ptys/astr 206planetary surfaces and interiors 2/20/07 for the next few weeks: terrestrial planets,...

PTYS/ASTR 206 Planetary Surfaces and Interiors2/20/07

For the next few weeks:

Terrestrial Planets, their Moons, and the Sun


Announcements

• Reading Assignment– Section 9-1 (pp 186-189) , 9-5 and 9-6 (pp 199-203)

• 3rd Homework is now posted on the course website (due Thursday 3/1)

• Term paper details are now posted on the website (due 4/17)

• Results from first exam will be briefly discussed today– Solutions will be posted soon

• Public Lecture next Tuesday (2/27) at 7:30PM in this auditorium– Prof. Bob Strom: “Global Warming”– There will be a sign-up sheet for students in this class. Sign in and

get 5pts extra-credit added to your in-class activities score for each one of these public lectures that you attend (including the first one, if you attended it)


First Exam Results

• Average: 69.1• Median: 70• High: 102-½ (5 scores over 99)• Total tests taken: 130

89 and above 18 (13.8%)

78 – 88 19 (14.6%)

67 – 77 38 (29.2%)

56 – 66 26 (20.0%)

55.5 and below 29 (22.3%)


First mid-term exam grade distribution

0

5

10

15

20

25

95-100 90-95 85-90 80-85 75-80 70-75 65-70 60-65 55-60 50-55 45-50 40-45 35-40 30-35 25-30

Series1


Approximate Curve

> 87.5 A (15.4%)

74 – 87 B (21.5%)

62.5 – 73.5 C (30.8%)

45 – 62 D (21.5%)

< 45 E (10.8%)


Today: Planetary Interiors and Surfaces


Review: Formation of terrestrial planets

• Small dust particles accreted to make “planetesimals”

• Planetesimals accreted (and collided with other planetesimals) to form protoplanets

• The protoplanets were at least partially molten– denser iron-rich material fell

to the center, bringing heavier metals with it, making an iron-rich core (differentiation)

– A terrestrial planet!


Internal structure of a terrestrial planet


All of the terrestrial planets, and Earth’s moon have a similar internal structure, but the relative sizes are different depending on how hot the interior got, how rapidly the object cooled, and how much mass it has

The largest factor in determining this structure is the object’s size


• Accretion– Conversion of gravitational energy of

incoming material to kinetic energy, into thermal energy

• Chemical Differentiation– Conversion of gravitational energy of

falling denser materials within the interior to thermal energy

• Radioactive decay of elements within the body’s interior– leads to a slight mass difference

between initial and final elements which is converted to energy E=Δmc2

Sources of heat


Cooling processes in terrestrial planets

• Mantle convection in the interior– Hot material rises, cold

material falls (like boiling water on a stove)

• Thermal conduction in the lithosphere– Like when a metal plate is

heated at one end – the other end will soon get hot too

• Radiative loss through the surface into space

• Volcanic eruptions


Probing the interiors of planets

• The only reliable way to accurately probe the interior of a planet is by analyzing seismic activity – like Earthquakes

• Another reasonable approach is to measure how the body rotates (by measuring its libration)

• The existence of a planetary magnetic field provides some basic information

• Theoretical and numerical models can be constructed, but these are not as definitive


Seismic Waves

• P and S waves (Primary and Secondary)– Move through the Earth’s

interior– Provide information

about interior’s structure

• Surface Waves– The rolling waves that

are felt on the surface– Like water waves


P- and S-waves

• P-waves– Compressional (or

longitudinal) waves– Can travel through

solids or liquids• S-waves

– Shear (or transverse) waves Cannot travel through liquids

P-waves, or compression waves

S-waves, or transverse waves

Water waves


• S-waves do not travel through the Earth’s core (creating a “shadow zone” as shown at below right)

• This proves that part of the core is liquid


• Another important tool for “probing” the interior of a planet

• Mercury – has a global magnetic field

– this is somewhat of a puzzle!

• Venus – does NOT have a global magnetic field

– Slow rotation rate

• Earth – has a very strong global magnetic field

• Mars – does not have a global magnetic field, but there is evidence that it had one in the past

Planetary Magnetic Fields


The Earth’s Dynamo

• Permanent magnets lose their field if raised to a temperature above about 500oC– The Earth is hotter than this

nearly everywhere

• Earth’s field is also known to change periodically– Pole reversals

• It must be generating its own internal magnetic field

• Need a circulating electric current – Circulation and convection of

electrically conductive molten iron in the Earth's outer core produces the magnetic field


Planetary Surfaces

• The appearance of a terrestrial planet’s surface is determined by internal geological activity, impacts with asteroids and comets, and erosion

• Examples of internal activity include:

– Plate tectonics and volcanism

• Atmospheres can give rise to both aeolian processes (wind erosion) and water erosion through precipitation and water flow, and other surface processes, such as the flow of glaciers


Evidence of Internal Activity: Volcanoes

• Volcanoes – either active or extinct – indicates whether the interior is currently active, or was so in the past

– Earth – active volcanoes – active interior

– Venus – possibly active volcanoes, (most are probably extinct) – active interior

– Mars – extinct volcanoes – no internal activity

– Mercury – no volcanoes – probably no internal activity

Olympus Mons, Mars


Evidence of Internal Activity: Plate Tectonics

• Plate tectonics is the shifting around of large crustal “plates” on the surface of a planet.– It is the result of convection in the

interior• Currently, only Earth has plate

tectonics

• One result of plate tectonics – huge mountain ranges


Aeolian processes

• Wind and water erosion have effected the surfaces of Earth, Mars, and Titan– Sand Dunes exist on all

three bodies– Dust devils exist on Mars

and Earth• Mars rover Sprit is still working

because of a dust devil! • Dust devils are much larger on

Mars

Dust devils on Mars

Sand dunes on Titan


Impact Cratering

• The dominant geological process in the solar system

• The number of craters on a body is a good indicator of the age of the surface

• Impacts have also affected the atmospheres of the gas giants


History of Craters

• Less than 100 years ago scientists felt that cratering due to impacts was unrealistic and highly improbable!– Volcanism was the

explanation (can see similar features associated with terrestrial volcanoes)

• 1960’s – consensus that impacts caused craters in solar-system bodies

“Crater” Elegante (Pinacate balsatic fields in Mexico, just below Arizona border)


Evidence for Craters due to

Impacts

• Shock effects in quartz (also seen in lunar rock samples) due to shock waves produced by the explosions associated with impacts

• Craters are nearly all circular in outline– raised rims, concentric inner terraces,

and other features that are seldom found in volcanic “craters”

– Circular also because of the shock waves associated with the explosion

• not elongated!

• Volcanic “craters” are usually very small and occur in a clearly volcanic environment.


Earth’s Impact Craters

• The Earth’s surface has probably received more impact events than the Moon– Earth is larger (but its

atmosphere burns up smaller ones).

• Only about 160 surviving craters

have been found thus far. So few because …– geological activity erases the

evidence (Earth’s surface is a young one)

– Earth’s atmosphere provides a shield against smaller asteroids Manicougan crater, Quebec, Canada

(5th largest crater in the world)


Earth’s Impact Craters

ptys/astr 206planetary surfaces and interiors 2/20/07 for the next few weeks: terrestrial planets,...

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