Presented here is a map of an alternate Earth from 542 million years ago.

enter image description here

It is 5% narrower in diameter and 20% lighter in mass than our Earth. A single day lasts 42 hours, a cycle that it must complete 827 times to make up its year. The only body orbiting it is a moon 3,474.2 miles wide and with a crust made entirely of ice. This "supermoon" orbits this Earth from a distance of 672,700 miles. It has an axial tilt varying from 109.7 to 118.3 degrees over a period of 58,000 years. Such a high axial tilt means that its two suns--both of which are G-type--rise from the west and set on the east. Even the oceans are arranged differently, courtesy of more active volcanic systems:

  • Shallow seas= 53%
  • Deep seas (1-4km)= 32%
  • Abyssal plains (5-6km)= 3%
  • Trenches and deeps (over 6km)= 12%

Once the (illegal) terraforming has been established, oxygen levels in the atmosphere are at 35% and carbon dioxide is at 2200 parts per million.

All of these details will be relevant in this series. Now the first question will be--how will any of these details affect the way the ocean currents flow?

  • $\begingroup$ In what sense is this an alternate Earth? $\endgroup$
    – rek
    Aug 11, 2020 at 4:30
  • $\begingroup$ With 53 % shallow seas the space for ocean currents seems rather small. $\endgroup$
    – Erik
    Aug 11, 2020 at 14:08
  • $\begingroup$ 42 hours day x 827 days year = 34.734 hours / year, 125 million seconds. The planet orbit at asteroid belt range, around 2.5 AU. Seems you increase the amount of CO2 to trap some heat but... at this level, is safe and friendly to liveforms? $\endgroup$ Aug 13, 2020 at 8:41
  • 1
    $\begingroup$ @RodolfoPenteado I did say "two suns". $\endgroup$ Aug 13, 2020 at 21:25

2 Answers 2


For one thing, the longer day will reduce any currents generated by coriolis effects. Coriolis effects are responsible for the clockwise rotation of ocean currents in the northern hemisphere and counterclockwise rotation in the south (see map below), mainly due to atmospheric pressure from air currents formed by coriolis flows. This means that the currents aren't very deep, so ocean depths play a very little role.

There will still be an appreciable coriolis effect, though, and lacking other influences (we will get back to that), these currents will be main ones, even if they are less strong than on Earth.

enter image description here Ocean Currents on Earth

The polar ocean on your planet (which has very little connection to the main ocean) will have a gyre rotating opposite your planet's rotation, like the arctic subpolar flow around Earth's Antarctic continent (see map).

Currents in your main ocean will be divided between north and south by an equatorial counterflow opposite the direction of rotation. The north will have a counterclockwise-flowing gyre, and the south a clockwise gyre (given your planet's direction of rotation). The vertical bay to the right of the middle of your map is unlikely to have any appreciable currents from the coriolis effect.

Now we get to the effects from lunar tides. Your moon has roughly 33 times the mass of our moon (assuming similar composition) and orbits at 2.8 times the distance. This makes the tidal influence 4.2 times as strong as the lunar tides on Earth. Solar tides are responsible for about one-third of the tidal force on Earth, but with no information about the mass and distance of your sun, we just have to assume that overall tides are roughly 4 times terran values. With no information about the eccentricity or inclination of your moon's orbit, I have to assume that both are insignificant.

Lunar tides make ocean levels rise on seas below the moon and on the opposite sides, while ocean levels decline on the 'sides' (both with a little lag, but that isn't important). The flows between high and low tides are hence likely to be the most significant on your planet. This will be especially important for your vertical bay, which will experience strong south-going currents between high and low tides (when water flows out) and north-going currents between low and high tides.

It is unclear if the high (retrograde) axial tilt you mention refers to your planet or its moon. Grammatically, the latter seems to be the case, in which case it has little effect on the ocean currents on your planet. If, however, your planet is tilted this way, seasons will be quite extreme, with polar circles extending to within between 20 and 38 degrees from the equator, leaving the greater part of the the planet in unbroken darkness for half a year, then in unbroken sunlight for half a year. I imagine that this would lead to a lot of evaporation on the summer side, falling as precipitation on the winter side, which would result in a small flow of water between the two sides, reversing as the seasons reverse. The magnitude of this effect would depend on the length of the year.

Regarding your moon: It is unlikely to have an icy crust at a distance from the sun that allows liquid water on your planet. The ice would sublimate and either escape into space or form an atmosphere of water vapor, which would soon heat the moon and melt the ice.

  • $\begingroup$ Don't axial tilts above 90 degrees just mean that the planet rotates counterclockwise? The axial tilt of Venus for example is 177.3 degrees. $\endgroup$ Aug 11, 2020 at 8:37
  • $\begingroup$ @TheDyingOfLight: It looks as if there are two conventions. The most common is to note that rotation is prograde or retrograde and provide an axial tilt between -90 and +90 degrees. However, some assume all rotations are prograde, which gives axial tilts between 0 and 360 degrees. I will update my post to reflect this. $\endgroup$ Aug 11, 2020 at 11:59
  • $\begingroup$ @KlausÆ.Mogensen I did a little math. 109.7-118.3 degrees are equal to 19.7-28.3 degrees, so for the lowest tilt, the summer insolation would be more extreme than Earth. And 827 rotations are equivalent to 1447.25 Earth days. I have to change the parameters a lot to make this more suitable for terraforming, don't I? $\endgroup$ Aug 11, 2020 at 21:43
  • $\begingroup$ @JohnWDailey: You're right in that I got my math wrong - it's ca. 20-38, nor 30 to 48 (I will correct that). However, since we continue from 90 at the north pole, the axial tilt corresponds to 90 minus 20-38 as seen from the equator. Compare with Venus, which as you say, has an axial tilt in the righ-hand system og 177.36 degrees - corresponding to 2.64 degrees in the retrograde notation. And yes, as it stands, it doesn't seem all that suitable for terraforming - but not impossible. $\endgroup$ Aug 13, 2020 at 8:17

So what factors would have an effect (from the most affecting to less affecting):

  1. Lower rotation speed - since currents are driven by Coriolis acceleration their speed would be lower. Polar curent would be much lower and even may not be there
  2. Lower diameter - all curent a closer to center of rotation, thus centrifugal and Coriolis accelerations are geting even smaller.
  3. axial tilt+long year - this planet is actually laying on it side so seasons would be very strong. There would be distinctive sesonal currents when half of the planet is frying under never-setting sun and other is covered with Earth-year icesheet. And then at spring/autumn all this ice melts... I'd be more vary about air currents on this planet - superhurricanes there would be as common as thunderstorm at spring.

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