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Number 6 - 2013
The surface of Mercury appeared im­
mediately as a succession of cratered
terrain and smooth plains with many
deep craters similar to those seen on the
moon (which exploration had ended
with the last Apollo mission in Decem­
ber 1972). The craters had formed when
meteors or small comets crashed into
the planet. Like the other terrestrial pla­
nets (Venus, Earth and Mars) Mercury is
made mostly of rock and metal. But to
any observer, Mercury's surface appears
to be like that of the moon. It refects
about 6% of the sunlight it receives, al­
most the same as the moon's albedo;
for, like the moon, Mercury is covered
by a thin layer of silicate particles.
Mercury is dry, extremely hot and al­
most airless as it is too small for its gravi­
ty to retain any signifcant atmosphere
over long periods of time. Its extremely
weak atmosphere contains hydrogen,
helium, oxygen, sodium, calcium and
potassium. Due to the heat of the pla­
net, the very thin vestigial atmosphere
is frequently blasted of its surface by
the solar wind and escapes into space.
Nevertheless Mercury's atmosphere
is constantly being replenished a fact
which indicates strongly that the planet
has an active inner core. Scans of Mer­
cury made by Earth-based radar show
that craters at Mercury's poles contain
water ice. The foors of the craters are
permanently shielded from sunlight,
so the temperature never gets high
enough to melt this ice.
Mercury has a very elliptical (oval-shaped) orbit. At peri­
helion (at its closest point) it is about 46 million km from the
Sun, but at aphelion (at its farthest point) it is 70 million km.
At its closest approach, Mercury is about 77.3 million km from
Earth. As a poetic note, the sun appears about 2 ½ times as
large in Mercury's sky as in the Earth's frmament.
A puzzling planet
The amount of information collected about Mercury over the
past decades has produced more questions than answers!
And the little we know is quite puzzling.
For instance, Mercury has an odd sense of time. It moves
around the sun faster than any other planet. Its year is there­
fore just a quarter as long as a year on Earth. With that sort
of speed, it would be easy to think that a Mercury day would
pass in a ficker. But with the planet's unusually slow rota­
tion, a Mercury day languidly passes once every six terrestrial
months! Mercury's orbital habits are strange and interesting,
indeed.
u
contêm gelo de água. Os pisos das cra­
teras estão permanentemente prote­
gidos contra a luz do sol, fazendo com
que a temperatura nunca fque alta o
sufciente para derreter o gelo.
Mercúrio tem uma órbita bastante
elíptica (oval). No periélio (no seu pon­
to mais próximo) ele está a cerca de 46
milhões de km do Sol, mas no afélio (em
seu ponto mais distante) está a 70 mil­
hões de km. Na sua maior aproximação,
Mercúrio está a cerca de 77,3 milhões
de km da Terra. Como uma nota poé­
tica, o sol parece ser cerca de 2 vezes e
meia maior no céu de Mercúrio, do que
no frmamento da Terra.
Um planeta enigmático
A quantidade de informações coleta­
das sobre Mercúrio durante as últimas
décadas tem produzido mais pergun­
tas do que respostas! E o pouco que
sabemos é bastante intrigante.
Por exemplo, Mercúrio tem um es­
tranho senso de tempo. Ele se move ao
redor do sol mais rapidamente do que
qualquer outro planeta. Seu ano é, por­
tanto, apenas um quarto de um ano na
Terra. Com essa velocidade, seria fácil
pensar que um dia em Mercúrio pas­
saria como em um lampejo. Mas com
a rotação invulgarmente lenta do pla­
neta, um dia em Mercúrio passa langui­
damente a cada seis meses terrestres!
Os hábitos orbitais de Mercúrio são de
fato estranhos e interessantes.
u
Estranhas características do solo de
Mercúrio reveladas pela sonda Messenger.
Strange features on the soil of Mercury revea-
led by the Messenger spaceprobe. © NASA
A sonda BepiColombo retratada em rota em direção
a Mercúrio, uma longa viagem de sete anos.
The BepiColombo spaceprobe depicted en route toward Mercury
on its seven-year long voyage. © Astrium