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Chapter 6
A Survey of the Universe
Alta High Astronomy
Ancient Astronomers
 Had knowledge of the
first 6 planets:
Mercury, Venus, Earth,
Mars, Jupiter, and
Saturn
 Galileo Galilee was the
first astronomer to use
a telescope and
publish his finding. He
did not however invent
it.
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19th Century Astronomers
 By the end of the 19th
century astronomers
had discovered
additional planetary
bodies including
 Uranus and Neptune
 They also discovered
Saturn’s rings,
comets, and many
“minor planets” or
asteroids
 Asteroid belt is
located between
Mars and Jupiter
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20th Century
 The 20th century
brought nonoptical astronomy
to the forefront,
including the use of
radar, and
spacecraft to map
our solar system.
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Comparative Planetology
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To organize all of the data we use
comparative planetology – a
comparison of the different
planets to the chemical and
physical properties of earth.
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The distance of each planet from
the Sun is known from Kepler’s
laws once the scale of the solar
system is set by radar-ranging on
Venus.
A planet’s (sidereal) orbital
period can be measured from
repeated observations of its
location on the sky, so long as
Earth’s own motion around the
Sun is properly taken into
account.
A planet’s radius is found by
measuring its angular size and
then applying elementary
geometry.
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Comparative Planetology
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The masses of planets with moons
may be calculated by application of
Newton’s laws of motion and gravity,
just by observing the moons’ orbits
around the planets.
The sizes of those orbits, like the
sizes of the planets themselves, are
determined by geometry.
We also look at density which can
be determined by d = m/v, where
v = 4/3 πr3.
Because the physical and chemical
properties of Mercury, Venus, and
Mars are somewhat similar to
Earth’s, the four innermost planets
are called the Terrestrial Planets.
The larger outer planets—Jupiter,
Saturn, Uranus, and Neptune—are
all similar to one another chemically
and physically They are labeled the
Jovian Planets The jovian worlds
are all much larger than the
terrestrial planets
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The Terrestrial Planets
Earth like
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The four terrestrial planets all lie within about 1.5 A.U. of the Sun. All
are small and of relatively low mass, and all have generally rocky
composition and solid surfaces. Beyond that, however, the similarities
end:
All four terrestrial planets have atmospheres, but the atmospheres
range from a near-vacuum on Mercury to a hot, dense inferno on
Venus.
Earth alone has oxygen in its atmosphere and liquid water on its
surface.
Surface conditions on the four planets range from barren, heavily
cratered terrain on Mercury to widespread volcanic activity on Venus.
Earth and Mars spin at roughly the same rate—one rotation every 24
(Earth) hours—but Mercury and Venus both take months to rotate just
once, and Venus rotates in the opposite sense from the others.
Earth and Mars have moons, but Mercury and Venus do not.
Earth and Mercury have measurable magnetic fields, of very different
strengths, whereas Venus and Mars have none.
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Exploration of Our Solar System
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Mercury –
Mariner 10 – 1973 – 3 flybys
March & September 1974, March
1975. 45% of surface mapped.
Venus –
Mariner 5 and 10 did flybys but
returned little data. Soviet
Venera probed did most of the
exploration. Venera 7 (1970)
landed and returned data for 23
minutes before being crushed by
atmospheric pressure. No Venera
lander has lasted more than 1
hour. Venera 15 & 16 were
orbiters which returned detailed
radar maps.
Pioneer Venus and Magellan are
US probes. Magellan mapped
98% of surface.
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Exploration of Our Solar System
 Mars –
 Mariner 4,6,9
photographed surface.
Paving the way for the
Viking landers 1,2 (1976)
returned a great dal of
data lasting until 1980
and 1982.
 Mars Global Surveyor  Pathfinder (orbiter),
Soujourner (rover) –
1997 return great
pictures.
 Two failures – current
spirit rovers.
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Asteroids and Comets
 Asteroids and
meteoroids reside in
the asteroid belt –
Between Mars and
Jupiter and they have
a rocky composition.
 Comets have an icy
composition and
resemble the moons of
the outer planets.
Since they are icy,
they vaporize when
they hit atmosphere.
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Jovian: Jupiter like
 Jovian planets all lie between the Asteroid belt and the
Kuiper belt
 They are all low density but high mass being composed
mainly of Hydrogen and helium gas and have no solid
surface. In fact, Saturn has a density less than water!
 With the exception of Uranus the Jovian planets have a
differential rotation which creates a banded weather
pattern on them. Both Jupiter have intense circular
storms and strong winds
 Uranus rotates 90 on it’s side and has no internal
energy. It also crosses paths with the dwarf planet Pluto.
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The Outer Planets
 Voyager
 Pioneer 10 & 11
scouted for Voyager 1
&2
 Voyager 1 – Jupiter
and Saturn, especially
Saturn’s moon Titan.
 Voyager 2 – Grand
tour visiting all of the
outer giants.
 Recent Missions
 Galileo to Jupiter
 Cassini to Saturn.
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Comparing planets
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http://www.sciencenetlinks.com/interactives/messenger/psc/PlanetSize.html
Alta High Astronomy