The Solar System-3.
There are 5 million billion tonnes of air surrounding the Earth.
This huge mass of air is pressing on each of us.
Without realising it, we live under this pressure.
On every square centimetre of our bodies, there is a force that is equivalent to a 1 kilogram object.
An average person is about a metre square in area.
The atmospheric pressure is the equivalent of a 10 tonne object pressing on us.
Just like the lobsters on the ocean floor, we have evolved to deal with this pressure so effectively,
we don’t even notice it is there.
The atmosphere not only provides oxygen for us to breathe,
but also protects us for the most powerful force in the Solar System: Our Sun.
Air pressure is due to billions of molecules in the atmosphere bouncing of us.
If a tennis ball is bounced of your face, it hurts because the change in direction of the ball,
requires a force to act.
The face provides that force.
Since every action there is an equal and opposite reaction, we feel the force in our face.
The molecules in the atmosphere are exactly the same as little tennis balls, only much smaller.
As they continually bounce off your body, they exert a force on you.
Pressure is defined as the force per unit area.
It doesn’t matter whether the square centimetre of your body is pointing up, down or sideways.
The number of air molecules that bounce of it will be the same.
The air pressure will act equally in all directions.
You can half fill a glass of water and carefully place a piece of paper over the top of the glass.
Holding the paper in place, you can turn the glass over, and then let go of the paper.
The pressure of the atmosphere pushing upwards on the paper, will hold the water in the glass.
Our bodies are actually completely open to the air.
There are no sealed air pockets inside us.
This means that we can exists quite easily at much higher or lower pressures,
than atmospheric pressure.
A scuba diver can descend to 40 metres below the surface of the ocean,
without any special equipment.
At a depth of 40 metres the pressure is 5 times the atmospheric pressure.
That’s equivalent to a 50 tonne object pressing on every square metre of the diver’s body.
As long as the diver keeps breathing and popping his ears, the pressure inside and outside the body, will remain in perfect balance, and therefore he will feel no ill effects.
The average temperature on Earth is 13 degrees.
It varies enormously across the planet.
The highest recorded temperature on Earth is 56.7 degrees in the Libyan desert.
The coldest is minus 89 degrees in the depths of Antarctica.
Compared to other places in the Solar System, our temperature swings are fairly gentle.
We are 150 million kilometres from the Sun.
The distance from this heat source sets the amount of energy being received,
which determines the temperature.
We would reasonably expect that a planet gets warmer when it gets closer to the Sun.
The difference between the day and night temperature, in the Namib desert in Namibia,
is 30 degrees.
The reason for this dramatic difference, is that the Namib desert is one of the driest places on Earth.
Where ever there is very little water vapour in the atmosphere,
there are large day to night temperature swings.
This is because the ability of the atmosphere to trap heat,
is directly related to the insulating effect of the water vapour.
In the dry environment of the desert the level of insulation is low.
So when the Sun disappears, the heat disappears quickly into space.
Greenhouse gases such as carbon dioxide, methane, nitrous oxide, also trap the heat of the Sun,
ironing out the differences between day and night temperatures.
Mercury is the smallest planet.
It is 58 million kilometres from the Sun.
It is difficult to see from Earth because of its proximity to the Sun.
It has the biggest temperature swings of all planets.
The temperature is 427 degrees in the day, and minus 173 degrees at night.
This is because Mercury has been stripped off the one thing that could protect it: its atmosphere.
Like the other rocky inner planets in the Solar System, Mercury had an atmosphere at formation.
In fact its thought that all 8 planets had a similar atmosphere,
when they were formed over 4 billion years ago.
Composed of lighter gases like hydrogen and helium and smaller amounts of heavier gases,
like oxygen and nitrogen.
Planets hold on to their atmosphere by the force of gravity.
The more massive the planet, the stronger the gravitational pull,
and the easier it is for the planet to hold on to its atmosphere.
The temperature of the atmosphere also effects this balance.
The hotter the atmosphere, the faster the molecules are moving around,
and harder for the gravitational force to hold on to them.
The giant planets of the outer Solar System, are Jupiter, Saturn, Uranus and Neptune.
They were large enough to exert the massive gravitational force,
needed to hold on to the lighter gases such as hydrogen and helium.
But on the inner, warmer and smaller rocky planets the story was very different.
The lightest gases would have gradually escaped into space from Mercury, Venus, Earth and Mars,
leaving behind a atmosphere richer in heavier gases like Oxygen and Nitrogen.
Earth has been fortunate to be able to hold on to these gases.
Mercury is tiny, compared to Earth.
Its circumference is 15000 kilometres.
Its mass is just 5% of that of Earth.
Coupled with high surface temperature, the gravitational force is not strong enough,
to hold on to the heavier gases.
So Mercury rapidly lost almost its entire atmosphere.
In Earth at sea level, in a volume the size of a sugar cube,
there are 25 billion billion molecules of gas.
On Mercury the same volume there would be 100 thousand, over 100 million million times less.
So Mercury was just too small and too hot to hold on to its atmosphere.
Atmospheres are a planet first line of defence.
With out them a planet like Mercury is at the mercy of our violent Solar System.
In 2008, in western Canada, an asteroid weighing 10 tonnes entered the Earth’s atmosphere.
It is not unusual for such rocks to hit the Earth, it happens about once a month.
There is a vast crater, the Chicxulub crater, over 180 kilometres in diameter, in Mexico.
It is thought to date back to the end of the Cretaceous period, 65 million years ago
It remains the primary candidate for the catastrophic event that wiped out the dinosaurs.
A 10 tonne rock travelling at 5 times the speed of sound, which hit Canada,
has an energy equivalent of 400 tonnes of TNT.
It is the atmosphere that causes such meteorites to slow down, and breakup.
The atmosphere protects us from such meteorites daily.
The Canadian meteorite, entering the atmosphere at 20 kilometres a second, a lump of rock and iron, which was originally the size of a desk,
would have immediately begun to compress the thickening atmospheric gases in front of it.
When air is compressed it heats up, and this in turn heats the meteorite until it is white hot.
This time it shined like a billion watt bulb.
After just 5 seconds, its billion year journey dramatically comes to an end.
It disintegrates in a series of explosions, peppering the fields below, with lumps of rock,
the size of golf balls.
Scientists hunted for the meteorite fragments in Canada.
These rocks had streamed through the atmosphere with such intensity, that their surfaces melt,
as it reaches temperature of 6000 degrees.
This is the surface temperature of the Sun.
This searing heat creates a tell-tale dark crust over the meteorite.
These rocks were part of a meteorite known as a Chondrite.
Chondrites are very old, having found at the beginning of the Solar System 4.5 billion years ago.
They have never being a part of a larger planet or moon.
They are pure wandering fossils of an earlier age.
If the meteorite had hit the ground intact, the explosion would have left a crater 20 metres wide.
It is Earth’s atmosphere which provides a protective layer from these meteorites.
In 2008, the Nasa Messenger space probe took images of Mercury’s scorched surface.
It confirmed that this tortured planet, is a battered, barren of rock.
The Messenger spacecraft is designed to deal with the specific problems,
of travelling to the inner Solar System.
Probes travelling to the outer planets have to accelerate to high speeds,
to travel across the vast distances in a reasonable time.
They do this by complex series of gravitational slingshots,
around the inner planets of the Solar System, to speed them up.
They must also carry enough fuel to slow them down, when they reach their destination.
However, Voyager swept past the outer planets, in a fleeting visit,
before entering into interstellar space.
Messenger had the opposite problem.
Because of mercury’s proximity to the Sun,
gravity causes Messenger to accelerate faster and faster as it approaches Mercury.
Messenger is designed to orbit Mercury.
This requires Messenger to slowdown enough to be captured by Mercury’s weak gravitational field.
To achieve this feat of spacecraft navigation, Messenger was on an extended and complex journey.
It flew by Mercury 3 times over 5 years, each time using Mercury’s gravity to slow down,
enough to enter orbit around Mercury, in 2011.
For the last 4.6 billion years, Mercury has been bombarded with countless asteroids and comets.
Unlike Earth, Mercury has no atmosphere to shield it.
When a meteorite hits naked Mercury, it strikes the ground intact, and at full speed.
The whole history of the planets violent past, is laid down on its surface.
It is a world pitted with hundreds of thousands of craters inside craters, inside craters.
Venus is 108 million kilometres from the Sun.
It is the brightest planet in the night sky.
It orbits the Sun every 225 days.
Venus is luminous enough to cast shadows on Earth,
as it reaches the maximum brightness before sunrise, or just after sunset.
Venus and Earth share many similarities.
We are next to each other, we were formed from the same material, we are roughly the same size,
and we also share a similar mass and gravitational field.
But that is where the similarities end.
Venus is a tortured world, where thick clouds of sulphuric acids,
are driven by high speed winds and temperatures at the surface are hot enough to melt lead.
It is not surprising that Venus is often known as Earth’s evil twin.
The reason for this hellish difference, is primarily Venus’s atmosphere,
which evolved along a very different path to Earth.
In 1919 the Magellan space probe began a four year mission in orbit around Venus.
It gave the first images beneath the shroud of cloud, that had hidden our view of Venus for centuries.
The images revealed a tortured landscape of volcanoes and impact craters.
Almost every image that Nasa has sent across space,
has relied on the work of mathematician and physicist Fourier.
The Fourier transform is a beautiful piece of mathematics,
that is originally developed with no practical application in mind.
Yet today Fourier’s work can be found in virtually every electronic image we see,
from our JPEG family photographs, to the images from the far reaches of the Solar System.
It is mathematics that drives our ability to compress huge amounts of information,
into files that are small enough to send around the world, or even around the Solar System.
In 1824, Fourier became the first scientist to describe an effect,
that is both crucial to our understanding of Venus, and vital for the future health of our own planet.
The idea of greenhouse effect originally came from Fourier.
He was the first scientist to suggest that gases in the Earth’s atmosphere,
might cause the planet to heat up.
In doing so Fourier paved the way for understanding our climate.
He also helped us understand the extreme effect of greenhouse gases in our sister planet Venus.
The greenhouse effect is basically a very simple piece of physics.
The gases in a planetary atmosphere observe the light of some wave lengths,
and allow others to pass through the surface unimpeded.
Earth’s atmosphere is mostly transparent to visible light, which is why we can see the Sun.
Fortunately for life on Earth much of the damaging shorter wavelength,
UV light is observed in the upper atmosphere by ozone.
The Sunlight allowed through by our atmosphere warms the Earth’s surface.
Earth then re-radiates this energy as infrared radiation.
Infrared light has a longer wavelength then visible and UV light.
Atmospheric gases such as carbon dioxide and water vapour are very good at absorbing it.
In other words, some of the heat radiation from the ground,
is prevented from escaping back into space by so called greenhouse gases.
This means the atmosphere gradually heats up, raising the temperature of Earth.
On Earth, the greenhouse effect is essential to our survival.
Without the greenhouse gases we have today, our planet would be on an average 30 degree’s colder.
It will be far too cold to support life as we know it.
A little greenhouse effect is a good thing.
If the concentration of greenhouse gases such as carbon dioxide are raised too much,
it has devastating consequences, like in Venus.
Venus’s atmosphere is flooded with greenhouse gases.
The rising temperatures would have long ago boiled away its oceans,
pumping water vapour into the atmosphere.
Carbon dioxide from thousands of erupting volcanoes added to the stifling mix.
Venus grow hotter and hotter.
Far hotter than its position closer to the Sun would merit.
The planet slowly chocked to death.
The Namib desert in Namibia stretches over 1900 kilometres.
It is been starved of rain for over 55 million years.
Its landscape is very much like Mars.
NASA’s viking probe landed in Mars in 1976.
It searched unsuccessfully for signs of life in Mars.
In 2004, the Opportunity Rover and the Sprit Rover landed on Mars.
They sent back exquisitely detailed images of the landscape.
Mars has vast sand dunes, giant ice sheets, river canyons, and enormous volcanoes.
It is a dry frozen version of Earth.
Its covered in red dust and sand, entirely familiar and yet entirely inhospitable to human life.
The barren landscape is due to the fact that today Mars has virtually no atmosphere.
The rovers have however found that Mars hasn’t always been this way.
In 2005, rover found a rock that turned out to be a nickel iron meteorite.
In 2009, it found another 10 times bigger than the first.
This makes it the biggest meteorite ever discovered on a planet other than Earth.
Its very existence makes no sense given what we know of the Martian atmosphere today.
Mar’s atmosphere is incredibly thin.
It consists of 95% carbon dioxide, 3% nitrogen and 1.6 percent argon,
with only traces of water and oxygen.
Compared to Earth the mass of the atmosphere is tiny.
It is 25 million million tonnes, compared to Earth’s 5000 million million tonnes.
The atmospheric pressure on Mars is less than 1% of the Earth’s surface atmospheric pressure.
This is the pressure we would experience at an altitude of 35 kilometres from Earth’s surface.
If the meteorite found by the Opportunity Rover had hit the planet today,
there would have been nothing to slow it down.
The Martian atmosphere is too thin to offer any breaking force.
A meteorite of this size would have been travelling so fast when it hit the surface,
it would have disintegrated on impact.
It simply shouldn’t be there.
The most likely explanation for this, is that at some point in the past, when the meteorite hit Mars,
the atmosphere was significantly denser.
Dense enough to slow it down so that it could land on the surface intact.
If this is correct, then why did Mars lose its thick atmosphere,
and become the barren planet we see today?
Atmosphere is a fragile ghost around the planet.
There are many ways for it to be disrupted and lost to outer space.
It is thought that the reason Mars lost its atmosphere is down to the red planet’s interaction,
with the powerful and far reaching influence of our Sun.
The solar wind is a stream of super heated, electrically charged particles,
that constantly flow away from the Sun at a million kilometres per hour.
This wave of smashed atoms may be invisible and appear innocuous to us here on Earth,
but it has the power to strip a planet of its atmosphere.
We are protected by an invisible shield that completely surrounds our planet,
known as the Earth’s magnetic field.
The Earth’s magnetic field is strong enough to deflect most of the Solar wind,
that comes our way, in stark contrast to Mars.
About four billion years ago, Mars had a molten core.
Mars was formed by the same processes as Earth, from the same material.
Its molten core would have generated a protective magnetic field.
There is however, one crucial difference between the planets.
Earth is 9 times bigger.
Mars total surface area is the size of the area of dry land on Earth.
This size difference is crucial because the larger the ratio of the surface area,
to the volume of the object, the quicker it will lose its heat.
Early the Mars history, the planet lost its internal heat through its surface and out into space.
Its core solidified.
Electric currents could no longer flow, and its magnetic field vanished.
Without these defences, the solar wind would have blasted Mars, and stripped its atmosphere.
With no atmosphere to insulate it, this Earth like world was transformed into the frozen desert,
we see today.
Although Mars has lost most of its atmosphere, what is left has the power to sculpt its surface.
Weather is a feature of every planet with an atmosphere, however tenuous and diffuse it might be.
Wind, storms, clouds and even rain can be found on other planets.
Wherever there is an atmosphere there is a delicate and complex interaction,
between the heat of the Sun, the surface of the planet, and the atmosphere.
We have discovered that it only takes the slightest trace of an atmosphere,
to produce extraordinary weather.
Jupiter is the largest planet in our Solar System.
This giant is over 140 thousand kilometres in diameter.
Over 13000 Earths can comfortably fit into Jupiter.
It has primarily hydrogen and helium.
Jupiter is almost all atmosphere.
The Jovian atmosphere is many thousands of kilometres thick.
It is in a constant state of seething motion, boiling with gigantic storms.
Yet this alien world shares a feature with Earth.
Jupiter crackles to the sound of electric storms.
Bolts of lightning, thousands of times brighter than lightning on Earth, illuminate the Jovian sky.
The forces that drive them are identical to the forces that drive storms here on Earth.
If there is warm moist air deep in the atmosphere, it will start to rise.
As it rises it cools and the moisture condenses out to form clouds.
The rising air leaves a gap beneath it, a low pressure area.
So even more warm moist air is sucked in, which fuels the beginning of a storm.
Here on Earth, the storm systems are driven by the power of the Sun.
It is the Sun heating the Earth that creates the convection currents,
that churn our atmosphere into action.
Jupiter, by contrast, is five times further away from the Sun.
This means it receives 25 times less solar energy, per square meter.
We might expect its storms to be considerable weaker.
Intriguingly, we have discovered that the opposite is true.
The storms systems in Jupiter are far more powerful and violent.
The secret to Jupiter’s storm tossed atmosphere, lies hidden deep within the gas giant.
On Earth we have clear boundaries between the gaseous sky, the liquid oceans, and the solid ground.
On Jupiter there are no such boundaries.
Jupiter is a giant ball of hydrogen and helium.
It is a planet built almost entirely of atmosphere.
As you go deep into Jupiter’s atmosphere,
something very strange and interesting happens to those gases.
Jupiter’s atmosphere is so dense, that 20000 kilometres beneath the cloud tops,
the pressure is 2 million times greater than the surface pressure on Earth.
Under these immense pressures the hydrogen gas is transformed into a strange metallic liquid.
When gases turn into liquids on this colossal scale, vast amounts of energy is released.
We have to put energy into liquid, for it to boil and turn to steam.
If we do the opposite and condense steam back into liquid water, energy must be released.
The same is true for gaseous and liquid hydrogen.
It is this energy force that creates the convection currents, that fuel some of the biggest storms,
in the Solar System.