Mercury is the closest planet to the Sun and the eighth largest. Mercury is smaller in diameter than
    Ganymede and Titan but more massive.

            orbit:    57,910,000 km (0.38 AU) from Sun
            diameter: 4,880 km
            mass:     3.30e23 kg

    In Roman mythology Mercury is the god of commerce, travel and thievery, the Roman counterpart of
    the Greek god Hermes, the messenger of the Gods. The planet probably received this name because it
    moves so quickly across the sky.

    Mercury has been known since at least the time of the Sumerians (3rd millennium BC). It was given
    two names by the Greeks: Apollo for its apparition as a morning star and Hermes as an evening star.
    Greek astronomers knew, however, that the two names referred to the same body. Heraclitus even
    believed that Mercury and Venus orbit the Sun, not the Earth.

    Mercury has been visited by only one spacecraft, Mariner 10. It flew by three times in 1973 and
    1974. Only 45% of the surface was mapped.

    Mercury's orbit is highly eccentric; at perihelion it is only 46 million km from the Sun but at
    aphelion it is 70 million. The perihelion of its orbit precesses around the Sun at a very slow rate.
    19th century astronomers made very careful observations of Mercury's orbital parameters but could
    not adequately explain them using Newtonian mechanics. The tiny differences between the observed
    and predicted values were a minor but nagging problem for many decades. It was thought that
    another planet (sometimes called Vulcan) might exist in an orbit near Mercury's to account for the
    discrepancy. The real answer turned out to be much more dramatic: Einstein's General Theory of
    Relativity! Its correct prediction of the motions of Mercury was an important factor in the early
    acceptance of the theory.

    Until 1962 it was thought that Mercury's "day" was the same length as its "year" so as to keep that
    same face to the Sun much as the Moon does to the Earth. But this was shown to be false in 1965 by
    doppler radar observations. It is now known that Mercury rotates three times in two of its years.
    Mercury is the only body in the solar system known to have an orbital/rotational resonance with a
    ratio other than 1:1.

    This fact and the high eccentricity of Mercury's orbit would produce very strange effects for an
    observer on Mercury's surface. At some longitudes the observer would see the Sun rise and then
    gradually increase in apparent size as it slowly moved toward the zenith. At that point the Sun would
    stop, briefly reverse course, and stop again before resuming its path toward the horizon and
    decreasing in apparent size. All the while the stars would be moving three times faster across the
    sky. Observers at    other points on Mercury's surface would see different but equally bizarre
    motions.

    Temperature variations on Mercury are the most extreme in the solar system ranging from 90 K to
    700 K. The temperature on Venus is slightly hotter but very stable.

    Mercury is in many ways similar to the Moon: its surface is heavily cratered and very old; it has no
    atmosphere; it exhibits no plate tectonics. On the other hand, Mercury is much denser than the Moon
    (5.43 gm/cm3 vs 3.34). Mercury is the second densest major body in the solar system, after Earth.
    Actually Earth's density is due in part to gravitational compression; if not for this, Mercury would
    be denser than Earth. This indicates that Mercury's dense iron core is relatively larger than Earth's,
    probably comprising the majority of the planet. Mercury therefore has only a relatively thin silicate
    mantle and crust.

    Mercury's interior is dominated by a large iron core whose radius is 1800 to 1900 km. The silicate
    outer shell (analogous to Earth's mantle and crust) is only 500 to 600 km thick. At least some of the
    core is probably molten.

    The surface of Mercury exhibits enormous escarpments, some up to hundreds of kilometers in length
    and as much as three kilometers high. Some cut through the rings of craters and other features in such a
    way as to indicate that they were formed by compression. It is estimated that the surface area of
    Mercury shrank by about 0.1% (or a decrease of about 1 km in the planet's radius).

    One of the largest features on Mercury's surface is the Caloris Basin (right); it is about 1300 km in
    diameter. It is thought to be similar to the large basins (maria) on the Moon. Like the lunar basins, it
    was probably caused by a very large impact early in the history of the solar system. That impact was
    probably also responsible for the odd terrain on the exact opposite side of the planet.

    In addition to the heavily cratered terrain, Mercury also has regions of relatively smooth plains.
    Some may be the result of ancient volcanic activity but some may be the result of the deposition of
    ejecta from cratering impacts.

    A reanalysis of the Marriner data provides some preliminary evidence of recent volcanism on
    Mercury. But more data will be needed for confirmation.

    Amazingly, radar observations of Mercury's north pole (a region not mapped by Mariner 10) show
    evidence of water ice in the protected shadows of some craters.
 
    Mercury has a small magnetic field whose strength is about 1% of Earth's.

    Mercury has no known satellites.

    Mercury is often visible with binoculars or even the naked eye, but it is always very near the Sun
    and difficult to see in the twilight sky.

            Mass (kg)                                                     3.303e^23
            Volume (1010 km3)                                     6.085
            Equatorial radius (km)                                2,439.7
            Mean density (gm/cm^3)                             5.42
            Mean distance from Sun (AU)               0.387
            Mean distance from the Sun (km)              57,910,000
            Equatorial surface gravity (m/sec^2)         2.78
            Equatorial escape velocity (m/sec)           4250
            Polar radius (km)                                         2440
            Volumetric mean radius (km)                     2440
            Escape velocity (m/sec)                             .4300

            Rotational period (days)                             58.6462
            Orbital period (days)                                   87.969
            Mean orbital velocity (km/sec)                   47.88
            Orbital eccentricity                                      0.2056
            Tilt of axis (degrees)                                   0.00
            Orbital inclination (degrees)                       7.004

            Ellipticity                                                       0.0000
            Magnitude (Vo)                                          -1.9

            Mean surface temperature                         179°C
            Maximum surface temperature                  427°C
            Minimum surface temperature                  -173°C

            GM (x 106 km3/s2)                                      0.02203
            Bond albedo                                                 0.056
            Visual geometric albedo                             0.11
            Visual magnitude V(1,0)                            -0.42
            Solar irradiance (W/m2)                              9214
            Black-body temperature (K)                       442.5
            Moment of inertia (I/MR2)                            0.33
            J2 (x 10-6)                                                     60.
            Largest known surface feature
                            -Caloris Basin (1350 km diameter)

            Atmospheric composition
                Helium                                                       42%
                Sodium                                                     42%
                Oxygen                                                     15%
                Other                                                         1%

 

	Mercury
Semimajor axis (106 km)	57.9
Sidereal orbit period (days)	87.969
Tropical orbit period (days)	87.968
Perihelion (106 km)	46.0
Aphelion (106 km)	69.8
Synodic period (days)	115.88
Mean orbital velocity (km/s)	47.89
Orbit inclination (deg)	7.00
Orbit eccentricity	0.2056
Sidereal rotation period (hrs)	1407.6
Obliquity to orbit (deg)	~0.1

 
 

Mercurian Magnetosphere

    Dipole field strength: 0.0033 gauss-Rh3
    Dipole tilt to rotational axis: 169 degrees
    Longitude of tilt: 285 degrees (from Mercury  I flyby)
                              115 degrees (from Mercury III flyby)

    Note: Rh denotes Mercurian radii, 2,439 km
 

Mercurian Atmosphere

    Surface Pressure: ~10-15 bar (0.001 picobar)
    Average temperature: 440 K (590-725 K, sunward side)
    Atmospheric composition: 98% Helium (He), 2% Hydrogen (H2)
 
 

Ice on Mercury
 
 

    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 K. 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 (1-3). 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.

How was the evidence for ice found?

    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 (1) 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 reflections were received by the National
    Radio Astronomy Observatories 26 VLA antennas. Calibration and processing of the radar returns
    showed radar-bright (high radar reflectivity) with depolarized signatures at the north pole. The
    Arecibo observations (2,3) 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.

Why are these radar-bright areas thought to be ice?

    Ice is highly radar reflective and the radar reflections off ice tend to be highly depolarized, unlike
    typical silicate rock which comprises the bulk of Mercury's surface. While not as highly reflective
    as other icy solar system objects, such as 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 (shown at left)
   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 so cannot be correlated with known craters.
   However, for the imaged areas at both poles most of the areas
   have been loosely correlated with known craters (3). Craters
   near the poles could provide the permanent, or near-permanent
   (see 5), 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 (6).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.

How can ice survive on Mercury?

    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 K. Additionally, Mercury has no ambient atmosphere and
    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, 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 which never rise above about 102 K (4)
    and that even flat surfaces at the poles would not exceed about 167 K (5). Other studies (6-7) also
    indicate that water ice in polar craters on Mercury could be stable over the age of the solar system.

How did the ice get there originally?

    There are only two significant sources for ice on Mercury: meteorite bombardment and planetary
    outgassing. Meteorites, especially in the past, potentially carried large amounts of water to
    Mercury's surface. Outgassing 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
    (6), such as a large comet impact.) Meteorites which impacted near the poles and water which
    outgassed in that region could have been easily trapped. Water originating away from the poles would
    behave as individual, randomly moving molecules, some of which could migrate to the poles and become
    trapped there (6). There are mechanisms for potential loss of ice, however. These include
    photodissociation, solar wind sputtering, and micrometeoroid gardening. The effects of these
    processes are not well-understood.

How can this discovery be tested?

    Direct observations of Mercury from Earth are difficult because Mercury is so close to the Sun.
    The only effective way to study the polar regions beyond radarobservations is to send a space probe
    equipped with an imager and spectrometry instruments. Missions to Mercury are difficult because
    the planet is deep in the Sun's gravitational well. No such mission is currently planned. The only
    mission to visit Mercury was Mariner 10 (shown at right) which had three flybys in 1974
    and 1975. Each of these flybys occurred when the same portion of the planet was lit by the Sun, so
    only about half the planet was imaged. By their very nature, the interiors of shadowed craters are
    too dark to image, so these pictures do not shed any light on whether or not ice exists inside these
    craters.
 

References

1) Mercury radar imaging: Evidence for polar ice, Slade et al., Science, v. 258, p. 635, 1992
2) Radar mapping of Mercury: Full-disk images and polar anomalies, Harmon and Slade, Science, v. 258, p. 640, 1992
3) Radar mapping of Mercury's polar anomalies, Harmon et al., Nature, v. 369, p. 213, 1994
4) Stability of polar frosts in spherical bowl-shaped craters on the Moon, Mercury, and Mars, Ingersoll et al., Icarus, v. 100, p.
40, 1992
5) The thermal stability of water ice at the poles of Mercury, Paige et al., Science, v. 258, p. 643, 1992
6) Mercury: Full-disk radar images and the detection and stability of ice at the north pole, Butler et al., Journal of Geophysical
Research, v. 98, p. 15,003, 1993
7) Near-surface ice on Mercury and the Moon: A topographic thermal model, Salvail and Finale, Icarus, v. 111, p. 441, 1994

Source of the Ice on Mercury report:
                Nine Planets by Bill Amett - (see Source)