The Inner Planets


The Average Inner Planet
Feature % of radius
Core 53.6%
Mantle 45.3%
Crust 1.1%
Atmosphere 1.9%

The Core
Every inner planet has a core. The cores of Mercury, Venus, and Earth are composed of iron-nickel. Mars's core is made of liquid iron sulfide mixed with some nickel.
Inner and Outer Cores
Earth and Venus have an inner core and an outer core; the inner core is solid due to the trememdous weight of the matter above it, and the outer core is liquid. Both inner core and outer core have the same composition.

The Mantle
Every inner planet has a mantle. Except for Mercury's, the mantle is a layer which is about of the planet's radius. Every planet's mantle is made of the same stuff: melted silicates mixed with some metallic oxides.

The Crust
Every inner planet has a crust. The crust ranges from about .5% of an inner planet's radius to about 2%. It is always made of rock silicates and metals in these inner planets.

The Atmosphere
Although Mercury's blanket of gas can barely be called an atmosphere, every inner planet has one. The atmosphere is where planets show the greatest range; Mercury's is helium at near-vacuum pressures, Venus's is extremely dense and acidic, ours is nitrogen-oxygen, and Mars's is a thin carbon dioxide atmosphere. They extend from the surface a distance of 2 to 3% of each inner planet's radius.


The inner planets formed about 4.5 billion years ago as huge, hot balls of solids and gases. The Solar Wind, coming from the unstable sun, washed away much of the atmospheres. Eventually, the planets' metals sank to form a core and created magnetic fields which blocked the charged particles.
Average, Minimum, and Maximum: The Inner Planets

Customary Metric

Average Minimum Maximum Average Minimum Maximum
Albedo Average: 32% Minimum: 11% Maximum: 65%
Axal Inclination Average: 12.9° Minimum: 0° Maximum: 25.2°
Average Distance to Sun 84,400,000 mi 36,000,000 mi 141,500,000 mi 135,800,000 km 57,900,000 km 227,700,000 km
Equatorial Diameter 5,674 mi 3,031 mi 7,926 mi 9,131 km 4,878 km 12,756 km
Escape Velocity 4.84 mi/sec 2.67 mi/sec 6.96 mi/sec 7.79 km/sec 4.3 km/sec 11.2 km/sec
Grav. Acceleration 21.2 ft/sec² 12.2 ft/sec² 32.1 ft/sec² 6.5 m/sec² 3.7 m/sec² 9.8 m/sec²
Mass 6.508• 1024 lbs 7.244• 1023 lbs 1.317• 1025 lbs 2.952• 1024 kg 3.286• 1023 kg 5.974• 1024 kg
Mean Density Average: 5.03• water density Minimum: 3.94• water density Maximum: 5.52• water density
Number of Moons Average: 1 (to nearest whole moon) Minimum: 0 Maximum: 2
Orbital Eccentricity Average: .081 Minimum: .007 Maximum: .206
Orbital Inclination* Average: 4.1° Minimum: 1.8° Maximum: 7°
Orbital Period Average: 341.23 earth days Minimum: 87.97 earth days Maximum: 686.98 earth days
Average Orbital Velocity 76,530 mph 53,980 mph 107,130 mph 123,160 kph 86,870 kph 172,410 kph
Polar Diameter 5,661 mi 3,031 mi 7,900 mi 9,110 km 4,878 km 12,714 km
Rotational Period Average: 75.93 earth days Minimum: 23 hrs 56 mins Maximum: 243.02 earth days
Minimum Temperature 40° F -350° F 835° F 5° C -210° C 445° C
Maximum Temperature 480° F 80° F 900° F 250° C 25° C 480° C
*Earth's orbital inclination is not counted because orbital inclination is defined as the angle formed by the plane of the given planet's orbit and the ecliptic, the plane of earth's orbit.

Presenting: The Inner Planets

Mercury is the smallest and innermost of the inner planets. It lacks an atmosphere and goes from freezing to boiling to freezing again every "day".
Venus is a mystery planet. With size, density, and mass all roughly equal to earth's, it was predicted that venus would be a lush jungle teeming with life. Instead, it is a planet even hotter than mercury whose rain is sulfuric acid.
Earth is the only planet in the solar system that we know supports life. Its orbit falls right in the middle of the zone in which life is possible, and it is covered with water.
Mars is the planet from which hostile life comes in almost all science fiction books and movies. When a scientist noted channels carved in Mars's surface, he used his own language-his word for channels was canals-prompting people to believe that they had been carved by sentient beings. Perhaps Mars once supported life, but now it is a frozen desert.
Structure of the Solar System