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the long-running debate about whether Mars ever had sustained liquid water on its sur-

face. Composition and textures showed that the rocks not only had been saturated

with water, but had actually been laid down under gently flowing surface water. For six

months beginning in June 2003, Opportunity examined deeper layers of rock inside a

stadium-sized crater about 700 meters (half a mile) from the landing site. The rocks

had all soaked in water, but textures in some showed that periods of dry, wind-blown

deposition alternated with periods when water covered the surface. After examining its

own heat shield and a nickel-iron meteorite near this crater, Opportunity drove more

than 3 kilometers (2 miles) southward to reach a different type of terrain in search of

rocks with different information to add about the region's wet history.

Mars Express (2003): This is a European Space Agency orbiter with NASA partici-

pation in two of its seven instruments. The spacecraft has been returning color images

and other data since January 2004 after entering orbit in late December 2003. It has

confirmed water ice in Mars' south polar cap and added information about how the

solar wind has been removing water vapor from Mars' atmosphere for billions of years.

Mars Express has found traces of methane in Mars' atmosphere. Scientists propose

that this gas would break down rapidly enough to be undetectable if there is not an

active source, either biological or non-biological, maintaining the amount. The orbiter

has also mapped variations in the concentration of water vapor in the lower portion of

the atmosphere. The spacecraft's June 2005 deployment of the antenna for a ground-

penetrating radar instrument prepares for a search for layers bearing ice and possibly

water beneath the surface of Mars.

Phoenix Mars Scout (2007): The first in a planned series of competitively selected

NASA Mars Scout missions, Phoenix will land in icy soils near the north polar ice cap

of Mars and explore the history of the water in these soils while checking for organic

chemicals and monitoring polar climate. It is scheduled to launch in August 2007 and

land in May 2008. The stationary lander will operate for up to three months. Its robotic

arm is designed to dig a trench up to half a meter (20 inches) deep and deliver sam-

ples from it to an onboard laboratory to analyze the samples' chemistry and physical

properties. The mission will serve as NASA's first exploration of this ice-rich region and

renew the search for carbon-bearing compounds, last attempted by the Viking landers

of the 1970s. The Phoenix mission plan, developed by a team led by a University of

Arizona scientist, was one of 25 proposals submitted to become the first Mars Scout

mission. NASA selected Phoenix from among four finalist proposals in 2003. A major

role for Mars Reconnaissance Orbiter in 2008 will be to serve as the primary communi-

cations relay for Phoenix.

Mars Science Laboratory (2009): This NASA mission will use precision landing

technologies to put a roving science laboratory at a selected site on Mars with a pay-

load of science instruments more than 10 times as massive as those of earlier Mars

rovers. The laboratory is designed to operate for more than a Martian year (687 Earth

days) and travel across a much greater range than previous rovers. To help scientists

assess whether the landing area ever had or still has environmental conditions favor-

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able to microbial life, the rover will analyze dozens of samples scooped from the soil

and cored from rocks. Instruments have been selected that could identify and inventory

the chemical building blocks of life in the samples and identify features that may show

effects of biological processes. The mission will mark major advances in measurement

capabilities and surface access. It will demonstrate technologies for accurate landing

that will be necessary for sending later missions to sites that are scientifically com-

pelling but difficult to reach. 

Additional Mars Scouts (2011 and later): Mars Scouts are competitively proposed

missions intended to supplement and complement, at relatively low cost, the core mis-

sions of NASA's Mars Exploration Program. Phoenix is the first. The next round of pro-

posals and selection targets a launch no sooner than 2011. 

Beyond 2011: For the second decade of this century, NASA proposes additional

missions to examine the geological record and current environment at selected sites

on Mars, possibly including extended surface operations, deep-drilling or the first mis-

sion to return samples from Mars. The flexible program includes many options.

Scientists and mission planners foresee technology development providing advanced

capabilities, such as life-detection investigations, drilling to reach underground sites, a

Mars ascent vehicle and a rendezvous in Mars orbit. 

In support of the Vision for Space Exploration, NASA has also begun preparations

for sending human explorers to Mars and beyond. An early step in realizing the vision

will be development of a crew exploration vehicle in the next few years; a crucial inter-

mediate step will be return of astronauts to the Moon by 2020, this time for a sustained

presence there. Meanwhile, plans envision precursor robotic missions to Mars between

2011 and 2020 that will pave the way for human exploration there by examining the

challenges to human missions and demonstrating solutions. Some challenges that

astronauts will face are environmental factors, such as dust and radiation. Others are

technological requirements, such as reliable precision landing and an adequate energy

supply. 

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