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This answer to Salinity of Martian water 3.5 Billion years ago in Astronomy says that most estimates of Mars' ocean's salinity are pretty high;

The article "Water Activity and the Challenge for Life on Early Mars" finds that the early martian ocean would have been acidic and almost as salty as the dead sea.

The estimates for salinity of the europan ocean vary widely, but most place the salinity above 50g/kg, more than that of sea water (about 45 g/kg).

I've commented there the possibility that:

  1. those estimates may change as more is learned about Mars
  2. there could be regional variations in surface water salinity since there is believed to have been rain, and tidal mixing may have been much less than on Earth

I support my amateur hypothesis in item 2 with the following

  1. Mars has no massive moon like the Earth's
  2. Mars's orbit is about 1.67 times further from the Sun than Earth on average, and tidal accelerations vary as $1/r^3$ putting the tidal acceleration at 22% of what Earth experiences at 1 AU.

However we should remember that Mars' surface gravity is only 38% of that on Earth, so the effect is not reduced as much as one might first think.

Question: How large were Mars' ocean tides and to what extent could they keep the oceans mixing and tend to keep salinity uniform? Can this at least be estimated, and has this been modeled?

Potentially related:

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    $\begingroup$ A brief search of Web of Knowledge finds Forward Modeling of the Phobos Tides and Applications to the First Martian Year of the InSight Mission (agupubs.onlinelibrary.wiley.com/doi/10.1029/2021EA001669), a paper from last year mostly about the internal structure of Mars that highlights the difficulty of distinguishing how much the moon Phobos contributes to Martian tides vs. the sun. I suspect estimates about an early Martian ocean would be complicated by not knowing what moons, if any, Mars had at the time; looks like there is still a debate about the history of Phobos and Deimos. $\endgroup$ Mar 14 at 17:07
  • $\begingroup$ @jeffronicus thanks for that! Currently "the Sun is the cause of the largest tides on Mars" and so an answer could simply address those, though it seems that this question will not have a tidy, conclusive answer. $\endgroup$
    – uhoh
    Mar 14 at 22:57
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    $\begingroup$ Not a direct answer to the question, but tidal mixing is estimated to provide "only" half of the 2 TW of energy required for ocean circulation on Earth, the rest being provided by wind and/or topography. So even in the total absence of tides on Mars, you could still have a decent amount of ocean mixing through other processes. $\endgroup$ Mar 15 at 14:13

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I will attempt a partial answer.

A search of Web of Knowledge finds that most of the papers addressing Martian tides either focus on atmospheric tides and their role in Martian weather or what current Martian surface tides can tell us about the interior structure of Mars.

One such paper from last year, Forward Modeling of the Phobos Tides and Applications to the First Martian Year of the InSight Mission, highlights the difficulty of distinguishing how much the moon Phobos contributes to Martian tides vs. the massive contribution of the sun. (Mars, of course, currently has two moons, Phobos and the much smaller Deimos, both much smaller than Earth's moon.)

Estimates about tidal mixing in an early Martian ocean would be complicated by a variety of uncertainties: When was there an ocean on Mars? How large was it? While the Sun's gravitational effects would have been constant, I don't think we yet know if Mars always had a moon or moons and how large were they at the time surface water would have been present. There is still a debate about the history of Phobos and Deimos. (See: A giant impact: Solving the mystery of how Mars' moons formed)

As you note in your question and Jean-Marie Prival noted in a comment, there are other drivers of ocean mixing other than tides, such as the water cycle of evaporation and precipitation, centrifugal force from planetary spin (in the posited north polar ocean), and winds (though Mars' atmosphere is thin). (See Ancient Mars May Have Had Slow-Moving Monster Waves.)

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  • $\begingroup$ Thank you for your answer! While Jean-MariePrival's comment points that out, my only reference to Martian weather was that "there could be regional variations in surface water salinity since there is believed to have been rain", something that could produce areas of lower surface salinity than the planetary average. These might be lakes, or perhaps in areas of high rainfall and low salt levels in surface material small, lower salinity seas? Basically I'm trying to explore the extent to which some bodies of water will have lower than average salinity and the extent to which low tides contribute $\endgroup$
    – uhoh
    Mar 15 at 19:08
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It's hard to model tidal influence of such minor moons....on a small planet While Mars had water in its ancient past....how much is a substantial mystery. Solar tidal actions would be even weaker....

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  • $\begingroup$ What about tides due to the Sun's gravity? $\endgroup$
    – uhoh
    Mar 14 at 22:52
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    $\begingroup$ Gravity is proportional to mass, which Mars is vastly smaller. If Mars had "oceans" the waves might be higher because of water slower rate of fall. $\endgroup$
    – LazyReader
    Apr 4 at 3:09
  • $\begingroup$ 1) tidal acceleration due to the Sun does not depend on the planet's mass, and tidal motion of oceans is not the same thing as gravity waves. $\endgroup$
    – uhoh
    Apr 4 at 3:24

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