Astronomy 1010Fall_2013Day-28

Planetary Astronomy

Course Announcements• Smartworks Chapter 8: Wed. Nov. 6• Smartworks Chapter 9, 10: Mon. Nov. 18• Smartworks Chapter 11, 12, 24: Fri. Dec.

6• Friday Nov. 1 – 3:00pm E306

• Dr. Douglas Tucker, Fermilab, “The Dark Energy Survey”

• Exam-3 will be returned on Friday• Exam-4: Monday Nov. 18 – Ch. 8, 9, 10

• Dark Night Observing: Monday, Nov. 4• Starts at 8:30 pm at the Observatory

Rocks returned from Moon missions (1969–1976) give ages through radioactive dating.

Almost all cratering happened in the first billion years of the Solar System.

Hadean Earth, Dawn of Life

Late Heavy Bombardment – ~3.9 Gyr ago Relatively quiet between formation and LHB Since then, protected by Jupiter

Sterilizing Impacts 350-400 km in diameter

Completely vaporize the oceansGlobal surface temperature rise 2000 C (3600 F)Last ~4.2-3.8 Gyr ago

Concept QuizThe Moon Long Ago

Imagine taking a picture of the Moon about 2 billion years ago. What would you expect to see?

A. It would have many fewer craters.B. It would have many more craters.C. It would have about as many craters as it does

now.

Rock layers are formed through sedimentation.

To find the ages of rock in these layers (or from Mars), scientists use radiometric dating.

Parent particles decay into stable daughter particles at a steady rate.

CONNECTIONS 8.1

Radioactive DatingSome elements can decay from one to

another (e.g., uranium to thorium).These changes take place at known rates.Parent element declines, daughter element

accumulates.Ratio of parent to daughter abundance gives

the age of the rock.Age = time since rock was last molten.

A radioactive isotope decays to half its original amount in a half-life.

MATH TOOLS 8.1MATH TOOLS 8.1

We model the Earth’s interior by studying earthquakes.

Seismic waves travel differently through different materials.

Some waves are surface waves; others travel through.

Primary waves travel through solids and liquids; they are longitudinal waves.

Secondary waves go through solids only; they are transverse waves.

Earth has a crust, mantle, and core (dense materials).

Produced by differentiation in the early Earth: Dense materials sink, low-density materials rise.

Current prevailing theory: Moon formed in large collision between Earth + Mars-sized protoplanet.

The material collected to form the Moon.

The composition of the Moon is like that of Earth’s crust.

The Moon and the other terrestrial planets have interiors similar to Earth.

Deeper in a planet means hotter and more pressure.

Formation energy and radioactive material help to heat the interior.

Smaller planets lose heat faster; large ones more slowly.

Large planets cool off more slowly than small planets.

If we assumed planets all began with the same amount of radioactive material and that the successive decay produced all the internal heat, then the rate of energy loss is dictated by how much energy there is (volume = 4/3 π R3) and where the planet loses the energy (surface area = 4 π R2) .

The amount of energy that could be lost divided by the area of loss is proportional to R/3, where R is the radius of the planet.

MATH TOOLS 8.2MATH TOOLS 8.2

Earth’s magnetic field acts like a giant bar magnet.

It originates from processes deep in the interior that are not understood fully.

Iron-bearing minerals tell us the orientation changes over time.

Earth and Mercury are the only terrestrial planets with a substantial magnetic field.

It is a puzzle why Venus and Mars do not have one.

The Moon had one long ago.

Concept QuizInternal Heat

What should be true about Earth’s internal heat?

A. It is produced by the magnetic field.B. It would have been greater long ago

when Earth was young.C. It currently is produced by differentiation.

Tectonism is the deformation of Earth’s crust. Earth’s crust is broken into lithospheric plates. Continental drift and plate tectonics describe

the movement of those plates.

Crustal plates are moved around by convection—the rising and falling of hot/cold material.

Earth has seven major plates and six smaller ones.

Plates can separate or collide. Because of these motions, most volcanoes and

earthquakes occur along plate boundaries.

Exam SadisticsMetric Ex-1 Ex-2 Ex-3Number 68 68 60Mean 63.5 58.6 55StdDev 13.2 16.1 14.7Median 65 58 55Mode 68 58 45High 94 97 100Low 30 27 23