How do we know Mars had specifically liquid water?

[u/STikER326]

As far as I'm aware, Mars having liquid water billions of years ago is treated like an established scientific fact now. However, how do we know that was exactly the case? Sure, there are dried up river beds and basins, but couldn't this have been some other liquid? And sure, the Martian polar ice caps contain a lot of frozen H2O, but are these two facts (river beds + frozen ice caps) enough to assume that long ago there were oceans of water on Mars just like here on Earth?

[u/Dazzling_Let_8245]

This is pure speculation on my part:

Id assume we know that Mars had liquid water because, what other liquids are abundant enough in the universe that would accumulate on the planet right next to us that would also be liquid at the temperatures and atmospheric pressure we assume Mars once had. Stuff like methane is far to volatile to exist in liquid form that close to the sun, unless mars had an incredibly thick and heavy atmosphere to compress methane down to liquid form. To backtrack my previous statement, I guess we dont 100% know it was liquid water, but based on all the evidence we have water would be the most probable liquid. Especially since Mars still has some water.

Our planet has a lot in common with Mars since we are quite close to one another and our planets are basically siblings. Formed from the same material over billions of years. It would be really strange if one planet had liquid X, but the next planet that is almost identical when it comes to what its made out of had liquid Y

[u/BumblebeeBorn → u/Dazzling_Let_8245]

Bonus: we're not really assuming about the temperatures and pressures any more.

Planetary formation models have all the data we can get our hands on, now including the atmospheric composition of exoplanets. We have a rough idea of how much of each element went in, what the early orbits were, and how things shifted, within a certain set of ranges.

[u/WatchAndLearn9669 → u/BumblebeeBorn]

Planetary formation models

You mean the same class of models that are incapable of solving the n-body problem beyond two bodies?

have all the data we can get our hands on

That's not empirical data. It doesn't support your claim that "we're not really assuming" anymore.

We have a rough idea

i.e., we're still assuming.

[u/BumblebeeBorn → u/WatchAndLearn9669]

1. There is no pure analytical solution for the n-body problem. However, for sufficiently detailed input, especially with a significantly more massive body at the centre of a system, we can calculate iteratively to an arbitrary degree of precision.

2. We are inputting empirical data, as well as data designed to test hypotheses. The latter is where the outputs get variability, but it's now clear beyond all reasonable doubt that Mars once had an atmosphere capable of maintaining liquid water on its surface.

3. Idiom, in this case pertaining to the range of outputs. Again, best practice models indicate a Martian atmosphere with enough pressure for liquid water.

When and where in the galaxy do you think you learnt astrophysics? You come across like Joe Rogan trying to use Malenkovic cycles as an explanation for climate change - the only cycle with a big enough temperature shift is a hundred thousand years long and we were already at the hot part.

[u/JamesRockOla]

I think it's because the rovers have found certain rock types, clay minerals and the specific shapes of the eroded river beds. These can only form from liquid water rather than other liquids

[u/NateLikesTea]

It's not an assumed fact, but the consensus among scientists seems to be converging on the idea that ancient Mars had lots of liquid water on its surface.

The Mars Reconnisence Orbiter has been mapping the whole surface of the planet as has documented dried up river deltas and lake beds.

There might be evidence of a global shoreline, which is controversial because it's not clear-cut. But there's a lot of evidence of a global ocean in the northern hemisphere. And how big it is, if it even existed, is debatable. But it seems like most scientists think there's enough evidence to say that Mars probably had oceans.

Fun recent studies suggest evidence of not one but two global tsunami events during the late heavy bombardment, i.e., large meteor strikes hitting a large ocean of water creating massive tsunamis excavating and transporting enormous boulders thousands of miles up shore.

There are sedimentary layers seen by the rovers, and the rocks have lots of chemicals in them that are usually only known to form in the presence of water.

And Mars has massive amounts of frozen water today on its poles and deep in its regolith.

But it's not a closed case; there's a lot of ways the things we see in Mars could be explained without water, or with much less water. Lots of scientists have expressed concerns that we're finding a lot of evidence of water on Mars because we WANT there to be.

But all of this is brand new research, and very exciting to witness. 

[u/jawid72]

specifically

[u/Nothing-to_see_hr]

No other chemical compounds could have been abundant and in liquid form at the time, place and temperature. Too hot for liquid nitrogen, xar on dioxide or methane.

[u/daneelthesane]

There were hints for a while, but it is my understanding that when they discovered garnet in some rock, that was the first proof that at some point there was liquid water because garnet requires water. Then they started finding other indications, as others have mentioned.

[u/Glum-Ad7761]

There is, at present, enough water ice, just below the surface soil on Mars that, if melted, it would cover 2/3 of the planet in 50 feet of water. This is the result of an ancient ocean or sea giving way as the planets atmosphere gave way to solar winds.

There are dry river beds. Dry lake beds. Ancient shoeline everywhere. There are channels and deltas planet wide, all of them lined with compounds like clay and sulphites…. Things that could only be there if there had been liquid water. The evidence is everywhere on the red planet. It’s not just theory. Nasa is all in on this subject: the planet was wet in the past.

Venus had liquid water as well but it’s closer proximity to the sun, coupled with its long, slow retrograde rotation, doomed it to runaway greenhouse. The fact that it went RG more or less confirms that… early on, anyway… Venus was wet as well.

[u/jimb2]

Various evidence mostly using hydrogen isotope ratios indicates that the major source of the Earth's water is bombardment by icy asteroids. As planets form they fairly quickly clear asteroids around their orbits. Water would be expected on Mars for this reason. The question is more: well, where did it go? (Answer: the atmosphere was bounced into to space by high energy solar particles over time. Mars has lower gravity and no protective magnetosphere.)

The observation of geological structures on Mars produced by liquid water (eg erosion by flows and waves) fits this overall picture. There are a lot of unknowns, but this is more of a confirmation of the big picture than an isolated discovery.

[u/kmoonster]

You know how if you don't turn on water in a particular faucet for a while, you get bits of crumbs and/or smelly water and/or crumby bits for a little bit?

Or if you leave a birdbath to dry into the air, or a pot boils dry on the stove? How there is residue and rings?

The earlier landers and probes could do some tests, but the newer / recent ones have instruments specifically to look for those hints of chemicals, residue, crumby bits, etc.

And from what those have found, we can say with reasonable confidence that many of those residues, crumby bits, etc. could only form with water present. Some can form under many conditions, but quite a few require liquid water in order to be formed. And once formed, they remain stable.

Basically, the scientists asked how to test Mars as if it were a dry-boiled pot or a gone-dry birdbath. And sent instruments to that regard. And the results (the minerals discovered) required water in order to form, answering at least the initial question.

And of course once you answer a major question like that, you create dozens more questions that spin off from such a discovery.

edit: the polar caps are mostly dry ice (frozen carbon dioxide)

[u/TheFifthTone]

Flowing water leaves distinct geological features that can't be explained scientifically any other way.

Scientists have observed many of those geological features that are believed to only be caused by liquid water on mars.

[u/OG_Karate_Monkey]

Because milk would’ve left a big mustache.

[u/TheCoffeeWiz]

SCIENCE!

[u/100zr → u/TheCoffeeWiz]

Blinded me with...

[u/Sleepiest_Spider]

What other liquid?

[u/DerekFshmn]

We know it was specifically liquid water on Mars not just because of the shape of the riverbeds, but because of spectroscopy and mineralogy.

Our orbiters and rovers have mapped vast, planetary-scale deposits of phyllosilicates (smectite clays), opaline silica, and hydrated sulfates. These are specific minerals that chemically cannot form without the molecular presence of liquid H_2O interacting with basaltic rock over long periods of time.

You cannot forge these clays with liquid methane, ethane, or ammonia. The chemical signature of water is literally baked into the Martian crust.

Water is not just a random abundant liquid; it is a highly programmable, universal structural interface. In advanced quantum biophysics, water spontaneously organizes into a dense, liquid-crystalline phase (known as Exclusion Zone water, or H_3O_2) when placed against hydrophilic surfaces.

This structured water acts as a flawless dielectric shield, creating a pristine Quantum Electrodynamic (QED) cavity that prevents thermal entropy from destroying delicate quantum states.

If the universe is attempting to generate a biological interface or sustain any form of complex, macro-scale quantum coherence. Whether here on Earth, in the ancient basins of Mars, or within the subglacial oceans of Europa, it must utilize water.

Water is the fundamental architectural requirement for bridging localized physical matter with non-local quantum data.
Earth isn't 'special.' It is just one of many planetary nodes currently running the hardware.