There is a Tide There is a Tide (continued)(continued)

Jupiter and the Sun

There is very little difference between Jupiter and a star. The composition of Jupiter is similar to that of a star. Jupiter formed in a mini-nebula, just like the solar nebula. During formation, Jupiter shined by gravitational

contraction, just like a star. Jupiter’s luminosity prevented light elements from

condensing on its inner moons, just like the Sun.

The only difference between Jupiter and a star is that Jupiter hasn’t been able to fuse hydrogen.

Jupiter’s Moons

The four Galilean moons of Jupiter show a range of properties:

Io is entirely molten, except for a thin crust. Volcanos are erupting all the time, covering the surface with lava.

Europa is warm enough under its surface to have liquid water.

Ganymede has rills and grooves on its surface, as if ice has been warmed and cooled.

Callisto is an old, cold moon, with no sign of evolution since it was formed.

Why the difference?

Jupiter and TidesThe tidal force of Jupiter on its moons is much stronger than the tides of the Earth-Moon system. These objects should be tidally locked to Jupiter. But …

Io, Europa, and Ganymede orbit in a 1:2:4 resonance. Io is constantly being perturbed by its neighbors.Io’s orbit is elliptical – its speed changes during its

orbit.

Io can’t become tidally locked!

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Heat and the Moons of Jupiter

As a result of Jupiter’s tides … Io is continually stressed by the tides of Jupiter. Its

interior is kept entirely molten. Europa feels some tidal stress as well. However, since it

is further away, the stress is less. Europa’s interior is probably warm enough to melt ice into liquid water.

Ganymede has been thermally stressed in the past, either by heat from Jupiter’s gravitational contraction, or by tides. The grooves in its surface are probably due to ice expansion and contraction. It is now tidally locked.

Callisto is far enough away from Jupiter to be thermally unaffected. It is a cold body.

The Roche Limit

The closer you get to a body, the stronger its tidal force. What happens when a body gets so close that the tidal stress is greater than the self-gravity holding it together?

If a moon gets within a planet’s Roche limit, it will be ripped apart by tidal forces. The rubble that is left will form a ring.

Planetary Rings

Jupiter Saturn

Uranus

Neptune

The Structure of Rings

When the Voyager satellites reached Saturn, astronomers were greatly surprised by the intricate structure of the rings. Some of the features are still unexplained. However, most can be attributed to the presence of small shepherd satellites.

Ring particles that get too close to an outer shepherd satellite lose energy, due to the gravity of the (slower moving) satellite. Particles which approach an inner satellite are whipped to a higher orbit.

Shepherd Satellites

Kepler’s laws ensure that the inner/outer moons will be moving

faster/slower than the ring particles.

Earth and SpaceEarth and Space

Sun-Earth Interactions

Since the Earth gets its energy from the Sun, any change in the Solar Constant has important consequences for our climate. But the Sun does change:

Over 5 billion years, the Sun has gotten brighter by ~ 75% Over periods of ~ 11 years, the Sun changes its brightness

by a couple of percent.

The Sun’s Differential Rotation

The Sun does not rotate as a solid body: its equator rotates once every 25 days, while regions near the poles rotate every 30 days.

The Sun’s Magnetic Field

Imagine the Sun as a bar magnet, with magnetic field lines cutting through it. The field lines are attached to the Sun.

After a while, differential rotation stretches and stresses the field lines. Kinks develop.

Stretching the Magnetic FieldThe magnetic field kinks appear on the surface as pairs of sunspots.

The spots appear dark because they are cooler than their surroundings – their energy is stored in the magnetic field.

Prominences and Flares

Eventually, something has to give. Just like a rubber band, the field lines will break and release their energy.

Solar Prominence

Solar Flare

The Sun in X-rays

Because the Sun’s temperature is about 6000°, it emits mostly at optical wavelengths. However, solar flares are extremely energetic explosions – they emit their energy in the x-ray part of the spectrum.

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The Sun and Earth

When the Sun has a lot of sunspots, solar flares, and prominences The Earth is warmed by all the additional energy The Earth is bombarded with cosmic rays, i.e., high energy

hydrogen and helium nuclei that are ejected from the Sun. (In other words, a stronger solar wind.) The Earth’s magnetic field and atmosphere protects us from these particles; those that get through are funneled into the atmosphere at the poles.

AuroraeWhen the solar wind hits the Earth’s atmosphere, the particles excite electrons bound to atoms of oxygen and nitrogen. When the electrons fall back down, they produce emission lines.

The Solar CycleOnce the magnetic field lines reconnect, the cycle begins again. Observations show that it takes about 11 years before the lines get stretched to the breaking point.

The sunspot cycle can also be seen by looking at the amount of Carbon-14 in tree rings.

The Maunder MinimumSunspots are easy to spot –you don’t need a telescope (just project the Sun through a pinhole). So good data on the Sun exists all the way to the time of Galileo.

In the 1600’s, the Earth went through a mini-ice age: Europe and Asia were abnormally cold. Apparently the solar cycle hasn’t always been as regular as it has been recently.

Other Sunspot MinimaThe data from Carbon-14 in tree rings (which indicates a good growing season), the geologic history of glaciers, and sunspot observations all show periods of low and high solar activity.

Pink – sunspots; Blue – 14C; Dots – glacier data

Example: the period between 1100-1300 was the medieval “warm” period with many aurora observations. But few aurorae were recorded between 1290-1340 and 1420-1500.

Small Bodies of the Solar System

Systems with more than 2 orbiting bodies are inherently unstable. Unless the orbits are well separated, the result will be gravitational interactions and chaotic orbits. Third bodies will either …

Be ejected completely out of the Solar System Be slung into highly elliptical (but still bound orbits) Crash into the Sun Crash into something else

Comets

Usually, comets are so small and so distant, they are invisible. However, if a comet’s orbit is gravitationally perturbed, it may enter the inner solar system in a highly elliptical orbit. It will then begin to evaporate, as radiation pressure and the solar wind blow material off.

Comets are dirty iceballs. Most reside either in The Kuiper Belt, a region beyond the orbit of Neptune

(between 30 and 50 A.U.) The Oort Cloud, a region about ~50,000 A.U. from the Sun

Comets

The tail of a comet consists of little bits of ice that have been blown off the comet by radiation pressure and the solar wind. The closer to the Sun the comet is, the larger the tail.

The Tail of a Comet

A comet actually has two tails: one of gas (blown off by the solar wind) and one of dust (released by the evaporating ice and blown by radiation pressure). Both always point in the direction opposite the Sun.

Long and Short Period Comets

From Kepler’s 3rd Law Kuiper belt objects have a period of ~ 250 yr. These are

“short period” comets Oort Cloud objects have a period of ~ 10,000,000 yr.

These “long period” comets will only be seen once.

But when these comets enter the inner solar system, their orbits may change!

Comet Period Comet Period

Halley 76 yr Swift-Tuttle 120 yr

Hale-Bopp 2,400 yr Encke 3.31 yr

Comets in the inner solar system will not last very long. They will evaporate!

Earth and the Comet’s TailsThe dust and ice particles blown off a comet continue to orbit the Sun (at least, until all the ice evaporates). When one of these tiny bits hits the Earth’s atmosphere, it quickly burns up: we see a meteor, i.e., a shooting star. When the Earth passes through the tail of a comet, we see a meteor shower.

Meteor ShowersEach year, the Earth passes through material blown off of comets. The closer to the comet, the higher the rate of meteors.

The name of the shower indicates the direction the meteors come from (i.e., the constellation to look in).

Major ImpactsIt is possible that a planet (like the Earth) will intersect the path of comet or an asteroid precisely. This last happened in July 1994, when Comet Shoemaker-Levy (after being tidally disrupted by Jupiter in a previous encounter) crashed into the planet.

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Major Impacts on Earth

Comets and asteroids do hit the Earth periodically. (But not nearly as often as they used to.) The last big impact was in Tunguska, Siberia in 1908.

Everything was destroyed within a 30 km radius. (And the object didn’t even reach the ground!)

The Dinosaur Killer

Frequency of Impacts

Next time -- Other Solar Systems