The Terrestrial Planets
Definition:
A terrestrial planet is a celestial body that is composed primarily of
silicate rocks or metals and has a solid surface. This distinguishes them from
gas giants, which are primarily composed of gases like hydrogen and helium,
water, and some heavier elements in various states.
The term terrestrial planet is derived from the Latin “Terra” (i.e.
Earth). Terrestrial planets are therefore those that are “Earth-like”, meaning
they are similar in structure and composition to planet Earth.
All terrestrial planets have approximately the same type of structure: a
central metallic core composed of mostly iron, with a surrounding silicate
mantle. Such planets have common surface features, which include canyons,
craters, mountains, volcanoes, and other similar structures, depending on the
presence of water and tectonic activity.
Terrestrial planets also have secondary atmospheres, which are generated
through volcanism or comet impacts. This also differentiates them from gas
giants, where the planetary atmospheres are primary and were captured directly
from the original solar nebula.
Terrestrial
planets are also known for having few or no moons. Venus and Mercury have no
moons, while Earth has only the one (the Moon). Mars has two satellites, Phobos and Deimos, but these are more akin to large asteroids than
actual moons. Unlike the gas giants, terrestrial planets also have no planetary
ring systems.
1.
Mercury
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Mercury is the closest planet to the sun.
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It is the smallest planet resembling our moon in both size
and appearance (because it is covered with circular craters left by impacts and
it also contains scarps which are thought to be left when the planet cooled and
shrank).
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Being the closest planet to the sun, mercury has the hottest
surface in the day with noon temperatures of about 923oC and the
coldest surface at night with temperatures reaching 173oc. The
extreme conditions result from the lack of an atmosphere to moderate the inflow
of heat from sun during the day and to retain heat during the night.
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The lack of the atmosphere on the surface of mercury is
because of its small mass which makes its gravity too weak to hold back gases
around it.
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Mercury is the fastest planet, completing one revolution
around the sun in 88 days but rotates very slowly making a complete rotation in
about 58 days.
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Average diameter: 4879.4 km
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Density: 5.427 g/cm3
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Surface gravity: 3.7 m/s2
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Natural satellites: 0
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Orbit period: 87.969 days
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Length of day: 4222.6 hours
web source: http://www.universetoday.com
2.
Venus
Venus
is the second planet from the Sun, and considered in many ways to be a twin
planet of Earth. It has a similar size, mass, density and gravity, as well as a
very similar chemical composition. In other ways, Venus is very different than
Earth, with its high surface temperature, crushing pressure, and poisonous
atmosphere.
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Venus orbits at an average distance of 108 million km from the Sun,
taking almost 225 days to complete one revolution around the Sun.
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One of the strange characteristics of Venus is that it is actually
rotating backwards from the rest of the planets. All of the planets rotate counter-clockwise,
but Venus turns clockwise on its axis. Even stranger is the fact that a day on
Venus lasts 243 days, which is longer its year.
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In terms of size, Venus is quite similar to Earth. Its radius is 6,052
km (95% the size of the Earth). Its volume is about 86% the volume of Earth,
and its mass is 4.87 x 1024 kg, which is about 82% the mass of the
Earth. The gravity on Venus is 90% the gravity on Earth, so if you could
actually walk around on the surface of Venus, the gravity would feel very
similar to Earth.
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In terms of atmospheric composition, Venus is very different from Earth.
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The temperature on the surface of Venus is a blistering 462 °C. That’s
hot enough to melt lead!
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The atmospheric pressure on the surface of Venus is 92 times Earth
pressure. (You would have to travel a kilometre down beneath the surface of the
ocean on Earth to feel that kind of pressure here.
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Venus’ atmosphere is composed almost entirely of carbon dioxide (97%),
and it’s this thick atmosphere that acts like a blanket, keeping Venus so hot.
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Venus has no water on its surface and very little water vapour in its
atmosphere.
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Most of the surface of Venus is covered by smooth volcanic plains, and is
dotted with extinct volcanic peaks and impact craters.
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Venus has no moons or rings.
3.
Mars:
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Mass is the fourth planet from the sun after Earth. The Romans named it after their god
of war, due to the red planet's bloody colour.
This red colour is due to iron-rich
minerals in its regolith (the loose dust and rock
covering its surface).
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Mars is
also smaller than Earth.
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Its
diameter at the equator is 6,794 kilometres, while Earth’s is 12,756
kilometres. If you measure Mars’s diameter from pole to pole, it is 6,752 km,
compared to Earth’s 12,720 km. These slight differences mean that both planets
are not quite perfectly spherical, and are slightly oblate. This slight
flattening is due to the rotation of each planet on its axis.
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Mars is
an Earth-like planet in many ways, but it does vary in size and gravitational
pull. From spacecraft and telescope observations, planetary scientists know
that it is smaller and less massive than Earth. Its mass is 0.107 Earth masses
and its gravity is about 62% less than Earth’s gravitational tug. That means
you would weigh less on Mars than you do on Earth.
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The
length of a Mars day is slightly longer than an Earth day. The Red Planet takes
24 hours and 40 minutes to turn once on its axis. By comparison, Earth’s takes
23 hours and 56 minutes.
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Mars’s
year is also longer than Earth’s. That’s because it takes 687 days to make one
trip around the Sun, as opposed to Earth’s 365.25-day-long year.
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The
Martian atmosphere is the very definition of “thin air”. Its atmospheric
pressure is about a hundred times less than Earth’s.
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Mars is
known as a rocky, “terrestrial” planet, which makes it very similar to Earth.
Recent data from the Mars orbiting missions show that the Red Planet has a
solid iron core, which helps generate the planet’s weak magnetic field. Other
modern studies of the planet suggest that the crust of Mars may have some
simple plates riding atop a solid mantle. Long-ago plate motions are thought to
be what began the formation of the Valles Marineris canyon system.
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The red
planet is home to both the highest mountain; (Olympus Mons) and the deepest, longest valley; (Valles Marinerisin)
the solar system.
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Olympus Mons is roughly 17 miles (27 kilometers)
high, about three times as tall as Mount Everest, while the Valles Marineris system
of can go as deep as 6 miles (10 km) and runs east-west for roughly 2,500 miles
(4,000 km), about one-fifth of the distance around Mars and close to the width
of Australia.
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Constant
volcanic activity from the three largest volcanoes on Mars built up a huge
region called the Tharsis Bulge (or Tharsis Rise) which may well have affected
the planet’s rotation rate. It has also been implicated in Mars’s plate
tectonics and possibly also changes in its climate.
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Mars
shows us a dry, dusty, rocky surface. The southern half of the planet is much
more rugged, with many more craters and highlands. The northern half of Mars
has more smooth basins and what appear to be dry lakebeds and sinuous
riverbeds. This difference in surface characteristics is called the Martian
dichotomy. Impact craters account for part of the dichotomy, and planetary
scientists suspect that long-gone oceans and lakes explain the smoothness of
some areas, while the motion of long-melted glaciers may have carved out
interesting terrain in other places. There are ice caps at both poles that grow
and shrink with the change of seasons.
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