Kurt Schwehr
5/17/93
schwehr _at_ cs stanford edu
Revised 8/19/93
(sections on Weather by David Chappell)

2.1.2 MARS SURFACE SCIENCE

This is an quick summary some of the major science questions that can be explored while on the martian surface. It includes objectives suitable for both human and robotic exploration. This list is a cross between goals of the Stanford International Mars Mission (SIMM) and a variety of published sources. (Ref. Carr and Pepin 1992, CNES 1992, Greeley 1990, Stoker 1989, Carr 1981, etc.) The objectives are summarized in Table 2.1.2-1.

SURFACE SCIENCE OBJECTIVES

TABLE 2.1.2-1

2.1.2.1 EXO-BIOLOGY

Exobiology is perhaps the most important goal of Mars exploration, leading with the question: Is there life on Mars and if it is not there now, was life ever present. A major focus of pre-human (precursor) missions should be the search for extant life and prebiological molecules, since the arrival of major organic contaminants from Earth will bring compounding problems to these experiments. For example, once humans arrive it may not be possible to distinguish whether humans were the source of any biological compounds. Also, if micro-organisms are found, and if they are not radically different, they could actually be Earth sourced life that has mutated and evolved rapidly in the very different martian environment. Discovery of extant life would most certainly be strong grounds for postponing any manned mission.

The question of extant life is still very open, as the Viking landers have left a large number of questions unanswered. Even what is known from these two sites is difficult to extrapolate to the rest of Mars from Viking landing sites. (Ref. Don Devanchency 1-20-93) The current best guess is that the martian surface is probably dead because of four factors: there is no liquid water at the surface - the pressure and temperature are near the triple point of water, so ice and frost go directly to gas; the surface is very oxidizing; the influx of UV light is very high due to the lack of a martian ozone layer; and finally, there is relatively small amount of N2 available for metabolic activity. (Ref. McKay, 2-1-93)

Because of this harsh environment, we have to look for places that are likely to harbor life from this world. There are several possible circumstances on Mars that may be harboring life or traces of life. One example is an area where ground ice interacts with the high thermal gradient of local volcanism. This interaction may produce stable water in the subsurface. Two other sites that may have had life in the past which could be preserved as frozen below the surface or as fossils are paleolakes and runoff channels that probably had periodic liquid water.

2.1.2.1.1 COMPLEX ORGANIC MOLECULES

There are many possible ways of looking for life. If there has been or is life, there may be remnants of complex organic molecules present that we may be able to detect. If these molecules exist on Mars, they will most likely be found preserved in a permafrost layer where they are protected from UV radiation and the heavily oxidizing surface.

2.1.2.1.2 Presence of micro-fossils in sedimentary rocks

If there was life on Mars, it is assumed that life started in a very small form and may or may not have evolved to larger, "macro", forms. Even if larger forms did not develop, it may still be possible to find remains of micro fossils. If martian organisms had any "hard parts" they may be preserved in paleo-lake, paleo-oceans, or other sedimentary environments. See Boardman et. al., 1987 for a recent work on paleontology. These hard body parts can be composed of mineralizations such as calcium carbonate, aragonite, chitin, or some metals. On Earth it took a long time to develop hard parts, as these hard parts are not seen until the pre Cambrian/Cambrian boundary - approximately four billion years after the formation of the Earth. Therefore it would be prudent to focus on looking for masses of micro-biological communities that leave behind structures known as stromatalites. These are some of the first evidences of life on Earth in the Archean eon and show up about 3.5 billion years ago - one billion years after the formation of the Earth. Currently we can see stromatalites forming in two areas on the Earth: salty lagoons in Shark Bay, Australia and in perennially frozen lakes in the dry valleys of Antarctica.

2.1.2.1.3 Presence and distribution of extant life

If we detect life on Mars it will be important to gain as much knowledge as possible before we seriously contaminate it since this will only be our second view of evolution in the universe. The jump in our understanding of life would be absolutely amazing. A large number of geologist are currently working hard on Earth to understand how groups of organisms work in present Earth environment since this information can often be used with fossils to tell us a lot about the environment in which now extinct organisms lived. This would open a vast new door to biology, beyond the table-top setups and theoretical concepts of current exobiology.

If life evolved early on in the martian environment, there is a chance that life could still be alive. One example of the hardiness of life is where pyrobacteria have been found living inside the turbine water in nuclear reactors. A better example comes from the dry valleys of Antarctica. Large mats of bacteria live at the bottom of the perennially ice covered lakes. Portions of the mat are transported up to the bottom of the ice cover where they are eventually brought to the surface of the ice over several years. The wind blows the exposed mat to a new site, where, if the bacteria find water, they start a new colony. (Ref. Andersen, 1993)

2.1.2.2 GEOLOGY

Geology is an area in which there is large amounts of work to do. It is a rapidly evolving field, that has not existed in its current form for very long. For example, plate tectonics not been in the text books for more than a decade or two. Being able to see another world will undoubtedly lead to major revolutions in thought about geologic process in a process called comparative planetology. It will be a tough job. On Earth, there have been thousands of modern geologist since the early 1900's. With Mars, were are still at the state we were at when the first geologic map of England produced in 1812: lots of well educated guesses and few solid answers. One of the most important tasks in geologic terms is to extend the set of geologic maps that cover as much area as possible and in as much detail as possible using ground truths to orbital observations. This section on geology includes the categories of geochemistry and mineralogy which are often separated out into their own topics. The martian geology may be able to provide resources for exploration. Materials for the production of consumables for fuel and for breathing air are present as well as other energy sources we do not yet know about. The known martian resources are water, O2 from CO2 and soil, Ar/N2 for breathing, and materials for various fuels. Some may be available in the subsurface as well as from the atmosphere.

2.1.2.2.1 Evolution and processes of polar caps

Understanding the polar ice caps is extremely important for two major reasons. The layers in the ice contain an extremely valuable record of the climatic history. Ice usually has bubbles that contain trapped gasses from ancient atmospheres. Stable isotopic ratios similar to that of O16/O18 may be able to tell us about martian climate through time, although they will be probably require different interpretations than measurements on Earth. Besides containing a record of the history, the polar ice caps themselves are a major control on climate. The CO2 mass in the atmosphere is modulated by how much dry ice in stored on the caps.

2.1.2.2.2 Formation of the valley networks

The valleys on Mars were thought to have been cut by large floods of water. The sediments on the valley floors may contain fossils of critters that lived in the water. The cuts have another important feature: since they are so deep, they may provide access to very ancient rock, both sedimentary and volcanic/plutonic, that have been buried. From these exposures, we may be able to build up a more complete stratigraphic record.

We should be able to see a wide range of the sedimentary process that occur on Mars within these valleys such as mass wasting. Canyon walls may be a good source of water. There may still be ice covered lakes on the valley floors which may be the last large scale liquid water bodies on the surface of planet (Ref. McKay, 2-1-93). The deposits in the valley floors are an excellent site to search for fossil remains. Some of these deposits may represent lake deposits. See section 2.2.2.3.4 for a description of Valles Marinares, the largest valley on Mars.

2.1.2.2.3 Northern lowlands/Southern highlands dichotomy

The Northern lowlands, Southern highlands dichotomy is a major break in the surface features encircling the planet approximately one third of the way down from the North pole to the South pole (around 35¡ North). The lowlands material is much young as seen by a much lower crater density. The cause of this break is not clear. There are a number of possible causes including: a large impact or a massive tectonic event. The North consists mainly of younger cratered and smooth plains with some ridges and domes. The South consists primarily of ancient heavily cratered highlands.

2.1.2.2.4 Paleontology

If there are fossils on Mars, it will be important to carefully collect as many as possible. With these fossils we can start to build up a progression of martian evolution. On Earth, fossils provide a valuable tool for dating sedimentary rocks from the age range of the organism as sedimentary rocks frequently do not contain material which can be radioisotope dated. This is possible only if life existed and left significant remains. Remains may still exist even if the organic material has been removed and/or replaced.

2.1.2.2.5 Stratigraphy

There are a number of scientific questions that fall under the title of stratigraphy, the sequencing and dating of rock features and events. One of the most important concerns is getting some hard numbers on the age of various features from radioisotope dating. Once a few basic ages are done, the relative ages from cratering can be put to an "absolute" time scale.

There are a number of layered deposits visible from orbit. However, as can be seen on Earth, there is a large amount of layering that is not visible from satellites because the change in proprieties is not observable by space born equipment or is just plain too small. Some examples are, changes in sedimentary structures like ripples or fossil progressions. As a result, a large amount of the work on stratigraphy must be done directly on the surface with direct access to features of all scales. As Mars exploration continues, we will be able to put together a continually better chronology of the events that have occurred on Mars since its formation.

2.1.2.2.6 Surface lithology, mineralogy, morphology, AND processes

Lithology, mineralogy, and morphology are some of the key clues to understanding the evolution of near surface crust. From this information, geologists can more fully understand the surface processes that occur on Mars. For example, the mineralogy around volcanoes can tell about composition and activity at great depths back in time.

2.1.2.2.7 Volcanic history and present activity

Mars shows a history of intense volcanic and tectonic activity into the past. Mars has the largest known volcano in the solar system, Olympus Mons. The volcanic areas present a window into the interior of the planet. Most of the material that erupts comes from great depths. Some may even come from the upper mantle. An example of this on the Earth are the olivine nodules that are brought to the surface in Hawaii and peridotites at the base of ocean crust sequences. The SNC meteorites found on Earth present evidence for volcanism on Mars as recently as 150 million years ago, if they are in fact actually from Mars (Ref. Carr, 2-1-93)

One of the key targets in assessing the present activity, is the search for geothermal areas. The heat combined with subsurface H2O may provide an area of liquid water which has the possibility of harboring life.

2.1.2.2.8 Volatile history

Only a few of the volatites present on and in Mars are visible the atmosphere and polar caps. The volatile history is of major importance to exobiology and mineralogy interests. The rocks on Mars, in particular the layered units, can tell us what liquids and gasses were present in the past. The sequences of rocks present the opportunity to understand the cycles that operate on Mars. Correlations with Earth will be important... it will be a fantastic opportunity to tell which events were local to one of the two planets and which were massive enough to effect both planets. Understanding what volatiles have been around is one of the best ways to gain understanding in the procession/history of the solar system.

There currently is evidence of large quantities of H2O on Mars. The most visible evidence is the numerous instances of erosion as in the river valleys. At some meteor impact sites there is ejecta that looks like it was mud when it was thrown from the blast area. Finally, there is different morphology of surface features at the high latitudes which appear to be unstable in the same manner that features on Earth that are laden with ice deform. The suspicion is that most of the water is in the old cratered terrain. (Ref. Carr, 2-1-93)

2.1.2.3 GEOPHYSICS

Since we have no way to directly look directly into the center of a planet, we have to resort to geophysical techniques to give us information on those hard to reach places.

2.1.2.3.1 Global and local magnetic fields

The global magnetic field can give us two kinds of information: the structure of mantel/core motion and time histories of surface rocks that contain magnetic minerals. Currently, Mars has a magnetic field that is in the neighborhood of 10-4 times the value at Earth (Table 2.1.2.3.1-1.) The field is extremely weak and it is debated whether there exists a field at all. The motion of the mantle or core is the only easy explanation for large planetary magnetic fields. Traces of the planetary field are often saved in rocks that contain magnetic minerals as these rocks form. If there was a large field in Mars' past, it will be important to look for magnetic reversals. If they occurred, these patterns represents an important tool for correlating ages of rock units. Once the large scale reversals are mapped and dated, reversals combined with other information give a better picture of when a sequence of events occurred. The magnetic field can also be used to determine at what latitude that the rocks were deposited. 

			Earth		Mars
Field At Equator:	35-40x103 nT	30 nT
Dipole Moment:		8x1015 Tm3	<2x1012 Tm3
MAGNETIC FIELDS TABLE 2.1.2.3.1-1

2.1.2.3.2 Planetary interior - structure and processes

It is important to understand the interior of Mars to work on our theories for the evolution of the solar system. We still do not know what the internal structure is beyond a mean density and volume of the planet. Therefore we can only guess. We would like to know if there ever was plate tectonics on Mars. If there is, why did it stop; otherwise why did plate tectonics not occur?

The available data on Mars are so limited that we can not confirm the existence of internal structures such as a core, mantle, and asthenosphere that are seen within the Earth, while the thickness of the crust has yet to be determined. Several models have been constructed of the internal state of the planet, but they make a number of assumptions about information that is known for the Earth but not Mars. An example of the range of results from the different models is that current estimates of the core range from 1/3 to 1/2 of the radius of Mars depending on what is assumed to the material content of the core. (Ref. MARSNET, 1992)

2.1.2.3.3 Tectonics

Currently we are limited to seismic data from one of the Viking landers, Viking 2, which had large problems with noise. From this data, we have one possible marsquake recorded. Recently, it has been predicted from orbital photos that there are marsquakes. Marsquakes give an indication of the stress that the crust is under and allow the interior of the planet to be imaged.

A probable source for a good number of Marsquakes, if they occur, is the Tharsis region. This whole area is a large bulge that encompasses a set of massive volcanoes. The major question is what is the cause of the bulge and how is it remaining stable or whether it is stable. Is it isostatically held up or is there an upwelling of low density mantle material? The other main source of seismic activity is probably the occasional meteorite impact. Mars is expected to be more seismicly active than the Moon. In has be predicted that marsquakes up to magnitude three should be common with occasional quakes up to magnitude five on the Richter scale (Ref. Trall, Golumbek, and Banerdt, 1992.)

2.1.2.3.4 Thermal history, heat flow, and temperature gradient

Heat flow measurements and temperature gradient knowledge will allow better prediction of the internal state of the planet. Using this data will allow a more accurate depiction of the temperature history of the interior of Mars.

2.1.2.4 METEOROLOGY

The atmosphere if Mars is much thinner than that of Earth, yet it is quite active meteorologically. Our current knowledge of the martian weather comes primarily from the two Viking landers in the Northern hemisphere and three orbiters (Mariner 9, Viking 1 and 2). Theoretical meteorology has had considerable success with Mars. With the exception of dust storms, martian weather can be predicted. See Michael Carr's The Surface of Mars , 1981 for more details on martian meteorology.

2.1.2.4.1 ATMOSPHERE

The atmosphere of Mars is quite different from that of Earth. The martian atmosphere is primarily composed of carbon dioxide, with nitrogen and argon as secondary components. Table 2.1.2.4.1-1 lists the abundances as measured by the Viking landers. As on Earth, convection is the prime mechanism that drives the martian atmosphere. 

		Component			Percent
		Carbon Dioxide (CO2)		95.3
		Nitrogen (N2)			2.7
		Argon (Ar)			1.6
		Oxygen (O2)			0.15
		Water (H2O)			0.03
		Neon (Ne)			0.0003
ATMOSPHERIC COMPOSITION TABLE 2.1.2.4.1-1

Both the pressure and temperature on Mars are much lower than on Earth. The surface pressure on Mars is about six millibars (0.006 atm), with maximum and minimum values of 8.9 and 1 millibars. An arbitrary "sea level" has been chosen where the pressure is 6.1 millibars. The recorded surface temperature ranges from 140 to 295 K. The Viking landing sites averaged 210 K (-63 C), while daily variations were 190 to 240 K. The typical temperature is below 0 C, but in some equatorial regions the temperature may climb above freezing.

Water vapor is only a minor component of the atmosphere because temperatures are almost always well below the freezing point. The Viking orbiters found that the amount of water vapor varies by season and location. Water vapor on Earth is is more than 200 times the amount measured on Mars. Nevertheless, the atmosphere is almost saturated with water, evidenced by the frost seen at the Viking landing sites. The low pressure and temperature keep the atmosphere from holding much water.

The martian atmosphere is generally clearer than that of Earth, with many fewer clouds, but several types of clouds form in the atmosphere of Mars. These include white water ice clouds, yellow carbon-dioxide clouds, and dust clouds (see Section 2.1.2.4.3). The water ice clouds are similar to those in the Earth's atmosphere. They often form around mountains, and low-lying H20 fog is also possible when the temperature is very low - just before sunrise. The canyons, such as Candor Chasma, may experience this morning fog. At high altitudes, the CO2 in the atmosphere can condense to form hazes of dry ice crystals.

The winter and summer weather patterns are quite different. The huge temperature difference between the equator and the winter pole dominates the winter hemisphere. Brisk westerly winds and strong low-pressure areas result. Cyclonic storms are common near the pole as winter approaches. The thin martian atmosphere allows more sunlight to reach the summer pole than on Earth. As a result, weather in the summer hemisphere is more stable with prevailing easterly winds. Dominant summer wind systems are caused by the expansion and contraction of the atmosphere as a result of daily solar heating. Wind speeds of 10 m/s are not uncommon. Surface winds are dominated by early morning downslope and afternoon upslope winds. There are also winds associated with the terminator.

2.1.2.4.2 CLIMATE HISTORY

Evidence from channels indicates that Mars may once have had large amounts of surface water. The presence of liquid water implies that the climate was different at such a time. Furthermore, the layering the polar regions suggests that Mars may undergo climatic changes similar to the ice ages on Earth with a cycle on the order of ten-thousand years. There also seem to be longer term trends. Several billion years ago, temperatures were warmer, the atmosphere was thicker, there may have been a greenhouse effect, and rain may have fallen. In those early days, the martian atmosphere more closely resembled that on Earth, and it would have been more favorable to the development of life.

Due to its small size (compared to Earth) and greater distance from the Sun, Mars eventually cooled and lost much of its atmosphere. The small gravity of Mars was unable to hold in lighter molecules of gas, and escape of the atmosphere gradually lowered the temperature. It eventually became cold enough that water froze out of the atmosphere, thus further reducing the atmosphere's ability to retain heat. This loss of atmosphere probably took place within a few hundred million years. Evidence from runoff channels indicates that rain probably has not fallen on Mars for at least three billion years. The result of these climatic changes is the cold, dry planet of today.

2.1.2.4.3 DUST STORMS

Another major feature of the martian weather is dust clouds that are raised by winds. Modern analysis reveals that the channels (sometimes misnamed canals) first seen by Giovanni Schiaparelli were actually patterns of dust clouds. These clouds can grow to cover a large fraction of the surface. Storms are common all over the planet. For example, 35 individual storms were seen on Mars in 1977, two of which developed into global storms. Various locations have different frequencies of dust storms. Candor Chasma, for example, has been seen to have a moderate number of storms.

Major dust storms occur in the southern hemisphere summer. They usually develop after Mars passes perihelion, but they can occur at any time. These storms seem to feed upon themselves as they grow. The dust storms spread outward across the surface and carry more and more dust high into the atmosphere.

The storms can occasionally envelop the entire planet with a featureless haze. During such periods, the dust clouds absorb most of the sunlight before it reaches the surface, and the atmosphere is heated from above. The troposphere disappears, planet wide temperatures tend to become uniform, and atmospheric circulation changes. After a month or two, the dust settles out, and the atmosphere returns to its normal status.

Besides being a strictly scientific interest, martian dust storms presents a hazard at worst and an annoyance at best to many Mars missions. Of particular interest is the dangers it posses to a manned presence on the surface of Mars.

The most likely problem is with communication. Dust will almost certainly block out any optical communications links. What is not obvious, is the potential for the dust to block out the radio frequency transmissions. If the dust is has a lot of iron in it, it may act in the same way that metal chaff does towards radar signals - seriously disrupting any signals that pass through the dust. If this is the case, the base can be equipped with a ELF system which should be to get through the dust, the tradeoff being a much lower bandwidth resulting in a lower data rate.

One problem is with fine particles in the atmosphere that can electrically charge a surface they contact. This is a hazard with helicopters on Earth, as friction on the vehicles surface builds up a very large charge. One other possible charge source is from tribuluminesence of clay particles under very low atmospheric pressures. A discharge has the potential to damage a suit or other sensitive equipment.

Simulations and field studies have given a base line that needs to be compared to the actual martian conditions before the arrival of humans. Under Mars like conditions, agitated sand has produced potentials of up to 5 kV/m (Ref. Heonig, 1975). The primary causes of charging are: contact electrification, frictional electrification, piezoelectric charges, and cleavage electrification (Ref. Greeley, 1978). Under the low martian pressure it is much easier to get arcing from these charges, so this could be a serious problem.

The final problem with dust storms is sand-blasting of exposed surfaces. It has been predicted that on Mars sand grains will begin saltating in about 25 to 30 m/s winds (Ref. Greeley, 1980). These grains will impact on any exposed surface with in a meter or two of the surface. If winds are below this start up speed, then this will probably not be a concern.

Kurt Schwehr / schwehr at cs stanford edu