Mercury looks like Earth's Moon. It has many craters and areas of smooth plains, no moons around it and no atmosphere as we know it. However, Mercury does have an extremely thin atmosphere, known as an exosphere. Unlike Earth's Moon, Mercury has a large iron core, which gives off a magnetic field about 1% as strong as that of the Earth. It is a very dense planet due to the large size of its core. Surface temperatures can be anywhere from about 90 to (−183 °C to 427 °C, −297 °F to 801 °F), with the subsolar point being the hottest and the bottoms of craters near the poles being the coldest.
Known sightings of Mercury date back to at least the first millennium BC. Before the 4th century BC, Greek astronomers thought that Mercury was two different objects: one able to be seen only at sunrise, which they called Apollo; the other that was only able to be seen at sunset, which they called Hermes. The English name for the planet is from the Romans, who named it after the Roman god Mercury, which they thought to be the same as the Greek god Hermes. The symbol for Mercury is based on Hermes' staff.
Even though Mercury is the closest planet to the Sun, it is not the warmest. This is because it has no greenhouse effect, so any heat that the Sun gives to it quickly escapes into space. The hottest planet is Venus.
Mercury is one of four inner planets in the Solar System, and has a rocky body like the Earth. It is the smallest planet in the Solar System, with a radius of . Mercury is even smaller than some of the largest moons in the solar system, such as Ganymede and Titan. However, it has a greater mass than the largest moons in the solar system. Mercury is made of about 70% metallic and 30% silicate material. Mercury's density is the second highest in the Solar System at 5.427 g/cm³, only a little bit less than Earth’s.
Mercury's surface looks similar to the surface of the Moon. It has plains that look like mares and has lots of craters. Mercury was hit by a lot of comets and asteroids 4.6 billion years ago. Mercury was also hit during a period called the Late Heavy Bombardment. Mercury has lots of craters because it does not have any atmosphere to slow objects down. Images gotten by "MESSENGER" have shown that Mercury may have shield volcanoes.
The surface temperature of Mercury ranges from 100 to 700 K (−173 to 427 °C; −280 to 800 °F) at the most extreme places. Even though the temperature at the surface of Mercury in the day is very high, observations suggest that there is frozen water on Mercury.
Mercury is too small and hot for its gravity to keep any thick atmosphere for a long time. It does have a thin exosphere that contains hydrogen, helium, oxygen, sodium, calcium, potassium. This exosphere is lost and replenished from lots of sources. Hydrogen and helium may come from the solar wind. Radioactive decay of elements inside the crust of Mercury is another source of helium, and also sodium and potassium.
Mercury has the most eccentric orbit of all the planets; its eccentricity is 0.21. Its distance from the Sun ranges from 46,000,000 to 70,000,000 km (29,000,000 to 43,000,000 mi). It takes 87.969 Earth days to go around the Sun. Mercury's axial tilt is 0.027 degrees which is best measurement of the axial tilt.
Many man-made satellites have been sent to Mercury to study it. They are:
The first spacecraft to visit Mercury was NASA's Mariner 10. It stayed in Mercury's orbit from 1974–1975. Mariner 10 provided the first close-up pictures of Mercury's surface. It showed many types of geological features, such as the craters. Unfortunately, the same face of the planet was day at each time Mariner 10 flew close to Mercury. This made close observation of both sides of the planet impossible. In the end, less than 45% of the planet's surface was mapped.
The Mariner 10 came close to Mercury three times. At the first time, instruments found a magnetic field, which surprised planetary geologists because Mercury's rotation was too slow to generate a magnetic field. The second time was mainly used to take pictures of Mercury's surface. At the third time, more information about the magnetic field were obtained. It showed that the planet's magnetic field is much like Earth's.
On March 24, 1975, just eight days after its final close approach, Mariner 10 ran out of fuel. Because its orbit could no longer be controlled, mission controllers instructed the probe to shut down. Mariner 10 is thought to still be orbiting the Sun.
The second satellite to reach Mercury is NASA's MESSENGER. It stands for MErcury Surface, Space ENvironment, GEochemistry, and Ranging. It was launched on August 3, 2004. It made a fly-by of Earth in August 2005. It made another fly-by of Venus in October 2006. It made its first fly-by of Mercury happened on January 14, 2008, a second on October 6, 2008, and a third on September 29, 2009. Most of the hemisphere not mapped by "Mariner 10" was mapped during these fly-bys. The satellite entered an elliptical orbit around the planet on March 18, 2011. The first image of Mercury orbiting the Sun was gotten on March 29, 2011.
MESSENGER was made to study Mercury's high density, the history of Mercury's geology, its magnetic field, the structure of its core, whether it has ice at its poles, and where its thin atmosphere comes from. "MESSENGER" crashed into Mercury's surface on April 30, 2015.
The European Space Agency and the Japanese Space Agency developed and launched a joint mission called "BepiColombo." It will orbit Mercury with two probes: one to map the planet and the other to study its magnetosphere. It was launched on October 20, 2018. "BepiColombo" is expected to reach Mercury in 2025. It will release the probe that will study the magnetosphere into an elliptical orbit. It will then release the probe the will make a map of Mercury into a circular orbit.
Mars
Mars is the fourth planet from the Sun in the Solar System and the second smallest planet. Mars is a cold terrestrial planet with polar ice caps of frozen water and carbon dioxide. It has the largest volcano in the Solar System, which is known as Olympus Mons, and some very large impact craters. Mars is named after the mythological Roman god of war because it appears of red color.
Space probes such as the Viking program landers are the main tools for the exploration of Mars.
It is sometimes known as the red planet
Mars is a terrestrial planet and made of rock. The ground there is red because of iron oxide (rust) in the rocks and dust. The planet's atmosphere is very thin. It is mostly carbon dioxide with some argon and nitrogen and tiny amounts of other gases including oxygen. The temperatures on Mars are colder than on Earth, because it is farther away from the Sun and has less air to keep heat in. There is water ice and frozen carbon dioxide at the north and south poles. Mars does not have any liquid water on the surface now, but signs of run-off on the surface were probably caused by water.
The average thickness of the planet's crust is about 50 km (31 mi), with a maximum thickness of 125 km (78 mi).
Mars has two small moons, called Phobos and Deimos.
The origin of Mars' moons is unknown and controversial. One theory is that the moons are captured asteroids. However, the moons' near circular orbits and low inclination relative to the Martian equator are not in agreement with the capture hypothesis.
Estimates of the mass ejected by a large Borealis-size impact vary. Simulations suggest that a body about 0.02 of Mars mass (~0.002 Earth mass) in size can produce a sizable debris disk in Martian orbit. Much of the material would stay close to Mars. There are several other large impact basins on Mars that could also have ejected enough debris to form the moons.
In the next billion years, Phobos will probably be reformed into a ring around Mars.