Astronomy Chapter 11-12

Is pushed by photons.

dust tail

From Earth-based telescopes, the rings look like solid, concentric sheets.

Earth-Based View of Saturn

Methane gas of Neptune and Uranus absorbs red light but transmits blue light. Blue light reflects off methane clouds, making those planes look blue

Methane on Uranus and Neptune

All jovian planets have substantial magnetospheres, but Jupiter's is the largest by far.

Other Magnetospheres

How does Io get heated by Jupiter?

tidal resonance

A layer rich in iridium and scoots tells us a huge impact occurred at this point in geological (and biological) history.

Iridium Layer

Asteroids are rocky leftovers of planet formation. The largest is Ceres, diameter ~1000 kilometers. There are 150,000 in catalogs, and probably over a million with diameter >1 kilometer. Small asteroids are more common than large asteroids. All the asteroids in the solar system wouldn't add up to even a small terrestrial planet.

Asteroid

Some large asteroids have their own moon. Asteroid Ida has a tiny moon named Dactyl.

Asteroids with Moons

Europa's surface appears heavily cracked even from a distance.

Europa

Iridium is very rare in Earth surface rocks but often found in meteorites. Luis and Walter Alvarez found a worldwide layer containing iridium, laid down 65 million years ago, probably by a meteorite impact. Dinosaur fossils all lie below this layer.

Iridium: Evidence of an Impact

Distance from the Sun= 30.1 AU Mass= 17M/Earth Density= 1.67g/cm^3 Composition= H compounds, rock, H and He

Neptune

Likely captured Kuiper belt object. Evidence of past geological activity.

Neptune's Moon Triton

Pluto will never hit Neptune, even though their orbits cross, because of their 3:2 orbital resonance. Neptune orbits three times during the time Pluto orbits twice.

Pluto's Orbit

A pair of small moons can force particles into a narrow ring.

Shepherd Moons

Adding mass to a jovian planet compresses the underlying gas layers. Greater compression is why Jupiter is not much larger than Saturn even though it is three times more massive. Jovian planets with even more mass can be smaller than Jupiter.

Sizes of Jovian Planets

Small moons (< 300 kilometer) - No geological activity Medium-sized moons (300-1500 kilometer) - Geological activity in past Large moons (> 1500 kilometer) - Ongoing geological activity

Sizes of Moons

These are far more numerous than the medium and large moons. They do not have enough gravity to be spherical: Most are "potato-shaped." They are captured asteroids or comets, so their orbits do not follow usual patterns.

Small Moons

The Cassini spacecraft revealed many concentric rings.

Spacecraft View of Ring Gaps

What Is the Weather like on Jovian Planets?

Strong winds and storms

What determines the direction of a comet's tail?

The tail extends away from the direction of the Sun.

Vesta is much wider across the equator than it is tall in part because a huge impact gouged out a crater near its South Pole

Vesta as Seen by the Dawn Spacecraft

Asteroids are cratered and generally not round.

What Are Asteroids Like?

Artist's conception of particles in a ring system. Particles clump together because of gravity, but small random velocities cause collisions that break them up. What we might see if we could get even closer...

Artist's Conception of Rings Close-Up

The Japanese spacecraft Hayabusa scooped up small amounts of the large asteroid Itokawa and returned it to Earth

Asteroids Up Close

"Classic" cratered iceball No tidal heating, no orbital resonances But it has a magnetic field!?

Callisto

Is the atmosphere that comes from a comet's heated nucleus

Coma

Formed beyond the frost line, comets are icy counterparts to asteroids. Nucleus of a comet is a "dirty snowball." Most comets do not have tails. Most comets remain perpetually frozen in the outer solar system. Only comets that enter the inner solar system grow tails.

Comet

Models suggest cores of jovian planets have similar composition. Lower pressures inside Uranus and Neptune mean no metallic hydrogen.

Comparing Jovian Interiors

A meteorite 10 kilometers in size would send large amounts of debris into the atmosphere. Debris would reduce sunlight reaching Earth's surface. Resulting climate change may have caused mass extinction.

Consequences of an Impact

Comet S L 9 caused a string of violent impacts on Jupiter in 1994, reminding us that catastrophic collisions still happen. Tidal forces tore it apart during a previous encounter with Jupiter. Artist's conception of S L 9 impact Impact sites in infrared light The black spot in this photo is a scar from the impact of an unknown object that struck Jupiter in July 2009. Impacts can extinguish life. But were they necessary for "life as we know it"?

Cosmic Collisions

Mission, to study nucleus of Comet Tempel 1. Projectile hit surface on July 4, 2005. Many telescopes studied aftermath of impact.

Deep Impact

Uranus and Neptune are denser than Saturn because they have less H/He, proportionately. But that explanation doesn't work for Jupiter....

Density Differences

Measuring the orbit of an asteroid's moon tells us an asteroid's mass. Mass and size tell us an asteroid's density. Some asteroids are solid rock; others are just piles of rubble.

Density of Asteroids

Objects in the solar system, other than the planets, which are round. May be similar to asteroids or to comets. Pluto, Eris, Makemake, Haumea, Ceres... Large objects, and their moons, in the Kuiper belt and asteroid belt

Dwarf Planets

Europa may have a 100-km-thick ocean under an icy curst. Rising plumes of warm weather may sometimes create lakes within the ice, causing the crust above to crack and explaining surface terrain that looks like a jumble of icebergs suspended in a place where liquid or slushy water froze.

Europa's Interior Also Warmed by Tidal Heating

Small impacts happen almost daily. Impacts large enough to cause mass extinctions happen many millions of years apart.

Frequency of Impacts

Largest moon in the solar system Clear evidence of geological activity Tidal heating plus heat from radioactive decay?

Ganymede

Some small moons create gaps within the rings Creates ripples as its gravity nudges particles that orbit faster than the moon (inside the gap), but slower (outside the gap).

Gap Moons

Jovian planets sometimes deflect comets toward Earth but send many more out to Oort cloud.

How do the jovian planets affect impact rates and life on Earth?

Why are there very few asteroids beyond Jupiter's orbit?

Ice could form in the outer solar system.

Asteroids and comets have hit Earth. A major impact is only a matter of time: not If but When. Major impacts are very rare. Extinction level events happen millions of years apart. Major damage happens tens to hundreds of years apart.

Impacts

Gravity of a jovian planet (especially Jupiter) can redirect a comet. Jupiter has directed some comets toward Earth but has ejected many more into the Oort cloud.

Influence of the Jovian Planets

High pressures inside Jupiter cause phase of hydrogen to change with depth. Hydrogen acts like a metal at great depths because its electrons move freely. Core is thought to be made of rock, metals, and hydrogen compounds. Core is about same size as Earth but 10 times as massive.

Inside Jupiter

No solid surface Layers under high pressure and temperatures Cores (~10 Earth masses) made of hydrogen compounds, metals, and rock The layers are different for the different planets.

Interiors of Jovian Planets

Saturn also radiates twice as much energy as it receives from the Sun. Energy probably comes from differentiation (helium rain). Neptune emits nearly twice as much energy as it receives, but the source of that energy remains mysterious.

Internal Heat of Other Planets

Io is the most volcanically active body in the solar system Volcanic eruptions continue to change Io's surface. Three large plumes lit by the Sun blanket the surface in the sulfur-rich snow and eruptions, evident in this infrared image, are frequent.

Io's Volcanoes

Much smaller than the terrestrial or jovian planets Not a gas giant like other outer planets Has an icy composition like a comet Has a very elliptical, inclined orbit Has more in common with comets than with the eight major planets

Is Pluto a Planet?

Jupiter does not have a large metal core like the Earth. How can it have a magnetic field?

It has metallic hydrogen inside, which circulates and makes a magnetic field.

Other jovian planets have cloud layers similar to Jupiter's. Different compounds make clouds of different colors.

Jovian Planet Atmospheres

Was Jupiter necessary for life on Earth?

Jovian Planets Affect earth Impacts

All four jovian planets have ring systems. Others have smaller, darker ring particles than Saturn.

Jovian Ring Systems

Distance from the Sun= 5.20 AU Mass= 318M/Earth Density= 1.33 g/cm^3 Composition= Mostly H, He

Jupiter

Visible clouds Gaseous hydrogen Liquid hydrogen Metallic hydrogen Core of rock, metals, and hydrogen compounds

Jupiter and Saturn

Hydrogen compounds in Jupiter form clouds. Different cloud layers correspond to freezing points of different hydrogen compounds.

Jupiter's Atmosphere

Reddish clouds of ammonium hydrosulfide blanket the entire planet at low altitude. Raising air forms white ammonia clouds at high altitude. Jupiter's strong Coriolis effect diverts the north-south air flow into strong east-west winds. Ammonia "show" falls from these high-altitude clouds, so descending air therefore lacks ammonia and ammonia clouds, allowing us to see down to the reddish clouds below. Red bands are lower-altitude clouds, which are warmer and therefore brighter in the infrared photo. White bands are higher-altitude clouds, which are cooler and therefore emit less infrared light.

Jupiter's Bands

Ammonium sulfide clouds (NH4SH) reflect red/brown. Ammonia, the highest, coldest layer, reflects white.

Jupiter's Colors

Is a storm twice as wide as Earth. Has existed for at least three centuries.

Jupiter's Great Red Spot

Jupiter radiates twice as much energy as it receives from the Sun. Energy probably comes from slow contraction of interior (releasing potential energy).

Jupiter's Internal Heat

Jupiter's strong magnetic field gives it an enormous magnetosphere. Gases escaping Io feed the donut-shaped Io torus.

Jupiter's Magnetosphere

On orderly orbits from 30-100 A U in disk of solar system Kuiper belt comets formed in the Kuiper belt: flat plane, aligned with the plane of planetary orbits, orbiting in the same direction as the planets

Kuiper belt

Geologists have found a large subsurface crater about 65 million years old in Mexico. A comet or asteroid about 10 kilometers in diameter approaches Earth.

Likely Impact Site

Fossil record shows occasional large dips in the diversity of species. Most recent was 65 million years ago, ending the reign of the dinosaurs.

Mass Extinctions

Almost all of them show evidence of past volcanism and/or tectonics. Ice fountains of Enceladus suggest it may have a subsurface ocean. Iapetus has a curious ridge around much of its equator.

Medium Moons of Saturn

They have varying amounts of geological activity—many unsolved mysteries! Miranda has large tectonic features and few craters (possibly indicating an episode of tidal heating in past).

Medium Moons of Uranus

Enough self-gravity to be spherical Have substantial amounts of ice Formed in orbit around jovian planets Circular orbits in same direction as planet rotation

Medium and Large Moons

The bright trail left by a meteorite

Meteor

Comets eject small particles that follow the comet around in its orbit and cause meteor showers when Earth crosses the comet's orbit. Meteors in a meteor shower appear to emanate from the same area of sky because of Earth's motion through space.

Meteor Showers

A rock from space that falls through Earth's atmosphere

Meteorite

A few meteorites arrive from the Moon and Mars. Composition differs from the asteroid fragments. A cheap (but slow) way to acquire Moon rocks and Mars rocks

Meteorites from Moon and Mars

Asteroids that come through our inner solar system and pose a possible danger to earth are known as?

Near Earth Asteroids

A "dirty snowball" Source of material for comet's tail

Nucleus of Comet

On random orbits extending to about 50,000 A U Oort cloud comets were once closer to the Sun, but they were kicked out there by gravitational interactions with jovian planets: spherical distribution, orbits in any direction

Oort cloud

Asteroids in orbital resonance with Jupiter experience periodic nudges. Eventually, those nudges move asteroids out of resonant orbits, leaving gaps in the asteroid belt.

Orbital Resonances

Every 7 days, these three moons line up. The tugs add up over time, making all three orbits elliptical. lo, Europa, and Ganymede share an orbital resonance that returns them to the positions shown about every 7 days and the recurring gravitational tugs make all three orbits slightly elliptical.

Orbital Resonances

Rocky planetesimals between Mars and Jupiter did not accrete into a planet. Jupiter's gravity, through influence of orbital resonances, stirred up asteroid orbits and prevented their accretion into a planet.

Origin of Asteroid Belt

Pluto's size was overestimated after its discovery in 1930, and nothing of similar size was discovered for several decades. Now other large objects have been discovered in the Kuiper belt, including Eris. The International Astronomical Union (I A U) now classifies Pluto and Eris as dwarf planets. Dwarf planets have not cleared most other objects from their orbital paths.

Pluto and Eris

Unchanged in composition since they first formed 4.6 billion years ago.

Primitive Meteorite

Younger; have experienced processes like volcanism or differentiation.

Processed Meteorite

Orbital resonance with a larger moon can also produce a gap.

Resonance Gaps

Jovian planets all have rings because they possess many small moons close in. Impacts on these moons are random. Saturn's incredible rings may be an "accident" of our time.

Ring Formation

Rock melts at higher temperatures. Only large rocky planets have enough heat for activity. Ice melts at lower temperatures. Tidal heating can melt internal ice, driving activity.

Rocky Planets Versus Icy Moons

Jovian planets are not quite spherical because of their rapid rotation. Gravity pulls material inward in all directions Rapid rotation flings material outward near the equator. Saturn isn't a perfect sphere. Compare its actual shape to the dashed circle.

Rotation and Shape

Distance from the Sun= 9.54 AU Mass= 95M/Earth Density= 0.71 g/cm^3 Composition= Mostly H, He

Saturn

Saturn's layers are similar, but deeper in and farther from the Sun (more subdued).

Saturn's Colors

We haven't seen it yet. Deflection is more probable with years of advance warning. Control is critical: Breaking a big asteroid into a bunch of little asteroids is unlikely to help. We get less advance warning of a killer comet....

The Asteroid with Our Name on It

Which explanation for the belt seems the most plausible?

The belt is where all the asteroids happened to survive.

Io is squished and stretched as it orbits Jupiter. But why is its orbit so elliptical? lo's elliptical orbit means continual changes in the strength and direction of the tidal force from Jupiter and the changing tides flex lo's interior and cause tidal heating.

Tidal Heating

Europa's surface appears heavily cracked even from a distance. Close-up photos show double-ridged cracks, best explained by an icy crust moving upon a soft or liquid layer below. Tidal stresses cause parts of Europa's icy crust to slowly slide past each other. Frictional heating expands ice here, forming the ridge and may melt ice here, collapsing the ridge center.

Tidal Stresses Crack Europa's surface Ice

Which large-sized moon of a jovian planet is the most similar to Earth?

Titan

Titan is the only moon in the solar system to have a thick atmosphere. It consists mostly of nitrogen with some argon, methane, and ethane.

Titan's Atmosphere

Huygens's probe provided the first look at Titan's surface in early 2005. It found liquid methane and "rocks" made of ice. Titan's north pole has lakes of liquid methane and a temperature of −180 degrees Celsius.

Titan's Surface

Distance from the Sun= 19.2 AU Mass= 14M/Earth Density= 1.24 g/cm^3 Composition: H compounds, rock, H and He

Uranus

Visible clouds Gaseous hydrogen Core: rocks and metals, water, methane, and ammonia

Uranus and Neptune

They are made up of numerous, tiny individual particles They orbit around Saturn's equator. They are very thin.

What Are Saturn's Rings Like?

Its moon Charon is nearly as large as Pluto itself (probably made by a major impact). Pluto is very cold (40 Kelvin). Pluto has a thin nitrogen atmosphere that will refreeze onto the surface as Pluto's orbit takes it farther from the Sun. New Horizons has revealed a surprisingly active geology.

What Is Pluto Like?

Only a tiny number of comets enter the inner solar system. Most stay far from the Sun.

Where Do Comets Come From?

They formed from dust created by impacts on moons orbiting those planets. Rings aren't leftover from planet formation because the particles are too small to have survived for so long. There must be a continuous replacement of tiny particles. The most likely source is impacts with jovian moons.

Why Do the Jovian Planets Have Rings?

Orbital resonances with Jupiter prevented planetesimals between Jupiter and Mars from forming a planet.

Why is there an asteroid belt?

According to Kepler's third law, the inner ring particles

move faster than outer ring particles

Is gas escaping from coma, pushed by the solar wind.

plasma tail