Mercury

**Mercury** is the smallest planet in the Solar System and the closest to the Sun. Its orbit around the Sun takes 87.97 Earth days, the shortest of all the Sun's planets. It is named after the Roman god of commerce and travel. It has a average (mean) distance of only 57,910,000 kilometres, and a diameter of 4,880 kilometres. Mercury's orbit is very elliptical and tilted towards the orbit of other planets. The space probe Mariner 10 was the first spacecraft to visit Mercury, in order to obtain information about Mercury, in 1974. The surface was similar to the lunar surface, and the atmosphere appears to be very thin.

## Physical characteristics

### Orbit, size and mass

Mercury is the closest planet in the Solar System, to the central star, Sun. The average distance to Sun is 48 million kilometres (about 70 million kilometres on furthest), as this reflects the elliptical shape of the orbit. The planet is very eccentric, in which it is one of the most elliptic orbits in the Solar System. Mercury rotates once in its axis in 59 Earth days, as Mercury orbits the Sun every 88 days. Mercury's axis of rotation is tilted 2 degrees to the plane of its orbit around the Sun.

Mercury is the smallest planet in the Solar System. With a diameter of 4,878 km, Mercury is 1/3 the size of Earth, and 1 1/2 larger than the Moon. Mercury has a mass of 3.3 x 1023 kg. This mass is contained in a volume of 60.8 billion cubic km (14.6 billion cubic miles). The mass and volume of Mercury is only about 0.055 times that of Earth. But because Mercury's small mass is enclosed inside of a tiny body, the planet is the second densest in the solar system, weighing in 98 percent of the density of our planet. Only Earth is denser. This high density in a planet that otherwise resembles the moon raises interesting questions about the composition of the planet's interior.

### Structure

Mercury is the second densest planet, after Earth, with a density of 5427 kg/m3. It has a large metallic iron core with a radius of about 1,289 miles (2,074 kilometers), about 85 percent of the planet's radius. There is evidence that it is partly molten, or liquid. Mercury's crust, comparable to Earth's crust (called the mantle and crust), is only about 400 kilometers (250 miles) thick and composed of silicate rock.

### Surface

Along with Venus, Earth and Mars, Mercury has many impact craters on the Mercury, similar to the Moon. The craters are called rupes. It was formed when the planet was born, about 4.5 billion years ago, the core of the planet was cooler. First images of Mercury was prepared in 1920, identifying various dark and light spots and cloud layers. The largest known crater is Caloris Planitia, or Caloris Basin, with a diameter of 1,550 kilometres. The impact that created the Caloris Basin was so powerful that it caused lava eruptions and left a concentric mountainous ring ~2 km tall surrounding the impact crater.

## Atmosphere

Instead of an atmosphere, Mercury possesses a thin exosphere made up of atoms blasted off the surface by the solar wind and meteoroids. Mercury's exosphere is composed mostly of oxygen, sodium, hydrogen, helium and potassium. Mercury's magnetic field, or magnetosphere, is offset relative to the planet's equator. Though Mercury's magnetic field at the surface has just one percent the strength of Earth's, it interacts with the magnetic field of the solar wind to sometimes create intense magnetic tornadoes that funnel the fast, hot solar wind plasma down to the surface of the planet. When the ions strike the surface, they knock off neutrally charged atoms and send them to the sky.

## Visibility

At its brightest, it can reach magnitude -1.9, brighter than Sirius, the brightest star in the sky. However, it more normally hovers at around magnitude 0, becoming roughly the fifth brightest object in the sky. Mercury can be seen rarely, because of its relative proximity to the Sun. It is visible only shortly before sunrise and after sunset. In Earth's latitudes, the planet can be observed most easily in the evening at spring, and in morning at autumn. However, Mercury's elongation is greatest, merely 28 degrees. Similar to the Venus and the Moon, Mercury has phases, waxings and wanings from full to new Mercury. This planet travels on a path across the solar disc, called transit, with approximately 12 times per century. A telescope would reveal Mercury as a small black disc in the front of our central star, the Sun. This is known as Mercury Transit. The next transit of Mercury will take place on 9 May 2016 (visible in Europe).

## Exploration

Reaching Mercury from Earth poses significant technical challenges, because it orbits so much closer to the Sun than Earth. A Mercury-bound spacecraft launched from Earth must travel over 91 million kilometres (57 million miles) into the Sun's gravitational potential well. Mercury has an orbital speed of 47.4 km/s (29.5 mi/s), whereas Earth's orbital speed is 29.8 km/s (18.5 mi/s). Therefore, the spacecraft must make a large change in velocity (delta-v) to get to Mercury and then enter orbit, as compared to the delta-v required for, say, Mars planetary missions.

The potential energy liberated by moving down the Sun's potential well becomes kinetic energy, requiring a delta-v change to do anything other than pass by Mercury. Some portion of this delta-v budget can be provided from a gravity assist during one or more fly-bys of Venus. To land safely or enter a stable orbit the spacecraft would rely entirely on rocket motors. Aerobraking is ruled out because Mercury has a negligible atmosphere. A trip to Mercury requires more rocket fuel than that required to escape the Solar System completely. As a result, only three space probes have visited it so far. A proposed alternative approach would use a solar sail to attain a Mercury-synchronous orbit around the Sun.

#### *Mariner 10*

The first spacecraft to visit Mercury was NASA's *Mariner 10* (1974–1975). The spacecraft used the gravity of Venus to adjust its orbital velocity so that it could approach Mercury, making it both the first spacecraft to use this gravitational "slingshot" effect and the first NASA mission to visit multiple planets. *Mariner 10* provided the first close-up images of Mercury's surface, which immediately showed its heavily cratered nature, and revealed many other types of geological features, such as the giant scarps that were later ascribed to the effect of the planet shrinking slightly as its iron core cools. Unfortunately, the same face of the planet was lit at each of *Mariner 10*'s close approaches. This made close observation of both sides of the planet impossible, and resulted in the mapping of less than 45% of the planet's surface.

The spacecraft made three close approaches to Mercury, the closest of which took it to within 327 km (203 mi) of the surface. At the first close approach, instruments detected a magnetic field, to the great surprise of planetary geologists—Mercury's rotation was expected to be much too slow to generate a significant dynamo effect. The second close approach was primarily used for imaging, but at the third approach, extensive magnetic data were obtained. The data revealed that the planet's magnetic field is much like Earth's, which deflects the solar wind around the planet. For many years after the *Mariner 10* encounters, the origin of Mercury's magnetic field remained the subject of several competing theories. 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 accurately controlled, mission controllers instructed the probe to shut down. *Mariner 10* is thought to be still orbiting the Sun, passing close to Mercury every few months.

#### *MESSENGER*

A second NASA mission to Mercury, named *MESSENGER* (MErcury Surface, Space ENvironment, GEochemistry, and Ranging), was launched on August 3, 2004. It made a fly-by of Earth in August 2005, and of Venus in October 2006 and June 2007 to place it onto the correct trajectory to reach an orbit around Mercury. A first fly-by of Mercury occurred on January 14, 2008, a second on October 6, 2008, and a third on September 29, 2009. Most of the hemisphere not imaged by *Mariner 10* was mapped during these fly-bys. The probe successfully entered an elliptical orbit around the planet on March 18, 2011. The first orbital image of Mercury was obtained on March 29, 2011. The probe finished a one-year mapping mission, and then entered a one-year extended mission into 2013. In addition to continued observations and mapping of Mercury, *MESSENGER* observed the 2012 solar maximum.

The mission was designed to clear up six key issues: Mercury's high density, its geological history, the nature of its magnetic field, the structure of its core, whether it has ice at its poles, and where its tenuous atmosphere comes from. To this end, the probe carried imaging devices that gathered much-higher-resolution images of much more of Mercury than *Mariner 10*, assorted spectrometers to determine abundances of elements in the crust, and magnetometers and devices to measure velocities of charged particles. Measurements of changes in the probe's orbital velocity were expected to be used to infer details of the planet's interior structure. *MESSENGER*'s final maneuver was on April 24, 2015, and it crashed into Mercury's surface on April 30, 2015. The spacecraft's impact with Mercury occurred near 3:26 PM EDT on April 30, 2015, leaving a crater estimated to be 16 m (52 ft) in diameter.

#### *BepiColombo*

The European Space Agency and the Japanese Space Agency developed and launched a joint mission called *BepiColombo*, which will orbit Mercury with two probes: one to map the planet and the other to study its magnetosphere. Launched on October 20, 2018, *BepiColombo* is expected to reach Mercury in 2025. It will release a magnetometer probe into an elliptical orbit, then chemical rockets will fire to deposit the mapper probe into a circular orbit. Both probes will operate for one terrestrial year. The mapper probe carries an array of spectrometers similar to those on *MESSENGER*, and will study the planet at many different wavelengths including infrared, ultraviolet, X-ray and gamma ray. *BepiColombo* conducted the first of its six planned Mercury flybys on October 1, 2021.

## See also

* Apparent magnitude
* Mercury Transit
* Mercury (element)