General Astronomy/Mercury

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Next Greatest Elongation, WEST: 2006 November 25

Next Greatest Elongation, EAST: 2006 October 17

Next transit of Mercury will begin at 19h:12m Universal Time on 8 November 2006.

Followed by that of 9 May 2016.

In Roman and Greek mythology, the planet Mercury played an important role in the religious life of many ancient civilizations. In Roman religion, Mercury was god of commerce and messenger of the gods, identified with the Greek Hermes. He was honoured at the Mercuralia, a festival held in May and attended primarily by traders and merchants.

Mercury is the closest planet to our sun lying at a mean distance of 57.909 million km and has an equatorial radius of 2,439 km, so that not only is Mercury slightly more than 1,367.6 km larger than the moon, it also looks very similar because the planet is covered with impact craters. The other unique attribute of Mercury is that the planet is almost entirely spherical since the planet’s polar diameter is the about the same as its equatorial diameter.

The planet is inclined on its axis at a mere 0.01º which is the lowest of all the planets in our solar system. Mercury is upright in its orbital path around the sun which takes only 87.968 days (115.88 Synodic days) to complete, as the planet rushes through space at 47.87 km/s.

The orbital inclination of Mercury is 7º. If it were possible to visit Mercury, the pole star would be the tenth magnitude star TYCHO 4215-996-1 in the northern constellation of Draco, the dragon.

The first attempts by amateur astronomers to measure the length of the planet’s solar day by observing and mapping surface markings, were quite wide of the mark; furthermore, only true measurements made by earthbound radar and spacecraft visiting Mercury, found the true figure to be 4,222.6 hours. Therefore, Mercury’s solar day is almost 176 earth days in length, and about twice as long as the planet’s year. Mercury rotates on its axis three times while it circles the Sun twice in synchronous rotation. The planet's sidereal day - the time it takes to rotate on its axis - is 1,407.6 hours, or 58.785 earth days, in length.

Prehistoric man probably saw Mercury; the first recorded observation was by Timocharis in 265 BC. The early Greeks believed that the east and west elongations of Mercury represented two separate objects which they called Hermes (evening star) and Apollo (morning star). When later the Greeks recognized that Mercury was one object, they designated it Hermes, the messenger of the gods, and god of twilight and dawn who announced the rising of Zeus. The ancient Egyptians however, first discovered that Mercury (called by them Sabkou) orbited the Sun.

To the Teutonic people Mercury was known as Woden, and our anglicized version of the midweek day Wednesday is derived from the original Woden's Day. The present name Mercury is derived directly from the Latin name Mercurius, which is the Roman designation for the Greek name Hermes.

Because Mercury and its neighbour the planet Venus are inferior planets - that is, they lie inside earth’s orbit - they are both able to display phases like our Moon, as they travel around the sun. The Italian astronomer Zupus first observed the phases of Mercury in 1639. Hevelius later observed them independently in 1644.

Another phenomenon associated with Mercury and Venus is a planetary transit, which takes place when Mercury (or Venus) passes between the Earth and the Sun. On such rare occasions, the inferior planet may be seen as a small black disk passing slowly across the Sun’s brilliant surface in transit. The transit of Mercury was first predicted by Kepler in 1630 and was observed by Gassendi.

The first recorded observations of the surface markings were by Schroter and Harding in 1800. In the same year, Schroter incorrectly measured a rotation period of 24 hours with a rotation axis inclined 70° to the orbital plane. Another incorrect rotation period of 88 days determined by Schiaparelli 80 years later was not corrected until the advent of recent radar observations, which in turn were confirmed by measurements made by the first Mercury space craft visitor Mariner 10.

It had been thought that the planet Mercury had no atmosphere whatsoever because of its low escape velocity of 4.3 km/s and its close proximity to the Sun, since the rapid movement of any atmospheric atoms would be greater than its escape velocity. However, visiting spacecraft, in particular Mariner 10, did detect a tenuous atmosphere - or "exosphere" - with a total mass of less than 1,000 kilograms. It has a measured composition of: 42% oxygen (O2), 29% sodium (Na), 22% hydrogen (H2), 6% helium (He) and 0.5% potassium (K). There were also very low traces of water, nitrogen and carbon dioxide, despite the average surface temperature of Mercury being 169 degrees Celsius. However, the atmosphere is still essentially a vacuum in every sense of the word.

Although Mercury is remarkably similar to the Moon, it is different from it in many respects. This was unexpected, based on observations from Earth prior the Mariner 10 mission. On the one hand it was known that Mercury reflects sunlight and radar waves in the same manner, as does the Moon.

This similarity combined with the probable absence of any appreciable atmosphere suggested a cratered surface and a lunar-like regolith of pulverized rock mantling the surface of the planet as the result of meteoritic bombardment. On the other hand, the bulk density of the planet was known to be almost the same as that of Earth and about 60 percent greater than that of the Moon, implying that Mercury was a body greatly enriched in the heavy elements and, like Earth, perhaps having an iron-rich core.

The surface of Mercury, like that of the Moon, was found to be pockmarked with impact craters. However, not expected was the discovery that Mercury, unlike the Moon, has a weak but nevertheless Earth-like magnetic field whose origin is undoubtedly related to a large iron-rich core. Paradoxically, Mercury has a Moon-like exterior and an Earth-like interior.

Craters and basins dominate the illuminated surface observed by Mariner 10 as it first approached Mercury. This region of Mercury shows a heavily cratered surface that at first glance could be mistaken for the lunar highlands. In marked contrast to this view of Mercury, the surface photographed after the flyby, as the spacecraft receded from Mercury, exhibited features totally different from those shown on the incoming views, including large basins and extensive relatively smooth areas with few craters.

The smooth surfaces are clearly younger than the heavily cratered ground seen in the incoming views of Mercury. The most striking feature in this region of the planet is a huge circular basin, 1300 km in diameter that was undoubtedly produced from a tremendous impact comparable to the event that formed the Imbrium basin on the Moon. This prominent Mercurial feature, named Caloris Planitia - or Caloris Basin - is filled with material forming a smooth surface or plain that appears similar in many respects to the lunar Maria.

Mercury, much like the Moon, can thus present two totally different faces; one is a heavily cratered surface like the highlands on the backside of the Moon, and the second shows a region of large basins filled with smooth plains similar to the front side of the Moon.

Both the heavily cratered regions of Mercury and the craters themselves, however, differ from their lunar counterparts.

Mercury's heavily cratered surfaces exhibit relatively smooth areas or plains between the craters and basins, whereas the lunar highlands display closely packed and overlapping craters. In many cases, these "intercrater" plains appear to predate that time when most of the large Mercurial craters were formed.

The lunar and Mercurial heavily cratered surfaces are probably different because the force of gravity on Mercury is twice that on the Moon. The ballistic range of material ejected from a primary crater on Mercury is less than that on the Moon and, consequently, covers, depending on the ejection velocity, an area from a fifth to a twentieth smaller for craters of the same size.

As a result, ejecta deposits and secondary craters on Mercury are confined more closely around the primary crater than on the Moon; thus, the early cratering record stored in the surface features of Mercury may be better preserved than on the Moon. Ejecta-forming secondary craters from the most recent large basin events on the Moon have been superposed on the earlier record of primary craters, increasing the density of craters and obliterating the earlier activity.

The difference in the gravity fields is also probably responsible for the variation in the geometry of craters of the same size on the two bodies. In both cases, the smallest craters are bowl-shaped and with increasing size exhibit central peaks and develop terraces on their inner walls.

At the larger sizes, the central peaks become complex structures and undergo a transition into an inner mountain ring that is concentric with the crater rim.

Although this progressive change in crater geometry is the same on both the Moon and Mercury, the change from one type to another occurs with smaller diameters on Mercury and apparently reflects gravitationally induced modifications to the original excavation crater.

An additional important difference between the heavily cratered surfaces of Mercury and the Moon are the lobate scarps or cliffs that are several kilometres high and extend for hundreds of kilometres across the Mercurial surface.

The scarp named Discovery is one of the best examples of this feature. Its shape and transection relationships suggest that scarps are thrust faults resulting from compressive stresses, perhaps due to cooling and shrinkage of the iron-rich core, and causing crustal shortening on a global scale.

Regardless of the mechanism for forming these escarpments, their presence in the large, well preserved craters establishes an approximate relative time scale for their age and eliminates the possibility that planet-wide melting or Earth-like movement of crustal plates has taken place since the heavily cratered ground was created.

The extensive areas of smooth surfaces or plains on Mercury have been classified into three types. The most widespread type forms a level to gently rolling ground between and around large craters and basins.

These "intercrater" plains are characterized by an extremely high density of superimposed small (5 to 10 km) craters, which are frequently elongate, shallow, and suggestive of being of secondary origin. A second type, "hummocky" plains, occurs within a broad ring that is 600 to 800 km wide and circumscribes the Caloris Planitia. These plains consist of low, closely spaced to scattered hills, and have been interpreted to be material ejected during the cratering event that produced the Caloris basin. "Smooth" plains are the third type and form relatively level tracts with a very low population of craters, both within and external to Caloris Planitia as well as in some of the smaller basins (e.g., Borealis Planitia in the Borealis quadrangle).

The smooth plains are similar to the lunar maria and, if analogous, result from extensive lava flows that would reflect an extended period of volcanism on Mercury after the Caloris event. In addition to the cratered surfaces and plains regions, several other distinctive topographic features occur.

A system of linear hills and valleys that extends up to 300 kilometres cuts through, or modifies, some parts of the heavily cratered areas in the Discovery quadrangle. These valleys are scalloped and range up to 10 km wide.

The best example of this type of feature extends more than 1000 km to the northeast from the mountainous rim, Caloris Montes, in the Shakespeare quadrangle. Both examples are similar to the so-called lunar Imbrium sculpture.

It is generally believed that this type of lineated surface feature resulted from excavations by secondary projectiles when the large basins were formed and, possibly, fracturing and faulting of the planet's crust during the basin formation. The basin associated with the lineations in the Discovery quadrangle is unknown, but it may be found in the darkened hemisphere that was hidden from Mariner 10's cameras.

Some of the most peculiar and interesting landforms seen on Mercury are in another region in the Discovery quadrangle that has been termed "hilly and lineated." The hills are 5 to 10 km wide and vary from a few hundred meters up to almost 2 km in height. This region included many old degraded craters whose rims have been broken up into hills and valleys. Similar surfaces are known at two sites on the Moon. In all three cases, the regions are antipodal to the youngest large basins (Imbrium and Orientale on the Moon and Caloris on Mercury).

For this reason, there could be a genetic relationship between the formation of the basins and the hilly and lineated terrain.

It has been suggested that seismic waves generated by the basin impacts are focused in the antipodal region and are the cause of the peculiar surfaces.

Well-defined bright streaks or ray systems radiating away from craters constitute another distinctive feature of the Mercurial surface, again in remarkable similarity to the Moon. The rays cut across and are superposed on all other surface features, indicating that the source craters are the youngest topographic features on the surface of Mercury.

Despite some differences, the striking duplication of surface features between Mercury and the Moon suggests that although an absolute time scale for the development of the Mercurial surface must remain uncertain, the relative sequence of events for the two bodies must have been very similar, if not contemporary. The greatest uncertainty in the Mercurial absolute time scale is: When did the heavy bombardment forming the heavily cratered surfaces (lunar highlands) and the large basins (lunar Imbrium and Orientale) come to an end'?

Within these uncertainties, Mercury's evolution can be divided into five stages or epochs. The first epoch includes the interval of time at the earliest stage of the solar system, condensation of the solar nebula into solids, and the accumulation of the solid material into the main mass of Mercury.

It is not known whether the planet accumulated heterogeneously or homogeneously; i.e., whether it formed directly as an iron core with a silicate crust, or whether the proto-Mercury was initially a mixture of iron and silicates which subsequently melted and separated into the core and crust configuration. Regardless of how the planet accumulated, all crustal melting must have been completed well before the craters in the heavily cratered surfaces were formed to have preserved their shapes and geometries to the present time.

Moreover, if Mercury ever had been enveloped in an atmosphere either during or immediately after accumulation, aeolian degradation of craters would have occurred, similar to that seen on Mars.

Because such degradation has not been recognized, any atmosphere must have disappeared before the oldest cratered surfaces were formed.

The second epoch following accumulation and chemical separation was a period of heavy bombardment by large objects from an unknown source that produced the heavily cratered surfaces and the large basins; this epoch was terminated by the time of the Caloris event. It is not certain whether this last period of heavy bombardment was the terminal phase of the accumulation of Mercury, or if it was the second episode of bombardment unrelated to the accretionary phase. The "intercrater" plains probably represent an older surface that predates this second epoch, or they may have been emplaced during the period of heavy bombardment. Because the lobate scarps are prevalent in the intercrater areas and sometimes pass through and deform some of the older craters, core shrinkage and crustal shortening may have occurred during the end of the first epoch and extended into at least the early part of the second.

The Caloris basin was a massive event marking the onset of the third epoch. It produced the mountainous ring Caloris Montes and the basin Caloris Planitia, as well as the ejecta deposits and sculpturing of the older heavily cratered surface that can be traced more than 1000 km from the ring of mountains. If the Caloris basin were contemporary with our Moon's two youngest basins, Imbrium and Orientale, an absolute time for the Caloris event would be about 4 billion years ago.

The start of the fourth epoch followed an indeterminate but probably short, period after the Caloris event. During this time broad plains were formed, most probably as a result of widespread volcanism grossly similar to that which produced the lunar maria. It has been suggested, however, that the smooth plains surrounding the Caloris Planitia (i.e., the Suisei, Odin, and Tir Planitia) are ejecta from Caloris that were melted by the impact. If the smooth plains are analogous to lunar maria, this fourth epoch may represent the period from four to 3 billion years ago. If the plains are impact melt, they must be contemporary with the Caloris event, about 4 billion years in age.

The fifth and final epoch in what can be recognized in Mercurial history probably extends from about 3 billion years ago to the present. Little has happened on Mercury during this period except for a light "dusting" of meteoritic debris that has produced many of the prominent rayed craters. The crater population on the smooth plains is very similar to that on the lunar Maria.

The apparent similarity in the sequence of events for the Moon and Mercury is especially significant for interpreting and understanding evolutionary processes of the terrestrial planets.

It is now clear that Mercury, in common not only with the Moon, but also with Mars, was subjected to an early, intense crater-producing bombardment (including basin events) that was followed by volcanism and, in turn, by a greatly reduced impact flux.

Ice on Mercury[edit]

Mercury would seem to be one of the least likely places in the solar system to find ice. The closest planet to the Sun has temperatures, which can reach over 700 °C. The local day on the surface of Mercury is 176 earth-days, so the surface is slowly rotating under a relentless assault from the Sun. Nonetheless, Earth-based radar imaging of Mercury has revealed areas of high radar reflectivity near the north and south poles, which could be indicative of the presence of ice in these regions. There appear to be dozens of these areas with generally circular shapes. Presumably, the ice is located within permanently shadowed craters near the poles, where it may be cold enough for ice to exist over long periods of time. The discovery of ice on the Earth's moon, and on Mars, can only serve to strengthen the arguments for ice on Mercury.

Ice is highly radar reflective and the radar reflections off ice tend to be highly depolarized, unlike typical silicate rock that comprises the bulk of Mercury's surface. While not as highly reflective as other icy solar system objects, such as on Jupiter’s moons Europa, Ganymede, and Callisto, these areas are still significantly more reflective than silicate material. Moreover, the depolarized nature of the reflections is also an indicator of water ice. The Arecibo results show that the radar reflective areas are concentrated in crater-sized spots. At the South Pole, the location of the largest area appears coincident with the large crater Chao Meng-Fu and the smaller areas with other identified craters. At the North Pole much of the area containing the radar bright spots was not imaged, and cannot be correlated with any known craters.

However, for the imaged areas at both poles most of the areas have been loosely correlated with known craters. Craters near the poles could provide the permanent, or near permanent, shading required for ice to exist on Mercury.

The radar results indicate the reflective areas are probably relatively uncontaminated ice. However, the lower reflectivity compared to pure ice features indicates the ice may be covered by a thin layer of dust or soil or else does not completely cover the crater floor. Note that no direct unequivocal detection of ice has been made.

The coincidence of the radar bright areas with large, possibly permanently shadowed, polar craters is strong circumstantial evidence for ice.

However, the radar reflections could be explained by an enhancement of some other radar reflective material, such as metal sulphides or other metallic condensates, or precipitated sodium ions

Investigations of Mercury were done from Earth using the Arecibo radio telescope, the Goldstone antenna, and the Very Large Array (VLA). The Goldstone/VLA study used the NASA Deep Space Network 70-m Goldstone dish antenna to transmit 8.51 GHz, 460 kW, right circularly polarized radar waves towards Mercury. The National Radio Astronomy Observatories 26 VLA antennas received the reflections. Calibration and processing of the radar returns showed radar-bright (high radar reflectivity) with depolarized signatures at the North Pole. The Arecibo observations were made by transmitting an S-band (2.4 GHz), 420 kW, circularly polarized coded radar wave at Mercury. The wave reflects off Mercury back to Earth. The wave is both transmitted and received by the Arecibo radio telescope.

Filtering and processing the return signal gives a radar reflectivity map of Mercury's surface with a resolution of approximately 15 km. About 20 anomalously reflective and highly depolarized features were observed at the north and south poles.

As mentioned above, all provinces on Mercury are exposed to the Sun for almost 90 earth-days at a time, and can reach temperatures over 700 °C. Additionally, Mercury has no ambient atmosphere and has very low gravity.

Water ice on the surface of Mercury is exposed directly to vacuum, and will rapidly sublime and escape into space unless it is kept cold at all times. This implies that the ice can never be exposed to direct sunlight.

The only locations on the surface of Mercury where this is possible would seem to be near the poles (Mercury’s South Pole is shown here), where the floors of some craters might be deep enough to afford permanent shading. Whether such permanently shadowed craters exist on Mercury is still problematic. The only close-up images we have of Mercury were taken by the Mariner 10 spacecraft on three close passes in 1974 and 1975.

The same hemisphere of Mercury was sunlit on each of these passes, so nearly half the planet has never been imaged, and no determination can be made of what polar areas, if any, are permanently shadowed. However, theoretical studies assuming typical crater dimensions show that craters near the poles should have areas that never rise above about 102 °C and that even flat surfaces at the poles would not exceed about 167 °C. Other studies also indicate that water ice in polar craters on Mercury could be stable over the age of the solar system.

There are only two significant sources for ice on Mercury: meteorite bombardment and planetary out gassing. Meteorites, especially in the past, potentially carried large amounts of water to Mercury's surface. Out gassing of water from the planet's interior could also provide a non-negligible flux of water to the surface, although this is speculative.

The permanently shadowed regions near Mercury's poles should act as "cold-traps" so that any water, which found its way to these regions, would freeze on the surface and remain. (The possibility that the ice is relatively uncontaminated may indicate that each deposit was laid down in one or a small number of rapid events, such as a large comet impact.) Meteorites, which impacted near the poles and water, which out gassed in that region, could have been easily trapped.

Solar Wind[edit]

The planet Mercury is the closest planet to the sun, our nearby star, which gives off a huge amount of light and heat. The Sun also has significant super hot plasma burst from its surface known as prominences, and upper atmosphere called coronal mass ejections.

The charged atomic particles then move at great speed through the solar system. Mercury therefore is the planet that is regularly in the line of fire, so that its surface is constantly bathed by the solar wind. While the planet’s atmosphere is too thin and rarefied to erode Mercurian surface features, the solar wind does appear to have an effect, albeit slowly, over eons of time.

Mercury's Physical Characteristics[edit]

Mass 0.3302 kg Volume 6,083 kg³ Equatorial Radius 2,439.7 km Polar Radius 2,439.7 km Ellipticity (Flattening) 0.0000 Mean Density 5.427 g/cm³ Surface Gravity 6 km/s Escape Velocity 4.3 km/s Visual Magnitude -0.42 (seen from Earth). Number of Moons None Planetary Ring system None Mean Orbital Velocity 47.87 km/s Orbital Inclination 7º Orbital Eccentricity 0.2056 Sidereal Axis rotation 1,407.6 hours Length of day 4,222.6 hours

Mercurian Magnetic Field

Dipole field strength 0.0033 gauss-Rh^3 Dipoles tilt to rotation axis 169º Longitude of tilt 285º (From Mercury-1 flyby) 115º (From Mercury-III flyby).

Transit of Mercury Phenomena[edit]

A transit is the passage of a planet across the Sun's bright disk. At this time, the planet can be seen as a small black disk slowly moving in front of the Sun.

The orbits of Mercury and Venus lie inside Earth's orbit, so they are the only planets that can pass between Earth and Sun to produce a transit. Transits are very rare astronomical events. In the case of Mercury, there are on average thirteen transits each century. A transit of Mercury occurs only if the planet is in inferior conjunction with the Sun (between Earth and Sun) and is also crossing through Earth's orbital plane (the Ecliptic). During the present period in Earth's history, Mercury's orbit crosses Earth's orbital plane in early May and early November each year. If Mercury is passing between the Earth and Sun at that time, a transit will be seen.

During the seven-century period 1601 to 2065, Earth experiences 94 transits of Mercury across the Sun. These events can be organized into two groups: All Transits = 94 = 100.0%. May (Descending Node) = 31 = 33.0%. November (Ascending Node) = 63 = 67.0%

Mercury's orbit is highly eccentric (e = 0.2056). This causes the planet's distance from the Sun to vary from 46 to 70 million kilometres. At perihelion, Mercury's orbital velocity (59.0 km/s) is over 50% faster than it is at aphelion (38.9 km/s).

Furthermore, the planet's orbit is tipped 7 degrees to Earth's around the Sun. Such a wildly varying and inclined orbit has important consequences on the characteristics and frequency of Mercury's transits.

During May transits, the apparent diameters of the Sun and Mercury are 1902 and 12 arc-seconds, respectively. Thus, Mercury appears to be 1/158 the size of the Sun. In contrast, the apparent diameters of the Sun and Mercury during November transits are 1937 and 10 arcseconds, respectively. So then Mercury appears to be 1/194 the size of the Sun. Consecutive transits of Mercury appear to be separated by 3.5, 7, 9.5, 10, or 13 years.

The pattern is rather complex because of Mercury's elliptical orbit. The shorter periods are a consequence of several longer harmonics between the orbital periods of Mercury and Earth. The 13-year period is of particular note because it corresponds to nearly 54 orbits of Mercury around the Sun (it falls short of a perfect fit by just 2.01 days). A longer period of 33 years (10 + 10 + 13) produces an even better fit, which corresponds to 137 orbits of Mercury minus 1.67 days. If one combines the 13 year and 33 year periods together, the 46-year total equals 191 orbits of Mercury plus only 0.34 days.

A useful way to organize Mercury's transits is by grouping them into series where each member is separated by 16,802 days or 46 years (= sum of 13 and 33 years). Thus, the transits of 1957, 2003 and 2049 belong to one series, while the transits of 1960, 2006 and 2052 belong to another series. May transits occur about a month after Mercury's aphelion passage so the planet is travelling at close to its minimum velocity?

The planet's relative position with respect to the Sun shifts by approximately 200 arcseconds with each transit. This rapidly evolving geometry means that transit series, which occur in May only, last about 10 cycles or 414 years. In comparison, November transits occur just a few days before Mercury reaches perihelion so the planet is travelling at nearly its maximum orbital velocity. Mercury's relative position with respect to the Sun shifts by only about 100 arcseconds with each transit, so November series last about twice as long as a May series. For example, Series 8 (November at Ascending Node) began in 1776 and will run through 2604 for a total of 19 transits spanning 828 years. For comparison, Series 9 (May at Descending Node) began just recently in 1957 and will run through 2371 for a total of 10 transits spanning 414 years. At any one time, there can be about six transit series running concurrently. But since the November series last twice as long as the May series, there are twice as many transits in November as there are in May. Mercury's transit series are quite analogous to the Saros series for solar and lunar eclipses although they are shorter and tend not have as many events in each series.

Ephemeris Data[edit]