Theia (Proto-Luna) was probably a gas planet

Luna's crust is made of the same rocks as the Earth's
crust, so it was part of the Earth long time ago.
Some astronomers believe that a collision between two
rigid bodies called Protoearth and Theia produced the
Earth and the Luna, but it could not have been
collision between two rigid bodies because:

1. Seismic velocity structures of the Moon's middle
and lower mantle are consistent with melting of the
upper mantle only.

2. The Moon's volatile elements are not depleted as
expected from the giant impact hypothesis. For
example, the Rb/Cs ratio of the Moon is lower than
the Earth's.

3. There is no evidence that the Earth ever had a
magma ocean.

I believe that these facts can be explained if Theia
was made mostly of hydrogen and helium. The collision
was between the Protoearth and Theia's atmosphere.
The gaseous Theia could not survive for a long time
close to the sun, so it seems likely that it was
originally located in the asteroid belt and was
knocked toward the Protoearth by Jupiter shortly
before the collision.

The existence of gaseous Theia in the asteroid belt
is supported by the Titius-Bode Law. If Theia was a
gas planet, it must have collided with the Protoearth
twice a within short period of time. Here is the proof:

1. At the time of the first collision the Protoearth
orbit was nearly circular because its planetary
neighbors (Venus and Mars) have nearly circular
orbits.

2. The first collision removed 25 km thick layer of
crust from 70% of the Protoearth surface. This means
that the first collision changed its circular or nearly
circular orbit into eccentric orbit. After the first
collision the Protoearth was inhospitable to life
because its eccentric orbit exacerbated temperature
differences between summers and winters. If the
Protoearth's orbit had been eccentric for a long time,
it would have perturbed Venus and Mars and it would have
probably collided with one of these planets.

3. The Earth orbit is nearly circular now, so something
must have changed its eccentric orbit. It was probably
another soft collision between the Protoearth and Theia's
atmosphere. When Jupiter hurled Theia into the eccentric
orbit, Theia became the biggest comet in the solar system.
It was loosing volatiles fast. When it collided with the
Protoearth for the second time, it still had enough gas
to ensure soft collision.

Probability of this scenario is so low that it could
not happen twice in the entire universe. If this is
the only way to make a planet that has both liquid water
and permanent continents, all extraterrestrial planets
are devoid of permanent continents. These planets may
have ephemeral islands, but these islands can neither
support advanced land species nor can they provide
abundant minerals for the ocean's surface. This means
that the extraterrestrial planets are probably devoid
of life except for small enclaves fertilized by
underwater vents and inhabited by blind animals.

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Australian astronomer Nick Hoffman was probably the
first person who noticed that the Earth is very rare
planet because it has both liquid water and large
continents. Here is his article:
http://www.spacedaily.com/news/life-01x1.html