Category: Astronomy

For most of human history, Mercury was a mystery, wrapped in superstition. As it orbits so close to the Sun, ancient astronomers could only see it when they had an open horizon, just after the Sun sets, or before it rises.

But now we’ve sent robotic spacecraft to all of the planets in the Solar System, including Mercury. Here’s a listing of the missions that have gone to Mercury, and a few that will be going shortly.

Mariner 10
Mariner 10 was launched on November 3, 1973 to fly past Mercury and Venus. It was the last spacecraft in the Mariner program (Mariner 11 and Mariner 12 were renamed to Voyager 1 and 2).

Its main objectives were to measure Mercury and Venus’ environment, atmosphere and capture images of their surfaces (the cloud tops of Venus).

It was the first spacecraft to take advantage of the gravity assisted slingshot maneuver, using the gravity of Venus to bend its flight path to bring it into a good trajectory to fly past Mercury. It also used light pressure from the Sun to make minor course corrections.

It made a flyby of Venus on February 5, 1974, and then its first Mercury flyby on March 29, 1974, getting within 703 kilometers of the planet’s surface. It completed a second flyby on September 21, 1974, and then a third and final flyby on March 16, 1975, passing as close as 327 km. Over the course of the three flybys, it was only able to map 40-45% of Mercury’s surface.

Mariner 10 is probably still orbiting the Sun.

MESSENGER
MESSENGER is NASA’s second mission to Mercury. It launched on August 3, 2004 to study the planet. This time, the spacecraft will actually be going into orbit around Mercury to study it until its electronics fail.

The spacecraft has greatly improved optics and electrons, and is capable of resolving features on Mercury’s surface down to 18 meters across. This is a vast improvement over the 1.6 km resolution of Mariner 10.

MESSENGER made its first flyby of Mercury on January 14, 2008, and will make a second on October 6, 2008, and a third on September 29, 2009. It will make its final orbital insertion around Mercury on March 18, 2011.

Bepi Columbo
This is a new mission under development by the European Space Agency and the Japan Aerospace Exploration Agency to the planet Mercury. It’s still in the planning stages, but ESA and JAXA intend to build a spacecraft that can split into two portions: a planetary orbiter and a magnetospheric orbiter. A planetary lander component was proposed, but the idea was scrapped for budgetary reasons.

Misiones a Mercurio

References:
NASA Solar System Exploration: Mariner 10
NASA Messenger Mission Page
NASA: Bepi-Colombo

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

References:
NASA Solar System Exploration: Mercury
Wikipedia
NASA: Mariner 10
NASA: MESSENGER Spacecraft

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

References:
NASA Solar System Exploration: Mercury
Wikipedia
cseligman.com

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.

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.

References:
NASA StarChild
Wikipedia
NASA Solar System Exploration: MESSENGER
NASA MESSENGER Mission Page

Superficie de Mercurio

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.

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.

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