Solar System
Mercury – Scientific Overview
Mercury is the smallest planet in the Solar System and the one closest to the Sun, orbiting at an average distance of 57.9 million kilometers (36 million miles). Its small size (diameter of about 4,880 km) and proximity to the Sun make it a unique and extreme world.
Orbit and Rotation
Mercury’s orbital period is just 88 Earth days, making its year the shortest of all planets. However, its rotation is peculiar — it takes about 59 Earth days to rotate once on its axis. Because of this 3:2 spin-orbit resonance, a day (from one sunrise to the next) lasts about 176 Earth days.
Surface and Composition
Mercury’s surface is heavily cratered, resembling our Moon. It’s covered with plains, ridges, and impact basins like the enormous Caloris Basin, which spans about 1,550 km. The planet has almost no atmosphere to erode or weather these features, so they remain preserved for billions of years.
Mercury’s composition is metal-rich — it has a large iron-nickel core that makes up about 85% of its radius, surrounded by a silicate mantle and crust. This unusually large core might be the result of a massive collision early in its history that stripped away much of its outer layers.
Temperature and Atmosphere
Because it’s so close to the Sun and lacks a substantial atmosphere, Mercury experiences extreme temperature changes: from about 430°C (800°F) during the day to -180°C (-290°F) at night.
Its atmosphere — or more accurately, exosphere — is extremely thin and consists mainly of oxygen, sodium, hydrogen, helium, and potassium. This exosphere is constantly being replenished by the solar wind and micrometeor impacts.
Magnetic Field and Interior
Mercury has a surprisingly strong magnetic field for such a small planet, about 1% as strong as Earth’s. It’s thought to be generated by a partially molten outer core. NASA’s MESSENGER mission provided valuable insights into its interior, confirming the presence of a solid inner core.
Exploration
Mercury is difficult to study from Earth due to its proximity to the Sun in the sky. Only two spacecraft have visited it so far: Mariner 10 in the 1970s, which mapped about 45% of its surface, and MESSENGER (2004–2015), which orbited Mercury and provided high-resolution mapping and detailed chemical analysis. The European Space Agency’s BepiColombo mission, launched in 2018, is currently en route and will arrive in 2025.
Scientific Significance
Mercury’s extreme environment makes it a natural laboratory for studying planetary formation and the effects of solar radiation. Its large core, thin mantle, and unique rotation also provide clues about the evolution of rocky planets.
Venus – Scientific Overview
Venus is the second planet from the Sun, orbiting at an average distance of 108.2 million kilometers (67.2 million miles). It is similar in size and mass to Earth — about 12,104 km in diameter — and is often called Earth’s twin. However, in terms of surface conditions, Venus is one of the most hostile environments in the Solar System.
Orbit and Rotation
Venus completes an orbit around the Sun every 225 Earth days. Its rotation is unusual — it spins backward (retrograde rotation) compared to most planets, meaning the Sun rises in the west and sets in the east. Even more peculiar, Venus rotates very slowly, taking 243 Earth days to complete one turn. This means its day is longer than its year.
Atmosphere
Venus has a thick atmosphere, composed mostly of carbon dioxide (96.5% CO₂), with nitrogen making up most of the rest. The surface pressure is about 92 times that of Earth, equivalent to being 900 meters (3,000 feet) underwater. The atmosphere traps heat through a runaway greenhouse effect, making Venus the hottest planet in the Solar System, with an average surface temperature of about 465°C (869°F) — hot enough to melt lead.
The clouds are made primarily of sulfuric acid droplets, which reflect sunlight efficiently, giving Venus its bright appearance in the sky.
Surface and Geology
Venus’s surface is hidden beneath its thick clouds, but radar mapping (such as from NASA’s Magellan mission) has revealed a world dominated by volcanic plains, mountains, and vast lava flows. Notable features include Maxwell Montes, the highest mountain, and Ishtar Terra and Aphrodite Terra, two large highland regions.
Evidence suggests Venus may still have active volcanoes. There are no oceans or liquid water — any water that once existed likely evaporated early in its history due to the extreme heat.
Magnetic Field and Interior
Unlike Earth, Venus lacks a global magnetic field, possibly due to its slow rotation and different interior dynamics. It is thought to have a core similar in size to Earth’s, but without the same kind of convection needed to generate a strong magnetic field.
Exploration
Venus has been visited by more than 20 spacecraft, making it one of the most studied planets. The Soviet Venera program in the 1960s–80s was the first to land probes on its surface, sending back images before being destroyed by the heat and pressure within minutes to hours. NASA’s Magellan (1989–1994) mapped its surface using radar. More recent missions, such as Japan’s Akatsuki orbiter, study its atmosphere and weather patterns. Upcoming missions — NASA’s VERITAS and ESA’s EnVision — aim to map the planet in even greater detail.
Scientific Significance
Venus is important for understanding climate change and atmospheric physics. Its runaway greenhouse effect provides a stark example of how a planet’s climate can evolve dramatically. Studying Venus also helps us compare Earth-like planets and assess their habitability elsewhere
Earth – Scientific Overview
Earth is the third planet from the Sun, orbiting at an average distance of 149.6 million kilometers (93 million miles). It is the only known planet to support life, with abundant liquid water, a breathable atmosphere, and a stable climate. Earth has a diameter of 12,742 km, making it the fifth-largest planet in the Solar System.
Orbit and Rotation
Earth takes 365.25 days to complete one orbit around the Sun, giving us our year. It rotates once every 24 hours, producing the day-night cycle. The planet’s axis is tilted about 23.5 degrees, which causes seasonal changes as Earth orbits the Sun.
Atmosphere
Earth’s atmosphere is composed mainly of nitrogen (78%) and oxygen (21%), with small amounts of argon, carbon dioxide, and other gases. This mixture is essential for supporting life and regulating temperature. The atmosphere is divided into layers: the troposphere (where weather occurs), stratosphere (containing the ozone layer), mesosphere, thermosphere, and exosphere.
The ozone layer protects life from harmful ultraviolet (UV) radiation, while the atmosphere as a whole moderates temperature and prevents extreme daily swings.
Surface and Hydrosphere
About 71% of Earth’s surface is covered by water, mostly in oceans. The remaining 29% is land, consisting of continents and islands. Earth’s surface features mountains, plains, valleys, deserts, forests, glaciers, and polar ice caps. Plate tectonics — the movement of large slabs of Earth’s crust — constantly reshapes the surface, creating earthquakes, volcanoes, and mountain ranges.
Liquid water plays a central role in Earth’s climate and ecosystems. It exists in all three states — liquid, solid (ice), and gas (water vapor) — enabling the water cycle that sustains life.
Magnetic Field and Interior
Earth has a strong magnetic field, generated by the movement of molten iron in its outer core. This magnetic field shields the planet from harmful solar wind particles and cosmic radiation. Earth’s interior is divided into a solid inner core, a liquid outer core, a mantle, and a crust.
Life and Biosphere
Earth’s biosphere includes all living things — plants, animals, fungi, and microorganisms — interacting with the atmosphere, hydrosphere, and geosphere. Life on Earth is supported by the availability of water, oxygen, nutrients, and energy from the Sun. The diversity of life is immense, from deep-ocean organisms to those in high mountains.
Exploration
Earth is continuously studied from the ground, in the air, and from space. Satellites monitor climate, weather, vegetation, and ocean currents. Humans have explored every continent, the deep ocean, and even traveled into orbit to study Earth from above.
Scientific Significance
Earth serves as the baseline for studying other planets. Understanding its climate systems, geology, and biosphere allows scientists to search for habitable worlds beyond our Solar System. Changes in Earth’s atmosphere, such as those caused by human activity, also offer lessons about planetary sustainability.