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Prize-winning question!!!! How does the Earth have oxygen? Fletcher, age 7, Class 2, Blackboys School. Answered by Alex Nicholls.
Alex Nicholls
What an interesting and thought-provoking question! Earth has more atmosphere than most of the other inner planets for two reasons: (1) gravity, and (2) volcanism. Being the most massive of all of the inner planets, Earth can hold more atmosphere to its surface by its gravity alone. Furthermore, volcanoes continually spew out gasses into the Earth's atmosphere at a rate faster than they escape into space. Unlike Venus, however, which has an even thicker atmosphere, Earth also has oceans where gasses in the atmosphere are removed and crystallized into rocks. Earth is a living planet with a balanced dynamic equilibrium.
Now, let us go back to Earth as it was a mere 3 billion years ago. No oxygen in the atmosphere, but lots of carbon dioxide, nitrogen and methane. Then, 2.45 billion years ago there is a very little oxygen in the atmosphere and tiny organisms known as cyanobacteria, or blue-green algae make their appearance. These microbes conduct photosynthesis: using sunshine, water and carbon dioxide to produce carbohydrates and, yes, oxygen. In fact, all the plants on Earth incorporate symbiotic cyanobacteria (known as chloroplasts) to do their photosynthesis for them to this day. It took another billion years, dubbed the "boring billion" by scientists for oxygen levels to rise high enough to enable the evolution of animals.
Whether or not a planet retains an atmosphere over geologic time (billions of years) depends on the mass of the planet, the radius of the planet, the temperature of the atmosphere and the composition of the atmosphere. The atmospheres of the Earth-like planets (Mercury, Venus, Earth and Mars) originated by out gassing, probably associated with volcanism, of the interior of the planet. Mercury which is less than 10% the mass of the Earth has almost no atmosphere, because any gases that come out of its interior are moving faster than the escape velocity. The escape velocity is the minimum velocity an atom (or rocket) has to have at the surface so that it can escape the gravitational clutches of a planet. On Earth, any Hydrogen or Helium that comes from volcanoes can easily escape the Earth. Hence we have very little H and He in our atmosphere. On the other hand, any atom or molecule of more than the atomic mass 20 (i.e. Neon), will be retained by the Earth, because the mean speed of the atom is less than the escape velocity. The escape velocity from Earth is 11.2 km/s. Venus, which is about the same size and mass as Earth has a thick atmosphere made up of carbon dioxide. Mars which is 15% the mass of Earth has a thin atmosphere, because it can only retain the more massive molecules like CO2. The pressure on Mars' surface is 100 times less than on Earth.
So to summarize, the principal factor as to whether a planet has a stable atmosphere is gravity. For massive planets, gravity overcomes the effects of thermal agitation and a molecule cannot escape the gravity field entirely. Small bodies like the MOON, have low gravity and the random thermal velocity that an atom has will send it zinging on its way into interplanetary space.
So there's your answer, its a long one!
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Our team of Scientists...
Lydia Samuel
As Director of Blast Science and a Primary Science Teacher for more than 15 years, Lydia has answered unending questions about Science from hundreds of children for over a decade and has a wealth of Primary Science classroom experience. She has a particular passion for Wizard Science, Chocolate Science and all things Gross!
Tom Holloway
​is a Primary Science Teacher and Space Enthusiast. Alongside being a full time teacher he runs Star Gazing evenings, is a Science Advisor, has established a Darwin Garden in his school in Caterham and won Primary Science Teacher of the Year in 2013/14. Watch the video here!
Steve Evans
​has recently joined Blast Science as a performer after working for several years at Herstmonceux Observatory. He knows a huge amount about physics, chemistry, space and.... Star Wars! So can answer all your intergalactic questions..
Keita Lynch
has helped Sussex 'stay curious' by coordinating the Brighton Science Festival for three years. She's passionate about sharing science in simple
and engaging ways; without all the big words.
Keita's also a keen supporter of campaigns
like Let Toys Be Toys that encourage toy makers and retailers to stop limiting children's imagination by branding things for 'girls' or 'boys'. Astronaut suits and dinosaurs for all!
Dr Sarah Newnham
I am 26 and have a degree and PhD in biochemistry. I enjoy learning new things and have spent 7 years at university studying and experimenting with molecules and microorganisms. I mainly like to play with bacteria and get them to do new things and produce different chemicals. I also enjoy helping with Science projects in schools and enthusing children about my subject.
Dr Matt Edwards
Dr Matt decided to become a doctor when he ran out of his own scabs to pick. He used to work in Accident and Emergency in Brighton, pulling broken bones back into place and sewing up cuts. He now works in Brisbane, Australia as a Family doctor. This involves cutting out lots of funny lumps and sticking his fingers in every hole that people have. In his time he has been covered in every body juice you can think of, and once juggled with three hearts. Ask him anything about bodies and Gross Science.
Jonathan Brown
(Engineer at Ricardo) Had a strange fascination with all things mechanical from an early age and would often spend time 'fixing' his Dad's car without telling his Dad first. Rarely did it end happily, so of course he decided to continue this into a career than now spans almost 2 decades of designing engines for big names the world over. Having studied Mechanical Engineering at university he has been working since at a world leading technology consultancy, principally creating models and simulations of engines big and small for everyone from McLaren to Volvo. These days you're as likely to find him on a ship or looking at a tidal turbine, but it's still engines that keep him running.
Ivan Teage
Ivan is a experienced all-round scientist. He studied astrophysics after a childhood of being obsessed with going to the moon, and now works at the Natural History Museum in London using technology to explain Science to young and old audiences. Ivan is interested in the science of music and is keen to help people understand the natural world, how it works, and what it has to offer. He is looking forward to your questions about nature, space, music, sound & the meaning of life.
With A-levels in Pure Maths, Applied Maths, Physics and Chemistry he went to Uni to read Nuclear Physics, but in the end switched to Law. He nevertheless retained a great interest in Science and has an encyclopedic general knowledge about nearly everything. Alex is also the linchpin of the Blast Science props dept but helps out answering complicated questions about Science for light relief from complicated carpentry.