[aksargige4 (OP)]

I'm writing a sci-fi that takes place on another planet. I know the composition would vary based on the rock composition of the planet, but, in general, are oceans inherently salty? I know the salt comes from the freshwater dissolving rocks on its way to the ocean and that landlocked seas can be excessively salty and landlocked lakes will be harder and saltier than other bodies of freshwater, but are oceans inherently salty or is there something unique about Earth's geology that makes it so here?

[Kryptid]

I think most oceans on most worlds would be more or less salty for the reasons you've described. Super-Earths or mini-Neptunes with a planetary ocean so deep that there is ice V on the ocean floor (a dense form of ice caused by water being under excessive pressure) would probably be much less salty because of just how many water there is.

[evan_au]

If there were a hot core to the planet, then water percolating down would pick up salts, and inject them into the sea water, like the "black smokers" at Earth's mid-ocean ridges.

Scientists are wondering whether such a mechanism is occurring on Europa, a moon of Jupiter.
See: https://en.wikipedia.org/wiki/Hydrothermal_vent
https://en.wikipedia.org/wiki/Europa_(moon)

To weather rocks rapidly, you need them continually exposed to some liquid which can erode them.
- In Earth's early history, the Moon was much closer, and would have produced enormous tides - the Moon's tidal pull would have been 1,000 times stronger than today. That would have produced massive erosion.
- 10,000-20,000 years ago, massive glaciers ground down rocks, during the ice age.
- Today, we have flowing rivers that erode rocks.
- Rivers and glaciers require the right temperature and pressure to evaporate water from an ocean, and then deposit it high up on mountains, as ice or water. This then flows downhill, causing erosion (including dissolving salts), which end up in the sea.
- The liquid on another planet does not have to be water - but water is very good at dissolving ionic substances like salts.
- For example, the hydrocarbon lakes on Titan would not dissolve salts.
See: https://en.wikipedia.org/wiki/Titan_(moon)

[Monox D. I-Fly]

Wait, wait, wait... Are you guys saying that salts are originally from the land in the rocks? I never knew that... I thought salt was exclusively sea product...

[Kryptid]

Are you guys saying that salts are originally from the land in the rocks?

Yep. You can still mine salt from the land in the form of halite.

[evan_au]

You can still mine salt from the land in the form of halite.

Halite is classed as an evaporite; it forms when an ocean or lake dries up, leaving a fairly pure layer of salts behind.

Now we have a circular argument: salt in the sea comes from the rock, which comes from the sea....

Are you guys saying that salts are originally from the land in the rocks?

Yes.
Apart from evaporites, salt is dispersed in very low concentrations in all rocks.
- fresh rainwater flowing over the rocks slowly erodes them, dissolving the salt.
- If the river doesn't run through a bed of halite, the salt concentration in the river is quite low - we would still call it fresh drinking water
- If the river flows into a dead-end lake, the water evaporates, producing a very saline lake
- Or the lake may evaporate entirely, leaving a layer of halite
- A good example is the Dead Sea

..and we are talking about a wide range of salts beyond the familiar Sodium Chloride.

See: https://en.wikipedia.org/wiki/Halite
https://en.wikipedia.org/wiki/Dead_Sea

[Kryptid]

Halite is classed as an evaporite; it forms when an ocean or lake dries up, leaving a fairly pure layer of salts behind.

Ah, I didn't know that. I thought that halite could be mined underground like other minerals.

[evan_au]

I thought that halite could be mined underground like other minerals.

It can. The dried up lakebed can be buried by other sediments, forming an underground salt deposit. This is where "rock salt" is mined from.

 Salt has a lower density compared than silicate rocks. With pressure from the overlying rocks, these flat salt deposits can migrate upwards and be deformed into a dome shape. This is very important in petroleum exploration, as oil and organic molecules don't dissolve salt, and so these salt domes can form traps for oil and natural gas.
https://en.wikipedia.org/wiki/Salt_dome

salts are originally from the land in the rocks?

If you want to go way back, all the elements of the Earth started as interstellar dust, blown into space by dying stars,
This formed tiny particles like candle soot which contained all the random atoms emitted by these stars, combining into a range of compounds that would be unstable on Earth. This includes the elements that make up salts, like sodium, magnesium, chlorine, etc.

This dust accumulated in the proto-Earth, which melted, with the denser elements sinking to the center (iron and nickel, and those elements which dissolve easily in molten iron). This is surrounded by less-dense silicon/oxygen compounds that make up rocks (and the elements which dissolve easily in silicates). Around this is even less-dense substances like water, nitrogen, methane, etc. This process is called differentiation.

The elements which form salts are too dense to appear in the atmosphere, and fit better into the rock layer.
Erosion and convection then spreads them around, blurring the pure density separation.

See: https://en.wikipedia.org/wiki/Planetary_differentiation#Gravitational_separation
 

[chiralSPO]

It is a safe bet that the oceans would be salty (assuming the oceans are water*, and on a rocky planet). Because physics and chemistry appear to be essentially the same everywhere in the universe, it is likely (certainly plausible) that Earth-like planets elsewhere will be Earth-like in most ways.

Metals that form cations of water-soluble salts are quite common in the universe (sodium, magnesium, calcium, and potassium together make up about 10% of the atoms in the universe that aren't hydrogen or helium). Chloride and sulfate anions are also going to be very common because of the abundance of oxygen, sulfur, and chlorine.
http://www.astronoo.com/en/articles/abundance-of-the-elements.html

Other metals may also form significant fractions of soluble salts, depending on other factors (iron in the +2 oxidation state is very soluble, while in the +3 state is much less so. Before photosynthetic organisms created all of the oxygen we now depend on, the seas were full of Fe²⁺ salts, which then were oxidized and precipitated out, forming a heavy rusty band in our geological history: https://en.wikipedia.org/wiki/Great_Oxidation_Event https://en.wikipedia.org/wiki/Banded_iron_formation)

*There are likely many planets out there that are too cold to have liquid water, but instead have oceans/lakes/rivers of ammonia or methane/ethane instead, as is the case with Saturn's moon, Titan: https://solarsystem.nasa.gov/moons/saturn-moons/titan/overview/
https://solarsystem.nasa.gov/moons/saturn-moons/titan/in-depth/#surface_otp

Good luck with the sci-fi!

[nicephotog]

Because nothing has ever been to a comparitively earthlike planet, here is a comparison that adds life to compare and some metal to salts existence.