Surface Conditions Atmosphere and Weather

Mercury has almost no atmosphere; its atmosphere is 1013 times less dense than ours (that is a million billion times less dense). Mercury is thought to have lost its atmosphere because of the high temperatures near the Sun, because of its weak magnetic field that allows the solar wind to strip away gases, and because of its relatively weak gravity. Because of its very thin atmosphere and relatively low gravity, the atmospheric pressure at the surface of Mercury is 10-15 atm, which is 1015 times less than Earth's atmospheric pressure. With such a thin atmosphere, Mercury has no weather in the common sense, but it is certainly bombarded by storms and material expelled from the Sun. The atmosphere consists partly of solar wind, mainly helium and hydrogen nuclei or atoms, but it also contains a significant proportion of sodium, potassium, and calcium, atoms that could only have come from its own crust. The atmospheric composition seems to change significantly with time, as a function of local time on Mercury, distance from the Sun, and level of solar activity.

The mosaic of images of Mercury's surface shown in the figure at right was taken by Mariner 10 as it passed from the dark side of the planet.The dark and bright parts of the planet lie in stark comparison, symbolizing the intense energy input the Sun provides the planet.

Mercury's average surface temperature is 243°F (117°C), but over the course of a Mercurian day, the surface temperature ranges from 889°F to -297°F (467°C to -183°C), the largest range in surface temperature of any planet. During a single day, the temperature rises high enough to melt lead and plunge low enough to freeze carbon-dioxide gas. The immense temperature range on Mercury is compared to those on other planets in the figure on page 108; no other planet approaches the temperature extremes of Mercury. The surface temperature of Mercury is also influenced by its strange orbital resonance: Since it orbits the Sun in a 3:2 resonance, Mercury has two spots on its equator that are especially hot because they face the Sun more often than the rest of the equator. Mercury also has, therefore, two especially cold spots, placed between the hot ones.

Mercury's ancient surface appears to have been most recently covered by volcanism about 3.7 billion years ago.The craters and scarps

This photomosiac image of Mercury was taken as Mariner 10 was outbound from the planet. The north pole is at the top of the image, and because of the angle of imaging, the equator is about two-thirds down the image. (NASA/JPL/Mariner 10)

Surface Temperatures

900 800

Temperature °F 300 200 100

-100

hj i

450 400 350 300 250 200 150 100 50 0 -50 -100 -150 -200

Temperature °C

Object

Average Surface Temperature °F (°C)

a

Venus

896 (480)

b

Mercury

day 800 (430) night -274 (-170)

c

Earth

60 (15)

d

Mars

-80 (-62)

e

Jupiter

-238 (-150)

F

Saturn

-292 (-180)

9

Uranus

-346 (-210)

h

Neptune

-364 (-220)

i

Pluto

-382 (-230)

The surface temperature ranges of each of the planets graphed here show that Mercury has by far the widest range of surface temperatures, though Venus has the hottest surface temperature, while Pluto, unsurprisingly, has the coldest.

resemble the ancient surface of the Moon. Like the Moon, Mercury has no atmosphere and so is naked to the radiative effects of the Sun. The planet's surface is bombarded by the solar wind and cosmic rays, heated intensely during the day and frozen at night.The high pressure, constant heat, and acidic environment of Mercury's neighbor Venus is an immense challenge to the engineers who design landing spacecraft, but Mercury's intense radiation and huge temperature swings may produce an even larger challenge. To date there has been no attempted landing on Mercury.

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