Introduction: Earth is a unique planet because it
has continents. It is very difficult to imagine any
geological or astronomical event that made the
continents. The most plausible explanation is a very
improbable collision between a very large comet and
the Moon. The chemical composition of the Moon, the
surface of Mercury, and the surface of Mars support
this explanation. The continents made it possible for
life to flourish and evolve into a technological
civilization. It is very unlikely that continents
exist on other planets, so intelligent life forms
probably do not exist there.
_______________________________________

The Earth's crust is made of two layers called sima
and sial. Sima is the lower and denser layer. It is
made mostly of basalt. Sima is 5 to 10 km thick and
covers the entire surface of the Earth. Sial is 5-70
km thick and covers only 30% of the Earth's surface;
this is the elevated part of the Earth's surface called
continents. The remaining 70% of the Earth's surface
is covered with oceans. Sial is sometimes called
granitic layer of the crust, because it is made mostly
of granite, which is made mostly of silica. The
easiest way to understand terrestrial continents is
to imagine them as granite islands floating on an
ocean made of basalt.

With the exception of iron depletion, the geo-chemical
composition of the Moon is similar to the composition
of the Earth. The similarity implies that the missing
sial was transported to the Moon. According to the
'giant impact theory' the Moon was made by a collision
between the Earth and a Mars-sized asteroid. There
are many theories attempting to explain the origin of
the Moon, but none of them explain how the sial was
selectively scooped up from 70% of the Earth's surface
and deposited on the Moon. A glancing collision with
a ball-shaped asteroid could not have scraped off sial
from 70% of the Earth's surface. Even a shovel-shaped
asteroid could not have scraped off so much sial. Any
collision between the Earth and a large solid asteroid
would have produced so much heat that the entire
surface of the Earth would have become perfectly round
(perfectly geoid) surface of liquid granite layer of
uniform thickness. There would have been no islands or
continents after the granite has cooled and solidified.

The temperature of the Earth increases about 36 degrees
Fahrenheit (20 degrees Celsius) for every kilometer
(about 0.62 miles) of depth. This means that the sial
is cool enough to be as hard as a rock.

Tectonic movements could not push horizontally, along
a gentle slope, one tectonic plate on top of another
tectonic plate because tectonic plates are too heavy.
The required pressure is on the order of 10 gigapascals
- much more than the compressive strength of the sial
(less than 270 MPa).

If a geological process removed the sial from 70% of
the Earth's surface, it must have taken place when
the sial was still hot and liquid because at present
the sial is not a pile of rocks, but a solid layer of
bedrock. This hypothetical process is not plausible
because it had to lift liquid sial up to 70 km
against the force of gravity.
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It is very difficult to imagine any geological or
astronomical event that selectively scooped up sial
from 70% of the Earth's surface. I believe that the
most probable event was a sequence of three collisions:

FACT: 2003EL61 is one of the strangest known objects
in the solar system. It is shaped like the american
football, spins end over end every 4 hours, and has
two satellites.

HYPOTHESIS: About 4.5 billion years ago the Kuiper
belt object that is now 2003EL61 collided obliquely
with another, unnamed, large Kuiper belt object.
Let us call the unnamed large Kuiper belt object
Orpheus. The oblique impact with the Orpheus caused
2003EL61 to spin rapidly and it transformed its shape
from a ball to the shape of american football. Some
of the debris coalesced into the satellites. The
probability that the impact was oblique is low, on
the order of 0.01, because the 2003EL61 is the only
large object in the solar system that spins rapidly
and has american football shape.

FACT: The absence of any planet in the place where
Ceres asteroid is now located is the only exception
of the Titius-Bode Law.

HYPOTHESIS: 4.5 billion years ago there was a bigger
asteroid in the place where Ceres is now. Let us call
it Theia. Ceres has rocky core overlain with icy
mantle. Theia had the same composition as Ceres but
it was larger. Orpheus was made mostly of water ice.
Orpheus was not broken into small pieces by the
impact with 2003EL61 because the impact was oblique.
The impact hurled Orpheus into a collision path with
Theia. When Orpheus hit Theia, the impact moved Theia
toward Jupiter and it melted most of the water ice.
The average distance between 2003EL61 and Ceres is
on the order of 5000 Gm (35 AU). The diameter of
Orpheus was probably on the order of 1000 km. The
diameter of Theia is unknown; let us assume that it
was 2000 km. The probability that Orpheus hit Theia
is on the order of 10^-13.

(/ 1.0 (expt (/ (* 5000.0 1000000000) 2000000) 2)) =
1.6e-013

HYPOTHESIS: The enormous gravity of Jupiter hurled
Theia toward the Earth. As Theia was moving toward
the Earth, its mantle of liquid water was vaporized
by the sunlight, creating watery atmosphere. Theia
became very large comet. Its rocky core collided with
the Moon, thus creating a new, hot Moon. A few hours
later Theia's watery atmosphere collided with the
Earth. It ablated some of the Earth's sima and all
sial except the back 30% of the Earth's surface. The
original crust was made of 20 km thick sial layer on
top of 10 km thick sima layer. Some of the original
sial layer near the back of the Earth was not ablated
by the collision, but it was pushed by the collision
towards the back of the Earth where it piled up and
formed the very thick sial layer that is now known
as the continents. Dust particles made from the sial,
the Moon, and the rocky core of Theia were suspended
in the atmosphere that enveloped the Earth and the
new Moon. The atmosphere was so large that it
enveloped Venus, Mercury and Mars. It was probably
shaped like a disk enveloping the ecliptic. Surface
temperature of Mercury, Venus, Earth, and Mars
dropped because the atmosphere scattered sunlight
away from the ecliptic. Some of the atmosphere was
captured by the new Moon. Large quantity of the dust
and larger debris fell on the Moon, Earth, Mercury,
Venus, and Mars over a period of several thousand
years. The surface of the Moon and the Earth were
hot due to the collision, so the dust melted as soon
as it fell. The Moon was cooled quickly by the
contact with the atmosphere, so a few hundred years
later it was so cool that the dust falling on the Moon
did not melt. When the collision separated Theia's
rocky core from its atmosphere, the atmosphere quickly
expanded due to the heat generated by the collision
and due to reduced gravity (no core). The expansion
reduced density of the atmosphere before the collision
with the Earth. Theia was quickly loosing its volatile
atmosphere after the collision with Orpheus because
much of its elliptic orbit was close to the sun.
If it was loosing its atmosphere at the rate of 0.1
meter per day, it had to collide with the Earth in about
10,000 years. The probability that the collision between
Earth and Theia took place within 10,000 years since the
collision between Theia and Orpheus is on the order of
10^-9.

(/ 1.0 (expt (/ (* 816620000 1000.0) 1000000) 2)) =
1.49955e-012

The probability that all these events occurred is
on the order of (0.01)*(10^-13)*(10^-9) = 10^-24.

One percent of stars in our galaxy, called Milky Way,
has Earth-like planets which have liquid water and
thus seem capable of supporting life. This means
that the probability that the Earth has oceans and
continents is on the order of 10^-26.

There are about 100 billion (10^11) stars in our
galaxy and about 7*10^22 stars in the entire visible
universe. The probability that another planet in
the entire visible universe has oceans and continents
is on the order of (10^-26)*(7*10^22) = 7*10^-4;
about one event in one thousand.
_______________________________________

FACT: Moon's Fe/Si ratio is equal to 0.22 as a whole
(crust + mantle + core). This is the lowest known
Fe/Si ratio of any object in the solar system.
(Source: O. L. Kuskov, "Constitution of the Moon: 4.
Composition of the mantle from seismic data" Physics
of the Earth and Planetary Interiors, vol. 102,
p. 239-257, 07/1997.)

FACT: Low density of the Moon suggests that it is
deficient in nickel. Large amounts of iron and nickel
are present in the Moon's crust.

FACT: Mark S. Robinson, a professor of geology in
Arizona State University's School of Earth and Space
Exploration: "Right now it looks as if Mercury formed
with a deficiency in ferrous iron." (Source:
http://www.sciencedaily.com/releases/2008/07/080703140703.htm)

FACT: Water was delivered to the lunar surface from
its interior in volcanic eruptions three billion
years ago, so it must have been present in the
Moon's interior when the Moon was formed. This means
that the Moon could not have been made by collision
of solid bodies because such collision would have
vaporized all its water. (Source: Marc Chaussidon,
"Planetary science: The early Moon was rich in water"
Nature, vol. 454, pp. 170-172, 10 July 2008.)

FACT: A troilite-rich nickel-iron particle found on the
Moon has surface erosion that, according to the authors
of the following article, is due to passage through a
cloud of hot gas and particulate matter. (Source:
David S. McKay, James L. Carter, and William R.
Greenwood, "Lunar Metallic Particle ("Mini-Moon"): An
Interpretation" Science, vol. 171, no. 3970, pp. 479-480,
5 February 1971.)

FACT: Tails of comets contain high concentrations of
ionized carbon monoxide gas.

FACT: Nickel carbonyl and iron pentacarbonyl form upon
treatment of the powdered metals with carbon monoxide
at room temperature. Both carbonyls are volatile
liquids at room temperature. Carbon monoxide forms
carbonyls with other metals, but, with the exception
of methylcyclopentadienyl manganese tricarbonyl, which
is a liquid at room temperature, these carbonyls are
either slightly volatile solids (dicobalt octacarbonyl,
triiron dodecacarbonyl, dimanganese decacarbonyl,
tungsten hexacarbonyl, and molybdenum hexacarbonyl),
or non-volatile solids at room temperature.

FACT: With the exception of nickel carbonyl and iron
pentacarbonyl, these solid carbonyls cannot be made
just by mixing metal powder with carbon monoxide at
room temperature. Chemical reactions that produce
these non-volatile carbonyls require either the
presence of other chemicals, or high pressure, or
high temperature.

FACT: Thermolysis of nickel carbonyl at 220-250°C
produces carbon monoxide and nickel. Thermolysis
of iron pentacarbonyl at approximately 130°C
produces carbon monoxide and iron. UV-photolysis
of iron pentacarbonyl produces nonvolatile diiron
nonacarbonyl. Thermolysis of diiron nonacarbonyl
at approximately 100°C produces carbon monoxide
and iron.

HYPOTHESIS: When Theia became a comet, it produced
lots of carbon monoxide. Soon after Theia's rocky
core collided with the Moon, the debris produced by
the collision was at room temperature and a chemical
reaction between iron, nickel and carbon monoxide
produced carbonyls. The carbonyl vapors were
suspended in the atmosphere surrounding the Earth
and the Moon until the atmosphere condensed on the
Earth. Some carbonyl vapors were captured by the
gravity of Venus. The gravity of the Moon was too
weak to capture the carbonyl vapors, so the vapors
removed iron and nickel from the Moon.

HYPOTHESIS: Some carbon monoxide and carbonyl vapors
were captured by the gravity of Venus. They reacted
with oxygen and thus enriched Venus with carbon
dioxide, iron, and nickel. Large quantities of carbon
dioxide are still present in the atmosphere of Venus.
The iron and nickel were buried under violently
recycled crust.

HYPOTHESIS: When Mercury was bombarded by the
debris, small pieces of its crust were thrown
above Mercury's surface and thereby were exposed
to the carbon monoxide, which reacted with the
crust producing carbonyl vapors. The gravity of
Mercury was too weak to capture the carbonyl
vapors, so the escaping vapors depleted iron and
nickel from the surface of Mercury.

FACT: Between 4.6 and 3.8 billion years ago Mars
had large amount of liquid water; enough to make
lakes and rivers. Sulfates found by NASA's twin
rovers, Spirit and Opportunity, in the Meridiani
Planum in 2004 are the evidence of water acting
in a highly acidic environment. (Source: Bethany
L. Ehlmann, John F. Mustard, Caleb I. Fassett,
Samuel C. Schon, James W. Head III, David J.
Des Marais, John A. Grant & Scott L. Murchie,
"Clay minerals in delta deposits and organic
preservation potential on Mars" Nature Geoscience,
vol. 1, no. 6, pp. 355-358, June 2008) Surface
water disappeared billions of years ago, but
frozen water remained deep below the surface.
Some of this water has been gushing out of
fissures onto the surface of Mars. The latest
eruption of water took place about 10 million
years ago.

FACT: The maximum surface temperature of Mars is
-5°C. The minimum surface temperature of Mars is
-87°C; only 8°C lower than the sublimation point
temperature of carbon dioxide. Mars is cold enough
to freeze all its water, but, with the exception
of its poles, it is warm enough to vaporize all
its carbon dioxide. Carbon dioxide is more
volatile than Water, so it seems that carbon
dioxide, rather than water, should vaporize to
outer space. The relative abundance of carbon
dioxide and the paucity of water implies that
unknown process either removed water or
replenished carbon dioxide.

FACT: Oxygen's bond with carbon is stronger than
its bond with hydrogen. If carbon monoxide and water
are involved in chemical reactions, hydrogen gas
is more likely to be released than oxygen gas.

FACT: Average density of Mars is similar to the
average density of Earth, and yet the surface of
Mars is covered with large amounts of heavy metal:
iron.

HYPOTHESIS: Mars had sufficient gravity to capture
the carbon monoxide and carbonyl vapors, so it was
enriched with iron and nickel. Mars has no plate
tectonics that could burry the iron and nickel, so
these metals are still abundant on its surface.
When young, wet Mars captured large amount of
carbon monoxide and carbonyl vapors, carbon monoxide
reacted with water, took away its oxygen, and
released hydrogen, which vaporized to outer space.
One of the products of these chemical reactions,
carbon dioxide, produced greenhouse effect which
melted the water ice. When the carbon dioxide
dissolved in the liquid water, it formed carbonic
acid, so the wet surface of Mars was acidic.
_______________________________________

Australian astronomer, Nick Hoffman claims that the
Earth is a unique planet because it has continents
(http://www.spacedaily.com/news/life-01x1.html).
He has not explained why there are no continents
on other planets.

I believe that the missing Earth's sial provides the
explanation and I agree with Hoffman that life cannot
evolve into a technological civilization on a planet
that is devoid of continents.

Marine life of our planet is confined to places that
have abundance of energy (either sunlight or chemical
energy), and abundance of essential minerals: iron,
nitrates, phosphates and silicates. The greatest
concentration of sunlight and minerals is on the ocean
surface near continents. The minerals are transported
there by rivers, winds, and by deep ocean currents
deflected upwards by continental shelf. Far from the
continents, the Ocean surface is almost devoid of life
because it is exceptionally deficient in iron. It
blooms after it has been sprinkled with iron.

Primordial oceans had a green tint due to large amount
of dissolved iron. Primordial atmosphere had lots of
methane, but no oxygen. The absence of oxygen was
beneficial for primeval forms of life because amino
acids can not form in an oxygenated atmosphere. Two
and one-half billion years ago cyanobacteria made lots
of oxygen as a byproduct of their photosynthesis. The
oxygen, over the course of millions of years, scrubbed
the iron and the methane, which is a greenhouse gas.
The oxidized iron settled on the ocean floor. At the
same time the free oxygen oxidized the Earth's crust.
When there was no methane left in the atmosphere, the
average temperature plummeted to minus 50 degrees
Celsius. Most of the surface of the Earth was covered
with a thick layer of ice. Some survivors of this
catastrophe eventually evolved to consume the abundant
oxygen and to produce carbon dioxide, which is a
greenhouse gas. Carbon dioxide freezes into dry ice
at minus 78 degrees Celsius. If the Earth had been in
a slightly more distant orbit, the temperature of its
poles would have plummeted to less than minus 78
degrees Celsius, so the carbon dioxide would have been
trapped there and the Earth would have remained an icy
wasteland.

In the absence of continents there would have been no
advanced forms of life on Earth because the entire
surface of the Earth would have been covered with
oceans and the only source of minerals for the marine
life would have been hydrothermal vents. The vents
provide both minerals and chemical energy, but they
cannot support great abundance and diversity of life,
which are necessary for speedy evolution of life.
If there are no continents, there are no land animals
that can make fire, smelt metals, and create
technological civilization.

If a planet's crust has no sial, tectonic movements of
its sima crust may produce ephemeral islands made of
sima. The Hawaiian Islands are such ephemeral islands.
Average Hawaiian island lasts a few million years
before it is submerged by gravity.

A giant asteroid impact may also create an island, but
the impact generates so much energy that the asteroid
is mixed with the crust, so the island has the same
density as the crust, and is submerged by gravity in
a few million years.

The missing sial leaves empty space between tectonic
plates and thus makes plate tectonics possible on
the Earth. Venus is good example of an Earth-like
planet that does not have plate tectonics. Without
plate tectonics to dissipate heat from its mantle,
Venus undergoes a cyclical process in which mantle
temperatures rise for a few hundred million years
until they reach a critical level that weakens the
crust. Then, over a period of a few million years,
subduction occurs on enormous scale, completely
recycling the crust. The subduction would have killed
all higher forms of life if they had been present on
Venus.

We are lucky to have massive Moon. Earth's obliquity
(the angle between the Earth's equator and the plane
of its orbit) is 23.5 degrees. If the massive Moon
had not existed, the Earth's obliquity would have
varied wildly between 0 and 80 degrees. Such
variation would have caused extreme climatic changes.

We are lucky to have plenty of water. If we had had much
less water, all our flora and fauna would have perished
during a snowball period. Terrestrial life barely survived
during the snowball periods under two kilometers thick
layer of ice (http://en.wikipedia.org/wiki/Snowball_earth).
Evolutionary rates were incredibly slow then.

We are lucky to have plenty of heavy elements (called
metals). According to Wikipedia:
(http://en.wikipedia.org/wiki/Metallicity)
"These youngest stars, including the Sun, therefore have
the highest metal content, and are known as "Population I"
stars. Across the Milky Way, metallicity is higher in the
galactic centre and decreases as one moves outwards."

We are lucky to be far away from the galactic center
and its high concentration of dangerous, exploding stars.

We probably survived cataclysms and close calls that
left no evidence that we can study.

My estimates are not precise but they do not have to be
precise to convey important truth: we are the only
civilization in the visible universe, so SETI is a waste
of time. There is another proof that planets having
continents are extremely rare: if they had been common,
extraterrestrial civilizations would have colonized our
galaxy and our planet billions of years ago.
_______________________________________

My explanation/understanding of plate tectonics of all
terrestrial planets of Earth size except the Earth:

Terrestrial (Earth-like) planets are made of high density
minerals covered with low density sial. The sial abounds
in silicates, so its physical properties are similar to
the properties of ceramics. Ceramics are brittle. Their
thermal conductivity and coefficient of thermal expansion
are low. The dense interior of the terrestrial planets
abounds in metals, so its physical properties are similar
to the properties of metals. Metals are ductile. Their
thermal conductivity and coefficient of thermal expansion
are high.

When a terrestrial (Earth-like) planet is young and hot,
its sial surface is liquid. When the planet cools, its
sial solidifies. Sial does not conduct heat well, so it
traps the heat that is generated in the interior by the
radioactive decay. The entire planet warms up and expands
because all its minerals have positive coefficient of
thermal expansion. The metallic interior expands more than
the ceramic sial because its coefficient of thermal
expansion is higher. Great tension builds up in the
ceramic sial until it shatters like a glass pane. We call
this event an earthquake. Liquid magma and volcanic ash
escape through the cracked sial into the atmosphere. We
call this event a volcanic eruption. When the magma cools
and solidifies, it seals the cracks in the sial and the
next cycle begins.

If the entire surface of a terrestrial planet is covered
with sial, enormous tension builds up in the sial over
millions of years. When the sial shatters, the earthquakes
and volcanic eruptions are so enormous that they kill all
all higher forms of life.