Having read that Phobos at 27 km long is around 30% porous, I am going to assume that there are a number of voids large enough for human entrance. If it turned out there was a void large enough and close enough to the surface somewhere on Phobos, it could be turned into a cheap but spacious habitat during a Mars exploration.

  1. How could a probe detect and map voids? Are there techniques for this? Ground-penetrating radar?
  2. Would the benefits of a pre-existing cave make it worthwhile to convert it into a habitat, or is that just more work than building something on the surface?


  1. 100m thick Regolith on Phobos
  2. Seismic instability inside a cave, being as Phobos is a pile of rubble and Mars exerts tidal forces
  3. There may be ice below the regolith - a void next to some ice would be great
  4. Cosmic radiation shielding inside the cave
  5. Retaining an atmosphere inside a cave
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    $\begingroup$ I am now super-excited about Phobos probe missions. $\endgroup$ – Russell Borogove Nov 21 '14 at 17:28
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    $\begingroup$ I'm going to go with a swarm of space drones. Mostly because it's cool. $\endgroup$ – NPSF3000 Nov 22 '14 at 3:20
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    $\begingroup$ I believe voids in Phobos and other bodies will be mapped with autonomous 3-D wall mappers. An example of such a vehicle is Ted Stone's DepthX. See ted.com/talks/bill_stone_explores_the_earth_and_space The whole vid is informative and entertaining but he starts talking about DepthX at around 7:50. Depth X is for underwater exploration but I believe it could be adapted to move about in a vacuum. $\endgroup$ – HopDavid Nov 22 '14 at 20:52
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    $\begingroup$ A big problem for a habitat in Phobos, which is not helped by a cave, is that it is a microgravity environment at about 1/2000 of the acceleration on Earth. Exploration would be fine, but multi-month habitation would require digging down a rotating habitat. Maybe the habitat is better placed in Mars-Phobos Lagrange point 1, only 2.5 km above Phobos' surface, where it is well shielded from radiation by the two bodies. $\endgroup$ – LocalFluff Dec 21 '14 at 14:09
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    $\begingroup$ The Wikipedia article suggests that "This void space is mostly on small scales (millimeters to ~1-m), between individual grains and boulders." -- which doesn't necessarily rule out the possibility of larger gaps. Reference. $\endgroup$ – Keith Thompson Dec 22 '14 at 19:20

How could a probe detect and map voids? Are there techniques for this? Ground-penetrating radar?

Radar, either GPR (Ground-Penetrating Radar) or SIR (Subsurface Interface Radar) would seem the obvious solution. However, the shortcomings of those techniques (as pointed out in @FraserOfSmeg's answer) may mean that they are of limited use for finding deeper caves. A technique similar to CONSERT might be the solution. However, at the moment CONSERT doesn't have good enough resolution to accurately map caves, just give a good indication of where to look.

For really deep mapping of voids we might turn to seismic or sonic based techniques such as seismic tomography, reflection seismology, or direct sonic mapping. Several small “Philea-class” probes could be landed to listen to Phobos. They could either listen for natural seismic activity, or an orbiter could bombard Phobos with impactors, rather like Deep Impact.

There's a good article on void detection and mapping by a company called EnviroScan that outlines most of the techniques used on Earth:

Large volume voids such as significant washouts, mine workings, tunnels, and karst-related cavities are excellent targets for microgravity surveys. The “missing” mass of the void creates a measurable disturbance in the earth’s gravitational field, with the magnitude of the disturbance directly proportional to the volume of the void. This relationship has allowed Enviroscan to use microgravity surveys to assist engineers in the effective design and accurate cost estimation of grouting programs. Other examples of Enviroscan’s application of microgravity are mapping of undocumented mine workings, location of abandoned or clandestine tunnels, and delineation of areas of unsupported floor slab. Dramatic Microgravity Anomaly Over a Cave System Beneath a Factory Since gravity surveys are immune to many common sources of electronic or acoustic noise that can impede many geophysical techniques, they are particularly suited to highly developed or industrial sites.

enter image description here

Smaller shallow voids can be detected using ground penetrating radar (GPR) or subsurface interface radar (SIR). Deeper voids are sometimes best detected and measured using seismic reflection. Deep, water-filled cavities (e.g. caves or tunnels below the below the water table) are particularly good targets for seismic shear wave imaging since water (or any fluid) cannot transmit shear waves - causing water-filled voids to appear as characteristic “blank spots” or shadows on a shear wave record.

Where boreholes are available, Enviroscan can perform crosshole seismic or electrical tomography. Beneath water, sub-bottom sonar, as well as gravity and sometimes radar can be used to detect voids such as tunnels or bridge pier scour features.

Would the benefits of a pre-existing cave make it worthwhile to convert it into a habitat, or is that just more work than building something on the surface?

Much harder to answer. But we can speculate on a few points;

The seismic instability, depending on severity, could render the direct conversion of a cave impossible, but that doesn't mean the cave couldn't still be useful. NORAD is constructed on giant springs to survive the shock of a nuclear explosion, and a similar technique could be applied to a habitat in a Phobian cave to help it survive Phobos' quakes.

enter image description here

This would help reduce the risk of seismic activity rupturing the habitat, but unless the seismic nature of Phobos becomes very well understood it would probably still be considered too dangerous.

If seismic activity isn't an issue the advantages to converting a cave are the same as anywhere else; cheap habitable space and protection from radiation. Depending on how deep the cave is it could provide ample shielding from solar radiation.

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    $\begingroup$ The seismic activity is diminished by the microgravity environment, isn't it? If you as an astronaut is stuck in a cliff on Phobos which is squeezing together by its intrinsic seismic activity, you could simply push it aside again with your muscle powered arms. A bit like Moses. Every ton of rock is like 500 grams. You just need to eat your spinach to overcome earthquakes there, simply jumping 50 meters high to box off the slowly tumbling rocks falling down on you. $\endgroup$ – LocalFluff Dec 22 '14 at 12:03
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    $\begingroup$ @LocalFluff You also have to consider that the main cause of the seismic activity would be tidal forces from Mars, so I'm not sure that just looking at local gravity would be enough to estimate how violent seismic activity would be. Moreover, the seismic activity only has to open a small crack in the habitat for everyone inside to be in serious trouble. $\endgroup$ – ForgeMonkey Dec 22 '14 at 14:56
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    $\begingroup$ Moving Phobosian mountains would just be a funny entertainment for your kids to play with as you let them out on a holiday afternoon to play in the cracks and surfaces of Phobos. Just look what they do if you give them a ski/sleigh on Earth at winter! Any kid is powerful enough to move any mountain on Phobos, just for the fun of it. That'll be the everyday life of a habitat on/in Phobos. Your depiction of heavy equipment works on Earth, but is not needed in microgravity Phobos. $\endgroup$ – LocalFluff Dec 22 '14 at 15:25
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    $\begingroup$ Ummmm, those mountains are going to have the same mass, regardless of their reduced weight. $\endgroup$ – Jerard Puckett Dec 23 '14 at 18:14
  • $\begingroup$ If there's any seismic activity strong enough to move mountain-sized chunks of rock, you're not going to be able to do anything by pushing back. The question is, in the microgravity environment of Phobos, is there any such activity? Tides might not be much of an issue, given that Phobos is in locked rotation in a nearly circular orbit. $\endgroup$ – Keith Thompson Jan 20 '15 at 18:29

I'll go with a longshot here, please bear in mind this is pure speculation and conjecture.

As @HopDavid said, probably 3D wall mappers could be a good choice, so I'd say a swarm of autonomous probes using some technology like Google's Project Tango could prove useful for doing this. Supposing they have been appropriately fitted, rugged, and protected from radiation.

-Edit: Funny I just came across this Google Tango and NASA collaboration on Spheres w/Tango:


Small voids could be detected using GPR (ground penetrating radar) or SIR (subsurface interface radar).

Big voids on Earth can be detected non-intrusively by micro-gravity surveying equipment, although I am not sure if this technique could be "fine-tuned" to work with Phobos' gravity.


I'm going to attempt an answer here; some of this is conjecture!

How could a probe detect and map voids? Are there techniques for this? Ground-penetrating radar?

From what I've read on ground-penetrating radar it's unlikely to be a great help. The depth of GPR is dependent on a lot of factors, but according to Wikipedia the chances of getting to more than 15 meters or so are slim, if the material is dry. However, since we're looking for potentially large openings, we don't really care too much about having a great resolution, so if we were to use an ultra low frequency we might have a chance.

An idea that struck me when thinking of this answer was the application of a 'swarm' of small orbiters. They could transmit signals into Phobos, and if that signal happens upon a a pore at the right interface angle, then the signal will be reflected! If you have enough of satellites in your swam, you could pick up this reflected signal and get the depth of the pore based on travel time of the signal. For reference:

Reflection Example

So you can see if the angle theta is greater than the critical angle then we could receive reflected signal.

Would the benefits of a pre-existing cave make it worthwhile to convert it into a habitat, or is that just more work than building something on the surface?

This question depends on a lot of other factors. To name a few:

  • How frequent and of what magnitude is the seismic activity
  • How much science we want done on Phobos itself
  • The cost, this seems like the kind of idea that would save money in the long run - the very long run
  • Technology when this idea is brought to funding stage

There's a lot that can be said about building under the surface of a moon. But personally I'd prefer we avoid digging up too much on Phobos. :)


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