Lecture 12 Overview of the Solar System Chapter 6

Lecture 12 Overview of the Solar System

Chapter 6 to be covered in another lecture

- read the chapter 1877 Mars was close to Earth

2 small moons discovered by Asaph Hall

Hubble Image of Mars Early photo and sketch of Mars

Schiapareli reported seeing “canali” - in Italian this means grooves or channels but was interpreted to mean canals Hubble image

Compare (same face)

Sketches Percival Lowell

Flagstaff AZ

He gave up his business in Boston to devote his life to astronomy and the search for life on Mars Novels (1917) about life on Mars inspired the young Carl Sagan and many others Northern Martian plains seen by the Phoenix lander

A lifeless planet

Red because of the iron content of the rocks - rust Panoramic view of Mars seen by the Opportunity Rover

We are still on a quest for life

- was there life on Mars in the past? Technology is the driver

Galileo’s telescope Earl of Rosse - 72 inch reﬂector the largest telescope in the world in the 19th century Exploration by Spacecraft 1972 Apollo astronaut prospecting in Mare Serenitatis The Solar System

Most information has been obtained in the last 40 years through spacecraft exploration Near circular orbits

but not perfect

All prograde

- they orbit in the same direction Ecliptic is the plane of Earth’s orbit the line is close to the ecliptic Sizes are drawn to scale

Earth about the size of the Great Red Spot on Jupiter

Densities

1410 kg / m^3 = 1.410 g / cm^3 better to use numbers near unity ? Components of the solar system

•! The Sun •! Mass/luminosity •! Solar Wind/Magnetic field

•! Planets and their moons and ring systems •! Terrestrial planets: Mercury, Venus, Earth, Mars •! Jovian planets: Jupiter, Saturn, Uranus, Neptune •! Dwarf planets: Pluto (Ceres, Eris)

•! Minor planets •! Asteroids: Asteroid Belt, Trojans, Near Earth Asteroids •! Comets: Kuiper Belt, Oort Cloud

•! Dust •! Zodiacal Cloud The Sun - mass

•! Vital stats: •! Mass = 1.989 x 1030 kg •! Radius = 6.95 x 108 m •! Mean density = 1410 kg/m3

•! Definition of the solar system is the material gravitationally bound to the Sun

•! Everything orbits the Sun on elliptical orbits with orbital periods of 1.5 tper = a years where a=semimajor axis in AU

•! The Sun’s influence extends out to ~100,000 AU (~0.5 pc), outside which galactic tides strip material from the solar system Kepler’s 3rd law a^3 / T^2 = constant

This gives us the ratios of the distances based on the Earth with a distance of 1 AU and a period of 1 year

For absolute numbers we need radar measurements of distances to nearby planets (Mercury and Venus) T = a^1.5

T = a^(3/2)

Square both sides of the equation

T^2 = a^3 Kepler 3

T must be in years

a must be in AU

because we are dealing with ratios and comparing with Earth Clues Comets have two tales Solar wind from one of ionized gas comets and one of dust The Sun – solar wind

•! First discovered because comet ion tails always point away from the Sun; caused by fast moving ions in the corona which escape the Sun’s gravitational field

•! It has a slow component (300-500km/s) at the equator (<150) and fast component (700-800km/s) at higher latitudes; at 1AU mean density is 7x106 protons/m3 (v ~ const, so ! " r-2); neutral, roughly solar composition

•! Interaction of charged particles with planet magnetospheres and atmospheres -> aurorae borealis

•! Solar wind interacts with the interstellar medium at the heliopause The Sun – solar wind

•! First discovered because comet ion tails always point away from the Sun; caused by fast moving ions in the corona which escape the Sun’s gravitational field

•! Interaction of charged particles with planet magnetospheres and atmospheres -> aurorae borealis

•! Solar wind interacts with the interstellar medium at the heliopause

Earth’s magnetic ﬁeld protects us from the solar wind 4. Planet formation Mercury, Venus, Earth Mars and Venus, Mercury, Jupiter, Saturn, Uranus and Neptune and Uranus Saturn, Jupiter, not to scale to not (a dwarf planet) dwarf (a + Pluto Pluto +

So, where did all these planetary systems come from? The planets – overview/mass

Mass Distance

Mercury 0.06 Mearth 0.39 AU Venus 0.82 M 0.72 AU earth Terrestrial

Earth 1.0 Mearth 1.0 AU planets

Mars 0.11 Mearth 1.5 AU

Jupiter 318 Mearth 5.2 AU

Saturn 98 Mearth 9.5 AU Jovian planets Uranus 15 Mearth 19.2 AU

Neptune 17 Mearth 30.1 AU Dwarf planet Pluto 0.002 Mearth 39.5 AU 24 -6 11 1 Mearth = 6 x 10 kg = 3x10 Msun , 1 AU = 1.5 x 10 m Sun is nearly one million times more massive than Earth Mercury

• Unusual rotation • Anomalous precession • Do Vulcanoids exist?

Anomalous precession of Mercury refers to the change in the orientation of the elliptical orbit

This could not be understood using Newton’s simple inverse square law of gravity

It was ﬁrst accounted for by Einstein’s theory of General Relativity The planets - rotation

•! Mercury is in a 3:2 spin-orbit Tper, yrs Prot, hrs Obliquity resonance, probably despun by Mercury 0.241 1407.5 0.10 solar tides Venus 0.615 5832.5 177.40 Earth 1.00 23.9345 23.450 •! Venus, Uranus and Pluto have Mars 1.88 24.623 25.190 retrograde rotation, possibly as Jupiter 11.86 9.925 3.120 result of collision with proto-planet during formation (Canup 2005; Saturn 29.46 10.656 26.730 Parisi et al. 2008) Uranus 84.00 17.24 97.860 Neptune 164.80 16.11 29.560 •! NB obliquity is measured relative Pluto 247.7 153.29 119.60 to orbital plane rather than ecliptic

Mostly prograde - two retrograde 3 to 2 Spin-orbit resonance

Mercury spins on its axis 3 times for every 2 orbits of the Sun

Produced by tidal braking of the spin caused by tides raised on Mercury by the Sun Venus

• Unusual rotation • Lack of small craters • Atmosphere

Carl Sagan - the greenhouse effect

Spin of Venus is retrograde and very slow (-243 days)

Also caused by tidal braking due to solar tides Earth

• Liquid water • Atmosphere • Life! • Few impact craters

Plate tectonics - the positions of the

continents change The Moon

• Formation? • Asymmetric faces • Synchronous • Receding • Water?

Mars

• Polar caps • Unusual rotation • Liquid water? • Life? • Eccentric orbit

Unusual rotation does not refer to the spin (about 24 hours) but to the large, slow changes in the orientation of it’s spin axis - these change the climate The planets - orbits Aphelion Perihelion ae Three things I will say about orbits: •! Semimajor axis, a (t =a1.5) per 2a •! Eccentricity, e •! Inclination, I •! Evenly spaced -> Titius-Bode’s law a=0.4+0.3(2i), predicted Ceres (1801) a, AU e I, deg and Uranus (1781) Mercury 0.39 0.206 7.0 Venus 0.72 0.007 3.4 •! La Grande Inequalite (JS near 5:2 Earth 1.0 0.017 0.0 resonance) and NP in 3:2 resonance Mars 1.5 0.093 1.9 •! All orbit in the same direction, in the Jupiter 5.2 0.048 1.3 same plane on roughly circular orbits Saturn 9.5 0.054 2.5 Uranus 19.2 0.047 0.8 •! Except Pluto, and possibly Mercury; Neptune 30.1 0.009 1.8 high e of JS also important for formation/evolution Pluto 39.5 0.249 17.1 The planets – internal structure Density, Terrestrial planets: kg/m3 •! iron core, rocky mantle, crust (differentiated in formation) Mercury 5427 •! Mercury’s high density -> large iron core (to 0.75Rpl), perhaps caused by massive impact (Asphaug et al. 2006) Venus 5204 Earth 5515 Jovian planets: Mars 3933 •! rocky/icy (liquid) core and metallic/molecular hydrogen (JS) Jupiter 1326 or mantle of ices and H/He/CH gas (UN) 4 Saturn 687 •! mass of Jupiter’s core is model dependent 0-11Mearth, but Uranus 1318 Saturn’s core does exist 9-22Mearth (Sauron & Guillot 2004) Neptune 1638 Pluto: rocky core, ice mantle, layer of frozen methane, Pluto 2060 nitrogen and carbon monoxide Density is a clue to composition but we must allow for the effects of compression Jupiter

• Gas giant • Role in orbital evolution • Satellite system • Ring system • Resonances! • Vulcanism!

Planetary satellites – Galilean Moons

Distance Mass

Io 422,000 km 0.015 Mearth

Europa 671,000 km 0.008 Mearth

Ganymede 1,070,000 km 0.025 Mearth

Callisto 1,883,000 km 0.018 Mearth

Io: strong vulcanism caused by 100 m tides from Jupiter (synchronous rotation) and orbital resonance with Europa; sulphur gives it a red/yellow colour

Europa: cracked icy surface; liquid water under surface; heated by tides; no craters

Ganymede: cratered; rock and ice; maybe water; largest moon in Solar System, and larger than Mercury and Pluto

Callisto: most heavily cratered; rock and ice Resonances

Ratios of orbital periods are ratios of small integers

Io / Europa / Ganymede = 1 / 2 / 4 Giant red spot and Galilean satellites

Europa

Ganymede

Callisto The new, close-up view from space

• • The space-age investigation of the solar system began in a cold war • competition between the Soviet Union, which launched the ﬁrst artiﬁcial • satellite, and the United States, which won the race to the Moon.

• The VoyagerVolcanic 1 and activity 2 ﬂyby on Io spacecraft transformed our understanding of • the four giant planets, Jupiter, Saturn, Uranus and Neptune, and revealed • fascinating, unexpected aspects of their moons and rings.

• The Giotto spacecraft was the ﬁrst to provide a close-up view of a comet, • showing that its nucleus is a black, city-sized chunk of water ice and dust • that emits sunward jets of water when passing near the Sun.

• Orbiting spacecraft have greatly increased the time for study of the planets • and moons, revealing ancient water ﬂow on Mars, vast outpourings of lava • on Venus, Jupiter’s volcanic moon Io and an ice covered ocean on its satellite • Europa, and Saturn’s marvelous rings, water-spewing satellite Enceladus, • and haze-shrouded moon Titan.

• Three rovers have explored the surface of Mars and provided evidence • for water ﬂow across its surface roughly 4.0 billion years ago.

• The Huygens Probe and radar from the orbiting Cassini spacecraft have • discovered rain, rivers and lakes of liquid methane on Saturn’s moon Titan. • • Massive eruptions continuously disﬁgure the surface of Jupiter’s satellite Io, • the most volcanically active body in the solar system. Water oozes out from Jupiter’s moon Europa

Is there an ocean below the ice?

Could it harbor life? Saturn

• Gas giant • Ring system • Satellite system • Resonances! • Vulcanism • Liquid water!

The new, close-up view from space

• The Voyager 1 and 2 ﬂyby spacecraft transformed our understanding of • the four giant planets, Jupiter, Saturn, Uranus and Neptune, and revealed • fascinating, unexpected aspects of their moons and rings.

• Three rovers have explored the surface of Mars and provided evidence • for water ﬂow across its surface roughly 4.0 billion years ago.

• The Huygens Probe and radar from the orbiting Cassini spacecraft have • discovered rain, rivers and lakes of liquid methane on Saturn’s moon Titan. Saturn's• realm • Massive eruptions continuously disﬁgure the surface of Jupiter’s satellite Io, • the most volcanically active body in the solar system.

The magniﬁcent rings of Saturn encircle the planet, never touching its cloud tops. From the outside in there are the bright A and B rings separated by the Cassini Division.

The narrow Encke Gap in the outer

A ring is also visible, as is the dark

C ring nearest to the planet. The planets – ring systems

None of the terrestrial planets (or Pluto) have ring systems but all of the Jovian planets do

Although recently Pluto suggested to form dust rings sporadically when KBOs collide with its moons (Stern 2006)

And there’s a large amount of space debris in orbit around the Earth…

They are interesting because: •! circumplanetary disk processes similar to circumstellar disk processes •! test of planetary system dynamics •! finding satellites •! origin and evolution The new, close-up view from space

• Three rovers have explored the surface of Mars and provided evidence • for water ﬂow across its surface roughly 4.0 billion years ago.

The magniﬁcent rings of Saturn encircle the planet, never touching its cloud tops. From the outside in there are the bright A and B rings separated by the Cassini Division.

The narrow Encke Gap in the outer

A ring is also visible, as is the dark

CSaturn’s ring nearest rings to open the planet. up

These images were taken from the Hubble Space Telescope during a four-year period, from 1996 to 2000 (left to right), as Saturn moved along one seventh of its 29-year journey around the Sun. 4 satellites of Saturn - all different Our solar system contains a huge variety of bodies

All have information on how the system has evolved

Cassini views of

Dione Telesto Hyperion Enceladus Tiger stripes on Enceladus

Saturn’s enigmatic moon Enceladus is a jumbled world of fresh snow plains (middle), old cratered terrains (top), and prominent tiger stripe fractures (bottom, false color blue).

The ﬁssures spray ice particles, water vapor and organic compounds outward, some of them forming Saturn’s E ring and others falling back on the moon. Enceladus vents water jets

Saturn’s enigmatic moon Enceladus is a jumbled world

of fresh snow plains (middle), old cratered terrains (top),

and prominent tiger stripe fractures (bottom, false color blue).

The ﬁssures spray ice particles, water vapor and organic

compounds outward, some of them forming Saturn’s E ring

and others falling back on the moon.

Dramatic plumes, both large and small, spray water ice particles, water vapor and organic compounds

out from many locations along tiger stripe fractures near the south pole of Saturn’s moon Enceladus. Radar images of hydrocarbon lakes on Titan taken by Cassini spacecraft

This movie, comprised of several detailed images taken by Cassini's radar instrument, shows bodies of liquid near Titan's north pole. Titan: Is this the most interesting satellite in the solar system? Uranus

• Unusual rotation • Narrow ring system • No resonances! • Inclination of Miranda • Orbital evolution

Neptune

• Lack of satellites • Retrograde Triton • Eccentric Nereid • Unusual ring system

Neptune’s Rings

• Radial conﬁnement? • Azimuthal conﬁnement? • Embedded satellites? • Stability

Pluto

• Large moon • Unusual rotation • Orbital evolution • Is it a planet? (No)

6.5 Interplanetary Matter

Pluto, once classified as one of the major planets, is the closest large Kuiper belt object to the Sun

Now known to have 4 satellites! - hard to keep up Satellite The planets - satellites count •! Terrestrial planets (and Pluto) have <3 moons Mercury 0 Venus 0 •! Jovian planets have >13 moons which fall into two Earth 1 categories: regular and irregular (Jewitt & Haghighipour Mars 2 2007) Jupiter 63 Saturn 47 Uranus 27 Neptune 13 Pluto 3

Also interesting because: Origin of satellites: •! Some are planets in •! circumplanetary disk during planet formation their own right (Estrada & Mosqueira 2006) •! Testbed of planetary •! circumplanetary disk following massive collision system dynamics •! captured minor planets •! Used to tell us about •! fragments of planet from collision interior of planets Planetary satellites – Irregular satellites Planetary satellites – Irregular satellites Planetary satellites – Irregular satellites Jupiter Saturn Uranus Neptune Jupiter Saturn Uranus Neptune Jupiter Saturn Uranus Neptune

Of Jupiter’s 63 satellites the majority are irregulars and rather than large satellites on circularOf Jupiter’s coplanar 63 satellites close-in theorbits, majority these are are irregulars smaller (D=2-200km) and rather than satellites large satelliteson eccentric on (e to Of Jupiter’s 63 satellites the majority are irregulars and rather than large satellites on 0.4)circular inclined coplanar (I to close-in500) more orbits, often these retrograde are smaller orbits (D=2-200km) at large distance satellites (>7x10 on9 meccentric for Jupiter) (e to circular0.4) inclined coplanar (I to close-in 500) more orbits, often these retrograde are smaller orbits (D=2-200km) at large distance satellites (>7x10 on 9eccentricm for Jupiter) (e to 0.4) inclined (I to 500) more often retrograde orbits at large distance (>7x109m for Jupiter) Wide field deep imagers -> large numbers -> find dynamical families and that number of irregularsWide field measured deep imagers to given -> large diameter numbers constant -> find for dynamicalall planets families(Jewitt &and Sheppard that number 2005) of Wideirregulars field measureddeep imagers to given -> large diameter numbers constant -> find for dynamical all planets families (Jewitt and& Sheppard that number 2005) of irregulars measured to given diameter constant for all planets (Jewitt & Sheppard 2005) Origin in capture from passing asteroids/comets, perhaps during formation, although also inOrigin collisions in capture (e.g., fromNereid passing and S/2002 asteroids/comets, N1 around Neptune, perhaps Gravduring et formation, al. 2004; Nesvornyalthough etalso al. Origin in capture from passing asteroids/comets, perhaps during formation, although also 2004in collisions -> collisions (e.g., withNereid protoplanetary and S/2002 N1 disk) around and dynamicsNeptune, ->Grav captured et al. 2004; objects Nesvorny released et in al. in2004 collisions -> collisions (e.g., Nereidwith protoplanetary and S/2002 N1 disk) around and Neptune,dynamics Grav-> captured et al. 2004; objects Nesvorny released et inal. 1002004 orbits, -> collisions Holman with et al. protoplanetary (2004) disk) and dynamics -> captured objects released in 100 orbits, Holman et al. (2004) 100 orbits, Holman et al. (2004) Follow-up involves, e.g., spectroscopy to determine origin (e.g., Vilas et al. 2006) showing Follow-up involves, e.g., spectroscopy to determine origin (e.g., Vilas et al. 2006) showing thatFollow-up JupiterThe involves, irregulars irregular e.g., from spectroscopy AB, satellites others mostlyto determine are from smallKB origin (e.g., satellites Vilas et al. that2006) showing that Jupiter irregulars from AB, others mostly from KB that Jupiterorbit irregulars at veryfrom AB, large others mostlydistances from KB from the planet Minor planets in the inner solar sytem

•! The Asteroid Belt is the belt of rocky asteroids orbiting 2-3.5 AU from the Sun (green) Jupiter

•! Some of asteroids in the Earth region (Near Earth Asteroids in red)

•! Another family of asteroids are the Jupiter Trojans at ± 60o from Jupiter

•! There are 20,000 numbered asteroids The Inner Solar System

Smalll bodies (planetesimals) left over from the formation of the large planets Asteroids - Trojans

•! Orbiting Jupiter’s L4 and L5 points, with more around L4 (though not sure if this is observational bias)

Asteroids - Trojans •! Size distribution n(D) D2.39 for D>5km and n(D) D1.28 for D<5km (Yoshida & Nakamura 2005) •! Orbiting Jupiter’s L4 and L5 points, with more •! There is just one known Trojan binary (617 around L4 (though not sure if this is observational Patroclus) bias) •! Origin is thought to be formation near Jupiter then capture while Jupiter was growing, possibly 2.39 with help of gas drag/collisions, though some seem •! Size distribution n(D) D for D>5km to be passing and n(D) D1.28 for D<5km (Yoshida & Nakamura 2005) •! Other planets also have Trojans (e.g., 4 at Mars)

•! There is just one known Trojan binary (617 Patroclus)

•! Origin is thought to be formation near Jupiter then capture while Jupiter was growing, possibly with help of gas drag/collisions, though some seem to be passing

•! Other planets also have Trojans (e.g., 4 at Mars) Asteroids – Near Earth AsteroidsAsteroids – Near Earth Asteroids

Near Earth Asteroids are those that come within 1.3AU of Earth: Amors (cross Mars), Apollos (cross Earth a>1), Atens (cross Earth a<1)

Origin is in the Asteroid belt: migration to chaotic region where eccentricities pumped up (consistent with cosmic ray exposure ages, size distribution, orbital distribution, impact rate on terrestrial planets, Bottke et al. 2002) Near Earth Asteroids are those that come withinColored red - cross the orbit of Earth and may (or may not) impact the planet 1.3AU of Earth: Amors (cross Mars), Apollos (cross Earth a>1), Atens (cross Earth a<1)

How many are there? How often do they hit Earth? Near Earth Asteroids are those that come within 1.3AU of Earth: Amors (cross Mars), Apollos (cross Earth a>1), Atens (cross Earth a<1)

0.1 m 1 km Dust: Zodiacal cloud

Dust created in collisions between asteroids in the Asteroid Belt spirals in toward the Sun meaning the Earth is enveloped in a dust cloud

This dust cloud is observable to the naked eye just before (after) sunrise (sunset) as the zodiacal light (sunlight scattered by dust)

It is also the brightest thing in the infrared sky outside the Earth’s atmosphere (e.g., Kelsall et al. 1998): the dust is heated by the Sun and reemits radiation in the IR Icy bodies beyond the orbit of Neptune

Kuiper Belt The Outer Solar System •! Now almost 1000 KBOs have been discovered

•! Argument over whether Pluto is a planet or a large KBO sparked by discovery of 2003 UB313 (now Eris) which at ~2860km is larger than Pluto (Brown, Trujillo & Rabinowitz 2005) Neptune

First TNO discovered in 1992 Oort cloud of comets

Beyond the Kuiper Belt

The giant cometary sphere surrounding the solar system is named the Oort cloud, after Jan Oort who postulated its existence in 1950. Comets arrive in the solar system from all directions, often from as far away as 100,000 AU. An AU, or Astronomical Unit, is the distance from earth to the sun. Pluto, for comparison, is 39 AU from the sun. Reminder

10^5 AU = 100,000 AU

1 AU = 1.5 10^8 km

10^5 AU = 1.5 10^13 km

1 Light Year is about 10^13 km

Nearest star to the Sun is 4.2 LY

Comets

• Origin? • Evolution? • Threat? • Density? • Water delivery?

Some Questions

• How did the solar system form? • How and where do planetary systems form? • What determines their dynamical evolution?Is there or has there been life in other parts of the solar system? • Is the solar system stable?

Is there life on other planetary systems?

When will we ﬁnd another Earth-like planet?