals there are disrupted. Almost all ejected from Solar System.

Simulations also suggest a few Mars-size objects formed in Asteroid Belt. Their gravity modified orbits of other planetesimals, before they too were ejected by Jupiter's gravity.

Asteroid Ida