| Teacher Background
Mars Pathfinder and Global Surveyor will be sending
back huge amounts of new images and data to NASA JPL, much of which
will be made available not only to scientists, but to students via
television, print, and the Internet. For students to understand this
data, they need some basic background about what is already known
about Mars. A good way to make this information interesting is to
compare and contrast conditions on Mars to those on our own planet
and/or evident in students' local or regional environments. The next
two Activities are intended to both engage and inform students. (See
Student Worksheet A.3 for basic Earth/Mars
Atmosphere and Hydrosphere
Abundant liquid water is what makes our home planet unique in the
solar system. Approximately three quarters of Earth's surface is
covered by it. Some of this water evaporates and condenses around
dust, salt, or pollen grains that are blown into the atmosphere,
and these condensation nuclei are the beginnings of clouds. Clouds
produce rain and snow and help trap the heat energy that's radiating
back from Earth's surface. Carbon dioxide also helps keep heat in
the atmosphere-which is known as the greenhouse effect. Clouds and
carbon dioxide help moderate the daily temperature fluctuations
on Earth, which are at their most extreme in deserts where there
is very little water vapor or clouds to trap heat.
The atmosphere of Mars contains very little water. Conditions on
Mars are far too dry for extensive water clouds to form, but even
this little amount can condense, forming high, thin, wispy clouds.
Early morning fog collects in valleys, and frosts may form on the
ground, but these rapidly dissipate as the morning temperature rises.
Since Mars is so cold, water is in the form of ice crystals.
The Martian atmosphere is too thin (equivalent to 100,000 feet
altitude on Earth) for carbon dioxide to hold in infrared radiant
energy and so it has no greenhouse effect as here on Earth. Mars
is heated only by the incoming solar radiation, and thus is subject
to great day-night fluctuations in temperature.
Storms on Mars are not rain storms as on Earth, but rather dust
storms. These occur when the southern hemisphere on Mars is in summer.
These dust clouds trap infrared energy and keep it from escaping
back into space and so help make Mars' atmosphere a little warmer.
(See MarsWatch for why dust storms are of great interest
to NASA's Mission Planners.)
Days and Seasons
The rate of spin of a planet (its rotation on its axis) determines
the length of its day-night cycle. Earth takes 24 hours to make
one complete rotation, which we call a "day". Mars takes 24 hours
and 37 minutes, which scientists call a "sol". If you were on Mars,
you'd sense a day-night cycle similar to that on Earth. Sojourner's
baseline mission is 7 sols, though scientists certainly hope it
will survive much longer.
The tilt of a planet's axis (relative to its orbit) determines
whether or not the planet has seasons and, if so, how severe they
might be. Earth's axis is tilted 23 1/2 degrees, and Mars about
25 degrees. Mars, just like Earth, has seasons. MPF will land on
July 4, summer in Earth's northern hemisphere and summer at the
planned Ares Vallis landing site on Mars.
The distance of a planet from the Sun and the nature of its atmosphere
also has a large effect on its weather and climate. Mars is almost
one and a half times as far from the Sun as Earth is, and takes
about twice as long to travel around the Sun. (A planet's revolution
around the Sun determines its year.) Consequently, Mars is colder
than Earth and its seasons last about twice as long as ours.
As students will soon discover, however, evidence written in surface
channels on Mars, and inferred from its giant volcanoes, make most
scientists pretty certain Mars was once quite different, with liquid
water on its surface and a thicker atmosphere protecting it from
destructive radiation. (See Activities 1.3 and 2.2) Now Mars is cold and dry; its surface too
cold for life and scoured by incoming UV rays. One key and fascinating
question that will take many missions over many years to answer
is whether life-dependent on water and a more clement climate-once
existed on Mars?