Orbit of Mercury

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.

While scientists have long known the orbital period of Mercury, they were wrong about its rotational period. Until the 1960s the prevailing theory held that the planet was tidally locked to the Sun and did not rotate at all. That was because when it was in a prime position for observation, the same portion of the planet was always visible. It wasn’t until 1965 that US astronomers, using the Arecibo Observatory, would prove conclusively that the planet’s rotational period was about 59 sidereal days. There is a direct correlation between the orbital period and rotational period. This is called spin-orbit resonance. For every 2 orbits of Mercury around the Sun, it rotates three times on its axis. This is known as a 3:2 spin-orbit resonance.

Despite the slow rotational period, Mercury does have a global magnetic field and both Mariner 10 and MESSENGER indicate that the strength and shape of the magnetic field are stable. Measurements taken by Mariner 10 led scientists to estimate that it is about 1.1% of the strength of Earth’s. Mercury’s magnetic field is dipolar and most likely generated by a dynamo effect. This dynamo effect would result from the circulation of the planet’s iron-rich liquid core. Mercury’s magnetic field deflects the solar wind creating a magnetosphere. The magnetosphere is strong enough to trap solar wind plasma contributing to the weathering of the surface. The Mariner 10 spacecraft detected this low energy plasma in the magnetosphere of the planet’s night side.

As you can see from this article, scientists know a great deal about the orbit of Mercury, but there are still things being discovered and waiting to be discovered. Mariner 10 provided a wealth of information, but left an incomplete picture. MESSENGER will fill in the blanks a little more and there is no limit on what may be revealed in the coming year.

We have written many articles about Mercury for Universe Today. Here’s an article about the rotation of Mercury, and here’s an article about the transit of Mercury.

If you’d like more information on Mercury, check out NASA’s Solar System Exploration Guide, and here’s a link to NASA’s MESSENGER Misson Page.

We’ve also recorded an entire episode of Astronomy Cast all about Mercury. Listen here, Episode 49: Mercury.

Órbita de Mercurio

NASA Solar System Exploration: Mercury
NASA: Mariner 10

Rotation of Mercury

The rotation of Mercury is a little strange to Earth bound creatures. It rotates on its axis very slowly compared to its orbital period. One rotation takes 56.85 Earth days, while one orbital period only takes 88 Earth days. This means that a single day on Mercury last about 0.646 times as long as a single year. The planet’s equatorial rotational speed is 10.892 km/h. These periods are given in solar days. In sidereal days Mercury rotates every 58.647 days and orbits twice during every three rotations.

At some places on Mercury’s surface, an observer could see the Sun rise about halfway, reverse its course, then set, all over the course of one Mercurial day. This happens about four days prior to perihelion, because Mercury’s angular orbital velocity is equal to its angular rotational velocity. This causes the apparent motion of the Sun to stop. Once Mercury achieves perihelion, its angular orbital velocity exceeds the angular rotational velocity and the Sun begins to move in reverse. That is a simplification of sorts, so here is another way to explain this, in a little more detail: During one Mercurian year, the average motion of the Sun is two degrees per day to the west(one-third of the stars’ motion of six degrees per day) making the day three times longer than the rotation period. At different times of that year, the motion varies. When nearing aphelion, the orbital motion is slower, and the net westward motion of the Sun is more than 150% its normal angular velocity, or more than three degrees per day. On the other hand, when approaching perihelion, the Sun slows, stops moving westward, moves a little over one diameter to the east, then starts slowly moving westward again, faster and faster, until the Sun is going more than three degrees per day to the west, at the next aphelion. At the same time that the Sun is changing speeds, it gets larger, and then smaller, because its apparent size depends upon how far away it is.

The rotation of Mercury was not discovered until 1965. Until then the most widely accepted theory had Mercury tidal locked to the Sun. Soviets scientists bounced radar signals off the planet’s surface in 1962 verifying that the planet rotated, but it wasn’t until scientists using the Arecibo Observatory verified the planet’s sidereal rotational period of 58.647 day.

We have written many articles about Mercury for Universe Today. Here’s an article about Mercury retrograde, and here’s an article about the size of Mercury.

If you’d like more information on Mercury, check out NASA’s Solar System Exploration Guide, and here’s a link to NASA’s MESSENGER Misson Page.

We’ve also recorded an entire episode of Astronomy Cast all about Mercury. Listen here, Episode 49: Mercury.

La rotación de Mercurio

NASA Solar System Exploration: Mercury

Composition of Mercury

Mercury is a terrestrial planet, like like all of the 4 inner planets: Mercury, Venus, Earth and Mars. It’s the smallest of them, and has a diameter of only 4,879 km at its equator. What is the composition of Mercury?

Astronomers have estimated that the Mercury composition is made up of approximately 70% metals and 30% silicate material. In fact, it’s only slightly less dense than the Earth, with 5.43 g/cm3.

Since Mercury is smaller than Earth, its gravity doesn’t compress it down as much, so it actually has much more heavier elements inside it. Geologists estimate that its core is very large, and mostly iron. The core probably accounts to 42% of Mercury’s volume, while Earth is just 17%.

The core itself is about 3,600 km across. Surrounding that is a 600 km thick mantle. And around that is the crust, which is believed to be 100-200 km thick. The crust is known to have narrow ridges that extend for hundreds of kilometers. Planetary scientists think that the ridges formed when the planet cooled and contracted underneath the crust, causing it to collapse down.

One reason to explain why Mercury has such a large core is that it was impacted by a large planetesimal early in its history. The impact would have stripped away much of the planet’s crust, leaving just the thick iron core. This is a similar process that explains the formation of the Moon. It’s also possible that Mercury formed before the Sun’s energy output settled down. If it had twice its original mass, the Sun could vaporize the surface of the planet, blasting it away with its power solar wind.

We have written many articles about Mercury for Universe Today. Here’s an article about the gravity on Mercury, and here’s an article about the color of Mercury

If you’d like more information on Mercury, check out NASA’s Solar System Exploration Guide, and here’s a link to NASA’s MESSENGER Misson Page.

We’ve also recorded an entire episode of Astronomy Cast all about Mercury. Listen here, Episode 49: Mercury.

Surface of Mercury

The surface of Mercury is marred by an impressive number of craters. The cratering is deeper than on most terrestrial planets because of its lack of an atmosphere. A thicker atmosphere would have slowed impacting bodies, making the craters more shallow. Less than half of the surface of Mercury was mapped until recent images from MESSENGER were interpreted by NASA.

The surface of Mercury displays some amazing extremes. The temperature may range by an astounding 590 K. While the planet is the closest to the Sun, there are places that are dark enough to contain water ice and there is water in the tenuous exosphere. Additionally, it is the smallest planet by surface area, but it is also one of the most dense. Its core accounts for 42% of the planet’s volume.

The surface of Mercury is believed to be geologically inactive and has been for billions of years. Telescopic observation has shown areas of vastly different reflectivity. This indicates that the surface of Mercury has dorsa(ridges), highlands, montes(mountains), planitiae(plains), rupes(escarpments), and valles(valleys). Mercury was heavily bombarded by asteroids and comets about 3.8 billion years ago during the Late Heavy Bombardment period. During this period the planet received an overwhelming number of impacts over its entire surface. At that time, the planet was volcanically active and basins like the Caloris Basin were filled with magma, produced the smooth plains. Data received from MESSENGER shows a very jumbled, heterogeneous surface.

Mercury’s craters range in diameter. Some are tiny bowl-shaped cavities, while others are multi-ringed basins that are hundreds of kilometers in diameter. They are in various states of degradation: some are relatively young rayed craters, others are barely visible remnants. Similar impacts here on Earth would have left smaller ejecta fields because of the higher gravity levels. Caloris Basin is the largest known crater(remember the entire surface has not been mapped) at 1,550 km. The impact created lava flows and left a ring around the impact zone that is 2 km tall. On the other side of the planet(antipode) there is a large hilly area called the ”Weird Terrain” that may have been pushed out by the shock wave created by the impact on the other side of the planet.

The surface of Mercury is a broken and distorted landscape. Mariner 10 sent us images of about half of the planet and MESSENGER is doing everything that it can to complete the picture, but everything indicates a barren landscape that never had a chance to be anything but desolate.

We have written many articles about Mercury for Universe Today. Here’s an article about the formation of Mercury, and here’s an article about the atmosphere of Mercury.

If you’d like more information on Mercury, check out NASA’s Solar System Exploration Guide, and here’s a link to NASA’s MESSENGER Misson Page.

We’ve also recorded an entire episode of Astronomy Cast all about Mercury. Listen here, Episode 49: Mercury.

NASA StarChild
NASA Solar System Exploration: MESSENGER

Superficie de Mercurio

Symbol for Pluto

The symbols of the planets, Moon and Sun are used in both astronomy and astrology. The symbol for Pluto is made from a monograph of the first two letters in the word PLuto. It’s also from the initials of the astronomer Percival Lowell, who predicted that the planet would be found through its gravitational influence on Neptune and Uranus. Of course, Pluto is not a planet anymore.

What Color is Mercury?

Unlike all of the other planets in the Solar System, Mercury is just bare rock. It does have a tenuous atmosphere, but ground and space-based observations see just the gray rocky color of Mercury. This gray color comes from Mercury’s molten surface that cooled and hardened billions of years ago after the formation of the Solar System.

There are no active tectonic or erosion processes happening on the surface of Mercury; it has remained unchanged for billions of years, reshaped only by the occasional meteorite impact. In the past, some of the basins were filled in by magma that flowed out of the planet when it still had an active geologic cycle. Geologists are fairly certain that there are no active volcanoes on Mercury any more, but it’s possible that there could still be the occasional lava flow. Fresh lava flows would appear as a different color on the surface of Mercury. Perhaps when NASA’s MESSENGER spacecraft enters orbit around Mercury, we’ll get a better idea of its colors. Certainly we’ll know more about its surface geology.

The photograph attached to this article provides one of the best true-color images of Mercury that we have. If you could fly over Mercury in your spacecraft, this is essentially what you’d see. The planet Mercury color is a dark gray surface, broken up by craters large and small. The color of Mercury’s surface is just textures of gray, with the occasional lighter patch, such as the newly discovered formation of crater and trenches that planetary geologists have named “The Spider”.

Mercury’s coloring is very similar to the Earth’s moon. In fact, when you’re looking at images of both objects, it’s very difficult to tell the two objects apart. Unlike the Moon, however, Mercury lacks the darker areas, or “seas”, that were created on the Moon by lava flows. Mercury’s color doesn’t have the variety that even the Moon has.

If you got here not asking what color is Mercury the planet, but what color is Mercury (the element), it’s silver, and a liquid at room temperature.

We have written many stories about Mercury here on Universe Today. Here’s an article about a side of Mercury never before seen by spacecraft, and here’s a story about the color of Mercury captured by NASA’s MESSENGER spacecraft during a recent flyby.

If you’d like more information on Mercury, check out NASA’s Solar System Exploration Guide, and here’s a link to NASA’s MESSENGER Misson Page.

We have also recorded a whole episode of Astronomy Cast that’s just about planet Mercury. Listen to it here, Episode 49: Mercury.

New Science Findings From Messenger’s Third Mercury Flyby
NASA Science: Surprises from Mercury
NASA Solar System Exploration

Discovery of Planet Mercury

Ancient people have known about the planets for millennia. It was only in the last few hundred years that new planets have been discovered that required a telescope to see. The earliest people thought of the planets as divine beings, moving across the heavens in unpredictable ways. If you’re wondering about the discovery of Mercury, though, it’s been known since prehistoric times, so there’s no way to really know who made the original discovery of planet mercury.

The five original naked-eye planets were Mercury, Venus, Mars, Jupiter and Saturn, and they had a significant impact on mythology, cosmology, and ancient astronomy. Any caveman could have made the Mercury discovery.

Mercury itself has been mentioned as early as the 2nd millennium BC by the Sumerians, and recorded by the Babylonians – they called the planet Nabu. The ancient Greeks associated Mercury with the god Hermes, who was thought to carry the Sun across the sky in his chariot. Later the Greeks called the planet Apollo when it was visible in the morning sky, and then Hermes in the evening sky. The Romans named the planet after the Roman messenger god Mercury, who was the same mythological figure as the Greek god Hermes.

The first telescopic observations were made by Galileo in the 17th century with his crude telescope; unfortunately, his crude instrument wasn’t powerful enough to see that the planet had phases, like Venus.

Mercury can occasionally be seen to pass directly in front of the Sun, as seen by Earth. This is called a transit. The last transit of Mercury happened in 2004, and was broadcasted worldwide across the Internet. But the first Mercury transit was seen in 1737 by John Bevis at the Royal Greenwich Observatory.

Until the 1960s, Mercury was thought to be tidally locked to the Sun, always facing one side towards our star. There were skeptics, who noted that if Mercury always faced one side towards the Sun, it should have a hot and a cold side, but research data didn’t back that up. Astronomers thought that maybe an atmosphere around Mercury kept the temperatures more even.

In 1962, Soviet scientists bounced the first radar signals off Mercury’s surface, and then American astronomers calculated that Mercury does rotate. It actually takes 59 days to turn once, and not the 88 days it takes to complete an orbit.

The first spacecraft to see Mercury up close was Mariner 10 back in 1974. Unfortunately, it was only able to see one hemisphere of the planet in total over the course of 3 flybys. Many of the missing pieces were filled in by NASA’s MESSENGER spacecraft, which completed its first flyby in January, 2008.

Descubrimiento del planeta Mercurio

NASA Cosmic Distance Scales
NASA Solar System Exploration: Mariner 10

Pictures of Mercury

We can’t just talk about Mercury. Sometimes you’ve just got to see it. Before NASA’s MESSENGER spacecraft, there weren’t a lot of Mercury pictures to choose from. But now the floodgates are open, MESSENGER is sending back more pics of Mercury with each flyby. So here are some of the best photos of Mercury taken so far. I also recommend you to read these amazing books for more information about the planet Mercury.

This first image of Mercury was actually taken by NASA’s Mariner 10 spacecraft, while the others were seen by MESSENGER. As you can see, the new images are so much better than the older ones.

This is one of the first close-up images of Mercury captured by NASA’s MESSENGER spacecraft just before its January 14th, 2008 flyby. It’s a full color image of Mercury, captured by the spacecraft’s Wide Angle Camera (WAC) filters in the infrared, far red, and violet wavelengths (red, green, and blue filters for this image.)

This image of Mercury was captured when the spacecraft was much closer to the planet. The prominent feature is crater Matisse, named after the French artist Henri Matisse. This same crater was imaged by Mariner 10, so this gives scientists a chance to see the difference.

Here’s an image of Mercury’s north pole, captured by MESSENGER during its January 14, 2008 flyby. It’s interesting to note that the planet’s southern regions are much more heavily cratered than its northern regions, which are relatively smooth in comparison. If you read the interesting facts about Mercury, you would know that there could be craters at the planet’s north pole that harbor deposits of ice.

This is a side of Mercury that had never been seen by spacecraft until NASA’s MESSENGER arrived to photograph it on January 14, 2008. Until now, astronomers had only made ground observations of this side of the planet. These images will help astronomers tune their methods and let them compare their ground observations to the close up images captured by spacecraft.

Here are some facts about Mercury.

Surface of Pluto

When you imagine cold, icy Pluto, orbiting in the distant regions of the Solar System, you imagine snowy white ball.

You can also look through these books from Amazon.com if you want more information about Pluto.

But images of Pluto, captured by the Hubble Space Telescope have shown that Pluto’s surface isn’t just pure ice. Instead, it has a dirty yellow color, with darker and brighter regions across its surface. Hubble studied the entire surface of Pluto as it rotated through a 6.4 day period.

The images revealed almost a dozen distinctive features never before seen by astronomers. This included a “ragged” northern polar cap cut in half by a dark strip, a bright spot seen to rotate around the dwarf planet, and a cluster of dark spots. The images also confirmed the presence of icy-bright polar cap features.

Some of the variations seen on Pluto’s surface could be topographic features, like basins and fresh impact craters. But most of them are probably caused by the complex distribution of frosts that move across Pluto’s surface during its orbital and seasonal cycles.

The surface area of Pluto is 1.795 x 107 square kilometers; about 0.033% the surface area of Earth.

When Pluto is furthest away from the Sun, gases like nitrogen, carbon monoxide and methane partially freeze onto its surface.

All will be revealed when NASA’s New Horizons spacecraft finally arrives at Pluto in 2015, finally capturing close-up pictures of Pluto and its moon Charon.

Who Was Pluto Named After?

You’re thinking about a certain Disney dog, aren’t you? Goofy’s pet dog? Nope, it was actually named after Pluto, the Roman god of the underworld.

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When Pluto was first discovered by Clyde Tombaugh in 1930, he was given the honor of giving it a name. Although they were calling it Planet X informally, they needed something that matched the rest of the planets in the Solar System.

The name Pluto was suggested by Venetia Burney, an 11-year old school girl in England. She was interested in ancient mythology, and thought that Hades, the Greek god of the underworld, made a good name. She suggested Pluto, to match the Roman god names given to the other planets.

Each astronomer in the Lowell Observatory was allowed to vote on a short list of names: Minerva, Cronus, and Pluto. Every one of them voted for Pluto. Venetia was given a 5-pound reward for providing the name.

In other languages, the name has been translated to names that match underworld god mythology, such as Yama, the Guardian of Hell in Buddhist mythology.