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Nature communications article "Mission to Earth's core — a modest proposal", suggests placing a large volume of liquid iron in a crack and let it sink all the way to the Earth's core, carrying along a probe that can transmit data using seismic waves.

It is an interesting idea. Although there might be many technical problems I feel no one seems insurmountable and it is worth a try.

However, I wonder why the author suggest using a liquid iron instead of a liquid lead. I would expect lead to be cheaper and also require a smaller volume to achieve the same fracture stresses (due to higher density). In addition, a lower melting point would facilitate the initiation of the fracture propagation near the surface.

Would lead dissolve in the surrounding magma? Would it chemically interact turning into a lighter compound?

As the original article is behind a paywall I copy here the main text:

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I've already commented on this before here. This will not work regardless of whether this is iron or lead or anything else. The fact that it was published in Nature does not mean it is true, always remember that.

A 108 kg ball of iron would have about 30 metres in diameter, which is not a lot, compared to Earth scales.

Here is why it will not work:

  1. We have very massive objects on the surface. They are called buildings. They don't sink. Even though they are not that dense, buildings stand on foundations that funnel their mass to a small area, creating very large pressures. The point of the foundations is to hold the building on solid rock so they do not sink.
  2. The author suggests to initiate the crack with explosives. Perhaps a nuclear explosion. We had many nuclear explosions in the past - they don't make cracks. They make craters.
  3. Crack propagation is only relevant in the lithosphere. Once you get to the asthenosphere, about 200 kilometres deep, cracks don't propagate any more and the equations the author presents are meaningless. Plastic behaviour of rocks is achieved much earlier anyway.
  4. Even if there is a crack, it will have smaller cracks radiating from it. The liquid iron, being liquid, will fill those smaller cracks and lose from the mass of the main liquid blob. With time, the blob will get smaller and smaller until it will not be able to do anything any more.
  5. The melting point of iron is more than 1500 °C, this higher than the temperature of the ambient rocks. It will lose heat quicker than it gains by gravitational potential and solidify.
  6. The iron will react with the rocks around it. Particularly if it is liquid, because these things are highly reactive. It will start dissolving things into it (which will counter point 5), but it will also lose iron to the surrounding rocks. It will dissolve oxygen, carbon, sulfur, possibly alkalis like sodium, maybe silicon. These are light elements that will overall lower the density of the ball of iron, bringing it to a stop.
  7. The oxygen fugacity of the crust and upper mantle is higher than iron-wustite, which will be imposed by the equilibrium with iron. Therefore, it will leach oxygen from the surrounding rocks, essentially turning some of the liquid iron into "rock". This will also decrease the mass of the blob of iron.

You were specifically asking about lead. All points above (except 5) are equally applicable to lead.

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  • $\begingroup$ I disagree with your points 1-3. And the others can be solved. The whole point of crack propagation is the extreme accumulation of stress in the tip of the crack. Therefore, your analogy with buildings is incorrect, both for the area and because is a solid, and here the key is that the stress is provided by hydrostatic pressure. In ice sheets, crack propagation can make a a crack under a surface pond propagate a couple kilometers in solid ice until it reaches the bed. That happen often in Greenland every summer, and the density contrast is much less than iron/crust. $\endgroup$ – Camilo Rada Apr 14 '19 at 2:14
  • $\begingroup$ Then, once the blob pass the layers with brittle behaviour, it don't needs cracks. It would sink just by density difference, as subducted slabs do, or similarly to the way magma diapirs rise. For the other points, you would just need a larger initial volume. $\endgroup$ – Camilo Rada Apr 14 '19 at 2:17
  • $\begingroup$ @CamiloRada what you just said just addresses point 1. But regardless of that, cracks only exist in the lithosphere which is the top ~200 km. Even if you could magically make a crack that goes down 200 km, it stops there. $\endgroup$ – Gimelist Apr 14 '19 at 2:17
  • $\begingroup$ @CamiloRada the original paper talks about how the crack propagates at 5 metres per second, making the mission take about a week. Once you're in the asthenosphere, timescales become hundreds of thousands of years. Which is more than enough time to chemically equilibrate the iron (or lead, or whatever) with the ambient mantle silicate and dissolve it away. $\endgroup$ – Gimelist Apr 14 '19 at 2:19
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    $\begingroup$ @CamiloRada yea - it doesn't really answer the question of why lead and not iron. No idea. My answer was mostly pointing out that it doesn't matter, both will fail. $\endgroup$ – Gimelist Apr 15 '19 at 3:58

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