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Somewhat of a two-part question. Firstly I'm interested to know if it is expected that a planet/asteroid has water/ice on it, what kinds of scientific instruments you need to identify the presence and the actual location of the liquid water/ice.

I am aware that this is more a geoscience question than directly space exploration. But, I was hoping someone might be able to shed some light on the relative benefits of just an orbiter mission, vs landing/analysis on the surface, vs a sample return mission.

EDIT: The focus here is on subsurface water, and so by location I mean the depth it occurs at. Additionally, the asteroid I'm actually interested in is Ceres. I kept the question general but any specific knowledge would also be appreciated.

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    $\begingroup$ This is a related but different question; it's specific to the case of certain measurements on Mars, but you might find it and the answers interesting What are the technologies that can identify, validate, and quantify water-rich locations on Mars? $\endgroup$ – uhoh Jan 6 at 9:23
  • $\begingroup$ If the planet is so small that it has no atmosphere the presence of liquid water is impossible anyway. For solid ice the temperature should be so low that a loss by sublimation is very, very slow. Sublimation is the direct transition from solid ice to gaseous water vapor. $\endgroup$ – Uwe Jan 6 at 12:18
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The easiest way to tell if a planet has water on it is through spectral data. By splitting the light into its component wavelengths, you can identify the composition of an object's surface. This can be done fairly easily by Earth-based telescopes. However, this has two major downsides: it can only identify what's on the surface and it can only analyze the spectrum of wide swaths of planet.

In the most recent discovery of water on Mars, done by the Mars Express probe, the instrument which made the discovery was MARSIS, a low-frequency radar device. It used radar to penetrate through the surface and look for subsurface features. Liquid water is highly reflective in one wavelength the probe uses, so the underground lakes showed up as a bright spot.

Another option for detecting subsurface water is through doppler shifts in a probe on the surface, which allow you to see very precise rotation data. From the data, you can infer quite a bit about the planet's internal structure. NASA is using the InSight lander to do this on Mars, and there have been proposals for sending a dedicated probe to Titan, which usually include the same thing. This has the advantage of not requiring any extra hardware (you can listen in on doppler shifts in the regular communications); however, this will only work for subsurface oceans like the ones suspected to exist on Europa, Titan, Enceladus, etc., so it wouldn't have found the underground lake that Mars Express did.

The Curiosity rover has science equipment for collecting detailed information on the composition of various rocks. It can record all the same spectral data as we can from on Earth, except on specific rocks instead of the surface of the planet as a whole. It also has access to some other methods that can't be used from Earth, like an Alpha Particle Spectrometer (shoot it with alpha particles and see what happens) for collecting even more detailed information.

As far as a sample returns go, looking for water or ice in rocks returned to Earth functions the same as looking for them in situ with a rover, you just don't have to be quite as selective about what instruments you use to analyze the rocks, though you have to be a lot more selective about which rocks you analyze.

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    $\begingroup$ I'm not sure about Doppler shifts of InSight in case of Mars. Mars have water ice deposits, some of them can be huge - kilometer thick and hundreds kilometers across.. But nevertheless their mass is tiny compared with total mass of the planet. According to this artile theverge.com/2018/7/25/17606966/… - InSight can help with question can underground water on Mars be liquid (by measuring internal heat), but not the question of presence of water or amount of water. $\endgroup$ – Heopps Jan 6 at 9:56
  • $\begingroup$ @Heopps I believe I mentioned those limitations on Doppler shifts in my answer, so I'm not sure what you're trying to say. $\endgroup$ – Jarred Allen Jan 6 at 17:49
  • $\begingroup$ Would the analysis using Doppler shift be taken between the lander and the orbiter or the lander and earth? Ie does the lander need to have very strong comms systems to communicate with earth or can the Doppler shift be calculated by using the comms between it and the orbiter? $\endgroup$ – Capeboom Jan 7 at 11:56
  • $\begingroup$ In the case of InSight, it communicates with the Earth directly (see lpi.usra.edu/meetings/lpsc2012/pdf/1721.pdf). However, this serves as not just an experiment but also a back-up method of communicating with Earth in case the orbiters fail. I see no reason why you wouldn't be able to run this experiment relative to a craft in orbit, though you'd need very precise data on the orbiter's velocity, which may be harder than just giving the lander an antenna that communicate with Earth directly. $\endgroup$ – Jarred Allen Jan 7 at 19:13

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