Astro 101Fall 2013Lecture 4

T. Howard

The Solar System

Ingredients?

● Planets

● Their Moons, Rings

● Comets

● Asteroids

● Meteoroids

● The Sun

● A lot of nearly empty space

Solar System Perspective

Orbits of Planets

All orbit in same direction.

Most orbit in same plane.

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

Exceptions:

Mercury Pluto (no longer a planet)

orbital tilt 7o orbital tilt 17.2o

eccentricity 0.21 eccentricity 0.25

Sun, Planets and Moon to scale

(Jupiter’s faint rings not shown)

Two Kinds of Planets

"Terrestrial"

Mercury, Venus,Earth, Mars

"Jovian"

Jupiter, Saturn, Uranus, Neptune

Close to the SunSmall

Far from the SunLarge

Few MoonsNo Rings

Main Elements Fe, Si, C, O, N

Mostly RockyHigh Density (3.9 -5.3 g/cm3)

Slow Rotation (1 - 243 days)

Mostly GaseousLow Density (0.7 -1.6 g/cm3)

Many MoonsRings

Main Elements H, He

Fast Rotation (0.41 - 0.72 days)

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

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 o ahead of and behind Jupiter. Some asteroids cross Earth's

orbit. Their orbits were probably disrupted by Jupiter's gravity.

11

4.2 minutes

34.8 min.

Asteroid BeltEarth (1 AU)

Mars (1.5 AU)

Jupiter (5.2 AU)

Solar wind

Kuiper Belt

Oort Cloud

THE OUTER SOLAR SYSTEM

1 1.5 5.2 9.5 19.2 30.1 39.4

.07 .58 1.2 4.02.5 5.3

zodiacal

cloudLight time from earth (hrs)

Orbital radius (AU)

12

CATALOGUED ASTEROIDS (C. 12/98)(VIEW ALONG ECLIPTIC PLANE)

ecliptic

13

CATALOGUED ASTEROIDS (C. 12/98)(VIEW FROM ABOVE ECLIPTIC)

Main belt

Trojan groupsJupiter

Source: Guide 7.0

Asteroid Eros (closeup)

(from the NEAR mission)

asteroid Mathilde (253)

Gaspra Ida and Dactyl

Total mass of Asteroid Belt only 0.0008 MEarth or 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.

• 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

Comets

Solar System DebrisComets

Comet Halley (1986) Comet Hale-Bopp (1997)

Short Period Comets Long 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

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.

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.

Comets

Comet Hale-Bopp(c. 1997)

Comet Halley(c. 1986)

CometsComet Hale-Bopp

(c. 1997)

Comet Halley(c. 1986)

Coma

Tail

Nucleus

Comet Hale-Bopp (1997)dust tail = white, ion tail = blue

Closeup --nucleus of

comet Halley seen by Giotto

spacecraft

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.

Debris spreads out along comet orbit.

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

atmosphere.

Fragmentation of Comet LINEAR

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

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 mainasteroid belt

• Known objects: ~1000• Estimated: ~ 70,000

• Several larger objects known

Green dots = KBOs Scale is in AU

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.

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)

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.

Debris spreads out along comet orbit.

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

atmosphere.

Fragmentation of Comet LINEAR

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

Russian Meteor 2/15/13

Russian Meteor 2/15/13

Impact Craters

Chicxulub craterYucatan

Chicxulub craterYucatan

Image: NASA

Comet Shoemaker-Levy, 1994

Image: NASA,HST, May 1994

Image: NASA – Galileo spacecraft, 7/22/94

Crater chain on Europa(from a similar event)

Image: John Spencer's Astronomical Visualizations.

Artist’s conception: one fragment (G) if it had hit Earth

Image: Steven Dutch, U. Wisconsin – Green Bay

Astroblemes – some known impact sites

Panspermia

Did life originate in space?

Asteroid Mining ? -- maybeFinding

Prospecting

Mining

Images: Planetary Resources

http://www.planetaryresources.com

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.

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.

A cloud of interstellar gas

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

a few light-years,or about 1000

times bigger thanSolar System

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.

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.

"momentum"

"angular momentum" (a property of a

spinning or orbiting object)

mass x velocity

mass x velocity x "size"

Well demonstrated by ice skaters . . .

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!

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.

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

a few large planetsresult

Hubble observation of disk around young star

with ring structure. Unseen planet

sweeping out gap?

Terrestrial - Jovian Distinction

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

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.

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.

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.

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