Science Questions

What will end life on earth first, the sun becoming a red giant or the magnetic core cooling?

Tue, 6th Sep 2016

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Paul James asked:

Hi Chris, Paul here from Malaysia. Naked Scientists is my favorite podcast and think it's great the show is free and hope it always will be.

My question is which is the following events will most likely cause the end of life on earth first.

1: When the sun becomes a red giant 2: when the inner core of our planet completely cools and the magnetic field disappears.

Kind regards,






Kat asked David Rothery to answer this, slightly depressing, question...Artist's impression of the structure of a red giant.

David - Paul is saying the magnetic core of the Earth is cooling down. I think what he’s getting at is when the outer part of the core is fluid and electrically conducting. That’s where the Earth’s magnetic field is generated and if a planet no longer has a magnetic field it is not protected from the solar wind and the atmosphere can begin to be stripped away. So, if that were to happen, it would be an uncomfortable place for us to live but Mars has no magnetic field and we will still regard it as habitable for microbes. So, even if this happened to Earth, it would not end life on Earth Paul. So, if you're a microbe, you can relax. The core is cooling down, it’s about 100 degrees per billion years. It’s got a way to go. It’s got several billion years before it would solidify. I can't tell you how long because we’re not quite sure what it’s made of so we don’t know what temperature it will solidify at. But the sun is going to swell up into a red giant in 4, 5, 6 billion years and that’s going to make the Earth intolerably hot for any kind of life. No more liquid water. So to answer the question, for any kind of life at all, the sun becoming a red giant is going to kill off life. If the question is about multicellular animal life at the Earth’s surface, maybe magnetic field collapse will do that first, but we don’t actually know.

Kat - Andrew…

Andrew - I've heard that the magnetic poles can reverse as well earlier.

Kat - Yeah, I was going to ask about that. It comes up in the news, like if the poles flip, our atmosphere will fly away and we’ll all be stuffed?

David - No. The Earth’s magnetic field is reversed many times. It gets weaker and then builds up again in the opposite direction. It happens every few hundred thousand years at irregular intervals. So it happened so many times and life is just carried on through. No signs of mass extinction events every time the poles flip.

Kat - I wonder about, what about things like our communications network and stuff like that? Is that going to cause a problem?

David - Well, they will cause a problem because yeah, satellites in space…

Kat - No Pokémon, no civilised life.

David - …won't be protected from the solar wind directly. Technological life could be very interesting in the few hundred years when the field is collapsing and rebuilding, yes.


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I believe that the sun will go red first. There are two reasons that the core is so hot:

1) residual heat left over from earth condensing and fractionating (heat from heavy stuff falling/sinking to core)

2) radioactive decay

The most of the heat from the 1st process was generated more than 4 billion years ago, and it is still cooling off (presumably taking billions more years to do so). The second is generating a significant amount of heat continuously, much of it derived from radioactive decay of uranium 238 (half life 4.4 billion years) and thorium 232 (half life 14 billion years). So the output of the nuclear decay will also last several billion years more.

The sun is expected to go all red gianty in about 5 billion years chiralSPO, Thu, 1st Sep 2016

But water all will go in <2 billion years ...

Dry would be sufficient to extinguish life , . RD, Thu, 1st Sep 2016

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