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What Would Happen If Earth and Mars Switched Places?

The views expressed are those of the author and are not necessarily those of Scientific American.


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Mars and EarthLast Saturday, at a workshop organized by the Foundation Questions Institute, Nobel laureate physicist Gerard ‘t Hooft gave a few informal remarks on the deep nature of reality. Searching for an analogy to the symmetries of basic physics, he asked the attendees to imagine what would happen to our solar system if you suddenly swapped Earth and Mars. He went on to discuss his ideas for explaining quantum mechanics, but I couldn’t get my mind off his question. What would happen?

Obviously, Martians would be delighted with the new arrangement. A fairly modest increase in Mars’s temperature would melt the polar caps and liberate gases from the soil, flipping the Martian climate into a new, cozier state nearly as warm as Earth. In an article for us in 1999, planetary scientist Chris McKay envisioned terraforming Mars by building factories to pump out greenhouse gases—proving that one man’s poison is another’s elixir—but moving the planet closer to the sun would certainly do the trick, too. Earthlings would get the short end of the deal. Sunlight would be half as intense and the planet would freeze over. On the plus side, we’d instantly be half as many years old.

In grand scheme of things, though, you might think that nothing would change. According to Kepler’s laws, the mass of a planet has almost no effect on its orbit; the mass of the sun is what controls things. Even though Earth is 10 times heavier than Mars, it would still trundle along Mars’s old path. Both Mars and Earth are perpetually falling toward the sun, and all falling bodies fall at the same rate.

But Kepler’s laws don’t account for the subtle gravitational perturbations that planets exert on one another. By rearranging the planets, you perturb these perturbations, and it’s not obvious what would happen. So I posed the question to planetary physicist Renu Malhotra of the University of Arizona, who was one of the first scientists to recognize that the planets migrated around early in the history of the solar system. Her initial guess was that Earth’s proximity would thin out the asteroid belt, but that the planets’ orbits would not be destabilized, at least not right away. She offered to run a computer simulation to check.

The results are a bit surprising. The planetary switch-a-roo makes the inner solar system strongly chaotic. Although none of the inner planets gets flung out of the solar system within the first 10 million years, all undergo large variations in their orbital distances. On occasion, Mars dips inward to become the second rock from the sun. To capture these variations, Malhotra found that she had to use a smaller time increment in the simulations than she had predicted, and consequently each computer run took nearly a day to complete.

To speed things up, she tried ignoring the planet Mercury—standard practice in perturbative calculations, on the assumption that Mercury is so piddling that its gravity is immaterial. Not in this case, though. Without Mercury, the other three inner planets went haywire in a few million years. Mars shot off into deep space. The sensitivity to Mercury’s absence is further proof that the altered system would be strongly chaotic.

Inner planet orbitsThe graph at left shows the actual solar system. For each planet, Malhotra plots the range of orbital distances: perihelion (closest approach to the sun), aphelion (farthest) and semimajor axis (midpoint). As Pierre-Simon Laplace showed in the late 18th century, our solar system is stable. The semimajor axes are constant, and the shapes of orbits vary modestly on a variety of periods, from tens of thousands to millions of years.

Inner planet orbits with Earth and Mars swappedThe next graph shows the altered system. Notice how wide the range of orbital distances for each planet has become. For Earth, that’s because it’s now closer to Jupiter; for Mars, because it’s the monkey in the middle. Venus changes hardly at all, while Mercury gets batted around like a pingpong ball. Malhotra’s simulation also included the outer planets, but I leave them off, because they lumber on as if nothing had changed.

These results support the emerging view, discussed in our pages by Doug Lin several years ago, that the solar system lives on the edge of chaos. It was probably unstable in its formative years. Planets got reshuffled or ejected until the survivors’ orbits were sufficiently well spaced. Any major change would push the system over the edge again. It’s analogous to a coffee cup. If you see a cup that is filled exactly to the rim, you can reasonably conclude that some coffee got spilled over the side, and anything you do to the cup would probably spill some more.

Malhotra has supported this viewpoint in the past, but cautions that the solar system is more stable than its age might imply, so the whole question remains unresolved. "Isn’t it interesting?" she wrote me. "This kind of thing is what attracted me to planetary dynamics."

Photo courtesy of NASA; graphs courtesy of Renu Malhotra





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  1. 1. Christine Gorman 2:50 pm 06/10/2011

    Sounds like an episode of Dr. Who to me!

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  2. 2. jgrosay 3:44 pm 06/10/2011

    Funny Sci-Fi, and an amazing evidence of the current computer’s power. What if Venus moved to the Mars orbit, or both moved somewhere closer to us and formed a double planet system ?. The possibilities of having fun from such kind of a game are enormous. Salut +

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  3. 3. Wizardwayne 4:08 pm 06/10/2011

    The real question is: what if Venus and Mars switch? I’d bet anything we’d then have three life-bearing planets in our solar system. Venus’ thick CO2 atmosphere would trap more solar radiation and Mars being closer to the sun would keep it out of the deep freeze!

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  4. 4. David Brin 7:49 pm 06/10/2011

    Dr. Musser assumed Earth would freeze permanently, if it orbited at the distance of Mars. May I switch from sci fi author to licensed astrophysicist? Sure, if Earth suddenly teleported out there, we’d suffer instant deep freeze. Yet, within a few million years, our planet would again have liquid seas and possibly life.

    Lots of people blithely talk about "goldilocks zones" around stars and whether extrasolar planets orbit in one. They assume this means a shell wherein light energy density is sufficient to heat a solid to a black body temperature in the liquid phase of water. This oversimplifies because of the greenhouse effect … and the Gaia Principle.

    The stronger, more "mystical" form of the Gaia Principle sees life creating homeostasis on worlds with liquid water, actively (if non-consciously) steering an equilibrium away from either freezing or dessication. The weaker version – calculated by eminent planetary atmosphericists like James Kasting – does not even need life. Two forces are at work:

    Seas evaporate. Rain erodes minerals from continents. Sea-suspended minerals pull CO2 out of the air which sinks as carbonate sediment. (Life accelerates this process; but it happens anyway.)  CO2 is replenished by volcanoes. Hence, the weak or strong Gaia Principles require both oceans and plate tectonics. A feedback loop follows. When CO2 falls too low, ice sheets spread and rainfall drops, weathering of continents declines and so does CO2 removal.  

    This isn’t just theory. It happened on Earth so severely – during the Kirschvink Glaciations – that the oceans were ice covered (raising reflective albedo, which made the cooling even worse) and life nearly failed.  It happened when the sun’s output was significantly lower.

      But when Earth was an iceball, plate tectonics continued. Volcanoes gradually filled the sky with CO2. In fact, CO2 levels overshot, resulting in a sudden, runaway melting followed by a steam bath planet!  When this happens, rainfail is immense and weathering sends so much mineral matter into the seas that CO2 is pulled out at an accelerated pace. Eventually, equilibrium is restored.

    Got it? Want me to write an article that explains how distance from the sun fits in? Perhaps I shall.

    With cordial regards,

    David Brin
    Author of EARTH, The Postman and The Transparent Society
    http://www.davidbrin.com
    blog: http://davidbrin.blogspot.com/
    twitter: http://twitter.com/DavidBrin1

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  5. 5. Quinn the Eskimo 1:02 am 06/11/2011

    If Earth and Mars switched places, I bet the Planetary Society would be pissed.

    Not to mention the front page full-color charts in USA Today. Oh, and Whiny Briny Williams at NBC would go live to show the devastation. Horrors!

    Katie might even return to CBS to interview the victims.

    But, then, I could be wrong.

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  6. 6. jgrosay 3:38 pm 06/11/2011

    The french philosopher Auguste Comte, that is put in a line that includes positivism and Karl Marx, once wrote: "It’s evident that the Solar System is badly designed". How many agree ?. Salut +

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  7. 7. skeam 4:13 pm 06/11/2011

    How come mercury was affected so much, just because earth moved farther away?

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  8. 8. ennui 9:57 pm 06/11/2011

    As long as we leave the clouds , that will be all around the globe, untouched, we may still live a good life. When people insist to see blue sky, the song is over. All the water will freeze or evaporate and make life impossible.
    Mars, being closer to the Sun, might evaporate the ice fields back into clouds again. Then we can slowly make new plans to emigrate to Mars.
    Since we will not be encumbered by Nasa we may have something to look forward too.

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  9. 9. dickrickert 10:33 am 06/12/2011

    Where is my copy of "When Worlds Collide"?

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  10. 10. JJJ1969 2:58 pm 06/12/2011

    "We won’t be encumbered by NASA"??? Why, who do YOU see springing up and sending spacecraft all over the solar system? We haven’t seen the ESA or the Russians do such things. I wonder why?

    Oh, that’s right – it costs money, and if America isn’t doing it, there’s no one’s coat-tails to ride on.

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  11. 11. ennui 3:31 pm 06/12/2011

    Hello JJJ1969,
    Do not forget that Nasa was offered the technology of the Flying Saucer, which would have enabled the Shuttles to reach the ISS in one hour, the Moon in a couple of hours and Mars within one day. THe cost would have been slashed dramatically.
    In the end it was rejected, it would make the Rocket Industry obsolete. Ever heard about fingers in the pie?

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  12. 12. mcarr 9:26 pm 06/12/2011

    Ummmm, ULA? Even NASA uses them to launch their satellites – it seems only a matter of time before they start doing the heavy stuff to deeper space.

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  13. 13. Ungolythe 1:43 am 06/13/2011

    JJJ1969, you’ll have to forgive ennui. He just can’t accept the fact that his "flying saucer technology" is already outdated by my teleportation device. Of course it’s only a prototype but like ennui it has been rejected by NASA. I’ll admit that the prototype doesn’t really work, not yet at least, but I expect a breakthrough soon. Now back to work on my perpetual motion machine….

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  14. 14. Ungolythe 1:49 am 06/13/2011

    I wonder if the oceans would completely freeze anyway. If there were still active volcanic vents at the bottom of the ocean do you suppose that the life that exists there now may not notice any difference?

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  15. 15. jtdwyer 2:38 pm 06/13/2011

    I suspect oceans would be very unlikely to freeze completely. Terrestrial surfaces are heated by the Sun only to superficial depths, while turbulent oceans are heated to great depths. A given surface area of land is heated by the Sun much faster than the same surface area of deep water. The constant motion of ocean water resists deep freezing as well as heating. Even if ice did form across the expansive oceanic surface, as envisioned in the ‘snowball Earth’ scenario, it seems highly unlikely that it would freeze all the way to the bottom.

    As I understand, the life surrounding mid-oceanic vents are highly localized, but must migrate to new vents as old ones extinguish. For that reason I’d expect vent life to continue to flourish as long as there was a survivable path to the next vent. Other circumstances might also negatively effect the survivability of vent life, though.

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  16. 16. ennui 8:14 pm 06/13/2011

    After I got the patent on the technolgy I offered it to Nasa. The technology used by the Flying Saucer was first rejected by the Propulsion Engineers in Cleveland, Ohio, as a Shuttle would not need rockets anymore but reach the ISS in one hour, the Moon in a couple of hours and Mars within one day. They were afraid that they would be out of a job. After the second Space Disaster they decided to look into it, did not contact me like I had urged, got the wrong setting for the Monopole HV Generator (that are these big spheres under a Flying Saucer) and blew the Power Transformer Station on their grounds to Kingdom Come. It caused the big black-out of 2003 in the North and East of the USA and Canada, which was deftly blamed on a poor, innocent little tree. Then they advised that the system used by a Flying Saucer was unsuitable for Space Travel. That is the story. Not your type of information.

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  17. 17. coolstar 11:47 am 06/15/2011

    Nice article, too bad it’s marred by one obvious mistake: "and all falling bodies fall at the same rate".
    Of course they don’t! The acceleration of gravity (due to the sun) at Mars orbital distance is just about
    1/(1.5)^2 = 0.444 that felt by the earth. What you should have said was something like "all bodies at nearly the same distance from……". I also seriously doubt that Laplace "proved" the stability of the solar system, at least with anything close to the modern meaning of that term. That’s because, as almost any first year science student should know, there’s no analytic solution to the many-body problem, and analytic solutions were the only ones available to Laplace.

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  18. 18. jtdwyer 6:15 pm 06/17/2011

    Good catch! I’m not sure what they author was trying to say (below), but if I understand correctly (not being a scientist), if the Earth was placed in Mars’ orbital position a year would be about 687 days long…

    The article states:
    "In grand scheme of things, though, you might think that nothing would change. According to Kepler’s laws, the mass of a planet has almost no effect on its orbit; the mass of the sun is what controls things. Even though Earth is 10 times heavier than Mars, it would still trundle along Mars’s old path. Both Mars and Earth are perpetually falling toward the sun, and all falling bodies fall at the same rate."

    I think that, on this subject, Kepler says: the further a planet is from the Sun (in a planetary system like our Solar system), its orbital velocity is diminished.

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  19. 19. bucketofsquid 4:24 pm 06/24/2011

    I wonder if the oceanic make-up would shift. Perhaps become somewhat more like the moons of jupiter that have a liquid ocean under a layer of ice according to the last article I read. I don’t think the main content is supposed to be H2O out there.

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  20. 20. whitegirl15 4:19 pm 04/20/2012

    - i wonder if we would die or not…. i dont know , ( what would really happen if we did?) and is mars before us or after …. i don’t know

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