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This is part one of a two part question. Part two can be found her (How to Cause a Hydrosphere Apocalypse Part II The Great Flood)

Desert planets are a staple of science fiction. And while we have worlds like Luna, Mars and Mercury to show that barren planets are decently common, one fascinating scenario I keep running into is the idea that once these dust balls were living breathing worlds until something happened long ago.

So imagine for one moment that intrepid human explorers found a semi arid life bearing world. Surface water is only at 12% concentrated in a single sea the size of the Mediterranean, fresh water lakes and hyper saline lakes surrounded by expansive salt-flats. The rest of the surface is taken up by sandy deserts, grassland like steppes, and trees only exist around fresh water oasis.

But everywhere the humans set up camp, everywhere the probes investigate, there is evidence of a completely different world. Fossils of alien fish and other aquatic creatures, the salt flats that were once sea beds, water carved landscapes where water hasn’t flowed in millions of years, and steep drop offs that were obviously once continental selfs.

All the evidence seems to be pointing to a single conclusion. Millions of years ago, there was a massive extinction level event in which somehow this once verdant planet lost most of its oceans and this dry arid world is still recovering from it.

So now everyone from the scientists on the expedition to the populace and politicians back home are all asking the same question.

What the hell happened to this planet?

What scientifically feasible scenario could happen that would cause a life supporting planet to dry up and loose it’s oceans?

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    $\begingroup$ Do you insist on an "event", or would you be satisfied with a gradual process taking a billion years or so? $\endgroup$
    – AlexP
    Feb 9, 2021 at 18:41
  • $\begingroup$ The time spans are critical. How long did the event take and how long ago was that? $\endgroup$
    – Slarty
    Feb 10, 2021 at 8:50
  • $\begingroup$ @Slarty Well it happened long enough ago that all the species on the planet is just used to be drier so let’s say 50 million years. As for how long the event took well let’s say it was somewhere between 1 years and 1 million years, so it was geologically quick. $\endgroup$
    – Jacob Badger
    Feb 10, 2021 at 9:45
  • $\begingroup$ Quick reference: Mars. It used to have a couple hundred meters of ocean, then it got Whacked, and then it misplaced its magnetic field and the few remaining dregs were blown away $\endgroup$
    – PcMan
    Feb 13, 2021 at 11:49

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Having a magnetic field ensures that the planet atmosphere is protected against the ionized particles of the stellar wind.

Those particles, by colliding with the atoms in the high atmosphere, can accelerate them until escape velocity. This causes, over astronomical times, a depletion of the atmosphere.

This is what is supposed to have happened on Mars, which happens to be a desertic planet.

Your explorers have found a planet in the process of being aresformed. Water is most likely to be depleted first because once its molecule is split by energetic photons, the hydrogen is pretty hard to keep inside the gravitational well of an rocky planet.

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  • $\begingroup$ The question asks for an "event". This answer describes very long process. (And I would wager a modest sum that Mars lost its air first and only afterwards its water.) $\endgroup$
    – AlexP
    Feb 9, 2021 at 18:43
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    $\begingroup$ @AlexP, the formation of a star is an "event". The formation of a planetary system is an "event". OP didn't ask for an instantaneous event. $\endgroup$
    – L.Dutch ♦
    Feb 9, 2021 at 18:48
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Water moves down. Hydrocarbons move up.

Deep inside this planet were huge deposits of abiogenic petrochemicals - from asphaltenes and long chain alkanes all the way to methane. Maybe they accumulated there during the formation of the planet.
Then, like an ice dam breaking and releasing a lake, an energetic event breached the impermeable geologic layer separating this deposit from the overlying ocean. And they switched places. This took several years; a blink of the eye in geologic time.

The hydrocarbons moved up and water moved down, occupying the deep spaces where the hydrocarbons had been. Your explorers might find this lake of tar on the far side of the planet. Probably they can smell it wherever they are.

The sudden drying of this ocean decreased the albedo of the planet, causing it to absorb more sunlight. More significantly the methane and volatile alkanes released into the atmosphere act as a greenhouse gas, heating the planet further. Surface waters everywhere began to dry up.

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Flares. The world might be in orbit around a red dwarf, which has a reputation for these, but you could suppose another star with a bad habit of flaring. (Maybe some interesting astrophysical event, possibly catastrophic or artificial in nature, caused a change in the star's activity for a short period of geological time?) Low magnetic field per L.Dutch would certainly combine with this. Together, they drive off the hydrogen of the water only, but the oxygen is very reactive and, as at Mars, will likely combine with some other mineral (such as iron)

Prevailing opinion is that this sort of apocalypse is very common, and usually people more fear it might affect some of our more beloved sci-fi stories waiting to happen like TRAPPIST-1. Nobody will bat an eye if you say this happened.

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