Collisions in Space Handout #2
Formation of the Solar system
The observations made of the Solar system must be accounted for in any theory regarding
its formation. A successful theory must explain the obvious properties including:
1. Planetary orbits lie approximately in a plane.
2. The planets orbit the Sun in the same sense (ccw viewed from above the Earth’s
north pole).
3. The Sun rotates in the same sense as the planets orbit.
4. Except for Venus and Uranus, the planets rotate in the same sense as they orbit the
Sun and as the Sun rotates.
5. Many planets have moons and rings (satellites); again, almost all of these orbit
their planet in a plane that passes through the planet’s equator and orbit in the
same sense as the planet rotates. (There are many exceptions though.)
6. There are two distinct types of planets: small rocky (terrestrial) planets near the
Sun and large gassy (giant or jovian) planets far from the Sun.
7. In addition to the large planets, there are many asteroids (rocks) and comets (rock
and ice) orbiting the Sun. Asteroid orbits mainly lie in the same plane as the
planets, but comet orbits can be out of the plane or in the plane. While the majority of the asteroids have orbits between Jupiter and Mars, most of the comets are
very far from the Sun, and many of them form a spherical halo around the Solar
system.
A good theory for the formation of the Solar system will explain these deep commonalities. It need not account for each individual fact in detail, since there can be individual
exceptions (e.g. point 4 above), but it must be able to explain the general properties (e.g.
the similarities between planets orbiting the Sun and satellites orbiting their parent planets). Understanding the exceptions from the general scheme is often a very interesting
way to verify the basic model itself.
The leading theory for the formation of the Solar system is that the planets formed from
the same collapsing cloud of gas and dust from which the Sun formed. We can see interstellar clouds of gas and dust as dark patches against star fields. These clouds contain
thousands or millions of solar masses of gas and dust. A cloud, or part of a cloud, can
collapse under its own gravitational “weight.” Whatever slight rotation the cloud may
have will increase as the cloud contracts (like an ice skater pulling in her arms during a
spin); as this happens, the cloud gently flattens into a disk, as pizza dough flattens when
spun. The proto-sun forms in the densest ball of gas at the center of this disk, continuing
to draw gas onto itself by gravitational attraction. This process of slowly building a large
mass by adding small masses is called accretion. At some stage, the mass of the Sun will
be large enough that fusion reactions that convert hydrogen to helium (individual atomic
nuclei smashing together and fusing into the heavier nuclei) start at the core. At this
stage, the Sun officially becomes a star.
Meanwhile, out in the disk, small particles of interstellar dust can grow. Interstellar dust
is composed of iron (a very common metal, relatively speaking) and silicates (combinations of silicon and oxygen; sand is a silicate). As the cloud surrounding the young star
cools, these materials will condense in this protoplanetary disk. As the temperature continues to fall, ice can condense and freeze as well. As you can guess, ices will preferentially form far from the Sun, which provides nearly all of the thermal energy in the Solar
system. The farther reaches of the Solar system are consequently rich in ices, while the
warmer regions near the Sun are proportionally rich in silicates and metals. Planets far
from the Sun probably have rocky and icy cores, but their gravity will also enable them to
accrete cool gas with high efficiency. By accreting gas from the protoplanetary disk,
these planets grow into gas giants with a large proportion of their mass in a deep gas atmosphere instead of in rocks and metals. This temperature difference in the formation
stages of the Solar system is believed to be the reason for the striking difference in the
nature of the planets: small rocky planets near the Sun, and big gassy planets far from the
Sun.
Collisions are extremely important in the intermediate stages of the formation of the
planets, a phase we have skipped over above. Somehow, the planets’ rocky cores must
form from the tiny grains of interstellar dust that condense from the nebula. The first
phase of this, forming “dust balls,” is still not well understood, but it is likely that static
electricity plays an important role here (the same effect as static electricity has in sticking
all of your socks together in a clothes dryer). As the “dust balls” grow by accreting sand
and dust grains, gravity becomes more important, accelerating the accretion. This is
something of a runaway process: as the balls grow in size, their gravitational attraction
increases, attracting more and more nearby material. Rapidly, the solar system fills with
thousands of planetesimals, objects just a bit smaller than our present-day Moon. These
collide frequently with each other, forming larger and larger objects (and sometimes a
spray of smaller bits and pieces). After a brief but violent period of collisions, the merging planetesimals form planets on stable orbits or are ejected from the Solar system. This
leaves a few planets on approximately circular orbits (if the orbits cross, then collisions
will occur and destroy the objects; this is not a stable situation in the long term). The bits
and pieces that do not form big planets can still form the asteroids and comets – in short,
all of the smaller rubble that’s in the Solar system.
Later collisions
This is not to say that the collisions ended when the major planets formed. We can see
the effects of collisions all through the Solar system, particularly on small quiescent objects like our Moon, other moons in the Solar system, Mercury, and on the surfaces of
asteroids themselves. Other than recording impacts, these surfaces have not changed
much since they first formed. By looking at the surfaces, we can see a history of impacts:
craters on top of craters on top of craters. Some areas on the surfaces are relatively
smooth, recording enormous lava flows that “repaved” the surface. Even in these regions, though, we can see craters left by colliding objects. We can get some estimate of
the size of impactors and the cratering rate through time (how many impacts came at dif2
ferent times) by counting the size and number of craters on relatively well preserved surfaces. From this, we can see that the bombardment rate was very high when the surfaces
were young, but that it then tapered off slowly to its present rather low rate.
Surface modification
The Earth must have been bombarded by many asteroids and comets early in its history,
but we don’t seem to see much trace of the era of impacts. Why does this happen? The
answer lies in a number of processes that go by the general name of geological activity,
indicating some kind of resurfacing action.
First, we may well see one effect that you don’t normally associate with an impact: we
have a pretty noticeable atmosphere. While there is still some debate on the relative importance of the two mechanisms, it is quite likely that both impacts by comets and the
Earth’s own volcanoes played important roles in forming the Earth’s atmosphere and
oceans. Comets, which contain a great deal of ice (water ice, methane ice, ammonia ice,
and other ices), may well have been a major contributor to the Earth’s early oceans and
atmospheric gases.
The Earth’s oceans and atmosphere, on the other hand, make it very difficult for the signs
of craters to survive. Erosion constantly modifies the Earth’s surface, tearing down crater
walls and filling in the basins with silt, stone, and dirt. Erosion is a fairly minor contributor to the modification of the Earth’s surface over long timescales, however. The motion
of the plates that form the Earth’s crust is a far more efficient way to recycle the surface.
The plates, which are rather stiff, brittle, and buoyant, float on the denser and plastic mantle region of the Earth. The mantle is an iron-rich rock that oozes and flows slowly – a
good analogy is steel piano wire, which seems very stiff, but slowly flows and stretches to
put the piano out of tune. The mantle sits on the Earth’s hot inner core. As heat flows
from the hot bottom layer to the cool top layer, convection currents carry mantle material
up in some regions, then horizontally across the surface, and then back down toward the
core; the same pattern (and happening for the same reason) that you can see in a pan of
soup with small noodles as you heat it on the stove. Regions with upwelling material are
spreading centers or rift zones. Material from the mantle adds to the edges of the crustal
plates where material comes up from the Earth’s depths, so the plates expand and move
away from the zones, floating across the Earth’s surface. The Earth has many rift zones,
with the center of the Atlantic Ocean being one of the most prominent.
At some stage, though, the edge of the crust far from the rift zone runs into another plate
edge spreading from a different zone. When the plates collide, something must happen:
one plate must be forced under the other. As the plate edge is forced down, it shatters and
snaps before it melts. These subduction zones are consequently severe earthquake zones.
One prominent region with colliding plates (and terrible earthquakes) is the Himalayas,
the Earth’s highest mountains (although erosion is working hard to tear them down). The
top of Mt. Everest is a huge piece of sea floor that’s been pushed 29,000 feet into the air
by the collision of two plates. Another region whose character is dominated by subduc3
tion zones is the Pacific Rim, a roughly circular region including the West Coast of the
US, Japan, the Philippines, New Zealand, and the coast of South America. We all know
these regions for their earthquakes, high mountains, and the volcanoes that form from the
crust as it melts under high pressure.
Formation of the Moon
In the scheme of things, the Earth’s moon is quite odd, and the question of how it formed
was a real puzzle for a long time. First, we are the only terrestrial planet with a large
moon; Mercury and Venus don’t have moons at all, and Mars has two tiny moons that
look like captured asteroids. Second, the Moon is far too big for the Earth’s size: while
the gas giants all have lots of moons, they are all much smaller than the “parent” planet.
The Earth’s moon is about a quarter of the diameter of the Earth, and the Moon isn’t that
much smaller than Mercury. This makes it too large to be captured or to easily form at
the same time and in the same orbit as the Earth.
Explaining the puzzle of the Moon’s formation is a relatively recent success story. There
were a number of things to explain with the theory:
1. The Earth, alone among the terrestrial planets, has a moon.
2. The Earth’s moon is proportionally very large.
3. Samples from the Lunar surface show that the Moon’s minerals are similar to the
Earth’s, and that it has the same basic structure: crust, mantle, core.
4. The minerals aren’t identical, though. The Moon looks like it was once “cooked”;
it has very little water (and anything else that evaporates easily) locked up in its
rocks.
The solution to this puzzle is a giant impact that took place early during the solar system’s formation. This impact dwarfs the little impact that we believe wiped out the dinosaurs: it was an impact between the forming Earth and a planetesimal about the size of
Mars. The impact ripped crust, mantle, and even some core from the Earth into a long
spray. Much of the spray was able to collapse again under its own gravity, and this
formed the Moon. This theory explains all of the items above, including the very high
temperature that the Moon apparently suffered at some stage early in its formation.
Other major impacts early in the Solar system’s formation
With such clear evidence from the Moon, it is tempting to look for other equally violent
collisions in other places in the Solar system. Two planets stand out. Of all the planets,
only Venus and Uranus rotate in strange directions: Venus rotates backwards (or is upside
down) and Uranus’ rotation axis lies nearly in the plane of the Solar system, instead of
nearly perpendicular like those of the other planets. It is hard to imagine that the planets
could have formed from the protoplanetary disk this way; it is more likely that enormous
collisions sometime during the late stages of planetesimal merging tipped them over to
their present orientation.
4