ASTR178Other Worlds

Week 2, August 11A/Prof. Orsola De Marco

9850 4241orsola.demarco@mq.edu.au

http://www.physics.mq.edu.au/current/undergraduate/units/ASTR178/

The Moon practical

• The first day when you can go out and look at the moonin the early evening is the 14th. DO NOT WAIT to do thepractical at the last minute as the Moon does not wait foryou.

Remember!

• Lecture notes for Week 1 and 2 are up on Blackboard.• Assignment 1 has been posted. Due back Fri 13th by 4PM in the assignment boxes. It will be returned in class on Mon 23 and Tue 24 August.

Help questions (For this class)

• 5,10 page 124. 26, page 125. 45-47, page 126.•1-6 page 180. 7, 9-21 page 181.

In last class• Galileo and his observations – the phases of Venus.• Newton and gravity.• Tides.• Lunar phases.• Lunar and solar eclipses.

In this class

• A few words on the Sun to start a tour of the Solar System.• What is light.• Comparing the planets.• The density of the planets.• The largest “satellites”.• What are planets made of.• The planetary atmospheres• The outer solar system.• Craters, volcanos and magnetic fields.

Moon phases and tides

Northern and Southern hemisphere – what is the difference?

Lunar eclipses

The eclipsed moon is red for the same reason why the sky is blue and the sunset is red!

Solar eclipses

Angular and Linear size Unexaminable

• The angular size of the Sun and Moon are very similar!• This allows us to study the solar corona.• Linear size = Distance x sin (angular size)• d = D sin ()• Distance to the Sun = 150 million km• Distance to the Moon = 400,000 km• Sun’s radius = 6.9 x 1010 cm• Moon’s radius = 1700 km = 1.7 x 108 cm• so …

http://sunearth.gsfc.nasa.gov/eclipse/eclipse.html

The Sun(Alexei Gilchrist will deal with the Sun in

depth – this is just a very short intro)

A few words about light

Light is not onlyvisible light.

We call light atany wavelength:electro-magnetic radiation

• Small dots:

The concept of temperature:

A body with temperature emits electro-magnetic radiation (light).Hotter bodies emit more light at higher energy = shorter wavelength (ultraviolet).A cooler body emit more light at lower energy = longer wavelength (infrared).

Temperature scales

An overview of the Solar System

The planets and the Ecliptic

• All planets orbit the Sun approximately on one plane.• From the point of view of Earth the Sun and planets all move on the same plane, called the Ecliptic.

Clementine Spacecraft

Saturn would float in water!

Mercury drawn to the same scale

The seven giant moons in the Solar System

Why do some planets have satellites and others do not?

Image from Cassini-Huygens (2005)

From spectroscopy we know that Titan’s atmosphere is made of methane and N2

• Gas giants: H2 and He• Gas, liquid and solid form.• Cold on their surface.

• What dictates what isIn a planetary atmosphere?

Why are planets almost spherical and asteroids are not?

What is the difference between:

• Asteroids• Meteors• Comets• Shooting stars … ?

Why Pluto is not a planet…

Are these volcanos?

The Moon and Mercuryhave a lot of craters.

Earth and Venus have few craters.

Mars has craters only on the highlands

Why do planets have craters?

Geologic inactivityAtmosphereSmall planets cool faster

Craters in the News (2009): Victoria Crater on Mars

Mars Reconnaissance Orbiter (launch: August 2005)New image of Victoria Crater taken 22 degoff vertical.

Craters in the News: Victoria Crater on Mars

Planetary magnetic fields: who has them, who doesn’t and why?

• Earth has a magnetic field.• Mercury has a weak magnetic field (1% of Earth’s).• Venus and Mars have no magnetic field.• Mars has detectable spotty magnetic fields.• What does it mean?!

Generating a magnetic field by “dynamo action”

Key Ideas

• The Nature of Light: Light is electromagnetic radiation. It has wavelike properties described by its wavelength and frequency , and travels through empty space at the constant speed c = 3.0 108 m/s = 3.0 105 km/s.

Key Ideas

• Bodies emit electromagnetic radiation. The hotter the body the shorter the wavelength of the radiation and then brighter the emission.

• A hot, dense object such as a blackbody emits a continuous spectrum covering all wavelengths.

• A cool, transparent gas in front of a light source that itself has a continuous spectrum produces dark (absorption) lines in the continuous spectrum.

Key Ideas

• Properties of the Planets: All of the planets orbit the Sun in the same direction and in almost the same plane. Most of the planets have nearly circular orbits.

• The four inner planets are called terrestrial planets. They are relatively small (with diameters of 5000 to 13,000 km), have high average densities (4000 to 5500 kg/m3), and are composed primarily of rocky materials.

• The four giant outer planets are called Jovian planets. They have large diameters (50,000 to 143,000 km) and low average densities (700 to 1700 kg/m3) and are composed primarily of light elements such as hydrogen and helium.

Key Ideas

• Satellites and Small Bodies in the Solar System: Besides the planets, the solar system includes satellites of the planets, asteroids, comets, and trans-Neptunian objects.

• Seven large planetary satellites (one of which is our Moon) are comparable in size to the planet Mercury. The remaining satellites of the solar system are much smaller.

Key Ideas

• Asteroids are small, rocky objects, while comets and trans-Neptunian objects are made of ice and rock. All are remnants left over from the formation of the planets.

• Most asteroids are found in the asteroid belt between the orbits of Mars and Jupiter, and most trans-Neptunian objects lie in the Kuiper belt outside the orbit of Neptune. Pluto is one of the largest members of the Kuiper belt.

Key Ideas

• Spectroscopy and the Composition of the Planets: Spectroscopy, the study of spectra, provides information about the chemical composition of objects in the solar system.

• The spectrum of a planet or satellite with an atmosphere reveals the atmosphere’s composition. If there is no atmosphere, the spectrum indicates the composition of the surface.

• The substances that make up the planets can be classified as gases, ices, or rock, depending on the temperatures at which they solidify.

Key Ideas

• Impact Craters: When an asteroid, comet, or meteoroid collides with the surface of a terrestrial planet or satellite, the result is an impact crater.

• Geologic activity renews the surface and erases craters, so a terrestrial world with extensive cratering has an old surface and little or no geologic activity.

• Because geologic activity is powered by internal heat, and smaller worlds lose heat more rapidly, as a general rule smaller terrestrial worlds are more extensively cratered.

Key Ideas

• Magnetic Fields and Planetary Interiors: Planetary magnetic fields are produced by the motion of electrically conducting liquids inside the planet. This mechanism is called a dynamo. If a planet has no magnetic field, that is evidence that there is little such liquid material in the planet’s interior or that the liquid is not in a state of motion.

• The magnetic fields of terrestrial planets are produced by metals such as iron in the liquid state. The stronger fields of the Jovian planets are generated by liquid metallic hydrogen or by water with ionized molecules dissolved in it.