My Science School

          Mercury is the innermost planet in the solar system. Since it is the closest to the Sun, Mercury is the most difficult planet to see because it is always seen quite near to the Sun in the sky and the Sun's glare or the bright sky usually overwhelms the planet's light.

          The only chance to see it is as a faint "star" in the morning or evening sky near the horizon, shortly before sunrise in the dawn or just after sunset in the dusk. So it has always been almost impossible to get any information about the surface of the planet by means of ground-based observations. The first, detailed images were obtained with the NASA Mariner 10 spacecraft which also procured most of our present information about Mercury's surface.

          Like our Moon, Mercury is small and its surface is scarred by craters that were formed by impacting rocks and asteroids, soon after the birth of the solar system. They smashed into the planet and blasted the material away from the surface. Mercury also has real cliffs, or scarps which formed when the young cooling planet shrunk like an old apple, with wrinkles on its surface.

          Mercury has the largest day-to-night temperature variation of all planets. The days are burning hot (about 400 °C) and the nights are freezing cold (about -200 °C). This is because it only has a very thin atmosphere.

         Mercury is one of the most heavily scarred objects in the Solar System. Thousands of meteor craters cover the planet, including the largest — the Calories Basin. This was formed when a piece of rock 100km (60 miles) wide collided with Mercury 3.6 billion years ago. Mercury is also shaped by wrinkles and cracks that formed when the surface of the planet cooled and shrank.

          The orbit of Mercury is the most eccentric of the planets in our Solar System. The planet has an orbital period of 87.969 Earth days. At perihelion it is 46,001,200 km from the Sun and at aphelion it is 69,816,900 km, a difference of 23,815,700 km giving it an eccentricity of 0.21. Mercury’s orbit is inclined by 7 degrees to Earth’s ecliptic. Mercury can only be seen crossing the face of the Sun when the planet is crossing the plane of the ecliptic and is between the sun and Earth. This happens about once every seven years.

Source: Orbit of Mercury – Universe Today

          A more precise value of the eccentricity of Mercury's orbit is 0.205 630. By comparison, the eccentricity of Earth's orbit is 0.0167086, and the eccentricity of the orbit of Venus is 0.006772.

          Mercury is locked in a 3:2 spin-orbit resonance making three rotations about its spin axis every two orbits about the sun. Because of this, if you were on the surface of Mercury, the Sun would pass overhead once every two orbits around the Sun, or 176 Earth days. In other words, one day on Mercury (sunrise to sunrise) takes two Mercury years. A Mercury year takes 88 Earth days, the length of time to orbit the Sun.

Source: Mercury’s Orbit

          So one solar day on Mercury is about 176 Earth days, and one "Mercury day" (a sidereal day or the period of rotation of Mercury around itself) is equal to approximately 58.7 Earth days.

         And there is also the precession of the perihelion of Mercury. The closest distance of Mercury from the Sun doesn't happen at the same place but moves slowly around the Sun. The other planets of the solar system have perihelion shifts, but classical mechanics did not give an accurate value of Mercury's perihelion precession. The General theory of Relativity was able to show and predict that Mercury's orbit shifts by about 43 seconds of arc per century.

          The planet Mercury is often cited as the most difficult of the five brightest naked-eye planets to see. Because it's the planet closest to the Sun, it never strays too far from the Sun's vicinity in our sky. It is often referred to as "the elusive planet." And there's even a rumor that Copernicus, never saw it, yet it's not really hard to see. You simply must know when and where to look, and find a clear horizon. And for those living in the Northern Hemisphere, a great "window of opportunity" for viewing Mercury in the evening sky is about to open up.

          Mercury is called an "inferior planet" because its orbit is nearer to the Sun than the Earth's. Therefore, it always appears from our vantage point to be in the same general direction as the Sun. In the pre-Christian era, this planet actually had two names, as it was not realized it could alternately appear on one side of the Sun and then the other.

          Mercury was called Mercury when in the evening sky, but was known as Apollo when it appeared in the morning. It is said that Pythagoras, about the fifth century B.C., pointed out that they were one and the same.

          Because of its proximity to the Sun, Mercury is a very difficult planet to explore. It is normally obscured by the Sun’s glare, which prevents even observatories such as the Hubble Space Telescope from peering at it because of the risk to light-sensitive equipment. Mariner 10 is the only probe to have visited Mercury, but it too could only photograph half the planet.

          Mercury’s diameter is 3,030 miles (4,878 km), comparable to the size of the continental United States. This makes it about two-fifths the size of Earth. It is smaller than Jupiter's moon Ganymede and Saturn's moon Titan.

          But it’s not going to stay that size; the tiny planet is shrinking. When NASA’s Mariner 10 spacecraft visited the planet in the 1970s, it identified unusual features known as scarps that suggest the world is shriveling. As the hot interior of the planet cools, the surface draws together. Since the planet boasts only a single rocky layer, rather than the myriad tectonic plates found on Earth, it pushes on itself to create scarps.

          A 2014 study of nearly 6,000 scarps taken by NASA’s MESSENGER spacecraft suggest that Mercury contracted radially as much as 4.4 miles (7 kilometers) since its birth 4.5 billion years ago. The discovery helped balance models of the planet's interior evolution with observations at its surface.

          “These new results resolved a decades-old paradox between thermal history models and estimates of Mercury’s contraction,” Paul Byrne, a planetary geologist and MESSENGER visiting investigator at Carnegie's Department of Terrestrial Magnetism, said in a statement. "Now the history of heat production and loss and global contraction are consistent.”

          The planet has a mean radius of 1,516 miles (2,440 km), and its circumference at the equator is 9,525 miles (15,329 km). Some planets, such as Earth, bulge slightly at the equator due to their rapid rotation. However, Mercury turns so slowly on its axis that astronomers once thought that the planet was tidally locked, with one side constantly facing the nearby sun. In fact, the planet spins on its axis once every 58.65 Earth days. Mercury orbits once every 87.97 Earth days, so it rotates only three times every two Mercury years. The slow spin keeps the planet's radius at the poles and the equator equal.

          Although mercury is the second smallest planet in the Solar System, it is heavier than Mars, and almost as heavy as Earth. The reason for this is that Mercury has an enormous core of iron —almost 3600km (2237 miles) in diameter.

          Despite being the closest planet to the Sun, often orbiting less than 60 million kilometres away from the star, temperatures on Mercury can drop below —180°C (-290°F). This is because Mercury is too hot and too small to be able to hold on to much gas. With no clouds to stop heat from escaping into space at night, temperatures on Mercury plummet.

          Mercury is the planet in our solar system that sits closest to the sun. The distance between Mercury and the sun ranges from 46 million kilometers to 69.8 million kilometers. The earth sits at a comfy 150 million kilometers. This is one reason why it gets so hot on Mercury during the day.

          The other reason is that Mercury has a very thin and unstable atmosphere. At a size about a third of the earth and with a mass (what we on earth see as ‘weight’) that is 0.05 times as much as the earth, Mercury just doesn’t have the gravity to keep gases trapped around it, creating an atmosphere. Due to the high temperature, solar winds, and the low gravity (about a third of earth’s gravity), gases keep escaping the planet, quite literally just blowing away.

          Atmospheres can trap heat, that’s why it can still be nice and warm at night here on earth. Mercury’s atmosphere is too thin, unstable and close to the sun to make any notable difference in the temperature.

          Space is cold. Space is very cold. So cold in fact, that it can almost reach absolute zero, the point where molecules stop moving (and they always move). In space, the coldest temperature you can get is 2.7 Kelvin, about -270 degrees Celsius.

          Sunlight reflected from other planets and moons, gases that move through space, the very thin atmosphere and the surface of Mercury itself are the main reasons that temperatures on Mercury don’t get lower than about -180 °C at night.

          The images revealed bright deposits on the floors of some craters — a discovery shrouded in mystery without higher-resolution images –are actually clusters of rimless pits surrounded by halos of reflective material.

          “The etched appearance of these landforms is unlike anything we’ve seen before on Mercury or the moon,” said Brett Denevi, a staff scientist at the Johns Hopkins University Applied Physics Laboratory, in a prepared statement. “We are still debating their origin, but they appear to be relatively young and may suggest a more abundant than expected volatile component in Mercury’s crust.”

          In other words, Mercury’s surface might look a lot like the moon. But evidence of recent volcanic history suggests the planet has more going on than scientists thought.

          Planets are born from the countless collisions of rocks and space debris that were part of the early Solar System. The heat from these impacts remains deep within the core of the planet, released through volcanic eruption. Mercury's cratered appearance shows that there has been no volcanic activity on the planet for billions of years. This makes Mercury a dead planet.

          Mercury is the closest planet to the Sun, and as a result is a dry, barren planet scorched by solar heat. Parts of Mercury's surface often exceed 450 °C (840 °F) when the planet is closest to the Sun. However, at night, temperatures can drop by over 600 °C (1,100 °F) and some scientists believe that there is actually ice in deep craters that never see the Sun. Radar imaging of the planet has revealed areas of high reflectivity near the planet's poles. This may be frozen water carried to Mercury by meteorites.

          This orthographic projection view provides a look at Mercury's North Polar Region. The yellow regions in many of the craters mark locations that show evidence for water ice, as detected by Earth-based radar observations from Arecibo Observatory in Puerto Rico. MESSENGER has collected compelling new evidence that the deposits are indeed water ice, including imaging within the permanently shaded interiors of some of the craters, such as Prokofiev and Fuller. The MESSENGER spacecraft is the first ever to orbit the planet Mercury, and the spacecraft's seven scientific instruments and radio science investigation are unraveling the history and evolution of the Solar System's innermost planet. In the mission's more than four years of orbital operations, messenger has acquired over 250,000 images and extensive other data sets. messenger's highly successful orbital mission is about to come to an end, as the spacecraft runs out of propellant and the force of solar gravity causes it to impact the surface of Mercury in April 2015.