The Solar System

Pat McCracken, NASA Why does the shadow point at the moon

Why 1960?

• Systematic observations (needed writing, Babylon 1500 BC)
• Model for the universe (needed observations, Ptolemy 100 AD)
• Collecting light (needed clear glass, telescope, Galileo, 1605 AD)
• Fundamental Physics (Needed calculus, Newton, 1666 )
• Saving light (needed chemistry, photography, 1860)
• Escaping the atmosphere (needed satellites, 1958)
• Synthesising the ideas (needed computers, 1960)
• Extending the spectrum (needed radar: Jansky radio-telescopes, 1955)
• Need to say a little more about the last:

Electromagnetic Spectrum

99.99% of our information about the universe comes as EM radiation

We see the spectrum from red to violet. The colour actually indicates the wavelength of the light (note we have to think about pure spectral colours: i.e. green is not a mixture of yellow and blue!)

• But light is part of the whole electromagnetic spectrum: wavelengths we can detect go from .000000000000000001 m to 10000m

• We "see" only one octave. Compare to sound, we hear (well, some of us!) 10 octaves (20Hz to 20000 Hz: middle C is at 252 Hz.)

• Why do we see so little of the spectrum? Answer lies in the transparency of the atmosphere. THis ignores extra problems like clouds and "twinkle".

Barbecues and Stars

• We know from barbecuing that colour of coals tells you how hot they are

  Gellert Ament/Istock

• Apparent temperature follows optical spectrum : i.e. the hotter the object, the "bluer" its colour

Gamma-rays: can use the atmosphere as a detector

HESS Gamma-Ray Telescope Credit: The HESS Collaboration

X-rays: must get above atmosphere (Einstein, Chandra, XMM satellites)

XMM Launched Drawing Credit: D. Ducros, XMM Team, ESA

Optical: High observatories (to avoid twinkle)

The VLT Interferometric Array Credit & Copyright: European Southern Observatory

Radio: Large dishes, many hooked together (Arecibo, VLA)

A Very Large Array of Radio Telescopes Credit: NRAO, NSF

Hubble

and above all, the Hubble (HST) which sees in the UV and IR and is above everything!

The Orbiting Hubble Space Telscope Credit: STS-103, STScI, ESA, NASA

Note that the effect of manned (or womanned) exploration has been miniscule (the Moon is a very dull place!)

So what has this brought us?

Mercury

The surface of mercury Similar to the moon but no mountains many scarps (probably formed by cooling of the planet) Very hot on sunward side (~500oC) Very cold on shadowed side (~-200 oC) No atmosphere

Degas crater

Orbital period of 88 days. Rotational period ∼ 56 days (Long thought to be ∼ 88 days: In fact, it is 2/3 of the orbital period).

Venus

Popular with writers: e.g C. S Lewis So does it look like this?

Almost featureless in optical. Usual picture is UV (upper) or infrared (lower) and only shows cloudtops. Venera, pioneer and radar showed surface for first time Year = 225 days. Rotation (i.e. 1 venus day = 243 days Retrograde (so sun "rises" in the west: unknown till 1961) Atmosphere very dense (pressure ∼ 100 × earth at surface). Mainly CO2.

Upper clouds rotate in 4 days (∼360 km hr-1) At surface, gentle winds, but temperature∼900 °C, Surface rocks basaltic, appear to be young

from Jim Imamura: http://zebu.uoregon.edu/~imamura/121/lecture-11/lecture-11.html

Radar maps show rough terrain as bright

Sapas Mons, a volcano 400 km across and 1.5 km high is on the western edge of Alto Regio. Note the lava flows extend for hundreds of km.

Mars

Very poular with writers: Bradbury did it best ("Sands of Mars") Lowell observed canals

Atmosphere: pressure ∼ 0.005 bar 95% CO2, rest O2, N2, Ar + very little H2O Temperature range 210 K->310 K

Two small, close, irregularly shaped moons. Phobos has very large impact crater.
Deimos Moons are probably captured asteroids.

VOLCANOES: Olympus mons: 25 km high, evidence of lava flows. Much larger than equivalent ones on earth (why?)
Candor Chasma Massive rift valley.

Many craters, at various stages of newness

The interesting problem: Does Mars have water? Sometimes it looks just as though it once did

This is the Newton crater

and what really look like arroyos in New Mexico

and "holes":deep caves where water could still exist. SO probably there was a lot of water, coiuld still be some underground

Hot off the press

Methane: tends to be created by living things (e.g. microbes ....)

NASA

Jupiter

Largest planet by far. Strongly banded appearance, corresponding to convective regions in atmosphere. Dark areas (bands) lie lower in atmosphere than light areas (zones).

Colours probably due to complex organic molecules: detected so far are: CH4 (methane), NH3 (ammonia), H2 (molecular hydrogen), C2H2 (acetylene), C2H5, PH3, H2O, G2H4 (germane), CO, HCN, H2S, top layer mainly ammonia.

Great Red Spot

: noted since 1600's ∼ 20,000 km × 50,000 km. Top of spot extends well above surrounding cloud tops. Note downstream eddies. Colour probably from organic molecules stirred up from below.

Speeds of rotation ∼ 500 km/hr Now clearly seen to be "hurricane" (lifetime not too surprising: 1000 × bigger than terrestial hurricanes, so lifetime could well be 1000 × longer!)

Cassini At Jupiter: Red Spot Movie Credit: CICLOPS, NASA, JPL, University of Arizona

Moons of Jupiter: Io

Jupiter has some of the oddest moons in the solar system. Four large easily visible with binoculars Can watch Io rotating
Io is in a state of continuous volcanic eruption: Volcanoes:Plumes to 250 km Vulcanism caused by "tidal pumping" by other moons.

Note surface is very unstable: no craters (age ∼ 106 yrs). "pimple" in centre is volcano seen frolm above

Europa:

Rock covered with ice, probably slushy since no impact craters.

Close-ups show odd crustal structures

Ganymede

Largest moon in the solar system: Ice on rock. Many craters, but with central pits, not peaks. Huge transverse faults

Saturn

Day 10 hr 14 mins. ATMOSPHERE similar to Jupiter, but less heating (internal &sun) so weather better

RINGS

First seen by Galileo as "Handles" Assumed to be solid, but Maxwell showed that tidal forces would have destroyed them... Spectrum consistent with small ice pellets and dust (moonlets).Voyager showed many thousands of ringlets, some rings elliptical F-ring very narrow & braided rings very thin (< 2 km) kept from dispersing by "shepherd" moons

Many Moons: Titan, Mimas, Tethys, Janus, and Enceladus.

Credit: Erich Karkoschka (University of Arizona Lunar & Planetary Lab) and NASA

• but the moons are weirdly different

• JANUS is two moons in orbit round each other.

Titan

Credits: ESA/NASA/JPL/University of Arizona

Touch down at 4.5 m/s, the saucer-shaped probe penetrated 15 cm. Surface consistency of wet sand or clay.

Titan Landscape Credit: ESA, NASA, Descent Imager/Spectral Radiometer Team (LPL)

Hyperion

Density about 1/2 water (!) suggests spongy texture!

Credit: Cassini Imaging Team, SSI, JPL, ESA, NASA

Iapetus

Half of moon is covered in material as black as coal

Enceladus

Giant striped snowball?

Hot (well actually very cold!) off the Press

Uranus

Pale green. Uranus rotates on side. Note no bands, deep clear atmosphere.

Ariel from a distance of 170,000 kilometers.

Miranda from 42,000 kilometers.

Ring system, probably 9 narrow dark rings (seen by occulting star)

THis is how it might look from Ariel

Neptune

Pale blue-green Large dark spot

1 major moon, Triton has an atmosphere, and a retrograde orbit (captured asteroid?). Other smaller moons. Appearance similar to outer moons of Jupiter: i.e. ice-covered rock.

Pluto

Pluto Originally found in a search for 9th planet, based on prediction due to the perturbations of Neptune's orbit. Sometimes closer than Neptune (till 1999!)

> Seen to be double planet: Pluto-Charon

Asteroids

Paths of the earth (green), Mars (red square) Jupiter (yellow) twelve brightest asteroids

Most asteroid orbits lie in plane of solar system, a few are very tilted. Most lie between Mars and Jupiter, maybe more beyond Jupiter (Centaur asteroids). two Centaur Asteroids (outer Chiron and 1991DA). Orbits clearly imply that they were never part of a single object which exploded

Gaspara

• and Ida and its moon, Dactyl.

Eros

Eros is a lump of rock

We have managed to land on it

the fourth object (after the Moon, Mars and Venus) in the Universe! (but a bit too hard!)

Meteors:

These are the Leonids

• and here is one at sunrise

  ---

  Can now see meteorites from the other side. These are Leonids seen from the space shuttle!

e.g Earth: this is the Manicougan crater in Northern Quebec: Note this is so large that it can only be seen well from space. ∼ 200 Million years old.

What killed the dinosaurs?

Alan Hildebrand (GSC for a while, now Calgary) found a 65 million year old, 112 mile wide ring structure (the Chicxulub crater) still detectable under layers of sediment of the Yucatan Peninsula region of Mexico. Crater is consistent with the impact of an object of sufficient size (6 to 12 miles wide) to cause the global disruptions.

Apollo asteroids: cross earth's orbit (and may collide occasionally!) Eros used to measure Earth-Sun distance very accurately All small < 30 km radius

Toutatis came within 1500000 km (4x distance of moon) in 2002. Was it really close? Expect major collision ~ 100000 years

Comets

The Hairy Stars: this is Arend-Roland

• "Dirty Snowballs" H2O + dust + CO2 Water boiling causes breakup? Some comets observed to have jets, which can alter orbits Low mass, cannot retain gas (this is Mrkos)

Breakup need not be just material being blown out behind: previous encounter can give rise to material in same orbit: this is Shumacher-Levy
e.g. Arend-Roland

Meteors are assumed to originate in comets which break up. e.g. the stuff left behind by Swift-Tuttle annually create the Perseids this is particularly bright every 33 years (tho. less so now). Perseid meteor in an aurora:

Credit <& Copyright: Jimmy Westlake, Colorado Mountain College)

This is comet LINEAR,

which was going to be quite spectacular

However it didn't work out that way!

Then it shattered into pieces!

• and more pieces

• and finally it has just evaporated!

Best close-up of a comet is of Wild: note the "outgassing". Diam is about 5 km

Going Wild Credit: Stardust Team, JPL, NASA

Most of the comets discovered with SOHO are "sungrazers". With the heat and tidal forces, they aren't going to survive their close passage.

and here is another one that isn't going to make it!

Where do comets come from?

Most have very long periods: Great Comet of 1858 had period of over 100000 years, very few have many repeats, (Halley's comet at 76 years and ∼30 repeats is an exception) Oort cloud 105 A.U. from sun Perturbations by passing stars can start comets falling towards sun. No angular momentum, so must start from rest.

(Parenthetical question: why are all comets about the same size: core of 10 km or so? Don't know!)

Quaoar is (almost) the most distant known object in the solar system

Sedna

No easy answer: conventionally we take original 8 as planets, and say everything else is not (i.e. Pluto isn't).

Cosmogony: Origins of the Solar System

Executive Summary of Observations

• Age ∼ 4.5 x 10⁹ years
• All planets lie in almost the same plane, and directions of rotation are same(except Venus).
  
• Spin of sun lies in ecliptic, all planets (except Uranus) close to it.
  
• Orbit shapes are nearly circular (little eccentricity)
  
• System repeated as
  • Sun => Planets and Asteroids,
  • Planet => Moons and Rings
• Comets and meteors have very eccentric orbits, at random angles ("inclinations")to the ecliptic.
  
• Most of mass the of solar system is in the sun. Most of the angular momentum of the solar system in the planets.
  
• Inner planets are terrestrial, outer planets are gas giants (jovian).
  
• Inner planets are closely spaced, outer planets are widely spaced.

The old theories:

Close encounter of a star with the sun pulls out a tongue of material

• Companion star explodes, material captured by sun condenses into planets

• Both dynamically impossible. No angular momentum!

Nebular Hypothesis. A rotating gas cloud, probably compressed by a nearby supernova shock wave, starts to collapse. The central part collapses to the sun.

• Conservation of angular momentum causes the outer part to speed up.

• The outer planets condense first. Gas and dust particles moving at an angle to ecliptic are more likely to interact and hence collapse into the plane normal to the rotation axis. Majority rule (less effective at the limits, hence Mercury & Pluto orbit further out of the plane.)

Intense solar winds remove hydrogen and helium from the inner part of the solar system. Terrestrial planets form from the left over refractory materials.

The orbits are circularized by collisions and tidal effects.

e.g. υ (say Upsilon) Andromeda

Where is Upsilon Andromedae? Credit and Copyright: Till Credner & Sven Kohle

has 3 planets, one very close

(New York Times)

• so it might look like this

  Drawing Credit & Copyright: Lynette Cook

• Total planets discovered:329
• Total planetary systems: 150

(as of today!). Special conditions required for the discoveries (can only see large planets, fairly close and orbit must lie close to our line of sight) suggest they are very common: probably 50% of stars have them.