Astronomy 121, Winter 2005

Exam 2 Review

Feb 3, 2005, jeb PRELIMINARY

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TOPICS FOR EXAM Venus Mars Jupiter Saturn

> Basic Properties of Venus, Mars, Jupiter, and Saturn Venus Mars Jupiter Saturn Discovery of Jupiter and Saturn Space Probes to Jupiter and Saturn Common Properties of Jupiter and Saturn Atmospheres of Jupiter and Saturn Rings of Jupiter and Saturn Magnetosphere and Internal Structure of Jupiter and Saturn Moons of Jupiter and Saturn

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Basic Properties of Venus, Mars, Jupiter, and Saturn

Planet order from Sun Orbital period Semi-major axis Mass Radius Density Surface Gravity Escape speed Rotational Period Inclination ..(axis tilt) Surface Magnetic Field Surface Temperature Number of Moons

Venus 2 0.62 years (225 Earth days) 0.72 AU 0.82 x Earth 0.95 x Earth 5240 kg/m3 0.91 x Earth 10.4 km/s -243 days 177.4° less than 0.001 x Earth 730 K 0

Mars 4 1.88 years (687 days) 1.52 AU 0.11 x Earth 0.53 x Earth 3930 kg/m3 0.38 x Earth 5.0 km/s 1.026 days 23.98° 1/800 x Earth about 150-310 K 2

Jupiter 5 11.9 years 5.2 AU 318 x Earth (0.1% of S.S.) 11.2 x Earth 1300 kg/m3 2.5 x Earth 59.5 km/s 0.41 days 3° 14 x Earth 124 K 61 and counting

Saturn 6 29.4 years 9.5 AU 95 x Earth 9.4 x Earth 690 kg/m3 1.1 x Earth 36 km/s 0.44 days 27° 0.67 x Earth 97 K 31 and counting

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Venus

Earth's Sister Planet

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lecture on Venus

Observing Venus from Earth

• Venus never separates from the Sun by more than 47^o, as viewed from the Earth
  • appears above the horizon
    • before sunrise and after sunset.
• Extremely bright
  • (10 x brightest star) (reflectivity = 70%)
  • 3rd brightest object in the sky, next to the Sun and the Moon

Venus - General Properties

• 2nd Planet from the Sun
• semi-major axis ("radius") = 0.72 AU
  • period = 225 days
• Radius = 6052 km = 0.95 R_Earth
• Mass = 4.9 x 10²⁴ kg = 0.82 M_Earth
• Density = 5240 kg/m³
• (Earth-like or "sister planet")
• Rotational Period = 243 days ( retrograde)
  • Why retrograde -> collision?
• Atmosphere
  • very dense => high altitude clouds of sulfuric acid block surface
• 90x as massive as the Earth's atmosphere
  • 96.5% CO₂ (by volume)
  • few % Nitrogen (very little water)
• surface temperature = 750K (T_Earth=300K, T_Mercury=700K)
• Greenhouse Effect
  • temperature uniformity - poles, equator, day, night

Venus' atmosphere is different from Earth's?

• On Earth, CO₂ is absorbed in the oceans and rocks.

Summary of Atmospheric Components

(percentages by volume)

CO2 Nitrogen Water Oxygen

Earth absorbed in rock free in atmosphere (78%) mostly condensed on surface product of life (21%)

Venus free in atmosphere (96.5%) free in atmosphere (3.5%) decomposed long ago, and hydrogen escaped no life to produce it

Evolution of Venus' Atmosphere

• During secondary atmosphere formation temperature of Venus high
  • no oceans form (Water is broken down and H escapes) and gases are not absorbed
  • gases in atmosphere trap heat (greenhouse effect)
  • temperature rises even more
  • runaway greenhouse effect

Venus' Surface

• A couple of continental sized features
  • Ishtar Terra (southern high latitudes)
  • Aphrodite Terra (near the equator),
  • with Earth-like mountains
• No oceans
  • rolling plains (65%)
  • lowlands (27%)
  • modest highlands (8%) (continents)
• No plate tectonic activity
• Lava flows -> volcanic origin
• Craters - mostly of volcanic origin
• The surface of Venus is rather young (less than 1 billion years) due to volcanic activity

Volcanism today?

Venus' Magnetic Field and Internal Structure

• Magnetic field - none detectable
  • this is consistent with slow rotation period of 243 days
• Model of internal structure is based on little hard data
  • Surface resembles surface of young Earth (at an age of about 1 billion years)
    • thin crust, volcanic activity,
    • internal convective processes not yet established
    • not yet developed plate tectonics
  • Why no development of plates?
    • Cooling slowly due to high surface temperature
      • crust remains too soft for Earth-style plates to develop
    • absence of water may also be important
      • no lubricating mechanism

Mars

A Near Miss for Life?

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lecture on Mars

General Properties of Mars

• Mars is the 4th Planet from the Sun
  
• semi-major axis ("radius") = 1.52 AU
• closest opposition every 17 years (in August/September) -> brightest
• orbital period = 687 days (1.88 years)
• Radius = 0.53 R_earth
• Mass = 0.11 M_earth
• => Density = 3900 kg/m³
• Rotational Period = 24.6 hours
• Inclination of 24.0° (23.5° for Earth)
  • tilt causes seasons

History of Observations from Earth

• 1877 - Mars is unusually close to Earth
  • Italy: Giovanni Schiaparelli
    • Observed network of linear markings (canali)
  • Percival Lowell, in US
    • Motivated by report of "canals", gives up successful Boston business to build observatory in Flagstaff, Arizona
    • devoted life to studying "canals" and Martian intelligent society

Space Probes to Mars

• Mariner 4 (1965)
  • First spacecraft to reach Mars
• Viking 1 and Viking 2 (1976)
  • landed on Mars and conducted biological experiments to search for evidence of life
• Pathfinder (& Sojouner, landed July 4, 1997)
  • landed at mouth of outflow channel
  • rocks deposited by flood water
• Mars Global Surveyer
  • launched November, 1996
  • start mapping, March, 1999
• Mars Exploration Rover Mission
  • Spirit landed on January 3, 2004, in a rocky landscape
  • Opportunity found rocks that had been very wet in the past, after landing on January 23, 2004.
  • Each continued to explore and send data back after one year, and having traveled about 4 (Spirit) and 2 (Opportunity) kilometers.

• Major results of these missions
  1. the surface of Mars contains much more iron that the surface of the Earth
     • less differentiation on Mars
  2. no evidence of life

Mars' Atmosphere

• Mars has a very thin atmosphere
  • (1/150 of Earth's atmosphere)
  • 95.3% CO₂ (by volume)
  • surface temperature:
    • noon, summertime 300K
    • night-time - 200K (much colder than on Earth)
      • (T_earth=300K)
• => dust storms (occasionally months-long)
• Composition of Mars' atmosphere similar to Venus, yet 10,000 times thinner
  • -> reverse greenhouse effect

Mars' Surface

• Huge volcanoes:
  • Olympus Mons (side view)
    • About 700 kilometers in diameter
      • would cover all of Washington and half of Oregon
    • Rises 25 kilometers above the surrounding plains
      • (82,000 feet)
    • (largest volcano in the Solar System)
  • Mariner Valley
    • crack in the surface of the planet
    • huge canyon running along the equator for about 4000 km -about 1/5 the circumference
• Vast dune fields:
• North-south asymmetry:
  • Northern Hemisphere: lava plains -> younger surface
  • Southern Hemisphere: heavily cratered highlands -> older surface
• THE MAJOR FEATURE:
  • Tharsis bulge
    • size of North America, near equator
    • rises 10 km above rest of surface
• Impact cratering:
  • wide spread cratering,
  • eroded by the atmosphere
  • appearance of craters (liquid splash) suggest permafrost (water ice) just below the surface
• Running water in the past:
  • the best evidence that water once existed in great quantities on the surface is the channels
    • runnoff channels: "http://physics.uoregon.edu/~jimbrau/astr121/Notes/Mars/runoff2.gif">runnoff channels:
      • southern highlands
      • 4 billions years ago
      • - thick atmosphere & surface water
    • outflow channels:(another view)
      • near equator
      • from south to north
      • enormous flow rates> 100xAmazon
      • about 3 billion years ago
  • No water today on the surface
    • a large amount of this water may now be locked in the polar caps and under the surface (permafrost)
• Polar Caps
  • north and south
  • primarily dry ice (CO₂)
  • seasonal variation
  • temperature remains below 150K

Mars' Magnetic Field and Internal Structure

• Mars' has a weak magnetic field
  • => about 1/800 of Earth
  • (but rotation period is similar to Earth 24.6 hours)
    • so -> core is non-magnetic, or has no liquid, or both
• planet probably never differentiated much
  • - large abundance of surface iron

Moons of Mars

• Two very small moons:
  • Phobos (fear) & Deimos (panic)
    • Irregularly shaped and cratered
    • Orbits at about 3 and 7 planetary radii
      • (Earth's Moon orbits at about 60 radii)
    • Both orbit synchronously
      • tidally locked, like the Moon
    • Both orbit prograde direction
    • Large (Stickney) Crater on Phobos
• Captured asteroids?
  • low density like typical asteroid

Martian Asteroids and the Evidence for Life on Mars

• About a dozen meteorites from the surface of Mars have been found on Earth
  • (identified by gases trapped in rocks)
• 1984 - scientists discoverd a rock ( ALH84001) on Antarctic ice field
• 1992 - detailed study of the rock - found (within a small region of the rock)
  1. organic molecules that might be associated with life
  2. several minerals characteristic of biological activity
  3. tiny tubular and egg-shaped structures resembling ancient single-celled bacteria found on Earth
• These observations were announced August 7, 1996
• This evidence for life on Mars has NOT been accepted as FACT
  • This evidence could all have been produced by chemical reactions without any kind of life
  • Could ALH84001 have been contaminated? (it sat on ice in Antarctica for 13,000 years
  • More evidence is need to resolve this
• NASA now plans a series of visits to Mars over the next decade to attempt to understand whether life did or did not exist on Mars in the past

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lecture on Jupiter

• Jupiter is the third brightest object in the night sky (after the Moon and Venus)
  
  
• Differential rotation
  • period of rotation at equator shorter than at higher latitudes
• Equatorial bulge
  • due to rapid rotation
• Prominent visual features
  • Bands: belts and zones
  • Great Red Spot
  • four prominent moons
    • Discovered by Galileo in 1609
• It takes over half an hour for light (or radio signals from a spacecraft) to get from Jupiter to the Earth
• Jupiter as a "failed star"
  • If Jupiter's mass were 80 times larger, the gravitational collapse would have ignited its hydrogen into thermonuclear fusion, creating a second star in the Solar System

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lecture on Saturn

• Not nearly as bright as Jupiter in the Earth's sky
  
• Due to its low density, it would float on water
• Total mass is about 1/3 of Jupiter's mass

Notable Characteristics:

Surface

• Featureless yellowish and tan cloud belts
• No prominent spots as seen on Jupiter, but faint, long-lived ovals are seen

Spectacular ring system

• Due to tilt of rotation axis (27^o) Saturn's appearance on Earth changes every few years.
  • Even appearing edge-on, as in this Hubble Space Telescope image of 1995, in which the rings nearly disappear, indicating they are very thin

Moons:

• Titan (largest, only large moon)
• 18 major moons named to date
• 13 smaller moons mentioned in textbook
  
  number of known moons around the Jovian planets is growing
  The count of Saturn's moons has reached 35

• Jupiter and Saturn have been known of since ancient time since they are bright in the night sky

• Pioneer 10 - (launched by USA, March 3, 1972)
  • flew by Jupiter on December 1, 1973
  • Carrying a gold, anodized plaque intended to communicate with extraterrestrials
• Pioneer 11 - (launched by USA, April 6, 1973)
  • flew by Jupiter on December 1, 1974
  • flew by Saturn in 1979
• Voyager 1 - (launched by USA, September 5, 1977)
  • Voyager 1 flew by Jupiter on March 5, 1979
  • flew by Saturn on Nov 13, 1980
• Voyager 2 - (launched by USA, August 20, 1977)
  Voyager 2 flew by Jupiter on July 9, 1979
  • flew by Saturn on Aug 27, 1981
  • flew by Uranus on Jan 30, 1986
  • flew by Neptune on Aug 15, 1989
• Galileo - (launched by USA and Europe, October 18, 1989)
  Galileo was designed to study Jupiter's atmosphere, satellites and surrounding magnetosphere for 2 years.
  • During the journey, Galileo made encounters with asteroids
    • Gaspra on October 29, 1991
    • Ida and moon Dactyl on August 28, 1993.
  • During voyage, had direct view of the Comet Shoemaker-Levy impacts on Jupiter (July, 1994)
  • Arrived at Jupiter, December, 1995
  • Significantly advanced our knowledge of the Galilean moons
• Cassini Mission to Saturn and its Moon Titan
  • Launch: October, 1997
  • Arrived at Saturn: 2004
  • Transmissions from Saturn: 2004-2008
  • includes Huygens Titan probe
    • Huygens landed on Titan on January 14, 2005
    • Images came back for analysis
  Mission: To explore the most distant planet that can easily be seen by the unaided human eye.
  • Saturn's interesting atmosphere and interior
  • the most spectacular of the four planetary ring systems
  • numerous icy satellites with a variety of unique surface features
  • a huge magnetosphere teeming with particles that interact with the rings and moons
  • the intriguing moon Titan, slightly larger than the planet Mercury with a hazy atmosphere denser than that of Earth.
  Many revealing photos have been taken
  Where is Cassini now?

• Atmospheres of Jupiter and Saturn
• Rings of Jupiter and Saturn
• Magnetosphere and Internal Structure of Jupiter and Saturn
• Moons of Jupiter and Saturn

Elements:
• 86.1% hydrogen {by number}
  
• 13.8% helium {by number}
  
• little of the early atmosphere escaped due to the strong gravity (large escape velocity)
  

Zones and belts appear as we see different depths of the atmosphere
• Zones(light warm, rising regions)
• Belts (dark ; cool, falling regions)

Great Red Spot
• Large hurricane in the south
  • about 2 times the size of Earth
  • at least 300 years old
• revolves with inner region of Jupiter
• rotates counterclockwise

Other storm related features
• gigantic lightning flashes and Auroral activity
• White ovals and Brown ovals
  • persist for many years or decades

• 92.4% molecular hydrogen {by number}
• 7.4% helium {by number}
  • missing helium (compare to Jupiter)
  • sank toward center (differentiation)?
• little of the early atmosphere escaped due to the strong gravity (large escape velocity)
  

• Three layers
• Thicker than Jupiter - blocking view into different layers
  • explanation for bland appearance of Saturn's surface

• As on Jupiter (east to west dominates) although much faster flow rates
  • more than 10 times hurricane speeds on Earth
• Large scale flows and small-scale storms powered by
  • convection in interior
  • rapid rotation

• fewer hurricane like storms than on Jupiter

• The rings are Saturn's most notable feature
  
  

• Rings have been labeled in order of discovery

  from the inside out D,C,B,A,F,E...

  Only C, B, and A are dense enough to be easily visible

• Table of Rings

• Structure of Rings

  Large collection of icy particles

  • ranging in size from fraction of millimeter to tens of meters
  • Very thin structure

• The Roche Limit

  Tidal forces will create a tidal bulge

  • At some close point to the planet the tidal forces will overcome the internal strength of the object, ripping it apart.
  • Occurs at about 2.4 times the planet's radius
  • E ring is outside Roche Limit (but not very dense)

• Cassini Division

  dark band about 2/3 out from inner edge discovered by Cassini in 1675

  not a void, just lower density (see width in table)

• Orbital Resonances

  Cassini Division due to 2:1 resonance with Mimas (the first medium-sized moon) Other resonances:

  • Outer edge of A ring -> 3:2 resonance with Mimas (Atlas)
  • others

• Shepherd Satellites

  F Ring - narrow and braid-like appearance

  Two small satellites (Prometheus and Pandora) orbit on either side

  • Shepherding the F ring
  • these moons confine the narrow F ring

  
  
• Ringlets (thousands) resolved by Voyager 2

Jupiter's Ring:

• Faint ring discovered by Voyager I
  Main ring lies above cloud layer and inside orbit of innermost moon
• Galileo spacecraft has explored the ring, and concluded that the ring system ( main ring + gossamer rings) has been formed by the dust kicked up when interplanetary meteoroids smash into the inner moons
  

• All jovian worlds have rings
• Rings are relatively short-lived (millions of years)
• Therefore: formation is a common event
  • otherwise we would not see rings on all 4 planets
• Local environment important:
  • Moons, catastrophic events, etc.

Jupiter's Internal Structure

• ---> internal heat built up during Jupiter's formation is slowly leaking out
  • gravitational energy

• gaseous hydrogen
• liquid hydrogen
• liquid metallic hydrogen (generates large magnetic field)
• rocky core (15 x's Earth's mass)

Jupiter's Magnetosphere

• surface magnetic field 14 times Earth's surface field
• field oriented opposite to Earth's

Saturn's Internal Structure

• Helium precipitation
• (Saturn in process of differentiating)

• Saturn gets denser and denser towards its interior)
  • liquid molecular hydrogen
  • liquid metallic hydrogen (generates large magnetic field)
  • rocky core (about 20 x's Earth's mass)

Saturn's Magnetosphere

• Intrinsic strength of magnetic field about 1,000 times Earth
    (about 1/20 of Jupiter)
  
• Saturn's surface magnetic field is roughly comparable to the Earth's (2/3 x Earth's)
• Field direction reversed relative to the Earth (just as with Jupiter)

61 known moons

16 major moons of greater than 10 km diameter
Four sets of four or five moons each
Tidally locked

1.) Inner moons (inside Galilean) = 4 small moons

2.) Galilean moons ( 4 moons)
• discovered by Galileo in 1609
• largest, similar in size to Earth's Moon

3., 4.) Outer moons ----> 2 sets of 4 and 5 known moons

• Each set appears to have been a single body,
  captured by Jupiter, then broken into 4 or 5 sections each.
  
Orbital motion
• inner 3 sets are prograde
• outer set is retrograde

Galilean Moons of Jupiter

all rotate synchronously (tidal lock)
Galileo spacecraft has been on a tour of discovery around the Galilean moons

• Most geologically active object in Solar System
  • Voyager's observed erupting volcanoes
• plasma torus produced by Io in Jupiter's magnetosphere
• volcanoes caused by Jupiter's tidal forces + Europa's influence

• surface network of lines :
  • cracks in ice cover?
  • Ocean?

• largest moon in Solar System (larger than Earth's Moon, Mercury or Pluto)
• ---> largely ice

• cratering and concentric rings

Moons of Saturn ( caption)

• Table of major moons

Three groups
• Titan (single large moon)
  • second largest satellite in the solar system
  • atmosphere denser than the Earth
• Medium-sized moons (6)
  • 400-1500 km in diameter
• Small moons (11)
  • irregularly shaped chunks of ice

• Featureless appearance in visible light (upper cloud deck)
• Large-scale features visible in infrared
• second largest moon of Solar System
• Atmosphere denser than Earth
  • Very active chemistry
  • surface covered by uniform smog-like haze
  • about 90% nitrogen
  • few percent methane (the source of the complex chemistry and smog)
• Dense atmosphere results from distance from Sun and cold evolutionary conditions
• Internal structure:
  • rocky core
  • thick mantle of water ice
  • density of 1900 kg/m³
• Below its cloud cover, Titan may be Earth-like in some respects

Mimas, Enceladus, Tethys, Dione, Rhea, Iapetus
• All densities about 1200 kg/m³
• largely rock and water ice, like Titan
• Interesting feature: Herschel crater on Mimas
• Water volcanism on Enceladus

Co-orbital satellites (Janus and Epimetheus)

Share one orbit - exchanging inner and outer paths every 4 years

Two (Telesto and Calypso) share the orbit of Tethys (a medium sized moon) -

Telesto 60 degrees ahead of Tethys and Calypso 60 degrees behind

• Lagrange Points

Helene is tied to Lagrange Point of Dione (medium-sized moon)