The Phoenix Mars Lander will investigate a site in the far north of Mars to answer questions about that part of Mars, and to help resolve broader questions about the planet. The main questions concern water and conditions that could support life.

The landing region has water ice in soil close to the surface, which NASA's Mars Odyssey orbiter found to be the case for much of the high-latitude terrain in both the north and south hemispheres of Mars.

Phoenix will dig down to the icy layer. It will examine soil in place at the surface, at the icy layer and in between, and it will scoop up samples for analysis by its onboard instruments. One key instrument will check for water and carbon-containing compounds by heating soil samples in tiny ovens and examining the vapors that are given off. Another will test soil samples by adding water and analyzing the dissolution products. Cameras and microscopes will provide information on scales spanning 10 powers of 10, from features that could fit by the hundreds into the period at the end of this sentence to an aerial view taken during descent. A weather station will provide information about atmospheric processes in an arctic region where a coating of carbon- dioxide ice comes and goes with the seasons.

Mars is a vast desert where water is not found in liquid form on the surface, even in places where mid-day temperatures exceed the melting point of ice. One exception may be fleeting outbreaks that have been proposed to explain modern-day flows down some Martian gullies. Today's arid surface is not the whole story, though. Previous Mars missions have found that liquid water has persisted at times in Mars' past and that water ice near the surface remains plentiful today.

Water is a key to four of the most critical questions about Mars: Has Mars ever had life? How should humans prepare for exploring Mars? What can Mars teach us about climate change? How do geological processes differ on Mars and on Earth? Water is a prerequisite for life, a potential resource for human explorers and a major agent of climate and geology. That's why NASA has pursued a strategy of "follow the water" for investigating Mars. Orbiters and surface missions in recent years have provided many discoveries about the history and distribution of water on Mars -- such as minerals that formed in wet environments long ago and liquid flows that are still active today in hillside gullies.

The landing site and onboard toolkit of Phoenix position this mission to follow the water further. The mission's three main science objectives are:

1. Study the history of water in all its phases.

On a time scale of billions of years, ice near the surface where Phoenix will land might be the remnant of an ancient northern sea. Several types of evidence point to plentiful liquid water on ancient Mars, and the northern hemisphere is low and smooth compared to the southern hemisphere. Much of the water that could have remained liquid when ancient Mars had a thicker atmosphere may now be underground ice.

On a time scale of tens of thousands to a few million years, ice near the surface where Phoenix lands might periodically thaw during warmer periods of climate cycles. The tilt of Mars' axis wobbles more than Earth's, and the shape of Mars' orbit also cycles over time, from rounder to more elongated. Currently, Mars is about 20 percent farther from the sun during northern summer than during northern winter, so the summers are relatively cool in the north. As the orbit varies, the northern ice cap will enjoy warm winters on a 50,000-year cycle. The wobble of Mars' axis may also cause the climate to change on a time scale of 100,000 to millions of years.

On much shorter time scales, the arctic ground "breathes" every day and every season, converting tiny amounts of ice into water vapor on summer days and condensing tiny amounts of frost from the atmosphere at night or in winter. In this way, the ice table slowly rises and recedes as the climate changes.

Phoenix will collect information relevant for understanding processes affecting water at all these time scales, from the planet's distant past to its daily weather.

2. Determine if the Martian arctic soil could support life.

Life as we know it requires liquid water, but not necessarily its continuous presence. Phoenix will investigate a hypothesis that some ice in the soil of the landing site may become unfrozen and biologically available at times during the warmer parts of long-period climate cycles. Life might persist in some type of dormant microbial form for millions of years between thaws, if other conditions were right.

The spacecraft is not equipped to detect past or present life. However, in addition to studying the status and history of water at the site, Phoenix will look for other conditions favorable to life.

One condition considered essential for life as we know it is the presence of molecules that include carbon and hydrogen. These are known as organic compounds, whether they come from biological sources or not. They include the chemical building blocks of life, as well as substances that can serve as an energy source, or food, for life. Phoenix would be able to detect even very small amounts and identify them. Two Viking spacecraft that NASA landed on Mars in 1976 made the only previous tests for organic compounds in Martian soil, and they found none. Conditions at the surface may be harsh enough to break organic molecules apart and oxidize any carbon into carbon dioxide. Phoenix will assess some factors in those oxidizing conditions, and it will check for organic chemicals below the surface, as well as in the top layer. Organic chemicals would persist better in icy material sheltered from sunshine than in surface soil exposed to harsh ultraviolet radiation from the sun.

Phoenix will also be checking for other possible raw ingredients for life. It will examine how salty and how acidic or alkaline the environment is in samples from different layers. It will assess other types of chemicals, such as sulfates, that could be an energy source for microbes.

3. Study Martian weather from a polar perspective.

In Mars' polar regions, the amount of water vapor in the thin atmosphere -- the humidity -- varies significantly from season to season. Winds carrying water vapor can move water from place to place on the planet. The current understanding of these processes is based on observations from orbit and limited meteorological observations from earlier Mars landers closer to the equator. Phoenix will use an assortment of tools to directly monitor several weather variables in the lower atmosphere at an arctic site.

Phoenix will measure temperatures at ground level and three other heights to about 2 meters (7 feet) above ground. It will check the pressure, humidity and composition of the atmosphere at the surface. And it will identify the amounts, altitudes and movements of clouds and dust in the sky above.

Over the course of the mission, this unprecedented combination of Martian meteorological measurements will help researchers evaluate correlations such as whether southbound winds carry more humidity than northbound winds; whether drops in air pressure are associated with increased dust; and how the amount of water vapor at the bottom of the atmosphere changes from late spring to mid-summer or later.