The 8 Planets – Part 4: Mars



A very belligerent planet. Mars. Named after the Roman god of war, Mars appears as a fiery battle-stained planet. Out of the seven planets (excluding Earth), Mars seems to be the most habitable planet. Although much smaller than Earth, Mars has an atmosphere (though thinner), ice caps (though mainly dry ice, or frozen carbon dioxide), and evidence that water once flowed on its surface. Astronomers have yet to find life on Mars, but sci-fi authors have long contemplated the possibility of life on Mars in the present or future (e.g. The Martian Chronicles). Don’t let the name fool you. Mars, the god of war, leaves no one alive while Venus, the goddess of love, often has a soft heart. In the solar system, Venus is a deadly planet while Mars is relatively benign.

The fourth planet from the Sun and the last terrestrial planet, Mars, the “Red Planet,” is actually the second smallest planet and the most explored aside from Earth. Distinguishable by its red color from an abundance of iron oxide, Mars has a thin atmosphere blanketing a surface filled with impact craters, volcanoes, valleys, deserts, and polar ice caps. Mars is also home to Olympus Mons, the tallest mountain in the solar system, and Valles Marineris, a large crack on Mars’ surface and one of the largest canyons. Dark patches on its surface suggest presence of large quantities of liquid water in the past. Though similar to Earth in surface features and climate, Mars is only about half the size of Earth, with 15% of Earth’s volume and 11% of mass. Because of its similar axial tilt, Mars has Earth-like seasons, though with a colder climate. Barren, Mars has the largest dust storms in the solar system. Mars is bigger than Mercury but less dense because of an iron sulfide core composed of lighter elements. The core is surrounded by a silicate mantle and the mantle by a thick crust. With no evidence of a global magnetic field, Mars, however, has some magnetized crust that has reversed polarity like Earth’s ocean floors. Mars’ geological history is split into three periods: Noachian period (4.5 – 3.5 billion years ago, oldest surfaces with impact craters and extensive flooding of water), Hesperian period (3.5 to 2.9 – 3.3 billion years ago, extensive lava plains), Amazonian period (2.9 – 3.3 billion years ago to present, few impact craters, Olympus Mons forms, lava flows). On Martian Soil, light silica-rich streaks appear on steep slopes, perhaps the dark underlying layers of soil exposed after dust avalanches. As Earth speeds Mars, Mars appears to move in a retrograde motion, or backwards with respect to the stars. Mars has two moons: Phobos and Deimos.


Phobos and Deimos

Mars’ two moons look more like asteroids than Earth’s moon. Named for Mars’ sons and attendants in battle, Phobos and Deimos mean “fear” and “panic,” respectively. Because of their low albedo, carbonaceous chondrite composition, and irregular shapes, scientists have proposed the capture theory. Stray asteroids from the Trojan belt may have been pulled into Mars orbit. Phobos is below synchronous position, so Mars’ tidal forces will eventually cause Phobos to crash into Mars’ surface, either forming an impact crater or a dust ring in 50 million years. On the contrary, Deimos is outside synchronous position, so the moon is slowly spiraling away from Mars. In about 50 million years, Mars may have no moons!

MISSIONS*: Mariner, Mars, Viking, Mars Global Surveyor, Mars Pathfinder, Mars Odyssey, Mars Express, Spirit, Opportunity, Rosetta, Mars Reconnaissance Orbiter, Phoenix, Dawn, Mars Science Laboratory

* Successful Missions


  • Order in Solar System: #4
  • Number of Moons: 2
  • Orbital Period: 687 days
  • Rotational Period: 24 hours, 39 minutes, 35 seconds
  • Mass: 6.4185 x 10^23 kg (0.107 Earths)
  • Volume: 1.6318 x 10^11 km³ (0.151 Earths)
  • Radius: 3,396 km (0.533 Earths)
  • Surface Area: 1.45 x 10^8 km² (0.284 Earths)
  • Density: 3.9335 g/cm
  • Surface Pressure: 0.636 kPa
  • Eccentricity of Orbit: 0.093315
  • Surface Temperature (Average): 210 K
  • Escape Velocity: 5.027 km/s
  • Apparent Magnitude: +1.6 to -3.0

The 8 Planets – Part 3: Earth


Earth, our home planet

Imagine a blank sphere floating in the middle of space. Now picture the whole sphere flooded by blue oceans, rivers, and lakes. And seven continents, defined by low elevation green patches, high elevation brown areas, and deserts golden brown. Add white ice caps capping the North and South Poles. And white swirling clouds in the atmosphere. Then tilt the whole sphere 23.5 degrees. There. Our home planet, Earth!

Third planet from the Sun and the only planet to support life, Earth, or the Blue Planet, formed 4.54 billion years ago from accretion of the solar nebula and first hosted life approximately 1 billion years ago. Though technically not named after any Gods, the Greek god Gaea is mother of the earth. Home to millions of species, Earth has the “Goldilocks Phenomenon” since all conditions including climate and temperature support life. Earth is in the “life zone,” where water exists in all three phases: gas, liquid, and solid. Earth’s surface is 30% land and 70% water. Collectively, the biosphere and the abundance of minerals support life. Earth’s atmosphere, specifically the ozone layer, and magnetic field blocks high-energy electromagnetic radiation harmful for life. The axis of the Blue Marble, the largest terrestrial planet, tilts 23.5 degrees, causing the four annual seasons. The hemisphere tilting toward the Sun is in summer and the other is in winter. In fact, Earth’s orbit is nearly circular and Earth is actually closer to the Sun in winter than in summer. Earth’s tectonic activity, or the sliding of tectonic plates, causes volcanic activity and earthquakes that renew Earth’s surface. A viscous liquid mantle and a rigid crust surround a solid core. Earth orbits the Sun once every 365.25 days and rotates once every 24 hours. Earth has one moon, or natural satellite.


The Moon

Earth only has one moon, called the Moon. Reflecting sun light, this natural satellite orbits the Earth once every ~29 days, seen in different phases throughout every month: New Moon, Waxing Crescent, First Quarter, Waxing Gibbous, Full Moon, Waning Gibbous, Third Quarter, Waning Crescent. Formed about 4.53 billion years ago, the Moon is an imperfect sphere bombarded by asteroids and comets during the Late Bombardment Period 3.8-4.1 billion years ago. In fact, the Near Side is much smoother than the Far Side (never observed from Earth), so one theory is that the Moon was actually two chunks of material that collided. However, the giant impact hypothesis indicates that a large object collided into Earth’s surface and while some mass fused with Earth, the rest formed the Moon. This theory explains why the Moon’s interior is similar to that of Earth. The Moon’s gravitational pull contributes the movement of ocean tides, stabilizes Earth’s tilt, and gradually slows the Earth’s rotation.


  • Order in Solar System: #3
  • Number of Moons: 1
  • Orbital Period: 1 year
  • Rotational Period: 1 day
  • Mass: 2.9736 x 10^24 kg
  • Volume: 1.08321 x 10^12 km³
  • Radius: 6,371 km
  • Surface Area: 5.10 x 10^8 km²
  • Density: 5.515 g/cm
  • Surface Pressure: 101.325 kPa
  • Eccentricity of Orbit: 0.0167
  • Surface Temperature (Average): 287.2 K
  • Escape Velocity: 11.186 km/s
  • Apparent Magnitude: N/A

The 8 Planets – Part 2: Venus



An inferno fireball on the inside, a smooth yellow marble on the outside. Venus, the two-faced planet known as “heaven and hell.” Beautiful yet dangerous, Venus is rightfully named after the Roman goddess of love and beauty. In modern culture, people associate Venus with beauty products… and Venus Williams, the world champion tennis player.

Shrouded by its thick sulfuric cloud atmosphere, Venus is the second planet from the Sun and the hottest planet on average in the solar system. Also known as the Morning Star or Evening Star, Venus reflects sun light strongly, with a high albedo. Because Venus’ size is similar to Earth’s, Venus is sometimes to referred to as “Earth’s twin” or “Earth’s sister.” Other than size, however, Venus and Earth have nothing in common. Venus’ atmosphere rains sulfuric acid on the dry dessert-like surface! Its thick atmosphere (90 times thicker than Earth’s) composed of mainly CO2 traps carbon dioxide (greenhouse effect) and maintains a searing temperature on Venus. Venus may have harbored water once, but rising temperatures evaporated all liquid water, leaving a volcanically active surface.  Mapped in 1990-1991 by Project Magellan, Venus’ surface comprises of 80% smooth, volcanic plains (70% plains with wrinkled ridges and 10% smooth plains) and 20% two highland “continents” Aphrodite Terra and Ishtar Terra. Venus has little impact craters but various volcanic features such as “novae” (star-like fracture systems) and “arachnoids” (spider-web-like fractures). Scientists know little about Venus’ interior without seismic data, but Venus’ size and density suggest an interior similar to Earth’s. Scientists have attempted to build probes to land on Venus’ surface, but all attempts failed (most only enter Venus’ atmosphere then burn up and crash). Venus’ clouds reflect and scatter 90% of sunlight, so scientists can only map its surface with radar. In fact, Venus’ atmosphere has an ozone layer and its clouds can produce lightning! Unlike any other planet, Venus spins from east to west, in a retrograde motion. Because Venus spins backward, its rotational period is longer than its orbital period; a day on Venus is longer than a year! Unlike Earth, Venus has a negligible magnetic field, unable to divert most solar wind. Like Mercury, Venus undergoes phases as seen from Earth. When Venus is in a crescent phase observers can actually see a mysterious ashen light. In the 17th century, Galileo proved the heliocentric theory with observations of Venus’ phases. Though Venus has no moons, scientists believe the planet had at least one that crashed into its surface. 10 million years after the collision, another impact changed Venus’ spin. Another possibility is that strong solar tides can disturb large satellites. Recently, the Transit of Venus occurred in June, when the planet crossed over the Sun.

MISSIONS: Venera, Sputnik, Mariner, Cosmos, Vega, Pioneer Venus, Magellan, Cassini, MESSENGER, Venus Express

*Many of these missions (Sputnik, Mariner) are series with only some successful and some only fly-bys; Venera is exclusive for Venus


  • Order in Solar System: #2
  • Number of Moons: 0
  • Orbital Period: 225 days
  • Rotational Period: 243 days
  • Mass: 4.8685 x 10^24 kg (0.815 Earths)
  • Volume: 9.28 x 10^11 km³ (0.866 Earths)
  • Radius: 6,052 km (0.9499 Earths)
  • Surface Area: 4.60 x 10^8 km² (0.902 Earths)
  • Density: 5.243 g/cm
  • Surface Pressure: 9.3 MPa
  • Eccentricity of Orbit: 0.2
  • Surface Temperature (Average): 735 K
  • Escape Velocity: 10.36 km/s
  • Apparent Magnitude: -4.9 (crescent) to -3.8 (full)

The Solar System: Basics

The Solar System


  • Eccentricity of Orbit: measures the ellipticity of orbit (ranges 0-1, with 0 as spherical and 1 as very elliptical)
  • Density: mass per unit volume; mass in grams and volume in cubic centimeters
  • Oblateness: measures how much the middle section of the planet bulges
  • Surface Gravity: the larger the surface gravity, the thicker the atmosphere as gravity pulls in more gases
  • Albedo: measures the fraction of light reflected compared to the amount of light received from the Sun; the higher the albedo, the more reflective the surface
  • Escape Velocity: minimum speed or velocity needed to escape the planet’s gravitational pull
  • Rotation: most planets rotate in counter-clockwise direction (prograde); others rotate in the clockwise direction (retrograde)
    • Rotational period is shortest for gaseous planets and longest for Venus
  • Roche Limit: about two and a half times the radius of the planet; within the Roche Limit, matter cannot accretes to form moons because the tidal force of the planet tears matter apart to form rings

Giant Planets: Giant planets have lighter elements such as hydrogen and helium in their atmospheres. They have stronger gravity and are at larger distances from the Sun. Jupiter, Saturn, and Neptune are stormy with great spots of lasting storms and belts and zones. However, Uranus is comparatively bland and uniform. All giant planets are home to convection, or hot gases rising and cold gases falling.

Terrestrial Planets: Terrestrial planets have heavier elements such as carbon, oxygen, and nitrogen. Mercury is most heavily cratered while Earth is least cratered. Larger terrestrial planets have plate tectonics. Earth has a sizable magnetic fields that can protect it from solar wind particles and Van Allen Belts. Earth has the “Goldilocks phenomenon,” or the right conditions for the development of life.


The Planets: Part I

“The Planets” sub-page under the “The Solar System” page has now been complete and updated with pictures and additional information. Part I includes the terrestrial planets: Mercury, Venus, Earth, and Mars! See the page here.