QUESTION: 
From a small child: How do scientists know that Mars was formed by the big
bang?

ANSWER from the Internet:
http://seds.lpl.arizona.edu/nineplanets/nineplanets/origin.html

The Origin of the Solar System
-by Frank Crary, CU Boulder

Here is a brief outline of the current theory of the events in the early
history of the solar system:

1. A cloud of interstellar gas and/or dust (the "solar nebula") is disturbed
and collapses under its own gravity. The disturbance could be, for example,
the shock wave from a nearby supernova.

2. As the cloud collapses, it heats up and compresses in the center. It
heats enough for the dust to vaporize. The initial collapse is supposed to
take less than 100,000 years.

3. The center compresses enough to become a protostar and the rest of the
gas orbits/flows around it. Most of that gas flows inward and adds to the
mass of the forming star, but the gas is rotating. The centrifugal force
from that prevents some of the gas from reaching the forming star. Instead,
it forms an "accretion disk" around the star. The disk radiates away its
energy and cools off.

4. First brake point. Depending on the details, the gas orbiting
star/protostar may be unstable and start to compress under its own gravity.
That produces a double star. If it doesn't ...

5. The gas cools off enough for the metal, rock and (far enough from the
forming star) ice to condense out into tiny particles. (i.e. some of the gas
turns back into dust). The metals condense almost as soon as the accretion
disk forms (4.55-4.56 billion years ago according to isotope measurements of
certain meteors); the rock condenses a bit later (between 4.0 and 4.5
billion years ago).

6. The dust particles collide with each other and form into larger
particles. This goes on until the particles get to the size of boulders or
small asteroids.

7. Run away growth. Once the larger of these particles get big enough to
have a nontrivial gravity, their growth accelerates. Their gravity (even if
it's very small) gives them an edge over smaller particles; it pulls in
more, smaller particles, and very quickly, the large objects have
accumulated all of the solid matter close to their own orbit. How big they
get depends on their distance from the star and the density and composition
of the protoplanetary nebula. In the solar system, the theories say that
this is large asteroid to lunar size in the inner solar system, and one to
fifteen times the Earth's size in the outer solar system. There would have
been a big jump in size somewhere between the current orbits of Mars and
Jupiter: the energy from the Sun would have kept ice a vapor at closer
distances, so the solid, accretable matter would become much more common
beyond a critical distance from the Sun. The accretion of these
"planetesimals" is believed to take a few hundred thousand to about twenty
million years, with the outermost taking the longest to form.

8. Two things and the second brake point. How big were those protoplanets
and how quickly did they form? At about this time, about 1 million years
after the system began to form, the star would generate a very strong solar
wind, which would sweep away all of the gas left in the protoplanetary
nebula. If a protoplanet was large enough, soon enough, its gravity would
pull in the nebular gas, and it would become a gas giant. If not, it would
remain a rocky or icy body.

9. At this point, the solar system is composed only of solid, protoplanetary
bodies and gas giants. The "planetesimals" would slowly collide with each
other and become more massive.

10. Eventually, after ten to a hundred million years, you end up with ten or
so planets, in stable orbits, and that's a solar system. These planets and
their surfaces may be heavily modified by the last, big collision they
experience (e.g. the largely metal composition of Mercury or the Moon).

ANSWER from Mary Urquhart on July 17, 1997:
Mars wasn't created by the Big Bang, but the stuff Mars is made of (and that
everything else in the Universe is made of) was.  A long, long time before
our solar system with all of its planets (including Mars) was made, the Big
Bang happened and the Universe was born.  The stuff in Mars had to be
reshaped into new types of stuff by spending time in stars before it became
part of Mars.  There are a lot of different ways astronomers know about the
Big Bang. We see all of the galaxies in the Universe all rushing away from
each other. We also see a type of light that fills the Universe (that you
can't see) that has been stretched quite a lot over time. These observations
suggest that the entire Universe is expanding (like the surface of a balloon
when you blow into it).  The event that began the expansion (and the
Universe) was the Big Bang.

Then about 4 and a half BILLION years ago (that's about 16 times longer ago
than when the very first dinosaur lived!) the Sun, Mars, and all of the
solar system formed from a piece of a giant cloud of gas and dust drifting
between the stars.  The Universe had already been around a long time by
then.  We know that all of the planets are the same age and when they formed
partially by studying the planets and the Sun and partially by looking at
meteorites.  Some meteorites are left over bits from when the planets
formed. We can also see clouds like the one we think our own solar system
formed from with baby stars inside.  Some of these baby stars even appear to
have planets starting to form around them!

You can see a gas cloud with baby solar systems inside at:
http://oposite.stsci.edu/pubinfo/PR/94/24.html