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I am currently reading the Cambridge Guide to the Solar System. In chapter 8, section 8.7, they explain that massive torrents of water created outflow channels on the surface of Mars in the past. The book goes on to state that these torrents would have been unstoppable and

Such discharges would not freeze even under present conditions on Mars.

Yet the atmosphere of Mars is under the triple point of water, so why is this the case?

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  • $\begingroup$ Are you referring to today's climatic conditions or those theorized for the past when the waters are assumed to have flowed ? $\endgroup$ – user34599 Aug 3 at 10:48
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    $\begingroup$ I suspect what is meant is that they would not freeze until after they had created outflow channels. I imagine they would be turbulent enough to both keep themselves warm, and disrupt any surface ice formation, they would lose some mass to evaporation (possibly enough to raise the partial pressure locally). When they reached the end of their flow they would freeze or evaporate over a period of time. $\endgroup$ – Steve Linton Aug 3 at 12:49
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... so why is this the case?

It may in fact NOT be the case!

Interestingly Phys.org's new item Early Mars was covered in ice sheets, not flowing rivers: study just showed up. It begins:

A large number of the valley networks scarring Mars's surface were carved by water melting beneath glacial ice, not by free-flowing rivers as previously thought, according to new UBC research published today in Nature Geoscience. The findings effectively throw cold water on the dominant "warm and wet ancient Mars" hypothesis, which postulates that rivers, rainfall and oceans once existed on the red planet.

and says later:

Grau Galofre's theory also helps explain how the valleys would have formed 3.8 billion years ago on a planet that is further away from the sun than Earth, during a time when the sun was less intense. "Climate modelling predicts that Mars' ancient climate was much cooler during the time of valley network formation," says Grau Galofre, currently a SESE Exploration Post-doctoral Fellow at Arizona State University. "We tried to put everything together and bring up a hypothesis that hadn't really been considered: that channels and valleys networks can form under ice sheets, as part of the drainage system that forms naturally under an ice sheet when there's water accumulated at the base."

and links to the new paper in Nature Geoscience Valley formation on early Mars by subglacial and fluvial erosion

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    $\begingroup$ Cool paper. And it shows that everything concerning the interpretation of the Marsian features is in flow, if I may say so. Geoscience principle: Never judge an outcrop from afar ! ;-) $\endgroup$ – user34599 Aug 4 at 8:22
  • $\begingroup$ @a_donda do I detect the formation of a pun? :-) $\endgroup$ – uhoh Aug 4 at 8:27
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Current conditions on Mars are between 1mbar and 10mbar with an average around 6mbar atmospheric pressure and surface temperatures between ~70°C and +20°C depending on place and time-of-day and season. The triple point of water is around 0°C and 6mbar pressure.

Thus the triple point of water is within the limits of what you find on Mars today - not everywhere, but in many places.

Of course it would eventually freeze and / or evaporate. However that also means that a significant amount of water would not freeze or evaporate immediately (there are rivers in Alaska and Siberia, too) should it suddenly emerge - but would have time to follow the gravity gradient and cause some erosion along its path.

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    $\begingroup$ I think it's important to add/note that just the water on the surface of any body would be at the triple point, all the water below it would be at a higher temperature and pressure. Thus, the surface water would sublimate or freeze, while the water immediately beneath it would be liquid and continue to flow / break through (think of a lava flow where the top freezes out but lava keeps breaking through below to flow). So you can still get large run-outs even today, IF such a large amount of water blows through. $\endgroup$ – Stuart Robbins Aug 3 at 16:25

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