PURDUE UNIVERSITY

EAS 105-THE PLANETS

Prof. Robert L. Nowack

 

Lecture 14

 

 

Atmosphere on Mars and the Search for Life

 

Viking 1 and 2 recorded and sent direct atmospheric measurements for Mars.  The atmosphere on Mars is 95.4% Carbon Dioxide, 2.7% Nitrogen, and 1.6% Argon.

 

 

 

 

Venus has similar atmospheric components as Mars (except for trace elements).   However, the atmosphere on Mars is very thin.  Because the atmosphere is so thin, temperatures of the lower atmosphere vary widely on a daily basis.  As a result, the convecting troposphere layer varies with location and season.  At night, it practically disappears.  Convection ceases at night and the atmosphere becomes very stable.

 

 

 

 

 

A Typical Weather Report:  "Light winds from the east in late afternoon, changing to light winds from southeast after midnight.  Maximum winds were 15 mph.  Temperature ranged from -122°F just after dawn to a maximum of -22°F.  Pressure steady at 7.7 millibars".

 

Several types of clouds can exist.  First, there are dust clouds raised by winds.  During certain times, these can cover large fractions of the surface.  Second, there are ice-water clouds.  These can form fog around mountains.  Finally, there are carbon dioxide clouds which form a high altitude haze of dry-ice crystals.  However, most of the time, the Martian atmosphere is much clearer than on Earth (except during major dust storms that occur during the southern hemisphere summer). 

 

Martian Atmosphere

 

 

 

 

The Martian atmosphere can be a dynamic place.  On June 27, 1997, the Hubble Space Telescope recorded a large cloud mass to the north of the Mars Pathfinder landing site, which is marked by a cross.  A localized dust storm in the Valles Marineris Canyon complex is also evident.  Clouds were less obvious by July 9th and the dust storm gone.  However a strong climatic front had developed near the North Polar Cap.

 

The main driver of global circulation is the exchange of carbon dioxide (CO2) between the atmosphere and the polar caps.  Atmospheric pressure can vary up to 20% with the seasons.

 

 

 

Water on Mars

 

Liquid water on Mars is largely absent as a result of the low pressure, as well as low temperatures.  In West Lafayette, water boils at 100°C (212°F) since at this temperature the upward pressure of the water vapor molecules leaving the liquid water in a pan just equals the pressure of the overlying atmosphere.  However, if we go to higher altitudes, (lower atmospheric pressures), the boiling temperature would decrease.  If we are at an altitude where the pressure is 0.0061 bars, the boiling temperature is 0°C (but this is also the freezing point).  At this pressure, liquid water can no longer exist.  Thus, even if the surface temperature gets warm enough, no liquid water would be on Mars (at current atmospheric pressures).  Water would immediately boil into the atmosphere.  On Earth, this also happens for carbon dioxide going directly from dry-ice - vapor.  On Mars, this happens to both; carbon dioxide and water.

 

Since the atmosphere of Mars is so cold and thin, it can hold very little water vapor.  In this sense, it is dry.  However, it holds about the maximum it can at its pressure and temperature.  So, “relative” humidity on Mars is very high.  It is thus common to see water vapor fogs to form at low elevations at night.

 

On Mars, water vapor is broken apart by ultraviolet radiation from the Sun to form hydrogen which diffuses up into the upper atmosphere and has been detected.  Still, much of the water must be tied up in a permafrost subsurface layer and on the northern ice cap.  If atmospheric pressures were higher in the geologic past of Mars, it might have been possible for liquid water to have once formed.  This might explain the runoff channels on the surface which cut the planets surface in the distant past.

 

            In January 2004, two robotic rovers, Spirit and Opportunity, landed on Mars to search for evidence of water.

 

            Spirit’s landing site was Gusev Crater which had old river channels going into it.  But, initial evidence was that the rocks were volcanic and dry.  Spirit then traveled several kilometers to the east to the Columbia Hills, and minerals there were detected that could suggest that the rocks were once altered by water. 

 

            Half a planet away, the rover Opportunity landed in a small crater on a vast flat plain.  Small spherules were found on the surface, nicknamed “Blueberries”, composed of hematite that could form by the precipitation of iron in water rich environments.  The outcrop rocks were rich in sulfur, chlorine, and bromine all suggesting the early presence of water.  The texture of the rocks with fine scale layering were also suggestive of sedimentary rocks formed in shallow water on Earth.

 

 

 

Detection of Life on Mars

 

The primary objectives of the Viking missions were to land on Mars and search for life using an automated miniaturized biological laboratory. The entire spacecraft was carefully sterilized to insure against contamination from Earth.  They chose to look for microorganisms.  (They also had cameras in case a giraffe might walk by.)

 

They first had to ask: "What is life?”  Even philosophers have had a surprisingly difficult time with this question.  The scientists chose to look for "life as we know it" - life based on complex carbon-based chemistry which breathed gas or consumes nutrients and leaves behind organic wastes.

 

 

 

Three Main Experiments of Viking 1 and 2

 

 

 

 

(2)  LR; Labeled Release Experiment: This followed a similar strategy, except the nutrients were tagged with radioactive atoms.  The release of these atoms into the atmosphere of the chamber could then be detected. 

 

 

 

 

(3)  PR; Pyrolitic Release Experiment: A more general strategy - maintaining soil sample in essentially a Martian environment.  (Martian micro-organisms might be killed by the water and nutrients.)

 

The soil sample was exposed to 14CO2, tagged radioactive carbon dioxide, and artificial sunlight.  After a period of time, the gas was removed and the soil was baked.  The idea was if living microbes in the soil breathed in the 14C rich air, they would incorporate 14CO2 into themselves.  Detectors would then be sensitive to radioactive 14CO2, which would be released when the soil sample was cooked. 

 

 

 

 

 

Viking Results

 

Much to the surprise of the scientists who designed these experiments, all 3 initially gave strong positive results!  Apparently, the Martian soil reacted strongly with the nutrients and gas in the test chambers.  However, this was different than expected for living organisms since the immediate activity was high, but declined with time.  This is just the opposite of that usually found for living organisms.  Were they being killed off ?

 

Finally, the gas chromatograph did not detect any organic chemicals in the soil, down to parts per billion!  How could life exist without organic chemicals?  Scientists concluded that life was not being detected.  Instead, the soil itself appeared to be much more chemically activity than Earth soils.  This might result from direct exposure of the surface on Mars to ultraviolet radiation - which breaks up carbon molecules.  It can also produce compounds called super-oxides in the soil which are highly reactive when brought in contact with water.

 

Is There Life on Mars?  These results seem to conclude no at this time (at least regarding life as we know it).  However, more exploration of the surface of Mars is required for a more definitive answer.

 

New clues have emerged recently from a meteorite retrieved from Antarctica thought to have originated on Mars.  The rock revealed organic molecules which might have resulted from bacterial action.  Although this is controversial, it might suggest that some form of life (at the micro level) could have existed on Mars in the distant geological past.

 

 

Mars Rocks on Earth

 

 

 

 

About 13,000 years ago, a 4.2 pound chunk of rock fell onto the ice of Antarctica.  It was picked up in 1984 by American scientists.  Someone eventually recognized it as a meteorite from Mars. 

 

Researchers identified organic molecules, minerals and cracks filled with carbonate globules that could be associated with bacterial life.  They even detected microscopic, worm-like structures that could be bacterial fossils.  Each of these findings alone could have non-biological explanations.  All of them together suggested to researchers that they might have found the first clear evidence that life is not unique to Earth. 

 

Skeptics caution against jumping to conclusions.  NASA urged other scientists to undertake even more rigorous tests.

 

 

 

Early History of Mars

 

Mars presumably formed with the rest of the planetary system about 4.6 billion years ago.  From the existence of heavily cratered uplands on Mars, the crust had formed in time for the late heavy bombardment period (about 3.9 billion years ago).  This period may well have been a period of enhanced erosion - Mars could have had just dense enough of an atmosphere to allow some liquid water.  However, the atmosphere slowly thinned.

 

The volcanic plains in the north formed near the equator about 3.1 to 3.9 billion years ago.  As the planet cooled, a bulge formed near the equator, creating the Valles-Marineris and the large volcanoes.

 

Nitrogen on Mars is highly enriched in its heavier isotope (similar to deuterium enrichment on Venus).  This suggests that nitrogen has been lost to space on Mars over geologic time.  Mars' atmosphere must have been denser in its geologic past.  

 

This figure compares Venus' atmosphere to a modified Earth and Mars atmosphere assuming no weathering, no life and no escape to outer space.

 

 

 

 

The early Mars atmosphere could have sustained a much warmer surface temperature from a greenhouse effect (and liquid water could possibly have flowed).

 

What happened?  Carbon dioxide could have dissolved.  As carbon dioxide was diminishing, nitrogen was escaping into space.  This resulted in decreasing surface pressure.  The planet's surface would begin to cool.  Water vapor would decrease in atmosphere further diminishing the greenhouse effect.  This could result in the opposite of a runaway greenhouse - a runaway refrigerator.

 

 

 

A Comparison of the Inner Planets

 

Table below compares the present atmospheres of the 3 inner planets and Jupiter.

 

 

 

 

For the inner planets, the following paths might have taken (note b means bars).

 

 

 

 

The upper left column lists estimates of the amounts of volatile species initially present in the terrestrial planets’ atmospheres.  The upper right lower columns show possible evolutionary paths, including estimated abundances for several gases at various critical times.

 

The critical atmospheric parameters for an inner planet appear to be the overall size of the planet (what gases can be retained), as well as the distance from the Sun (heat flux from the Sun). 

 

 

 

 

The Earth lies in the habitable zone of our Sun.  Venus and Mercury are on the hot side of the habitable zone.  Mars sits on the cold fringe.  The other planets are well into the cold side.  Distances from the Sun are given in astronomical units.  (1 A.U. = 93 million miles)

 

We can then define a zone about the Sun where the heat flux is just right for a possible habitable zone for life as we know it (also liquid water is possible).

 

 

 

 

Interior of terrestrial planets as deduced from a wide range of observations.  The crust, mantle and core of a planet are distinguished from one another on the basis of their geochemistry.  It is not clear whether the Moon and Venus have discrete cores, nor does Mercury necessarily have a chemically distinct crust.

 

 

 

 

Age distribution of present planetary surfaces.  The thickness of the curve at a given time is an estimate of the percentage of the planet having the indicated age.  For the Moon, 80% of the present surface was emplaced in its first 600 million years and hardly any materials were formed after 3 billion years ago.  Mercury appears comparable.  Most of Mars’ surface was formed in the first ˝ of its history with waning volcanism continuing to the present.  Venus is uncertain with indications of both old and young units.  On Earth, the only planet with demonstrable plate tectonics, over 2/3 of the surface (ocean basins) was formed less than 200 million years ago and surface rocks older than 3.5 billion years are rare.