Mars' orbit is significantly elliptical. One result of this is a temperature variation of about 30 C at the subsolar point between aphelion and perihelion. This has a major influence on Mars' climate. While the average temperature on Mars is about 218 K (-55 C, -67 F), Martian surface temperatures range widely from as little as 140 K (-133 C, -207 F) at the winter pole to almost 300 K (27 C, 80 F) on the dayside during summer.
The atmosphere of Mars contains very little water
vapor. The level of water vapor averages about 0.016 percent, compared to the earth’s
average level of about 2 percent. The water content of the atmosphere on Mars varies
seasonally and by location and can form clouds and even frost. Six major types of clouds
form in Mars’s atmosphere. The polar hood is a haze of water and perhaps carbon
dioxide ice that forms over the polar regions in the fall and can cover much of the
northern plains. Wave clouds form on the sheltered side of large obstacles, such as
craters, and have very distinct ridges. Convective clouds form in high areas at midday.
Orographic clouds form when air lifts over large-scale objects like Olympus Mons, and are
most common in spring and summer when the water vapor content of the air is highest.
Ground hazes occur in low areas at dawn and dusk and probably consist of water ice. Wispy
high-altitude clouds sometimes occur just at dawn and dusk. The Viking 2 lander recorded images of water-ice frost during
the winter.
In general, the atmosphere
of Mars is transparent. Dense clouds like those on Earth would not be expected in a
tenuous carbon-dioxide atmosphere, and under normal conditions the surface features can be
seen clearly, without obscuration. However, clouds do occur, and are of two types
(telescopic observations have been confirmed by data from Mariner
9). The so-called 'white clouds' lie at a high altitude and are not
uncommon. It has been suggested that they are made up of ice or solid carbon
dioxide crystals. They are especially pronounced at Martian sunrise and
sunset. These clouds are distinct from what are often called 'violet layer' or 'blue
haze'. This is a haze in the Martian atmosphere which is virtually permanent.
It cannot be seen through a telescope, but it reflects light of short wavelength (the blue
end of the spectrum) and shows up when Mars is photographed in blue light.
Photography in other lights enables observers to see features on Mars that may be obscured
by this haze, since orange, yellow or red light penetrates it. Occasionally, the
haze clears in patches, allowing the surface of Mars to be photographeed in blue light as
well, giving results of finer detail. The 'yellow clouds'
are quite different. They seem to occur at low levels in the atmosphere, and are
generally interpreted to be clouds of dust. During some oppositions of the planet,
major dust storms have been observed. Typical cases were the storms of 1909, 1924,
1937, 1956, 1971, all of which were perihelic oppositions.
The Martian atmosphere is less than one one-hundredth that of earth's at sea level and is primarily composed of carbon dioxide. It is persistently hazy due to suspended dust and particles that make the sky took gray-yellow.
The atmosphere contains little water vapor because water condenses at low temperatures. Most of the water supply is held in the polar ice caps or near the equator in soil grains on the surface. Carbon dioxide is the chief oxygen-bearing gas on Mars, though some trace amounts of atomic oxygen and ozone appear in the atmosphere as a result of chemical reactions with ultraviolet light. No sulfur-containing gases have been detected in the atmosphere but they may have been released by volcanoes in the past and became particles that eventually formed sulfate minerals. When water is released into the soil oxygen is released, showing us that the soil contains an oxidant, probably a peroxide. Because the atmosphere of Mars has much less ozone than that of Earth, it lacks the equivalent of a stratosphere. In comparison with the Earth, and for the purposes of human survival, the Martian atmosphere is negligible. According to results from the Russian spacecraft, at an altitude of over 300 km the main constituent of the atmosphere is atomic hydrogen, and, in fact, the upper atmosphere of Mars is more like Venus than that of Earth.
The low temperatures of Mars is a result of the large amount of carbon dioxide, which is able to radiate away the energy that reaches the planet from the sun. The thin atmospheres does little to help with the transport of heat on the planet, which is why carbon dioxide condenses on the surface of the poles as dry ice during the winter. This decreases the pressure at the poles during the winter; the pressure may drop about 25% from fall to winter. The entire surface is very responsive to direct sunlight, so the temperature may drop tens of degrees after the Sun sets. When winds on the surface of the planet reach 50 to 100m per second sand grains about 100 microns across start a skipping motion called saltation. They strike other smaller dust grains that may remain suspended in the atmosphere for months before coming down again. Winds of some 150 kph are estimated to be the minimum for the initiation of particle movement. Because of these high speeds particles have a greater erosion capability than they would have here on Earth. A local dust storm can grow to global proportions in a matter of weeks, enveloping the entire planet in a dust shroud. After a while there is too much dust in the storm and the winds and storm begin to decay. They develop during the spring and early summer in the southern hemisphere, when the planet is closest to the sun and the winds are stronger.
Four years, (or two Mars years') worth of Hubble observations show that
the Red Planet's climate has changed since the mid-1970's. "The Hubble results show
us that the Viking years are not the rule, and perhaps not typical. Our early assumptions
about the Martian climate were wrong," said Philip James of the University of Toledo.
"There has been a global drop in temperature. The planet is cooler
and the atmosphere clearer than seen before," said Steven Lee of the University of
Colorado in Boulder. "This shows the need for continuous monitoring of Mars. Space
probes provided a close-up look, but it's difficult to extrapolate to long-term conditions
based upon these brief encounters."
The researchers attribute the cooling of the Martian atmosphere to
diminished dust storm activity, which was rampant when a pair of NASA Viking orbiter and
lander spacecraft arrived at Mars in 1976. Two major dust storms occurred during the first
year of the Viking visits,
which left fine dust particles suspended in the Martian atmosphere for longer than normal.
Warmed by the Sun, these dust particles (some only a micron in diameter, about the size of
smoke particles) are the primary source of heat in the Martian atmosphere.
"Hubble is showing that our early understanding based on these
visits is wrong. We just happened to visit Mars when it was dusty, and now the dust has
settled out," Lee said. "We are going to have to look at Mars for many years to
truly understand the workings of the climate,"
said Todd Clancy, of the Space Science Institute, Boulder, Colorado.
Knowledge about the Martian climate has been limited by the
fact that ground-based telescopes can only see weather details when Earth and Mars are
closest -- an event called opposition -- that happens only once every two years. Though
Hubble has observed Mars only for four years, the observations are equivalent to 15 years
of ground- based observing because Hubble can follow seasonal changes through most of
Mars' orbit.
Though the Mariner and Viking series of flyby, orbiter and lander
spacecraft that visited Mars in the late 60's and 70's provided a close-up look at Martian
weather, these were snapshots of the planet's complex climate. Hubble provides the
advantage of a global view - much like the satellites that monitor Earth's weather, and
can follow martian seasonal changes over many years. When Mars is closest to Earth, Hubble
returns near-weather satellite resolution.
Although there has been concern about a lack of ozone (a form of
molecular oxygen created by the effects of sunlight on an atmosphere), dubbed the
"ozone hole" over Earth's poles, there are no ozone holes on Mars. By contrast,
the planet has a surplus of ozone over its northern
polar cap, as first identified by the Mariner 9 spacecraft in 1971. (However the Martian
atmosphere is different enough from Earth's that few parallels can be drawn about
processes controlling the production and destruction of ozone.) Hubble's ultraviolet
sensitivity is ideal
for monitoring ozone levels on a global scale. The Martian ozone is yet another indication
the planet has grown drier, because the water in the atmosphere that normally destroys
ozone has frozen-out to become ice-crystal clouds. Spectroscopic observations made with
the Faint
Object Spectrograph (FOS) show that ozone now extends down from Mars' north pole to mid
and lower latitudes. However, the Martian atmosphere is so thin, even this added ozone
would offer future human explorers little protection from the Sun's harmful ultraviolet
rays.
The fourth planet from the Sun, Mars is one of the most intensely
scrutinized worlds because of its Earth-like characteristics. Mars is tilted on its axis
by about the same amount Earth is, hence Mars goes through seasonal changes. However,
because Mars' atmosphere is much
thinner than Earth's, it is far more sensitive to minor changes in the amount of light and
heat received from the Sun. This is intensified by Mars' orbit that is more elliptical
than Earth's, so it's range of distance from the Sun is greater during the Martian year.
Mars is now so distant, the sun is nearly 25% dimmer than average. This chills Mars'
average temperature by 36 degrees Fahrenheit (20 degrees Kelvin). At these cold
temperatures, water vapor at low altitudes
freezes out to form ice-crystal clouds now seen in abundance by Hubble.
"Clouds weren't considered to be very important to the Martian
climate during the Viking visits because they were so
scarce," says Clancy. "Now we can see where they may play a role in transporting
water between the north and south poles during the Martian year." Seasonal
winds also play a major role is transporting dust across Mars' surface, and rapidly
changing the appearance of a region. This gave early astronomers the misperception that
Mars' shifting surface color was evidence of vegetation following a season cycle.
As clearly seen in the Hubble images, past dust storms in Mars'
southern hemisphere have scoured the plains of fine light dust and transported the dust
northward. This leaves behind a relatively coarser, less reflective sand in the southern
hemisphere.