120 Encyclopedia of the Solar System
Although both sodium and potassium are probably de-
rived from the surface of Mercury, the mechanism by which
they are supplied is not well understood. The sodium and
potassium in the Mercurian exosphere could be released
from sodium- and potassium-bearing minerals by their in-
teraction with solar radiation, or impact vaporization of mi-
crometeoroid material. Both sodium and potassium show
day-to-day changes in their global distribution.
If surface minerals are important sources for the
exosphere, then a possible explanation is that their
sodium/potassium ratio varies with location on Mercury. A
possible explanation for some of the K and Na variations is
that Na and K ion implantation into regolith grains during
the long Mercurian night (88 Earth days), and subsequent
diffusion to the exosphere when the enriched surface ro-
tates into the intense sunlight. At least one area of enhanced
exospheric potassium emission apparently coincides with
the Caloris Basin whose floor is highly fractured. This exo-
spheric enhancement has been attributed to increased dif-
fusion and degassing in the surface and subsurface through
fractures on the basin floor, although other explanations
may be possible.
4. Polar Deposits
High-resolution, full-disk radar images of Mercury obtained
from both the Arecibo and the linked Goldstone—Very
Large Array radar facilities discovered unusual features at
Mercury’s poles. The radar signals show very high reflectiv-
ities centered on the poles. The reflectivity and ratio values
are similar to outer planet icy satellites and the residual po-
lar water ice cap of Mars. Therefore, Mercury’s polar radar
features have been interpreted to be water ice. The radar
characteristics are consistent with the ice being covered by
a few centimeters of regolith. It has also been proposed
that the radar characteristics are the result of volume scat-
tering by inhomogeneities in elemental sulfur deposits. In
this case, it is proposed that sulfur volatilized from sulfides
in the regolith wascold-trappedat the poles.
Mariner 10images of Mercury’s polar regions show
cratered surfaces where ice or sulfur could be concentrated
in permanently shadowed portions of the craters. Radar
studies have shown that the anomalies are indeed concen-
trated in the permanently shadowed portions of these polar
craters (Fig. 2). The south polar radar feature is centered at
about 88◦S and 150◦W and is largely confined within a crater
(Chao Meng-Fu) 150 km in diameter, but a few smaller fea-
tures occur outside this crater. In the north polar region, the
deposits reside in about 25 craters down to a latitude as low
as 72◦(Fig. 2). Because the obliquity of Mercury is near
0 ◦, it does not experience seasons, and, therefore, tempera-
tures in the polar regions should be<135 K. Water ice can
be stable in the interiors of craters even down to 72◦lati-
tude if covered with only a few centimeters of regolith, or if
it is relatively new. This means that water ice would still be
present in its perpetually shadowed craters or even in illu-
minated craters at high latitude, if covered with a veneer of
regolith. In permanently shaded polar areas (i.e., the floors
and sides of large craters), the temperatures should be less
than 112 K, and water ice should be stable to evaporation
on timescales of billions of years when covered with a thin
veneer of regolith. The problem with sulfur being the de-
posits on Mercury is that sulfur is stable at much higher
temperatures than water, and there are no highly radar-
reflective deposits in the polar regions where temperatures
are within the stability range of sulfur. A 1-m- thick layer of
water ice is stable for one billion years at a temperature of
− 161 ◦C while sulfur is stable at a considerably higher tem-
perature of− 55 ◦C. Much of the region surrounding per-
manently shadowed craters is less than− 55 ◦C, but there
are no radar-reflective deposits there.
The deposits are concentrated only in the freshest
craters, and even in some craters less than 10 km in diame-
ter. Degraded craters do not show the highly radar-reflective
deposits, probably because there are no permanently shad-
owed regions in these low-rimmed and shallow craters. In
fact, the permanently shadowed cold traps are essentially
full. Furthermore, the strong radar signal indicates that the
material is relatively pure. The thickness of the deposits
has been estimated to be between∼2 and maybe 20 m.
The higher value is, in fact, arbitrary because the radar data
cannot place upper limits on the thickness. The area cov-
ered by these deposits (both north and south) is estimated
to be∼30,000 km^2. This would be equivalent to 4× 1016 to
8 × 1017 grams of ice, or 40–800 km^3 for a 2–20 meter thick
deposit. Each meter thickness of ice would be equivalent
to about 10^13 kilograms of ice.
If the deposits are water ice, then they could originate
from comet or water-rich asteroid impacts that released
the water to be cold-trapped in the permanently shad-
owed craters. Because comets and asteroids also impact the
Moon, similar deposits would be expected to occur in the
permanently shadowed regions of lunar craters. The neu-
tron and gamma ray spectrometers on theLunar Prospector
spacecraft discovered enhanced hydrogen signals in perma-
nently shadowed craters in the polar regions of the Moon.
This has been interpreted as water ice with a concentration
of only 1.5±0.8% weight fraction.5. Interior and Magnetic Field
Mercury’s internal structure is unique in the solar system.
It also imposes severe constraints on any proposed ori-
gin of the planet. Mercury’s mean density of 5440 kg/m^3
is only slightly less than Earth’s (5520 kg/m^3 ) and larger
than Venus’ (5250 kg/m^3 ). Because of Earth’s large inter-
nal pressures, however, its uncompressed density is only
4400 kg/m^3 compared to Mercury’s uncompressed density