Mars Polar Lander Science Goals

In the last thirty years, our knowledge of Mars has been revolutionized by spacecraft missions and by studying martian meteorites found on Earth. Some of the most intruiging discoveries have been that liquid water, and perhaps even life, were possibly present once on Mars' surface. The study of water on Mars - its availability for life, its role in the weather and climate, and its usefulness as a resource for future human exploration - is the crucial issue in the future of Mars exploration, and is a central focus of NASA's Mars Surveyor Program missions.

Mars Polar Lander will touch down in a unique region of Mars near the border of the southern polar cap at a latitude of about 76 degrees south. The lander is the only spacecraft planned by any space agency to study an area of Mars this far south or north.

Mars has polar caps at both its north and south poles. Both caps include a permanent or residual cap visible year-round, and a temporary or seasonal cap that appears in winter and disappears in summer. In the north, the permanent cap is water ice, while in the south the perma- nent cap is mostly carbon dioxide ice with perhaps some water. The north's permanent cap is 10 times larger than the south's; it remains a mystery as yet why the caps differ so. The south's seasonal cap is larger than the north's, which is caused by the fact that the southern winter takes place when Mars is farthest in its orbit from the Sun.

Both poles show signs of an unusual layered terrain, whose alternating bands of color may contain different mixtures of dust and ice. Like growth rings of trees, these layered geological bands may help unravel the mystery of past climate change on Mars and help determine whether it was driven by a catastrophic change or merely a gradual evolution in the planet's environment. One of the lander's primary science objectives is to conduct a visual survey of this largely unknown dome of ice and dust, and characterize the mineralogical makeup of the layered terrain.

As the next lander mission in the Mars Surveyor Program, the Mars Polar Lander will focus primarily on Mars' climate and water. The "Volatiles and Climate History" theme for the 1998 Mars Surveyor missions was recommended by the Mars Science Working Group and is aligned directly with NASA''s Mars exploration strategy for the next decade focusing on: Evidence of past or present life, Climate, and Resources. The Lander will carry the Mars Volatiles and Climate Surveyor (MVACS) instrument suite, which will perform in situ investigations to address the science theme "Volatiles and Climate History", the Mars Descent Imager (MARDI), and a LIDAR instrument supplied by the Russian Space Agency. The Lander will search for near-surface ice and possible surface records of cyclic climate change, and characterize physical processes key to the seasonal cycles of water, carbon dioxide and dust on Mars. The duration of the landed science phase is expected to last no more than approximately 90 days. Piggybacking on the Mars 98 lander are two small microprobes. Separating from the lander just prior to entry into the Martian atmosphere, the two microprobes will slam into the surface of Mars at a velocity of 200 meters per second. The aeroshell on each probe will shatter to release the science package which will penetrate up to 2 meters into the soil. The microprobes will determine if water ice is present in the Martian subsurface, and will also measure the temperature and monitor the local Martian weather

Primary science objectives of the Mars Polar Lander mission are:

• Image the polar terrains during descent to related surface and orbital images.

• Image the landing site in many colors to determine soil types & soil mineral composition.

• Trench into the surface to look for annual layers (less than millmeter-resolution)

• Analyze soil samples for evidence of water, ice, hydrates and other aqueously deposited minerals --> ancient climate

• Measure wind, temperature, pressure & humidty at the polar site

• Detect ice fogs, surface frost & snow, and ground ice formation & loss

An Expedition to the South Pole

Weather, climate, and the water cycle on Earth are studied by comparing the results of thousands of individual studies conducted using a vast variety of instruments in many different locations. Exploration of other planets requires a much more selective process. With each rare opportunity to study Mars, this being only the fourth in human history, scientific investigators must carefully decide what instruments to send and where to send them. For the study of volatiles and climate, the polar regions of Mars are ideal sites . The poles are where the seasonal extremes occur, where the volatiles interact with the surface, where the climate changes are recorded, and where buried volatiles are most likely to be found. The polar environment is more severe than the landing sites of previous missions and less is known about the site to ensure the safety of the spacecraft, but the exploration of this new territory will be an exciting and significant step in Mars research.

Volatiles: Substances Subject to Change

The goals of the MVACS payload, as its name suggests, are to study volatiles and climate on Mars. But what are volatiles? From a purely physical point of view, planets can be thought of as large, self-contained collections of atoms and molecules. The laws of chemistry and physics operating through time have sorted these substances into their present forms and locations. For example, on Mars, substances with high freezing points are solid, like basaltic rock on the planet's surface and interior. Substances with low freezing points are vapor, like argon gas in Mars' atmosphere. But if a substance has a freezing point that lies within the range of daily or seasonal extremes of temperature, it will change form, or phase, over time. On Mars, water and carbon dioxide fall into this category and technically are called volatiles. The very fact that volatiles are affected by changes in daily, seasonal, and climatic changes in temperature make them extremely interesting to monitor.

The Search for Water in All its Forms

Many of the MVACS investigations focus on water. The complete water cycle involves exchanges between the surface and atmosphere, exchanges between different forms of water, and transport within the atmosphere. Of these, previous spacecraft missions have observed only the latter. While the total amount of atmospheric water is extremely small (ten thousand times less than in Earth's atmosphere), it has a strong seasonal variation. The Mars Atmospheric Water Detection (MAWD) experiments on each of the Viking orbiters revealed how water vapor is released from near-surface ice each spring. MVACS will measure, for the first time, all the important aspects of the water cycle at one point on the surface. Where is it? In what forms does it exist? What is the present water cycle, and how may it have differed in the past? MVACS will look for answers to these questions by measuring the amounts of water in the form of vapor, ice, and water-bearing minerals. MVACS will also measure the physical parameters that control water, like pressure, temperature, and wind. These direct measurements from a point on the surface will complement the present global view of the atmospheric water cycle derived from orbiting spacecraft.

Climate: A Brief Look at the Martian Almanac

Before describing our current understanding of water and carbon dioxide on Mars, it is useful to review the seasonal and climatic changes that affect them. The weather (day-to-day temperature, winds, pressure, and precipitation) and climate (long term trends in weather) of a planet are set by factors like the distance of a planet from the Sun, its rotation rate, and the properties of its atmosphere. Some of these factors are surprisingly Earth-like. Mars has a 24-and-a-half hour day and is tilted on its axis 25 degrees, just 2 degrees more than Earth. Unlike Earth, however, Mars orbits the Sun is 668 days and follows a quite elliptical path. While the seasons change as the direction of the planet's tilt revolves around the Sun, the distance from the Sun also changes. As a result, summer in Mars' northern hemisphere, which occurs when Mars is far from the Sun, is longer and cooler than summer in its southern hemisphere, which occurs when Mars quickly passes perihelion, the closest point in its orbit to the Sun. Temperatures on Mars, in summary, are always colder than on Earth and vary with an asymmetric seasonal cycle.

Climate change occurs as the factors which influence the weather change over time scales of thousands or millions of years. On Earth, the advances and retreats of continental glaciers are believed to be linked with cyclical changes in the Earth's axial tilt (called its obliquity) and the ellipticity of Earth's orbit (called its eccentricity), ultimately due to the tidal forces of Jupiter and the other planets. Layering found in the Greenland ice cap and in high mountain glaciers provides a record of Earth's climate changes. Mars is also subject to the tidal forces of the other planets and may be expected to have similar climate fluctuations. In fact, Mars may experience more severe changes because it does not have the protection of the Moon, which for Earth acts as a flywheel, moderating changes to Earth's orbit. The strongest evidence for such changes are images of Mars' polar regions, which show extensive layering. These layers may be formed as different amounts of ice and dust are deposited in different climate extremes. It remains to be conclusively shown, however, that this layering is in fact due to astronomically driven climate changes. Even if it does occur, this type of cyclical climate change probably cannot produce the conditions needed to support the volume of liquid water on Mars that carved the flood channels seen on the surface. An explanation for such major climate change remains elusive.

Carbon Dioxide: The Air of Mars

The composition of Mars' atmosphere results in a very unfamiliar seasonal effect. Carbon Dioxide, which makes up 99% of the air on Mars, turns to solid, or "dry ice", when it freezes at 148 ° Kelvin (-193 °F). When these temperatures are reached in the atmosphere or on the surface during winter, the atmosphere itself begins to freeze onto the ground. Each winter, a seasonal polar cap of carbon dioxide is deposited at the pole (see the figure below). In addition, a permanent polar cap of carbon dioxide exists at the south pole, its surface temperature never exceeding 148 Kelvin. If long-term climate change occurs on Mars, this deposit would grow or diminish, adding or removing some volume of Mars' atmosphere. An even larger deposit of carbon dioxide may be stored in Mars' porous soil, or regolith, by a chemical bonding process called adsorption. MVACS will measure the amounts of carbon dioxide in the atmosphere, adsorbed in the subsurface, and trapped in carbon dioxide-bearing minerals.

---