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