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Given the physical characteristics of Mars (the crust's thickness, its composition, gravity, etc.) how deep a trench (that could be expected to continue to exist indefinitely, or rather, for periods of time measurable in the lifespan of continents on earth) could theoretically be dug?

Assuming that the sides of such a "trench" where optimally angled for the durability of the feature.

Mars planet's crust is about 50 km

Previous question on depth for a specific atmospheric density

Edit: Comments in other questions suggest a possible hard limit of something like 30 km due to the weight of rock on that below.

So what you're saying is that at a depth of over 30 km (ballpark figure of course) the rock at the bottom of our "trench" will begin to flow & well up into the bottom of the trench until it's only 30 km deep again

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  • $\begingroup$ Good question. Just be aware that it could be years before the answer is known. But don't despair, the HP3 probe on InSight may help us find an answer! $\endgroup$
    – DrSheldon
    Dec 4, 2018 at 1:30

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What I should have been looking at is the Geoid of Mars, more properly called the Areoid on Mars, & the depth from it to the mantle

The thickness of the crust varies between 50 km & 22 km due to the Geography & features (rock layers) above the Areoid while the distance from it to the mantle should be reasonably uniform.

I presume the putative Geoid of Mars is the altitude used for measuring atmospheric density.

According to some sources the crust of Mars is 10 km thick at its thinnest points which is presumably to be found in those places furthest below the Areoid like the Hellas Planitia . which means the mantle is around 17 km below the Areoid & may mean we can't go much deeper than 7 km without lava flows.

So the crust beneath the Hellas Planitia is probably only 10 km thick.

I found this KSP compatible Mars Heightmap

enter image description here

& this map of the topography of Mars (useful as you can zoom in & out on features)

The Interactive Mars map at the bottom of WikipediA's 'Atmosphere of Mars' page is also useful as it names features you hover the cursor on & links you directly to their page if you click on them.

I find the fact there appear to be no features with a bottom closer than 10 km from the mantle suggestive that this may be the hard limit .. so, anywhere the crust is 50 km thick we should be able to dig a hole as much as 40 km deep if we want (well, 30 km, anything deeper will fill back to 30km due to the rocks flowing under the weight of rock above).

It also means we probably can't achieve a depth below the standard atmospheric pressure of Mars of much more than a few kilometers beyond 7 km without being in danger of getting magma .. the 0.168 PSI of Hellas Planitia (air pressure at the summit of Mount Everest (the highest point on Earth) is 4.89 PSI) is probably not that far from as good as we can get on Mars by digging holes .. not what I hoped.

The peak (presumably a post impact volcano) at the centre of at least one crater close to this depth (Lyot) seems to support the idea a point on the crust thinner than 10 km is likely to pop.

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    $\begingroup$ I don't like answering my own questions but it's been 12 months & I just dug up some stuff on one of my many journeys down the rabbit hole which I think may provide an answer .. so here it is. $\endgroup$
    – Pelinore
    Nov 29, 2019 at 2:21
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    $\begingroup$ There is no shame in performing research and then taking the time to organize it, with sources, and share it with others. There's usually pride ;). $\endgroup$
    – Magic Octopus Urn
    Nov 29, 2019 at 3:02
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    $\begingroup$ You do make a few unsourced assumptions though- but I cant refute them! $\endgroup$
    – Magic Octopus Urn
    Nov 29, 2019 at 3:04
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    $\begingroup$ @MagicOctopusUrn : By my count two unsourced assumptions (the mantle is relatively uniform & 0 on the topographical maps, Mars Geoid & the elevation used to measure average atmospheric pressure are all the same thing) plus two extrapolations derived from them (the mantle is 17 km below the Geoid & the crust will likely pop (or at least leak) if much thinner than 10 km) .. I think the assumptions are sound .. but I'd be more than happy if someone refuted them & (more to the point) my conclusion, it's not the answer I wanted ;) $\endgroup$
    – Pelinore
    Nov 29, 2019 at 3:59
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    $\begingroup$ I wonder if this can also be applied to How were Mars' zero elevation datum defined? What are their shapes? $\endgroup$
    – uhoh
    Nov 29, 2019 at 5:15

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