A lesson in making the wrong scientific assumptions
school experiment are designed to demonstrate a particular scientific
principle. They have been done many, many times and they
are nearly always made to work, (even if your teacher helps you
cheat!). Science in the real world is not like this, however.
Scientists work at the very edge of knowledge and they often probe
into the unknown. This is what science is all about and
what makes it exciting. Sometimes of course, scientists
get it wrong. They make incorrect assumptions, and if the
errors are not detected this can lead to all sorts of problems.
we can do an experiment, which on the face of it looks simple
and straightforward, though the assumptions are wrong. If
you take the results at face value, they could be interpreted
that there is no life on Earth! The experiment demonstrates
how things can go wrong, and how careful you need to be in drawing
conclusions from an experiment.
The project simulates a life test experiment of the type that
might be done with samples of martian soil on board a robot lander
in 1976, when the Viking
spacecraft landed on Mars, one of the tests that was carried out
was to detect gas given off by any martian microbes that
might be lurking in the soil - the Gas
Similar tests could be conducted on Earth to see if life
can be found here! The Viking life tests caused a
lot of problems in their interpretation. At first, the results
appeared to be positive, but then doubts appeared and arguments
about the results have continued ever since. The generally
accepted view is that life was not detected. False readings
caused by normal chemical - non-organic - processes gave the positive
readings. However there are some notable
people who still dispute this, and claim the readings were
due to life processes.
can undertake a simplified simulation of the test using everyday
products. You should get positive life results where there
IS life. You
may also get negative reading where there IS
life and we will show you how positive readings can be obtained
where there is NO life.
On Earth we would be able to carry out other tests to help confirm
the result, but when conducting such tests at a distance of several
million miles, by remote control, it can be difficult. You
cannot change anything or try new tests that you might have devised.
task is to find out which of 5 soil samples contains living organisms
by detecting the gas given off by their metabolism.
getting the results you should re-evaluate the experimental assumptions
to see if they are valid. Then devise further experiments
to test the assumptions.
critical. From your own knowledge, do the
results make sense? Are they reasonable? Could there
be other explanations for the results? Did you get negative
results where you expected positive ones? If you got positive
results, are you sure they are life signs and not from a non-biological
help in this read up on the subject.
Life processes produce gases through their metabolism. Detecting
the gas tells us something is alive. Microbes we cannot
see in soil samples can be detected by this means.
Take sensible precautions such as washing
hands after handling soil and pond water. Do not put these
substances in your mouth.
We want to find out if there are living
organisms in the soil. As life processes release gas, it
should be possible to capture that gas. If you can capture
the gas, it may be possible to inflate a balloon with it.
To do this you must seal the sample into an airtight environment,
such as a 35 mm film canister. The soil samples can be given
nutrients to promote life activity and reproduction.
will need the following items:
35 mm film containers
soil from the your yard, garden or a field
- ideally all the same color
small bowl of water
Collect half a cup of some soil from the
your yard, garden or a field. Try and ensure it contains
some decaying matter (eg leaves).
Use the film containers as the sealed environment incubation chambers.
You will only need about two teaspoons of each soil sample in
each container. You do not need to fill the film pots!
Prepare your five soil samples as follows:
two identical Mars soil samples as described in The
Make-up of Mars.
two identical Earth soil samples from the soil you collected
one sample that consists entirely of sand.
Label your pots on the inside, so you will not know which pot
is which when you put the balloons on. However you should
record the soil and nutrient contents of each for later reference.
Put each sample into a canister, recording which container contains
Prepare nutrient by dissolving two teaspoon of sugar in half a
cup of warm water. Allow this to cool.
Add a teaspoon of nutrient to:
Mars soil sample
Earth Soil sample
Record which container has nutrient added. 9
Place a balloon on each container, so it can inflate
a little as gas is given off. Ensure the balloon is absolutely
flat and uninflated as you do this, so it flops over the side
of the container.
Leave the samples at room temperature for at least half an hour
(you can leave the samples for several days if you wish).
Now inspect what has happened? (If nothing has happened wait at
least four hours)
Have any balloons partially inflated?
Using the amount of inflation as your guide, write down which
sample you think is which. Also record which and approximately
by how much, each balloon has inflated.
Now remove the balloons and see if you predictions are correct.
You may well have discovered that only one balloon has inflated
- the one with "Mars" soil and nutrient. Now, three others
had living material in them. The exception being the (sterile)
sand. So why no balloon inflation? Maybe it was the lack
of nutrient. But nutrient was added to an Earth soil sample
too. So why was there no inflation? Can we conclude
there is no life on Earth? No, of course not. But
we can conclude that there was no life in our particular soil
If you inspect it visually, you may be able to see living things
in it, with the naked eye - algae, worms or mites for instance.
Under a microscope you can probably see more living organisms.
In fact, it is likely that the sample is full of life. So
why did our test fail?
Before we look into that, lets do a little test with the last
Take the final film canister and add half a tea spoon of baking
powder. The next part will have to be done very quickly.
You may want someone to help you. Take a teaspoon of vinegar
and add it to the film pot. Now fit the balloon over the
top, very quickly.
the balloon inflate? Is this the sign of life we were looking
for? Is the mixture alive?
Your balloon should have inflated, at least a little. Yes,
balloon inflation is the life sign we were looking for.
But no, the mixture is not alive. The gas given off, which
is carbon dioxide and the same as that produced by the yeast in
the "mars" soil sample, is made by a normal chemical reaction,
not by living entities.
We must therefore conclude, that although the chemicals and materials
in each experiment were totally different the result was the same.
If we accept our initial assumption, then not only do living things
appear not to be alive, there are circumstances where non-living
materials DO appear to be alive.
assumptions must be wrong. Totally wrong. There are
two main flaws in our assumptions.
in the assumptions and what should be concluded
here on Earth, you were able to check the tests by other means.
But on Mars, it would be a different matter. First, we are
not completely sure what the Mars soil is made of. Secondly
we have no means of visually inspecting any test sample with a microscope.
(CO2) can be given off by non-life processes as well
as life processes
all life processes produce a net increase in gas volume in their
local (in this case, sealed) environment. Yeast, which
was used to simulate Martian life, is in fact unusual in this
respect. That is why it is used to aerate bread and other
example here was very simple, and the flaws were easily found
when a little thought and book research was applied to the problem.
But for the Mars Mission designers in 1974, when the Viking mission
was being planned, the situation was different. There were
no books to check and the true composition of Mars soil was only
guessed at .
built four test experiments that could cross check each other,
and there was a video camera for visual inspection. The
Viking mission specialists did the best they could with the available
data, both before and after the mission. You can only hope
to do the same. But be aware that on occasions your assumptions
at the beginning of your experiment could be wrong. This
is often how new scientific breakthroughs come about.
Teachers' Notes for
Classroom - Planets in a Bottle
Life on Mars
Life on other planets
1999 Satellite Events Enterprises Inc.