Exploration of the Solar System

Topic 9, Week 7

Ancient Mars, the Viking Spacecraft, and the Mars Rock

The Viking Spacecraft

Last time, I mentioned that the only successful “biology spacecraft” to date was the US Viking spacecraft (2 of them) which landed on Mars in 1976. A picture of the Viking lander is shown below.

Each of the Viking landers carried three, independent biology experiments, which analysed samples of Martian soil.

The idea of the mission was to have all three experiments give either a “thumbs up” or a “thumbs down” for life on Mars. What actually happened was that there were two “thumbs down” and one “thumbs up” . So, based on a strict observation of the mission’s protocols, the experiment was inconclusive.

It is interesting to talk a little about the experiment which gave the “yes” signal, which was called the Labeled Release Experiment. In this experiment, a sample of Martian soil was taken into the spacecraft. A sample of liquid nutrient was then added to the soil, which would presumably stimulate microbial activity. This nutrient was special, however, in that much of the carbon in the nutrient was Carbon 14, an unstable radioisotope which emits easily detectable radiation.

The idea was that any microbes present in the soil would ingest the nutrients and begin metabolic activity. In the process of respiring, they would emit carbon dioxide and other carbon-carrying gases into the air above the sample. This air was drawn off into another chamber in the spacecraft and observed for radioactivity.

The signal of life present in the soil would be increasing radioactivity in the air above the sample after the nutrient had been added. The instrument was tested on numerous sites on Earth before launch on Viking.

The data returned from the Viking lander is shown below.

It shows a signal essentially identical to that when it was tested on Earth.

Most scientists involved in this research believe that the two negative experiments were giving the correct answer, and that the experiment described above was responding to highly oxidizing conditions in the soil. However, it is notable that the principal investigator on the “positive” experiment never accepted this, and has remained adamant to the present day that his experiment gave the correct answer. If you want to read his side of the story, a 1997 article is available at

http://www.biospherics.com/mars/spie/spiehtml.htm

In any case, it will be highly desirable to send another “biological spacecraft” to Mars, and such was the intent with the European “Beagle 2” spacecraft on Mars Express, which was lost. We now have a much better idea of the geological history of Mars, and much better ideas on where to land such spacecraft (Spirit and Opportunity being an example).

The Mars Rock

The textbook makes the interesting case that we may learn if life existed on Mars in the remote past before we learn whether or not it is still there.

One of the most interesting developments of the recent decade has been the study of a rock named ALH84001. It is a meteorite that was discovered in Antarctica in 1984. About 10 years later it was determined that this meteorite came from Mars.

The reason for believing that this rock (and about 12 other such meteorites) came from Mars is that there is gas trapped inside the rock which has the chemical composition of the atmosphere of Mars, not the Earth. If you wish to read more about the description of this remarkable object, the Mars rock has its own homepage at

http://www.lpi.usra.edu/lpi/meteorites/The_Meteorite.html

From an analysis of this rock, we have been able to deduce an amazing amount about its history.

1.It formed 4.5 billion years ago, and so is a rock from the earliest days of the solar system. (Question: How do we know this?)

2. At a later time, estimated to be about 4.0 billion years ago, the rock was fractured and shocked. Since this was in the middle of the age of bombardment , it is reasonable to suppose that the fracturing event was a crater-producing impact near ALH 84001.

3.After this fracturing event, it was evidently submerged in water, since there are deposits in the fractures that show water seeped in. Note that this fact in itself favors the warm, wet Mars history.

4. At a much later time (perhaps 10 million years ago), ALH 84001 was blasted off the surface of Mars by another impact, traveled through the solar system for a few million years, and finally landed in Antarctica, where it was discovered in 1984.

An analysis of the interior of the rock (where the water had entered) yielded some very interesting features which were claimed (in 1996) to indicate the action of past life. At the present, it is not completely clear whether these minerals are the result of ancient (about 4 billion years ago) biological activity, or inorganic chemistry. We probably won’t know for sure until we get more samples of Martian rocks.

Recent Developments

Now is the golden age of Martian exploration. Since January of 2004, the two Mars Exploration Rovers spacecraft on the surface of Mars. Both have returned data which strongly support the view that at least part of the surface of Mars was submerged in water for periods of time long enough to influence mineral formation. One of the Rovers, Opportunity, will soon crawl out of a feature called “Opportunity Crater”, in which it examined rock formations much older than those on the surface. Since the best evidence for the “warm wet Mars” conditions comes from the oldest periods in Martian geological history, the results will be very interesting. Stay tuned.

Another interesting development comes from the European “Mars Express” mission. Data from Mars Express showed evidence for the gas methane in the Martian atmosphere. On Earth, methane is primarily generated by life (microbes). Scientists speculate that this methane may be generated by communities which have survived since the early days of the planet. In support of this possibility, the European scientists claim that the methane signature is strongest at locations where there is evidence for subsurface water.

The Future

NASA has an ambitious plan for future space research. This is described in its Mars Missions web site at

http://marsprogram.jpl.nasa.gov/missions

The most intriguing one will be the Mars Sample Return Mission, which is intended to land, collect rocks, and return them to Earth. NASA hopes to launch this around 2014, with spacecraft return several years later.