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In this scenario, there is a rocky planet that is 230% the width of our Earth, covering a total area of a billion square miles, 55% of which consists of three continents separated by a single ocean that averages in at less than one mile. However, what really makes this stand out is that the core is lighter in density, resulting in more Earthlike gravity.

Now let's get a few things out of the way and summarize the consequences of a standard super-Earth.

  1. There is a disagreement as to how the geology works on a super-Earth. Some say that geological activity would be higher, with more vigorous tectonic movements due to thinner crusts under higher stress. However, others say that strong convection currents will thicken the crust, preventing the magma from breaking it into plates.
  2. This is speculation at the moment, but it's believed that super-Earths would have stronger magnetic fields.
  3. Thicker atmospheres, which retain more heat and more moisture, thus rendering super-Earths warmer and flatter (meaning that mountains won't be as dominant as they are back home.)

Now all this is from a standard super-Earth, but this one has a lighter core. Apart from the lighter gravity, would a lighter core on a super-Earth affect the three listed factors in any way?

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  • $\begingroup$ @DWKraus Don't answer in the comments, also there is no observable link between gravity and atmospheric density, Venus has less gravity than Earth and an atmosphere that makes the bottom of the ocean look like a picnic spot. $\endgroup$
    – Ash
    Sep 15, 2021 at 23:17
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    $\begingroup$ If you replaced the iron core with something lighter, you should not expect your point number 2 to hold! $\endgroup$
    – JDługosz
    Sep 16, 2021 at 0:00
  • $\begingroup$ @JohnWDailey The ability of a planet to retain its atmosphere depends on its escape velocity, not its surface gravity, as well as its magnetic field. Your question only mentions "gravity", without specifying surface gravity or escape velocity, and doesn't state how "Earthlike" the 'gravity" is - 75 percent, 100 percent, 125 percent, or some range of values, or whatever. That certainly makes it harder to calculate the density of your planet. $\endgroup$ Sep 16, 2021 at 19:38

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This is all fairly speculative but:

  1. Geology: If the mantle/core is sufficiently light to result in earth-like gravity at the surface then it cannot contain much in the way of dense metals such as iron. In that case it presumably contains more light elements. If there is a significant amount of potassium then its decay would likely increase the internal temperature of the super-earth relative to our planet. Even if that is not the case, then the increased volume/surface-area ratio would have reduced the ability of the core/mantle to cool over geological time. So the mantle is likely hotter, which would result in significantly more volcanic activity on the planets surface. The hot core, plus larger radius would also lead to an increase in plumes and convection currents in the mantle so probably more continental drift rather than less. Result: a surface with lots more volcanos and more (and larger) mountains than earth.

  2. Magnetic field: I believe that the likely reduction in volume of a molten metallic outer core would actually reduce the magnetic field (assuming teh metal is replaced with rocky materials. Result: lower magnetic field and also less shielding from the local sun's solar winds.

  3. Atmosphere: If the planet has similar surface gravity there is cause for a thcker, denser, or moister atmosphere. On the other hand, increased vulcanism may result in a lot more CO2 in the atmosphere. To add to the difficulty in calculating this, the constituents of earth's atmosphere are dominated by biological influences over the last couple of billion years. So density/constitution could probably be chosen as you want. But the much larger dimensions of the planet would allow a lot more 'weather' to happen. One possibility would be a greater degree of atmospheric banding - i.e. regions of fast east/west flows than on earth, so potentially extrtemely strong winds and/or jet-streams, and much larger, more powerful, and long lived cyclones and hurricanes. Much more space for large dry desert regions to develop (especially with 55% land coverage). With only 45% surface covered by much shallower water, the oceans would have less of a stabilising influence on climete so expect greater seasonal and latitudinal climate variation. (although shallow oceans are possibly not consistent with increases vulcanism and continental drift). Result: Chose atmosphere how you want it, but expect the weather/climate and desertification to be extreme.

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A super earth with earthlike gravity is going to have to be primarily composed of light elements from the first two rows of the periodic table, transition metals are going to be relatively rare. What transition metals there are will sink towards the core quite rapidly as the hot young world is going to have a very low viscosity and high rate of convection in it's Hadean phase making the crust almost devoid of metals. A larger world should stay hotter for longer due to retained formation heat but with a low thorium percentage it will get less heat from radioactive decay in the core.

Geology - The minerals that are going to form the iron depleted crust of this world are going to have a much lower melting point than the rocks we're used to seeing on earth, but they do form the dominant volcanism in one place so we know something about their chemistry and behaviour. This crust is going to be relatively thin per unit of cooling time because it melts at half the temperature of the basalt that makes up most modern lava, especially oceanic crust. The low melting temperature results in very fluid lavas so all volcanoes are going to be shield type with large spatter cones around active vents. The combination of thin crust, low mantle melting temperature and rapid convection is going to mean volcanoes everywhere even without plate tectonics. In fact tectonics may not be a factor on this world unless there are large-scale mantle currents as the crust is simply too thin to resist fracturing and build up cratons around which plates accrete. As many of the minerals that make up the resulting lava flows are relatively soft and water soluble erosion rates will be very high resulting in a world of generally low relief landscapes. The low viscosity of the magma means that to spite having high percentages of dissolved gas explosive eruptions are almost unheard of, gas can and does migrate too easily through the melt.

Magnetic field - With the all the ferromagnetic material migrating to the core this super earth could potentially have quite a strong magnetic field for a long time if the rate of core rotation is high even if the core is relatively small because it will stay fluid longer than the core of a larger world due to reduced depth/pressure rate and better insulation. The magnetic field may be more prone to fluctuations because it's source is deeper in the planet and also because it is relatively smaller.

Atmosphere - The formation and evolution of planetary atmospheres is something we don't really understand, if it were purely a matter of gravity we should have a thicker atmosphere on Earth than that on Venus this is not so. There are several other factors that we similarly think should have an effect on the atmosphere, like the rate of solar stripping that don't explain the realities we see in different places around the solar system. Having said that there are some things we can know about this world that will effect the atmosphere:

  • The planet will have relatively large amounts of water in it, the minerals in the crust and mantle have relatively high water of crystalisation compared to what we find in basaltic mantle rocks.

  • It will have relatively little surface water for a given percentage of total water mass, the crust is going to be highly permeable and water will soak to depth rapidly leaving the surface dry. If you want oceans then the planet is going to need a lot of water as a total percentage of it's mass.

  • Water and other gases are going to pour into the atmosphere continuously from volcanic vents densely scattered all over the world. Even if the planet perhaps shouldn't be able to hold onto an atmosphere it will have one for long time.

  • The sea(s) are going to be shallow, the erosion rate for carbonatites, especially by water, are simply too high for basins of any great depth to form faster than they fill.

  • The ocean(s) will also be extremely rich in dissolved minerals and you would expect to see direct solution deposition of some rocks, Chert (flint), Glocanite, and Limestone have a history of such deposition on Earth, this world would probably see many more forms as well.

As a side note the high percentage of carbon and the highly convective mantle are going to make Diamonds more common than useful metal ores in the crust. Due to their durability, especially compared to most of the crustal rocks of this world diamond sand is going to be a major constituent of the soils and beaches.

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