Daily Astronomy Picture of the Day from NASA

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Mercury, the first planet from the Sun, has a diameter of about 4,879km (3,032 miles). Mercury is about 57.9 km (36 million miles) from the sun. It orbits the sun once every 88 days.  Mercury is the smallest planet in our solar system. The planet’s crust is pockmarked from crater impacts. The crust is made up of basaltic rocks from eruptions on the planet. The planet has an iron core surrounded by a silica-based mantle and crust. (Ridpath 2006) To date there have been two spacecraft to study Mercury. The Mariner 10 spacecraft did three flybys of Mercury in 1974 and 1975. The second spacecraft, MESSENGER was launched on August 2004 and completed three flybys of Mercury before settling into an orbit around the planet on March 2011 and remained in orbit until the end of its mission in 2015. There is a third spacecraft enroute to Mercury, the BepiColombo mission sponsored by the European and Japanese Space Agencies will reach Mercury sometime in 2025.

Citations

Ridman I. (2006) Eyewitness Companions Astronomy., 90-91 p DK Publishers, London, New York, Munich, Melbourne, Delhi

Venus is the second planet from the Sun and is often called Earth’s "sister planet" because of its similar size, mass, and composition. However, Venus is very different from Earth in many ways.

### Key Characteristics of Venus:

1. **Atmosphere:**

   - Venus has a thick atmosphere composed mainly of carbon dioxide (CO₂), with clouds of sulfuric acid.

   - The atmosphere creates a runaway greenhouse effect, trapping heat and making Venus the hottest planet in the solar system, with surface temperatures around 467°C (872°F).

2. **Surface:**

   - The surface of Venus is rocky, with mountains, valleys, and vast plains covered in volcanic rock.

   - The planet has over 1,600 major volcanoes, more than any other planet, although it's unclear if they are currently active.

   - The surface pressure on Venus is about 92 times that of Earth, equivalent to the pressure found 900 meters (3,000 feet) underwater on Earth.

3. **Rotation:**

   - Venus has a very slow and retrograde rotation, meaning it spins in the opposite direction to most planets, including Earth.

   - One day on Venus (a complete rotation on its axis) lasts 243 Earth days, which is longer than its year of 225 Earth days.

4. **Exploration:**

   - Venus has been explored by several spacecraft, including NASA’s Magellan, which mapped the planet’s surface using radar.

   - The Soviet Union's Venera missions were the first to land on Venus and send back images from its surface, but the harsh conditions quickly destroyed the landers.

5. **Visibility:**

   - Venus is the brightest natural object in the night sky after the Moon, often visible just after sunset or before sunrise, earning it the nickname “the Evening Star” or “the Morning Star.”

### Interesting Facts:

- Venus lacks a magnetic field, likely due to its slow rotation.

- The planet has no moons or rings.

- The surface of Venus is relatively young, geologically speaking, estimated to be about 300–500 million years old.

Venus's harsh environment, thick clouds, and extreme conditions make it a challenging place to study, but it remains a focus of interest in planetary science and future exploration missions.

Planet Earth is the third planet from the Sun and the only known place in the universe where life exists. It is a unique planet with a combination of conditions that support a diverse range of life forms.

### Key Characteristics of Earth:

1. **Atmosphere:**

   - Earth's atmosphere is composed mainly of nitrogen (78%) and oxygen (21%), with trace amounts of other gases like carbon dioxide and argon.

   - The atmosphere protects life by filtering out harmful solar radiation and maintaining a stable climate through the greenhouse effect.

   - It also contains the ozone layer, which absorbs most of the Sun's harmful ultraviolet radiation.

2. **Surface:**

   - Earth’s surface is 71% water, with oceans covering the majority of the planet. The remaining 29% consists of continents and islands.

   - The planet has a variety of landscapes, including mountains, valleys, plains, deserts, and forests.

   - The Earth's crust is divided into tectonic plates that move and interact, causing earthquakes, volcanic activity, and the formation of mountains.

3. **Climate and Seasons:**

   - Earth experiences seasons due to its axial tilt of 23.5 degrees relative to its orbit around the Sun.

   - The planet's climate ranges from polar to tropical, with significant variations in temperature, precipitation, and weather patterns.

4. **Magnetic Field:**

   - Earth has a strong magnetic field generated by the movement of molten iron in its outer core. This field protects the planet from solar wind and cosmic radiation.

   - The magnetic field also causes phenomena like the auroras near the poles.

5. **Life:**

   - Earth supports a vast range of life forms, from microorganisms to large mammals. Biodiversity is found in nearly every environment, from deep ocean trenches to the highest mountains.

   - The biosphere, which includes all living organisms and their interactions with the land, water, and atmosphere, is a key feature of Earth.

6. **Orbit and Rotation:**

   - Earth orbits the Sun at an average distance of about 93 million miles (150 million kilometers), completing one orbit every 365.25 days, which defines a year.

   - The planet rotates on its axis once every 24 hours, resulting in day and night.

### Interesting Facts:

- Earth has one natural satellite, the Moon, which influences tides and stabilizes the planet’s axial tilt.

- The planet is approximately 4.5 billion years old.

- Earth is often called the "Blue Planet" because of its appearance from space, due to the abundant water on its surface.

Earth's unique combination of water, atmosphere, and suitable climate has made it a cradle for life, setting it apart from all other known planets in the solar system.

The Moon is Earth's only natural satellite. It orbits our planet and is about 1/6th the size of Earth. It’s made up of rocky material and has a surface marked by craters, mountains, and plains called maria. The Moon affects Earth in several ways, including the tides through its gravitational pull. It also plays a role in stabilizing Earth's axial tilt, which helps maintain a relatively stable climate.

Mars is the fourth planet from the Sun in our solar system, often called the "Red Planet" because of its reddish appearance, which comes from iron oxide (rust) on its surface. It's about half the size of Earth and has a thin atmosphere primarily composed of carbon dioxide.

Mars has the largest volcano in the solar system, Olympus Mons, and a vast canyon system, Valles Marineris, which is over 4,000 kilometers long. The planet has seasons, polar ice caps, and evidence of past water flows, suggesting that it may have once had conditions suitable for life. Mars is a key target in the search for extraterrestrial life and human exploration.

The asteroid belt is a region in space located between the orbits of Mars and Jupiter, where most of the solar system's asteroids are found. It is roughly 2.1 to 3.3 astronomical units (AU) from the Sun (one AU is the average distance between Earth and the Sun, about 93 million miles or 150 million kilometers). 

The asteroid belt contains a vast number of small rocky bodies, ranging in size from tiny dust particles to large asteroids like Ceres, which is also classified as a dwarf planet. These objects are remnants from the early solar system, believed to be material that never coalesced into a planet due to the gravitational influence of nearby Jupiter. The asteroid belt is an important area of study for understanding the formation and evolution of our solar system.

Jupiter is the largest planet in our solar system, known for its massive size and strong gravitational pull. It’s the fifth planet from the Sun and is classified as a gas giant. Here are some key features of Jupiter:

- **Size**: Jupiter's diameter is about 86,881 miles (139,822 kilometers), making it more than 11 times wider than Earth.

- **Atmosphere**: It has a thick atmosphere composed mainly of hydrogen and helium, with traces of methane, ammonia, and other gases.

- **Great Red Spot**: Jupiter is famous for its Great Red Spot, a massive storm that has been raging for hundreds of years.

- **Rings and Moons**: It has a faint ring system and a large number of moons, with four major ones known as the Galilean moons: Io, Europa, Ganymede, and Callisto. These moons are among the largest objects in the solar system outside the Sun and the planets.

- **Magnetic Field**: Jupiter has a strong magnetic field, which is the largest of any planet in the solar system. It creates intense radiation belts and has a significant effect on its moons and the surrounding space environment.

Jupiter's immense size and complex atmospheric dynamics make it a fascinating subject of study in planetary science.

Saturn is the sixth planet from the Sun and is renowned for its spectacular ring system, which is the most developed and complex in the solar system. Here are some key features of Saturn:

- **Size**: Saturn is the second-largest planet in our solar system, with a diameter of about 72,367 miles (116,464 kilometers), making it nearly 10 times wider than Earth.

- **Atmosphere**: It has a thick atmosphere primarily composed of hydrogen and helium, with traces of methane, ammonia, and other gases. This atmosphere features strong winds and storm systems.

- **Rings**: Saturn's rings are made up of countless ice and rock particles, ranging in size from tiny grains to large chunks. They are divided into several main groups and are visible from Earth with even a small telescope.

- **Moons**: Saturn has a large number of moons, with over 80 known. The largest moon, Titan, is larger than the planet Mercury and has a thick atmosphere of its own. Another notable moon, Enceladus, has geysers that suggest the presence of a subsurface ocean.

- **Magnetic Field**: Saturn has a strong magnetic field, though not as intense as Jupiter's. This field, along with its rings and moons, creates a complex and dynamic magnetosphere.

Saturn's rings and moons make it one of the most visually striking and scientifically intriguing planets in our solar system.

Uranus is the seventh planet from the Sun and is known for its unique characteristics. Here’s a brief overview:

- **Size**: Uranus is the third-largest planet in the solar system, with a diameter of about 31,518 miles (50,724 kilometers). It is slightly smaller than Neptune but more massive.

- **Atmosphere**: The atmosphere of Uranus is composed mainly of hydrogen and helium, with significant amounts of methane, which gives the planet its blue-green color. The methane absorbs red light and reflects blue and green light.

- **Tilt**: One of Uranus's most distinctive features is its extreme axial tilt. The planet is tilted on its side by about 98 degrees relative to its orbit, which means it rotates almost perpendicular to its orbital plane. This unique tilt causes extreme seasonal variations.

- **Rings**: Uranus has a faint ring system composed of dark, narrow rings. There are 13 known rings, which were discovered in the 1970s by the Voyager 2 spacecraft.

- **Moons**: Uranus has 27 known moons, named after characters from the works of William Shakespeare and Alexander Pope. The largest moons include Titania, Oberon, Umbriel, Ariel, and Miranda.

- **Magnetic Field**: The planet has a tilted and offset magnetic field, which is not aligned with its rotational axis.

Uranus is less studied compared to other planets, partly due to its distance from Earth and its faint appearance. The Voyager 2 spacecraft, which flew by Uranus in 1986, provided much of the current knowledge about the planet.

Neptune is the eighth and farthest planet from the Sun in our solar system. It’s known for its striking blue color and its dynamic atmosphere. Here are some key details about Neptune:

- **Size**: Neptune is the third-largest planet by diameter, about 30,598 miles (49,244 kilometers). It’s slightly smaller than Uranus but more massive.

- **Atmosphere**: Its atmosphere is composed mainly of hydrogen and helium, with a significant amount of methane, which gives Neptune its blue color. The methane absorbs red light and reflects blue and green light.

- **Weather**: Neptune has one of the most active atmospheres among the planets, with strong winds and storm systems. The Great Dark Spot is a prominent storm system similar to Jupiter's Great Red Spot, though it has been observed to change over time.

- **Rings**: Neptune has a faint and incomplete ring system made up of dark, narrow rings. These rings are difficult to see and were first observed by the Voyager 2 spacecraft.

- **Moons**: Neptune has 14 known moons, with Triton being the largest. Triton is unique because it has a retrograde orbit, meaning it orbits Neptune in the opposite direction to the planet’s rotation. Triton also has geysers that suggest a subsurface ocean.

- **Magnetic Field**: Neptune has a strong magnetic field that is tilted relative to its rotational axis and offset from the planet's center. This complex magnetic field contributes to the planet’s intense space weather.

Like Uranus, Neptune was first observed indirectly through mathematical predictions before being directly observed through telescopes and spacecraft. The Voyager 2 mission in 1989 provided much of the current detailed information about Neptune.

Pluto is a dwarf planet located in the outer reaches of the solar system, specifically in the Kuiper Belt, a region of space beyond Neptune filled with icy bodies. Here are some key details about Pluto:

- **Size**: Pluto has a diameter of about 1,473 miles (2,370 kilometers), making it smaller than Earth's Moon. It's one of the largest known objects in the Kuiper Belt.

- **Atmosphere**: Pluto has a thin atmosphere composed mainly of nitrogen, with traces of methane and carbon monoxide. This atmosphere can freeze and collapse onto the surface as Pluto moves farther from the Sun, then expand again as it approaches.

- **Surface**: Pluto's surface is a mix of ice and rock, with large areas of frozen nitrogen and methane. It features diverse landscapes, including mountains made of water ice, vast plains, and dark spots. The most prominent feature is Sputnik Planitia, a large heart-shaped plain of nitrogen ice.

- **Moons**: Pluto has five known moons: Charon, the largest and closest moon, as well as Styx, Nix, Kerberos, and Hydra. Charon is about half the size of Pluto, and the two bodies are sometimes referred to as a double dwarf planet system due to their relatively large size compared to each other.

- **Orbit**: Pluto has an elliptical orbit that takes it inside Neptune's orbit at certain points, although their orbits are such that they do not collide. It takes about 248 Earth years to complete one orbit around the Sun.

- **Status**: Pluto was discovered in 1930 and was originally classified as the ninth planet. However, in 2006, it was reclassified as a dwarf planet by the International Astronomical Union (IAU) due to its inability to clear its orbit of other debris, one of the criteria for full planetary status.

Pluto was visited by NASA's New Horizons spacecraft in 2015, providing detailed images and data that significantly advanced our understanding of this distant world.

The sun is about 4.6 billion years old and is the largest item in our solar system. Its size is massive when compared to the earth. Its diameter is 1.4 million kilometers (864,000 miles) and is about 149 million kilometers (93 million miles) from the earth.  The sun’s surface temperature is 5,500 degrees Celsius on the surface (9,930 Fahrenheit). At the center of the sun the temperature is about 5 million degrees Celsius or 127 million degrees Fahrenheit (Ridpath 2006).

The is made up of hydrogen and helium with about 75% hydrogen and 25% helium. It is gravity that keeps these gases together (Ridpath 2006)  The sun has several layers to it. These layers are the corona, transition layer, chromosphere, photosphere, convection zone, radiative zone, and core (Zell, 2017). The corona is only visible during a solar eclipse. The transition layer is a layer between the chromosphere and the corona. This layer has a large in temperature as you go out from the chromosphere to the corona. While the chromosphere is the only sphere of the sun that gets hotter as you move away from the center. The photosphere is the sphere that is visible from earth. The convection zone is where energy gets carried by convection cells to the surface. The radiative zone takes the energy from the core in the form of photons and moves it to the convection zone. Finally, the core of the sun is where the nuclear reactions occur. (Ridpath 2006)

Citations

Ridman I. (2006) Eyewitness Companions Astronomy., 84-85 p. DK Publishers, London, New York, Munich, Melbourne, Delhi

Zell,H. (2017) Layers of the Sun., https://www.nasa.gov/mission_pages/iris/multimedia/layerzoo.html