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Lecture 4.06: Mars -- The Viking experiement

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Наука о Солнечной системе

Калифорнийский технологический институт

4.9 (оценок: 333) | Зарегистрировано учащихся: 20K

Learn about the science behind the current exploration of the solar system in this free class. Use principles from physics, chemistry, biology, and geology to understand the latest from Mars, comprehend the outer solar system, ponder planets outside our solar system, and search for habitability in our neighborhood and beyond. This course is generally taught at an advanced level assuming a prior knowledge of undergraduate math and physics, but the majority of the concepts and lectures can be understood without these prerequisites. The quizzes and final exam are designed to make you think critically about the material you have learned rather than to simply make you memorize facts. The class is expected to be challenging but rewarding.

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Получаемые навыки

Solar Systems, Chemistry, Geology, Astrobiology

Рецензии

4.9 (оценок: 333)

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JA

Dec 07, 2017

Excellent Course! Very informative and gives a brief overview of the history of the Solar System and some of the questions driving new and current missions. Thank you.

AJ

Jun 13, 2017

And excellent, deep yet accessible introduction into what we know about the solar system, the science behind our knowledge, and the big questions under investigation.

Из урока

Unit 4: Life in the solar system (week 1)

Lecture 4.01: Introduction to life 10:50

Lecture 4.02: Photosynthesis 8:34

Lecture 4.03: Water 9:54

Lecture 4.04: Alternative energy sources 11:15

Lecture 4.05: History of life on Earth8:36

Lecture 4.06: Mars -- The Viking experiement 9:12

Lecture 4.07: Mars -- Microbial hitchhikers 8:58

Lecture 4.08: Mars -- Methane?16:09

Lecture 4.09: Mars -- Methane!!5:19

Lecture 4.10: Mars -- a habitable environment18:46

Преподаватели

Mike Brown
Professor

Текст видео

Only one time have we ever sent

instruments to another place in the solar system,

whose sole purpose was to determine whether

or not there was life in that location.

That location, was the surface of Mars.

Those instruments were on the Viking landers.

Remember the Viking landers?

The Viking landers landed way back in 1976.

That is coming up on 40 years ago.

That was the last time.

The Viking landers, the story that I even remember at the time from the

Viking landers life experiment, was that they

proved that the surface of Mars was sterile.

There was no life on Mars, and if anything, they made the idea

of searching for life on Mars, fall out of favor for many decades.

Let's go back and look at those results

a little more carefully and see what we think.

First, let's look at what really happened, so

Mars life experiment really consisted of three separate experiments.

And, this is going to be pretty cool.

They were actually designed specifically to look for things that

we would think, that we would understood, that life would do.

What does life do?

It's the metabolism, it's those waste products,

it's those gases that they give off.

Let's see how it works.

First, was the labelled release experiment.

Labelled is a word that geochemists like to use.

Labelled is where you take something normal like CO2, but you label it by

taking the C and turning it into a different isotope, carbon 14, for example.

If you label all of it as carbon 14, and you can

then look and see where that specific molecule has, of carbon has gone.

So, what do they do?

Well, they have a, a tiny little half of a cubic

centimeter sample here, and, a little dropper of water over here.

It dropped on it, onto this little sample of

Martian soil from, scooped up, put into this thing.

In this water, were organic compounds, labelled with carbon 14.

There's very little naturally occurring carbon 14 on the surface of Mars.

Then the idea was, to see if there were microbes inside here, that took that

organic material which had the carbon 14, used it as fuel, and then expelled a gas.

As a waste product.

What gas?

Well, if it were us, we would be expelling CO2 with 14 C.

But, we know other organisms could give off CO or methane.

So, there was a detector that sat right here, that

was looking, specifically for this product with, labelled carbon atom.

Now, simply finding these gasses, doesn't really tell

you that there are microbes in there doing it.

There's, could be some sort of chemical reactions that

are going on that are, that are purely abiotic.

And so, a very part of the ex, experiment was to repeat this entire experiment

after this sample had been heated, to extreme

temperatures that would sterilize anything inside of there.

Okay, so what happened?

The nutrients were put in there, and indeed.

Labelled gases came off.

Something was going on.

Heat the sample up to sterilize it.

Do it again.

Not the same sample.

Take a new sample, heat it up, sterilize

it and add the labelled nutrients in there.

And look to see what happens.

Nothing happens.

If you were the person who designed this experiment, you might at this

point consider jumping up and down and saying you just discovered life on Mars.

And in fact, that's what this person's been doing for most of the last 40 years.

There's a big problem though, which happened in an

experiment that was not part of the biology experiment.

It was just an experiment that was design to look for organics on Mars.

Organics, makes it sound like it's life, but

organics are just any of these carbon compounds.

Those, those things that we were talking about, those long carbon chain things.

And the idea is that all of these, you would

need to have organics if eating organics, like the stuff

that was in the labelled release here, if that was

an important part of the food sources of these putative microbes.

And the experiment designed to look for organics found absolutely nothing.

It found so little in fact, that it was, it was a shock.

organics.

Organics are those things like the

carbon that come in those carbonaceous chondrites.

Carbonaceous chondrites hit the surface of Mars.

Even that small amount of chondritic material.

Carbonaceous chondritic material coming from the meteorites, should have given

enough organic material, that there should be some trace amounts.

And yet, the detection was, there really is none.

This is the result that led people to say, that Mars is sterile.

The actually biology experiments themselves,

at this point, didn't much matter.

If there are no organics, there are no microbes.

There were two other experiments in the biology part.

And they had more ambiguous results, so we're not going to worry about those.

But we're just going to continue to focus on the labelled release

experiment, and on the experiment that showed that were no organics.

After the putative success of the labelled release experiment,

the the samples were allowed to rest for a while.

And by samples I'm now talking about, this was done

twice, this was done on Viking one and on Viking two.

The samples were allowed to rest for a

while, and then, more of the nutrient was added.

If this were a terrestrial sample, if you took

dirt, add the nutrient again, gas would come off.

Probably even more gas, because you had grown that community

and more gas would come off it at that point.

And the gas would go away.

In this experiment, when this, it was done the second time,

well the first time, nutrient was added, labelled gas came off.

The second time nutrient was added, nothing happened.

What could possibly be going on?

Well, it could well be, that this is.

Indeed just a chemical process.

You add the nutrients the first time.

All the chemical reactions that are going to take place, take place.

You add it the second time, there are no more chemical reactions to take place.

Very different from biology, which uses those nutrients to create

a bigger community, which would then have an even bigger impact.

And this is the way the field has stood for about 30 years.

Mars is sterile, because there were no organics found on it.

And, the labelled release experiment must

have just been looking at chemical reactions,

because A, there are no organics, and B, behave like a chemical reaction.

An interesting thing happened though, with the Phoenix

lander, to the North Polar region of Mars.

You remember the Phoe, Phoenix Lander was that one that scooped down.

Found those, that subsurface ice.

It also found, that the surface of Mars, at the

place that it landed, was covered in a chemical called perchlorates.

A perchlorate is something attached to a chlorine and four oxygen atoms,

you could have something like hydrogen

perchlorate, this would be perchloric acid.

You could have ammonium perchlorate.

And if you did, you would have a very nice rocket fuel.

These are what perch, perchlorates look like.

One of the nice things that perchlorates can do, is to decompose organic materials.

Interestingly, they don't decompose organic materials at

the temperatures on the surface of Mars.

They only decompose organic materials when you heat them up a little bit.

And in fact, this experiment, looking for organics, took the soil,

heated it up a little bit, and looked for the organics.

If this soil had perchlorates in it, and it

had organics in it, the organics would have been destroyed.

The fact that this experiment did not find organics, which has

been really sort of a mystery for a long period of time.

The fact that they didn't find organics, suddenly makes

sense, if the chlorates are a common thing on the

surface of Mars, which we now know there are,

then we would not expect this experiment to find organics.

So woah, suddenly Mars could have.

Organics on the surface.

Not huge quantities, but I think the number

that has been quoted is that it could

be 1.1% of the soil could be organics,

and this experiment would not have detected them.

So if we suddenly throw this experiment out, it's important

to go back and rethink what this labelled release experiment meant.

And when we do that, I think we have to come down on

the side of the problem with addition of the nutrients that second time.

If the first time was due to biological activity, adding a second

time should we continue that biological activity as much or more so.

It should not be diminished.

If this were simply chemical activity, then it should diminish.

The one strange thing, you remember though, the

sterilized sample had no release in it whatsoever.

So, how could sterilization affect something that's, that's abiotic?

The answer is, perchlorates.

Again, if you heated the procloids and there were

organic materials in through here, they could react with these.

Those perchlorates would destroy the organic materials,

and we would end up having no effect.

At best, the final results from Viking have to be labelled inconclusive.

The experiment that was designed to look for organics, turned out to not

be capable of seeing the organics

because of the unexpected presence of perchlorates.

The specific experiments, looking for metabolism, did not obviously see any.

And while we now we now better understand those viking results finally,

nearly 40 years later, we also know that because they were inconclusive, the

idea that the surface of Mars is, is totally sterile, we now know

that we don't actually have evidence to know whether that's true or not.

And, the idea of perhaps looking for some sort of

a metabolic activity on Mars, is now coming back into favor.

It does not seem like a bad thing to be doing, and, as we'll see in the next

lecture, there are some grand planetary scale experiments that

we can use to address these questions in the meantime.

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