Mariner 10 and Mercury

Mariner 10 mission

Mariner 10 was a robotic space probe launched on November 3, 1973 to fly by the planets Mercury and Venus. It was launched approximately 2 years after Mariner 9 and was the last spacecraft in the Mariner program (Mariner 11 and 12 were redesignated Voyager 1 and Voyager 2). The mission objectives were to measure Mercury's environment, atmosphere, surface, and body characteristics and to make similar investigations of Venus. Secondary objectives were to perform experiments in the interplanetary medium and to obtain experience with a dual-planet gravity-assist mission.

Design and trajectory:

Mariner 10 was the first spacecraft to make use of an interplanetary "gravitational slingshot" maneuver, using Venus to bend its flight path and bring its perihelion down to the level of Mercury's orbit. This maneuver, inspired by the orbital mechanics calculations of the Italian scientist Giuseppe Colombo, put the spacecraft into an orbit that repeatedly brought it back to Mercury. Mariner 10 used the solar radiation pressure on its solar panels and its high-gain antenna as a means of attitude control during flight, the first spacecraft to use active solar pressure control.

Instruments:

Mariner 10 instruments included:

1. Twin narrow-angle cameras with digital tape recorder
2. Ultraviolet spectrometer
3. Infrared radiometer
4. Solar plasma
5. Charged particles
6. Magnetic fields
7. Radio occultation
8. Celestial mechanics

Departing the Earth/Moon system:

During its first week of flight, Mariner 10 tested its camera system by returning 5 mosaics of Earth and 6 of the Moon. It also obtained photographs of the north polar region of the moon where prior coverage was poor. These provided a basis for cartographers to update lunar maps and improve the lunar control net.

Cruise to Venus:

A trajectory correction maneuver was made on November 13, 1973. Immediately following this maneuver the star-tracker locked onto a bright flake of paint which had come off the spacecraft and lost lock on the guide star Canopus. An automated safety protocol recovered Canopus, but the problem of flaking paint recurred throughout the mission. The on-board computer also experienced unscheduled resets occasionally, which would necessitate reconfiguring the clock sequence and subsystems. Periodic problems with the high-gain antenna also occurred during the cruise. In January 1974 Mariner 10 made ultraviolet observations of Comet Kohoutek. Another mid-course correction was made on January 21, 1974.

Venus flyby:

The spacecraft passed Venus on February 5, 1974, at a closest range of 5768 km at 17:01 UT. Using a near-ultraviolet filter, it photographed the Cytherean chevron clouds and performed other atmospheric studies. It was discovered that extensive cloud detail could be seen via Mariner's ultra-violet camera filters. Venus's cloud cover is nearly featureless in visible light. Earth-based ultra-violet observation did reveal some indistinct blotching even before Mariner 10, but the detail seen by Mariner was a surprise to most researchers.

First Mercury flyby:

The first Mercury encounter took place at 20:47 UT on March 29, 1974 at a range of 703 kilometres (437 miles).

Second Mercury flyby:

After looping once around the Sun while Mercury completed two orbits, Mariner 10 flew by Mercury again on September 21, 1974 at a more distant range of 48,069 km (29,870 mi).

Third Mercury flyby:

A third and final encounter, the closest to Mercury, took place on March 16, 1975 at a range of 327 km (203 mi).

End of mission:

With its maneuvering gas just about exhausted, Mariner 10 started another orbit of the Sun. Engineering tests were continued until March 24, 1975, when the final depletion of the nitrogen supply was signaled by the onset of an un-programmed pitch turn. Commands were immediately sent to the spacecraft to turn off its transmitter, and radio signals to Earth ceased.

Presently, Mariner 10 is still orbiting the sun, although its on-board electronics have probably been damaged by the sun's radiation.

Discoveries:

During its flyby of Venus, Mariner 10 discovered evidence of rotating clouds and a very weak magnetic field.Mariner 10 flew past Mercury three times in total. Owing to the geometry of its orbit — its orbital period was almost exactly twice Mercury's — the same side of Mercury was sunlit each time, so it was only able to map 40-45% of Mercury’s surface, taking over 2,800 photos. It revealed a more or less moon-like surface. It thus contributed enormously to our understanding of the planet, whose surface had not been successfully resolved through telescopic observation. The regions mapped included most or all of the Shakespeare, Beethoven, Kuiper, Michelangelo, Tolstoj, and Discovery quadrangles, half of Bach and Victoria, and small portions of Solitudo Persephones, Liguria, and Borealis.Mariner 10 also discovered that Mercury has a tenuous atmosphere consisting primarily of helium, as well as a magnetic field and a large iron-rich core. Its radiometer readings suggested that Mercury has a night time temperature of -183°C (-297°F) and maximum daytime temperatures of 187°C (369°F).

Transit of Mercury

Transit of Mercury on November 8, 2006 with sunspots 921, 922, and 923

A transit of Mercury across the Sun takes place when the planet Mercury comes between the Sun and the Earth, and Mercury is seen as a small black dot moving across the face of the Sun. On November 8, 2006, the planet Mercury could be last seen going across the sun. The best place to have observed the transit on that date was in Hawaii.Transits of Mercury with respect to Earth are much more frequent than transits of Venus, with about 13 or 14 per century, in part because Mercury is closer to the Sun and orbits it more rapidly.Transits of Mercury can happen in May or November. November transits occur at intervals of 7, 13, or 33 years ; May transits only occur at intervals of 13 or 33 years. The last three transits occurred in 1999, 2003 and 2006 ; the next will occur in 2016.During a May transit, Mercury is near aphelion and has an angular diameter of 12" ; during a November transit, it is near perihelion and has an angular diameter of 10".

Close-up of Mercury during the Nov. 8, 2006 Transit

Grazing transits of Mercury:

Sometimes Mercury only grazes the Sun during a transit. In this case it is possible that in some areas of the Earth a full transit can be seen while in other regions there is only a partial transit (no second or third contact). The transit of November 15, 1999 was such a transit, and the previous one before that was on October 28, 743. The next such transit will occur on May 11, 2391.It is also possible that a transit of Mercury can be seen in some parts of the world as a partial transit, while in others Mercury misses the Sun. Such a transit last occurred on May 11, 1937, and the previous one was on October 21, 1342. The next such transit will occur on May 13, 2608.

Simultaneous transits:

The simultaneous occurrence of a transit of Mercury and a transit of Venus is extremely rare, and will next occur only in the years 69,163 and 224,508. The last simultaneous transit occured in 373,173 BC. On September 13, 13,425 an almost identical event is predicted: a transit of Mercury and a transit of Venus will follow one after the other, in an interval of only 16 hours.The simultaneous occurrence of a solar eclipse and a transit of Mercury is very rare. The last time this happened was August 27, 11436 BC. The next solar eclipse occurring during a transit of Mercury will be on July 5, 6757, and will be visible in Eastern Siberia.

Ground-based telescopic research for Mercury

Size comparison of terrestrial planets (left to right): Mercury, Venus, Earth, and Mars

The first telescopic observations of Mercury were made by Galileo in the early 17th century. Although he observed phases when he looked at Venus, his telescope was not powerful enough to see the phases of Mercury. In 1631 Pierre Gassendi made the first observations of the transit of a planet across the Sun when he saw a transit of Mercury predicted by Johannes Kepler. In 1639 Giovanni Zupi used a telescope to discover that the planet had orbital phases similar to Venus and the Moon. The observation demonstrated conclusively that Mercury orbited around the Sun.

A very rare event in astronomy is the passage of one planet in front of another (occultation), as seen from Earth. Mercury and Venus occult each other every few centuries, and the event of May 28, 1737 is the only one historically observed, having been seen by John Bevis at the Royal Greenwich Observatory. The next occultation of Mercury by Venus will be on December 3, 2133.

The difficulties inherent in observing Mercury mean that it has been far less studied than the other planets. In 1800 Johann Schröter made observations of surface features, claiming to have observed 20 km high mountains. Friedrich Bessel used Schröter's drawings to erroneously estimate the rotation period as 24 hours and an axial tilt of 70°. In the 1880s Giovanni Schiaparelli mapped the planet more accurately, and suggested that Mercury’s rotational period was 88 days, the same as its orbital period due to tidal locking.This phenomenon is known as synchronous rotation and is also shown by Earth’s Moon. The effort to map the surface of Mercury was continued by Eugenios Antoniadi, who published a book in 1934 that included both maps and his own observations. Many of the planet's surface features, particularly the albedo features, take their names from Antoniadi's map.

In June 1962 Soviet scientists at the Institute of Radio-engineering and Electronics of the USSR Academy of Sciences lead by Vladimir Kotelnikov became first to bounce radar signal off Mercury and receive it, starting radar observations of the planet. Three years later radar observations by Americans Gordon Pettengill and R. Dyce using 300-meter Arecibo Observatory radio telescope in Puerto Rico showed conclusively that the planet’s rotational period was about 59 days.The theory that Mercury’s rotation was synchronous became widely held, and it was a surprise to astronomers when these radio observations were announced. If Mercury were tidally locked, its dark face would be extremely cold, but measurements of radio emission revealed that it was much hotter than expected. Astronomers were reluctant to drop the synchronous rotation theory and proposed alternative mechanisms such as powerful heat-distributing winds to explain the observations.

Italian astronomer Giuseppe Colombo noted that the rotation value was about two-thirds of Mercury’s orbital period, and proposed that a different form of tidal locking had occurred in which the planet’s orbital and rotational periods were locked into a 3:2 rather than a 1:1 resonance.Data from Mariner 10 subsequently confirmed this view. The 3:2 resonance results from Mercury's eccentric orbit, as the Sun raises higher tides on the planet at perihelion which, combined with the planet's high velocity then, make the planet spin faster. This also means that Schiaparelli's and Antoniadi's maps were not "wrong". Instead, the astronomers saw the same features during every second orbit and recorded them, but regarded those seen in the meantime, when Mercury's other face was toward the Sun, as spurious.

Ground-based observations did not shed much further light on the innermost planet, and it was not until space probes visited Mercury that many of its most fundamental properties became known. However, recent technological advances have led to improved ground-based observations. In 2000, high-resolution lucky imaging observations were conducted by the Mount Wilson Observatory 1.5 meter Hale telescope. They provided the first views that resolved some surface features on the parts of Mercury which were not imaged in the Mariner missionsLater imaging has shown evidence of a huge double-ringed impact basin even larger than the Caloris Basin in the non-Mariner-imaged hemisphere. It has informally been dubbed the Skinakas Basin.Most of the planet has been mapped by the Arecibo radar telescope, with 5 km resolution, including polar deposits in shadowed craters of what may be water ice.

Orbit and rotation of mercury

Orbit of Mercury (yellow)

Mercury has the most eccentric orbit of all the planets; its eccentricity is 0.21 with its distance from the Sun ranging from 46 to 70 million kilometers. It takes 88 days to complete an orbit. The diagram on the left illustrates the effects of the eccentricity, showing Mercury’s orbit overlaid with a circular orbit having the same semi-major axis. The higher velocity of the planet when it is near perihelion is clear from the greater distance it covers in each 5-day interval. The size of the spheres, inversely proportional to their distance from the Sun, is used to illustrate the varying heliocentric distance. This varying distance to the Sun, combined with a 3:2 spin-orbit resonance of the planet’s rotation around its axis, result in complex variations of the surface temperature.

Mercury’s orbit is inclined by 7° to the plane of Earth’s orbit (the ecliptic), as shown in the diagram on the right. As a result, transits of Mercury across the face of the Sun can only occur when the planet is crossing the plane of the ecliptic at the time it lies between the Earth and the Sun. This occurs about every seven years on average.Functionally, Mercury’s axial tilt is nonexistent,with measurements as low as 0.027°.This is significantly smaller than that of Jupiter, which boasts the second smallest axial tilt of all planets at 3.1 degrees. This means an observer at Mercury’s equator during local noon would never see the Sun more than approximately 1/30 of one degree north or south of the zenith. Conversely, at the poles the Sun never rises more than 2.1′ above the horizon.

Orbit of Mercury as seen from the ascending node (bottom) and from 10° above (top)

At certain points on Mercury’s surface, an observer would be able to see the Sun rise about halfway, then reverse and set before rising again, all within the same Mercurian day. This is because approximately four days prior to perihelion, Mercury’s angular orbital velocity exactly equals its angular rotational velocity so that the Sun’s apparent motion ceases; at perihelion, Mercury’s angular orbital velocity then exceeds the angular rotational velocity. Thus, the Sun appears to move in a retrograde direction. Four days after perihelion, the Sun’s normal apparent motion resumes at these points.

Magnetic field and magnetosphere of Mercury

Graph showing relative strength of Mercury's magnetic field

Despite its small size and slow 59-day-long rotation, Mercury has a significant, and apparently global, magnetic field. According to measurements taken by Mariner 10, it is about 1.1% as strong as the Earth’s. The magnetic field strength at the Mercurian equator is about 300 nT. Like that of Earth, Mercury's magnetic field is dipolar in nature.Unlike Earth, however, Mercury's poles are nearly aligned with the planet's spin axis.Measurements from both the Mariner 10 and MESSENGER space probes have indicated that the strength and shape of the magnetic field are stable.

It is likely that this magnetic field is generated by way of a dynamo effect, in a manner similar to the magnetic field of Earth.This dynamo effect would result from the circulation of the planet's iron-rich liquid core. Particularly strong tidal effects caused by the planet's high orbital eccentricity would serve to keep the core in the liquid state necessary for this dynamo effect.

Mercury’s magnetic field is strong enough to deflect the solar wind around the planet, creating a magnetosphere. The planet's magnetosphere, though small enough to fit within the Earth,is strong enough to trap solar wind plasma. This contributes to the space weathering of the planet's surface.Observations taken by the Mariner 10 spacecraft detected this low energy plasma in the magnetosphere of the planet's nightside. Bursts of energetic particles were detected in the planet's magnetotail, which indicates a dynamic quality to the planet's magnetosphere.

Physical characteristics of Mercury

Image from MESSENGER's second Mercury flyby. Kuiper crater is just below center. An extensive ray system emanates from the crater near the top.

physical characteristics:

Mean radius	2439.7 ± 1.0 km
Flattening	<6000
surface area	7.48×107 km2
volume	6.083×1010 km3
Mass	3.3022×1023 kg
mean density	5.427 g/cm³
Equatorial surface gravity	3.7 m/s²
Escape velocity	4.25 km/s
sidereal rotation period	58.646 day
Equatorial rotation velocity	10.892 km/h
Axial tilt	2.11′ ± 0.1′
North pole right ascension	281.01°
North pole declination	61.45°
Albedo	0.119 bond
Apparent magnitude	up to −1.9
Angular diameter	4.5" – 13"
surface temperature (0°N, 0°W)	minimum 100k
surface temperature (0°N, 0°W)	medium 340k
surface temperature (0°N, 0°W)	maximum 700k
surface temperature (85°N, 0°W)	minimum 80k
surface temperature (85N, 0°W)	medium 200k
surface temperature (85N, 0°W)	maximum 380k

Atmosphere of Mercury

First high-resolution image of Mercury transmitted by MESSENGER (false color)

Composition :

42% Molecular oxygen
29.0% sodium
22.0% hydrogen
6.0% helium
0.5% potassium

Trace amounts of argon, nitrogen, carbon dioxide, water vapor, xenon, krypton, & neon.

Surface pressure :trace

Internal structure of Mercury

picture:
1. Crust—100–300 km thick
2. Mantle—600 km thick
3. Core—1,800 km radius

Mercury is one of four terrestrial planets in the solar system, and is a rocky body like the Earth. It is the smallest planet in the solar system, with an equatorial radius of 2439.7 km.Mercury is even smaller—albeit more massive—than the largest natural satellites in the solar system, Ganymede and Titan. Mercury consists of approximately 70% metallic and 30% silicate material.Mercury's density is the second highest in the Solar System at 5.427 g/cm³, only slightly less than Earth’s density of 5.515 g/cm³.If the effect of gravitational compression were to be factored out, the materials of which Mercury is made would be denser, with an uncompressed density of 5.3 g/cm³ versus Earth’s 4.4 g/cm³.Mercury’s density can be used to infer details of its inner structure. While the Earth’s high density results appreciably from gravitational compression, particularly at the core, Mercury is much smaller and its inner regions are not nearly as strongly compressed. Therefore, for it to have such a high density, its core must be large and rich in iron.Geologists estimate that Mercury’s core occupies about 42% of its volume; for Earth this proportion is 17%. Recent research strongly suggests Mercury has a molten core.

Surrounding the core is a 600 km mantle consisting of silicates.Astronomers have postulated that, early in Mercury’s history, a giant impact with a body several hundred kilometers across stripped the planet of much of its original mantle material, resulting in the relatively thin mantle compared to the sizable core.

Based on data from the Mariner 10 mission and Earth-based observation, Mercury’s crust is believed to be 100–300 km thick.One distinctive feature of Mercury’s surface is the presence of numerous narrow ridges, some extending over several hundred kilometers. It is believed that these were formed as Mercury’s core and mantle cooled and contracted at a time when the crust had already solidified.

Mercury's core has a higher iron content than that of any other major planet in the Solar System, and several theories have been proposed to explain this. The most widely accepted theory is that Mercury originally had a metal-silicate ratio similar to common chondrite meteors, thought to be typical of the Solar System's rocky matter, and a mass approximately 2.25 times its current mass.However, early in the solar system’s history, Mercury may have been struck by a planetesimal of approximately 1/6 that mass.The impact would have stripped away much of the original crust and mantle, leaving the core behind as a relatively major component.A similar process has been proposed to explain the formation of Earth’s Moon (see giant impact theory).

Alternatively, Mercury may have formed from the solar nebula before the Sun's energy output had stabilized. The planet would initially have had twice its present mass, but as the protosun contracted, temperatures near Mercury could have been between
2500 and 3500 K (Celsius equivalents about 273 degrees less), and possibly even as high as 10000 K.Much of Mercury’s surface rock could have been vaporized at such temperatures, forming an atmosphere of "rock vapor" which could have been carried away by the solar wind.

A third hypothesis proposes that the solar nebula caused drag on the particles from which Mercury was accreting, which meant that lighter particles were lost from the accreting material.Each of these hypotheses predicts a different surface composition, and two upcoming space missions, MESSENGER and BepiColombo, both aim to make observations to test them.

Iron 'snow' helps maintain Mercury's magnetic field

Mariner 10 also discovered that Mercury has a weak magnetic field, about one percent as strong as Earth's. NASA

May 8, 2008

James E. Kloeppel/University of Illinois

New scientific evidence suggests that deep inside Mercury, iron "snow" forms and falls toward the center of the planet, much like snowflakes form in Earth's atmosphere and fall to the ground.

The movement of this iron snow could be responsible for Mercury's mysterious magnetic field, say researchers from the University of Illinois and Case Western Reserve University. In a paper published in the April issue of Geophysical Research Letters, the scientists describe laboratory measurements and models that mimic conditions believed to exist within Mercury's core.

"Mercury's snowing core opens up new scenarios where convection may originate and generate global magnetic fields," said University of Illinois geology professor Jie (Jackie) Li. "Our findings have direct implications for understanding the nature and evolution of Mercury's core, and those of other planets and moons."

Mercury is the innermost planet in our solar system and, other than Earth, the only terrestrial planet that possesses a global magnetic field. Discovered in the 1970s by NASA's Mariner 10 spacecraft, Mercury's magnetic field is about 100 times weaker than Earth's.

Made mostly of iron, Mercury's core is also thought to contain sulfur, which lowers the melting point of iron and plays an important role in producing the planet's magnetic field.

"Recent Earth-based radar measurements of Mercury's rotation revealed a slight rocking motion that implied the planet's core is at least partially molten," said Illinois graduate student Bin Chen, the paper's lead author. "But, in the absence of seismological data from the planet, we know very little about its core."

To better understand the physical state of Mercury's core, the researchers used a multi-anvil apparatus to study the melting behavior of an iron-sulfur mixture at high pressures and high temperatures.

In each experiment, an iron-sulfur sample was compressed to a specific pressure and heated to a specific temperature. The sample was then quenched, cut in two, and analyzed with a scanning electron microscope and an electron probe microanalyzer.

As the molten, iron-sulfur mixture in the outer core slowly cools, iron atoms condense into cubic "flakes" that fall toward the planet's center, Chen said. As the iron snow sinks and the lighter, sulfur-rich liquid rises, convection currents are created that power the dynamo and produce the planet's weak magnetic field.

Mercury's core is most likely precipitating iron snow in two distinct zones, the researchers said. This double-snow state may be unique among the terrestrial planets and Earth-like moons in our solar system.

"Our findings provide a new context into which forthcoming observational data from NASA's MESSENGER spacecraft can be placed, "Li said. "We can now connect the physical state of our innermost planet with the formation and evolution of terrestrial planets in general."

Mercury:First planet from the sun

MESSENGER image of Mercury

Mercury is the innermost and smallest planet in the solar system,orbiting the Sun once every 88 days. Mercury is bright when viewed from Earth, ranging from −2.0 to 5.5 in apparent magnitude, but is not easily seen as its greatest angular separation from the Sun is only 28.3°. It can only be seen in morning or evening twilight. Comparatively little is known about it; the first of two spacecraft to visit Mercury was Mariner 10, which mapped only about 45% of the planet’s surface from 1974 to 1975. The second is the MESSENGER spacecraft, which mapped another 30% during its flyby of January 14, 2008. MESSENGER will make one more pass by Mercury in 2009, followed by orbital insertion in 2011, and will then survey and map the entire planet.

Mercury is similar in appearance to the Moon: it is heavily cratered, has no natural satellites and no substantial atmosphere. However, unlike the moon, it has a large iron core, which generates a magnetic field about 1% as strong as that of the Earth.It is an exceptionally dense planet due to the large relative size of its core. Surface temperatures range from about 90 to 700 K (−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.

Recorded observations of Mercury date back to at least the first millennium BC. Before the 4th century BC, Greek astronomers believed the planet to be two separate objects: one visible only at sunrise, which they called Apollo; the other visible only at sunset, which they called Hermes.The English name for the planet comes from the Romans, who named it after the Roman god Mercury, which they equated with the Greek Hermes. The astronomical symbol for Mercury is a stylized version of Hermes' caduceus.

Information's about mercury:

Semi-major axis	0.387098 AU
Aphelion	0.466697 AU
Perihelion	0.307499 AU
Eccentricity	0.205630
Orbital period	87.9691 days
Synodic period	115.88 days
Average orbitalspeed	47.87 km/s
Mean anomaly	174.796°
Inclination	7.005°
Longitude of ascending node	48.331°
Argument of perihelion	29.124°
Satellites	none

MESSENGER's second Mercury flyby

photo: The spectacular image shown here is one of the first to be returned and shows a WAC image of the departing planet taken about 90 minutes after the spacecraft’s closest approach to Mercury. NASA/JHU APL/Carnegie Institution of Washington.

When Mariner 10 flew past Mercury three times in 1974 and 1975, the probe imaged less than half the planet. In January, during MESSENGER's first flyby, its cameras returned images of about 20 percent of the planet's surface missed by Mariner 10. On October 6, at 4:40 a.m. EDT, MESSENGER successfully completed its second flyby of Mercury, and its cameras captured more than 1,200 high-resolution and color images of the planet — unveiling another 30 percent of Mercury's surface that had never before been seen by spacecraft.

"The MESSENGER team is extremely pleased by the superb performance of the spacecraft and the payload," said MESSENGER Principal Investigator Sean Solomon of the Carnegie Institution of Washington. "We are now on the correct trajectory for eventual insertion into orbit around Mercury, and all of our instruments returned data as planned from the side of the planet opposite to the one we viewed during our first flyby. When these data have been digested and compared, we will have a global perspective of Mercury for the first time."

On October 7, at about 1:50 a.m. EDT, MESENGER turned to Earth and began transmitting data gathered during its second Mercury encounter. The spectacular image on the right — one of the first to be returned — was snapped by the Wide Angle Camera (WAC), part of the Mercury Dual Imaging System (MDIS) instrument, about 90 minutes after MESSENGER's closest approach to Mercury, when the spacecraft was at a distance of about 17,000 miles (27,000 kilometers).

The bright crater just south of the center of the image is Kuiper, identified on images from the Mariner 10 mission in the 1970s. For most of the terrain east of Kuiper, toward the edge of the planet, the departing images are the first spacecraft views of that portion of Mercury's surface. A striking characteristic of this newly imaged area is the large pattern of rays that extend from the northern region of Mercury to regions south of Kuiper.