Mercury 119
at aphelion. As a consequence, Mercury’s orbital velocity
averages 47.6 km/s but varies from 56.6 km/s at perihelion
to 38.7 km/s at aphelion. At perihelion the Sun’s apparent
diameter is over three times larger than its apparent diam-
eter as seen from Earth.
Mercury’s rotation period is 58.646 Earth days, and its or-
bital period is 87.969 Earth days. Therefore, it has a unique
3:2 resonance between its rotational and orbital periods: It
makes exactly three rotations on its axis for every two orbits
around the Sun. This resonance was apparently acquired
over time as the natural consequence of the dissipative pro-
cesses of tidal friction and the relative motion between a
solid mantle and a liquid core. As a consequence of this res-
onance, a solar day (sunrise to sunrise) lasts two Mercurian
years or 176 Earth days. Theobliquityof Mercury is close
to 0◦; therefore, it does not experience seasons as do Earth
and Mars. Consequently, the polar regions never receive
the direct rays of sunlight and are always frigid compared
to torrid sunlit equatorial regions.
Another effect of the 3:2 resonance between the rota-
tional and orbital periods is that the same hemisphere al-
ways faces the Sun at alternate perihelion passages. This
happens because the hemisphere facing the Sun at one per-
ihelion will rotate one and a half times by the next perihe-
lion, so that it faces away from the Sun; after another orbit,
it rotates another one-and-a half times so that it directly
faces the Sun again. Because the subsolar points of the 0◦
and 180◦longitudes occur at perihelion they are calledhot
poles. The subsolar points at the 90◦and 270◦longitudes
are calledwarm polesbecause they occur at aphelion. Yet
another consequence of the 3:2 resonance and the large
eccentricity is that an observer on Mercury (depending on
location) would witness a double sunrise, or a double sun-
set, or the Sun would backtrack in the sky at noon during
perihelion passage. Near perihelion Mercury’s orbital ve-
locity is so great compared to its rotation rate that it over-
comes the Sun’s apparent motion in the sky as viewed from
Mercury.
Although Mercury is closest to the Sun, it is not the
hottest planet. The surface of Venus is hotter because of its
atmospheric greenhouse effect. However, Mercury experi-
ences the greatest range (day to night) in surface temper-
atures (650◦C= 1170 ◦F) of any planet or satellite in the
solar system because of its close proximity to the Sun, its pe-
culiar 3:2 spin orbit coupling, its long solar day, and its lack
of an insulating atmosphere. Its maximum surface temper-
ature is about 467◦C (873◦F) at perihelion on the equator;
hot enough to melt zinc. At night just before dawn, the
surface temperature plunges to about− 183 ◦C(− 297 ◦F).
3. Exosphere
Although Mercury has an atmosphere, it is extremely tenu-
ous with a surface pressure a trillion times less than Earth’s.
TABLE 1 Mercury’s Main Exospheric ConstituentsaConstituent Vertical Column Abundance (atoms/cm^2 )Hydrogen (H) ∼ 5 × 1010
Helium (He) ∼ 2 × 1013
Oxygen (O) ∼ 7 × 1012
Sodium (Na) ∼ 2 × 1012
Potassium (K) ∼ 1 × 1010
Calcium (Ca) ∼ 1 × 107aThe Earth’s atmosphere has∼ 2 × 1018 molecules/cm 2The number density of atoms at the surface is only 10^5 atoms
cm−^3 for the known constituents (Table 1). It is, therefore,
an exosphere where atoms rarely collide; their interaction
is primarily with the surface.Mariner 10’s ultraviolet spec-
trometer identified hydrogen, helium, and oxygen and set
upper limits on the abundance of argon, neon, and carbon
in the exosphere. The hydrogen and helium are probably
derived largely from the solar wind, although a portion of
the helium may be of radiogenic origin, and some hydro-
gen could result from the photodissociation of H 2 O. The
interaction of high-energy particles with surface materials
may liberate enough oxygen to be its principal source, but
breakdown of water vapor molecules by sunlight could also
be a possible source.
In 1985–1986, Earth-based telescopic observations de-
tected sodium and potassium in the exosphere, and sub-
sequent observations have detected calcium (Table 1).
Sodium and potassium are also found in the Moon’s ex-
osphere. Both sodium and potassium have highly vari-
able abundances 10^4 –10^5 Na atoms/cm^3 and 10^2 –10^4 K
atoms/cm^3 near the surface on timescales of hours to years.
Their abundances also vary between day and night by a
factor of about 5, the dayside being greater. Often bright
spots of emission are seen at high northern latitudes or
over the Caloris Basin. The temperature of the gas is about
500 K, but a hotter more extended Na coma sometimes
exists. Observed variations in the abundances of these ele-
ments are consistent with the photoionization timescale of
120 minutes for sodium and∼90 minutes for potassium.
Photoionization of the gas will result in the exospheric ions
being accelerated by the electric field in the planetarymag-
netosphere.Ions created on one hemisphere will be ac-
celerated toward the planetary surface and recycled, but
ions on the opposite hemisphere will be ejected away and
lost. The total loss rate of sodium atoms is about 1.3× 1022
atoms per second, so the atoms must be continuously sup-
plied by the surface. The total fraction of ions lost to space
from the planet is at least 30%. The atmosphere, therefore,
is transient and exists in a steady state between its surface
sources and sinks.