Why is Earth Magnetized and Venus Not?

A new analysis reveals that the gigantic impact that led to the Moon's formation might have also switched on Earth's magnetic field.

Interiors of Venus and Earth

Based on their bulk density, Venus and Earth have cores that take up about half of their radius and roughly 15% of their volumes. Researchers don't know if Venus has a solid inner core, as Earth does.
Don Davis / The New Solar System (4th ed.)

Planetary scientists don't really know what to make of Venus. Although it's a near twin of Earth in size, mass, and overall rocky composition, the two are worlds apart (so to speak) in many ways. One obvious difference is our sister planet's dense, cloud-choked atmosphere. This enormous blanket of carbon dioxide has triggered a runaway greenhouse effect, trapping solar energy so well that the planet's surface temperature has rocketed to roughly 460°C (860°F).

Dig deeper, and the differences become even starker. Based on its density alone, Venus must have an iron-rich core that's at least partly molten — so why does it lack the kind of global magnetic field that Earth has? To generate a field, the liquid core needs to be in motion, and for a long time theorists suspected that the planet's glacially slow 243-day spin was inhibiting the necessary internal churning.

But that's not the cause, researchers say. "The generation of a global magnetic field requires core convection, which in turn requires extraction of heat from the core into the overlying mantle," explains Francis Nimmo (University of California, Los Angeles). Venus lacks any of the plate tectonism that's a hallmark of Earth — there's no rising and sinking of plates to carry heat from the deep interior in conveyor-belt fashion. So for the past two decades Nimmo and others have concluded that the mantle of Venus must be overly hot, and heat can't escape from the core fast enough to drive convection.

Now a new idea has emerged that attacks the problem from a wholly new angle. As Seth Jacobson (now at Northwestern University) and four colleagues detail in September's Earth and Planetary Science Letters, Earth and Venus might both have ended up without magnetic fields, save for one critical difference: The nearly assembled Earth endured a catastrophic collision with a Mars-size impactor — the one that led to the Moon's creation — and Venus did not.

Jacobson and his team simulated the gradual build-up of rocky planets like Venus and Earth from countless smaller planetesimals early in solar system history. As bigger and bigger chunks came together, whatever iron they delivered sank into the completely molten planets to form cores. At first the cores consisted almost completely of iron and nickel. But more core-forming metals arrived by way of impacts, and this dense matter sank through each planet's molten mantle — picking up lighter elements (oxygen, silicon, and sulfur) along the way.

Over time these hot, molten cores developed several stable layers (maybe as many as 10) of differing compositions. "In effect," the team explains, "they create an onion-like shell structure within the core, where convective mixing eventually homogenizes the fluids within each shell but prevents homogenization between shells." Heat would still bleed out into the mantle but only slowly, via conduction from one layer to the next. Such a stratified core would lack the wholesale circulation necessary for a dynamo, so there'd be no magnetic field. This might have been the fate of Venus.

Magnetic field of Earth

Thanks to churning convection in its liquid outer core, Earth has a substantial magnetic field. Blue arrow indicates pole direction; yellow arrow points toward the Sun.
NASA-GSFC Scientific Visualization Studio / JPL / NAIF

On Earth, meanwhile, the Moon-forming impact affected our planet literally to its core, creating turbulent mixing that disrupted any compositional layering and creating the same mix of elements throughout. With this kind of homogeneity, the core started convecting as a whole and drove heat readily into the mantle. From there, plate tectonism took over and delivered that heat to the surface. The churning core became the dynamo that created our planet's strong, global magnetic field.

What's not yet clear is how stable these compositional layers would really be. The next step, Jacobson says, is to grind through more rigorous numerical modeling of the fluid dynamics involved.

The researchers note that Venus certainly endured its share of big impacts as it grew in size and mass. But apparently none of them hit planet hard enough — or late enough — to disrupt the compositional layering that had already settled out in its core. By contrast, the team concludes, "Earth was struck violently at the end of its growth, simultaneously creating its Moon and homogenizing its core." If they're right, then the divergence of Earth and Venus becomes a classic story of planetary "haves" and "have nots."

14 thoughts on “Why is Earth Magnetized and Venus Not?

  1. Robert-LaPorta

    If this is proven to be true, it constrains even more tightly the probablility of life and intelligent life. It is highly unlikely manyEarth size planets are struck with Mars size bodies. This would mean very few Earth size planets have appreciable magnetic fields. Solar particles would not be deflected and heavy radiation would reach the surface, such as occurs on Mars. This could prevent the development of complex molecules and development of life.

    Thus the Earth and life become even more special and rare.

    1. Jon-Groubert

      That’s a good point, Robert, but it ain’t necessarily so. We humans could not evolve on such a planet that was bathed in radiation. But other organisms, like the now famous water bears, as well as cockroaches, and certain flour beetles, have an inherent resistance to radiation. It’s always a case of “life-as-we-know-it”.

      An as Star Wars loves to display, whether rightly or wrongly, intelligent life may be able to develop from any of a wide variety of life. Chewbacca is a very intelligent dog; Admiral Ackbar is some sort of evolved trout; Jabba is a big, smart slug. We can’t definitively state that intelligent life could not evolve from lesser species that happen to be radiation-resistant. We only have a sample size of one, and that’s not enough to extrapolate from.

    2. Genac

      Methinks ye underestimate the vastness of space. There are 1,000,000,000,000,000,000,000,000 stars. Essentially our comprehension yields an infinite universe and given infinite iterations of mass-containing systems, the likelihood of countless earth clones is near certainty.

    3. Anthony BarreiroAnthony Barreiro

      I find Robert’s argument compelling. We know of one planet where life has evolved: our own Earth. It seems quite significant that Earth has an anomalously large Moon, orbiting, not around the planet’s equator as do all the other known moons, but in Earth’s ecliptic plane. The Moon serves as an orbital outrigger, keeping Earth’s poles from wobbling too much and thus our seasons consistent, and giving us those lovely tides and biologically rich tide pools. And now we learn, perhaps the Moon’s creation mixed up our core and thus has sustained our protective magnetic field for billions of years!

      Many people who have read a lot of science fiction assume that the universe must teem with life, and try to put the burden of proof on skeptics to show that life doesn’t exist out there. But it is more philosophically and scientifically reasonable simply to acknowledge that we don’t yet know whether life has evolved anywhere else in the universe. We should keep looking, fully accepting that we might not ever find any sign of extraterrestrial life. And, regardless of our personal biases about the probability of extraterrestrial life, we all need to do a much better job of caring for this one biosphere where we shall all survive or go extinct.

      By the way, Kelly, this is a fascinating article!

      1. Lindsay

        As always Anthony, I find your comments insightful, education and compelling.
        In addition to what have been listed in response to Kelly’s article, there are probably many more reasons why planets do not generate magnetic cores. I have read that the core must have the right nickel content to generate a magnetic core: see doc: 10.1038/ncomms16062

        With regards to intelligent life on the hundred billion or so planets in our Milky Way, why haven’t the SETI folks been receiving extra-terrestrial radio signals from these aliens?

        1. AB

          This presumes that the aliens bothered to create radio signals or need them to communicate? Given a very different planet, intelligent inhabitants might communicate by telepathy, or pheromones, or a planet-wide conscious link, or any number of things we can’t even imagine. They might wonder about other intelligent life out there and be trying to connect with it by holding large group efforts to contact alien consciousnesses through their concept of a spirit world. Who knows???

          … Chewbacca’s a DOG….???? XD

  2. John-Sheff

    It’s a complicated situation. What we have to remember is that Venus is the only planet whose rotation is retrograde. Its axial inclination is 177° to its orbital plane; it’s rotating almost exactly upside down. It’s hard to come up with an explanation for this that doesn’t involve a huge impact by a large body early in the history of the solar system. So Venus, like Earth, must have gotten whacked. But of course, the devil is in the details…

    1. Kelly BeattyKelly Beatty Post author

      hi, John — great observation. so I asked the research’s main author, Seth Jacobson about this. his response: “To my knowledge, giant impacts are associated with rapid spin. They more-or-less reset the spin state of the planet, and it’s just unlikely that the spin state will be reset to zero. Small impacts are the opposite. They tend to drive the spin state in a specific way. I don’t see an inconsistency between an early last giant impact on Venus and a slow spinning Venus at the end of planet formation.” note the “early” in his last sentence — the layered core Jacobson envisions doesn’t remain stable if Venus takes a big *late* hit — which is what happened on Earth.

    2. mike

      There has been a great deal of work going all the way back to the late 1960s that can explain Venus’ retrograde rotation. I refer you to the work of Dobrovolskis & Ingersoll (1980) and more recently Correia & Laskar (2001) who show that Core-Mantle Friction along with Tidal Torques on Venus’ thick atmosphere are sufficient to explain its present day rotation period. In fact Dobrovolskis & Ingersoll mention the possibilty that w/o a thick atmosphere Venus could have even been tidally locked to the sun (as the Moon is to the Earth). A more recent paper by Rory Barnes has even shown that this could have also come true for Earth if it’s initial spin state was about 1/3 of what it is today.

  3. MickeyJ65

    Also, I don’t think it’s necessarily unlikely for Earth size objects to be hit by Mars size objects, especially during the early formation of the solar system ( s ).

  4. james-ball

    How much would convection currents be reduced by the simple fact that the surface temperature is 800+F on Venus versus 70F on Earth? Simple physics will say that the larger the temperature gradient the greater the convection current will be. If Venus can not vent its internal heat out to space easily, then the convection will be reduced. Also just having the Moon in orbit around the Earth will increase the “mixing” of the liquid core through gravitational tidal forces, which Venus also doesn’t have.

  5. Old Kid

    Some very interesting and well-considered ideas. What I always wonder about, is that since Venus is exposed to twice as much solar radiation and solar wind than Earth, why does it have an atmosphere at all? In the billions of years that have passed, why hasn’t it been stripped away, lacking the protections of the magnetic field Earth has?

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