29:50 Modern Astronomy
Fall 1999
Lecture 38 ...December 1, 1999
Mars Continued

Future Opportunities to See Mars Oppositions of Mars occur about every two years. There was one this April. The next one will be in June of 2001, and Mars will be much closer then.
Oppositions of Mars
Explore Mars on your own with the Mars Global Surveyor Website. I will be using it extensively.
Mars Global Surveyor Homepage

MARS ...The Angry Red Planet

Within a few years it was realized that the seasonal variations in the surface features of Mars were not due to vegetation, but rather planet-wide dust storms which occurred every summer.

Thus by the early 1970's it looked as though Mars more resembled the Moon than the Earth, and seemed to be extremely unpromising as an abode for life. As it turned out, however, the early Mariner spacecraft had painted too pessimistic a picture. Fortuitously, they had passed over the quite barren southern hemisphere of the planet, which appears most Moon-like. Around 1971 the spacecraft Mariner 9 went into orbit around Mars. It painted a more ``diverse'' planet, which once again kindled hopes that there might be restricted areas where life might be possible.

Martian Geological Features. In what follows, we will take a guided tour of Mars, based on findings from the Mariner and Viking orbiter pictures. It turns out Mars has a number of interesting features. By the way, look in your book, which has wonderful illustrations of the surface of Mars.

Volcanos: Mars has the largest volcanos in the Solar System
Martian Volcanos
An interesting illustration in your textbook, Figure 11-13 on p230, shows the size of Olympus Mons compared to Mauna Loa; see also Figure 11-14.
Olympus Mon is also frequently seen to have clouds associated with it, as is the case with terrestrial volcanos. Some of these clouds are inferred to be water clouds rather than clouds. These volcanos are not active; they probably have not undergone eruptions for billions of years.
Valles Marineris The surface of Mars is dominated by a fissure called Valles Marineris, which can be called a vast canyon which runs for about 4000 kilometers. Pictures are shown in your book on p233. Valles Marineris appears to be a large crack in the crust of the planet.
Picture of Valles Mareneris with fog in the bottom, MGS website Valles Marineris

Similar fog is seen in the bottom of craters sometimes, and is part of the reason people think there is a large amount of subsurface water. The picture at the beginning of the Chapter presents a neat artist's conception of one of these canyons with fog.
Water Channels. In my opinion the most fascinating geological surface features on Mars are the water channels. These are large, now-dry stream beds that were formed by flowing water. Mars and Jupiter's moon Europa are the only solar system objects other than the Earth where there was, or is, liquid water on the surface.
Dead Martian Rivers
MGS website showing water channels. Most of these appear to have been huge runoff events, although there is an indication that some of them were formed in river-like bodies, and there is also evidence of ``lakes'' that lasted for a long period of time.

Question for audience: Any guesses about how long ago this happened? Hint: Does Figure 11-22 give you any ideas?

The results of the later Mariner spacecraft made Mars look like a much more interesting place from the point of view of life in space. Although there were no big civilizations left, perhaps microbial life had developed.

To give a feeling for the surface of Mars, we can show a video combining many of the Mariner and Viking images of the surface.
Video of Mars
For this reason, in the 1970's the United States sent two Viking spacecraft to Mars. The had soft landers, and each had three highly sophisticated biological laboratories to search for the existence of life on Mars. I recall the discussions before the spacecraft arrived. It was agreed upon that all three would have to give a thumbs-up or thumbs-down indicator to definitively determine if life were present or not present on the planet.

Pictures from Mars pathfinder website
Live from the Surface of Mars

The three experiments were the Labeled Release, the Pyrolytic Release, and the Gas Exchange experiment. The nature of these experiments can be indicated by a description of the Labeled Release. The labeled release took in a sample of Martian soil, which might contain Martian microbes. A nutrient was added to the soil which contained , a radioactive tracer which makes it easy to see where the nutrient went. If microbes were present in the soil, they would ingest this solution, then exhale it into the air in the sample. The labeled release experiment consisted of taking samples of air above the soil sample, and testing for the presence of . As a test, the same experiment would be undertaken on a soil sample which had been sterilized by heating.

When the experiment was tested on Earth, it worked in a proper fashion. On Mars, the labeled release gave a ``positive'' signal indicating the presence of life. However, the other two experiments gave thumbs-down.

For this reason, the ``majority viewpoint'' in the scientific community is that the LR experiment was responding to an inorganic chemical reaction on the surface of Mars, and that the Viking data as whole indicate that Mars lacks life of any sort at the present time. As stated in your textbook: ``The consensus among the scientists who carried out the Viking biology experiments was that the results showed that the soil of Mars is chemically but not biologically active''. However, the textbook also conveys, in a way most textbook do not, that there is still some controversy about this topic. It is interesting to note that as of ten years after the Landers (there has been no new information since then), the principal investigator of the LR experiment maintained that his experiment had shown evidence for life on Mars.
Transparency; abstract of article by Levin.

Going on, even if Viking did show that life was not present at the places studied by the Viking Landers, (which were not particularly auspicious from a biological point of view) it does not mean that there are not other places on Mars, perhaps in the foggy bottoms of Valles Marineris, where it might be found, or that it might have existed in the remote past when we know water flowed on the surface of the planet.

It is interesting to note that the Mars pathfinder spacecraft landed in one of these ancient water channels.

In this context, I would recommend to you an article by McKay and Stoker.
Abstract of article ``The Early Environment and its Evolution on Mars: Implications for Life'', Reviews of Geophysics, Vol.27, p198, 1989. This McKay was the one who this summer announced the possible detection of fossil life in a Mars rock. This article should be accessible to a large number of people in this class who have studied subjects such as biology and geology extensively.

Transparency with abstract of McKay and Stoker article. The main point of this article is that (as a result of Viking orbiter and lander observations) we know that the atmosphere of Mars was dense, and that liquid water was present for a period of about a billion years after the formation of the planet. On Earth, this was sufficient for the development of the first forms of life. The question is whether it was also sufficient time for life to have formed on Mars.

There is the old statement of a picture being worth a thousand words, and in this article McKay and Stoker give a pictorial representation of the geological histories of Earth and Mars.
Transparency of histories of Earth and Mars. Note that the same expanse of time is represented in both diagram. The crucial aspect of this diagram is that on Mars, a period of warm, moist conditions, with clear indications of surface water, existed up to 3.5 billion years ago, and perhaps somewhat later. On Earth, there is definite evidence for the existence of life by that time.

There are even places on the Martian surface where conditions favorable for life may have persisted for longer than 3.5 billion years ago. They discuss a feature called Hebes Chasma (latitude 0degrees S, longitude 75 degrees west).
MGS images of Hebes chasma.
Hebes Chasma: Ancient Martian Lake
There is a mesa in the middle of the chasma which shows a layered structure. This is interpreted as sedimentary deposits formed when the chasma had standing water.

McKay and Stoker present all of this as good arguments for further unmanned spacecraft missions to Mars. The Viking spacecraft landed exactly 20 years ago, and incorporated technology designed several years before that. With modern technology, and the information obtained from previous Viking landings, we could make a large step forward at the present time. Obviously, clear discovery of evidence for fossil life on Mars would be one of the most profound discoveries of all time. However, even a negative discovery would not be meaningless. McKay and Stoker end their article with an interesting essay;
Text on ``importance of a negative result''.
Oceans on Mars
Martian Topography

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