Transcript Earth and Space

There is a Tide
(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 Tides
The 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.
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Io can’t become tidally locked!
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 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 Field
The 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 Flare
Solar Prominence
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.
Solar Flares
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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.
Aurorae
When 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 Cycle
Once 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 Minimum
Sunspots 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 Minima
The 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
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
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
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 Tails
The 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 Showers
Each 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 Impacts
It 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