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This question discusses what you'd look at from space, but mostly focuses on how you could identify the weather.

This question discusses what resources a colony would need, but doesn't discuss how to find them.

We use absorption spectroscopy to identify liquid water on foreign planets, but how can we tell whether the water is usable? Although it is possible to remove most contaminants (salts, ammonia / nitrates, metals, etc.), a population of almost any size will prefer to have a fresh water source for drinking, agriculture, etc.

On Earth, bright green color indicates significant plant life, which tends to go along with animal life and potential for gathering or growing food. However, green sand deserts, producers that are other colors, etc. could throw this off.

What techniques could be used to identify large amounts of biomass from orbit?

Is there any way to identify metals or metal ores near the surface?

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This question asks for hard science. All answers to this question should be backed up by equations, empirical evidence, scientific papers, other citations, etc. Answers that do not satisfy this requirement might be removed. See the tag description for more information.

  • $\begingroup$ Your question body is completely different from your question title. Edit your question to reflect what you want - is it ores that you want to identify from orbit, or habitable locations? $\endgroup$ – Aify Jul 14 '16 at 4:39
  • $\begingroup$ Ah, good point. What is a good word for places that can are not merely survivable, but that humans can do well? $\endgroup$ – Jon Takagi Jul 14 '16 at 4:41
  • $\begingroup$ otherwise unusable due to contamination - pee is usable water or not usable due to contamination? Just try to clarify, what unusable means in this Q. $\endgroup$ – MolbOrg Jul 14 '16 at 6:41
  • $\begingroup$ The reason I focused on the weather in the first related question is because everything interacts with everything. So you plant life will be dependant on your annual weather patterns. So as someone pointed out, you may scan and find green plants using IR, land and start building your settlement but then the seasons change and you end up in a desert for 6 months of the year. The terrain influences the climate but also influences the plantlife. The plantlife in turn can influence the climate Eg cloud forests, amazon forest, warm deserts. $\endgroup$ – EveryBitHelps Jul 14 '16 at 10:46
  • $\begingroup$ My edit took more than 5 minutes :( I believe in order to make a proper decision on where to place a settlement you need to know the entire annual cycle. Or at least the broad outline. $\endgroup$ – EveryBitHelps Jul 14 '16 at 10:53
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I’m going to attempt to answer all three of your questions (this is a long answer ~2500 words, so be prepared). Both Colony and Emergency Scenarios.

(1) Basic overview of Remote Sensing techniques (2) You found your PlanetZ, now what! How to find a suitable location for a settlement taking into account biomass and mining options from orbit? (I assume you mean mining as you mentioned locating metals and metal ores).


REMOTE SENSING. It’s a real existing thing. Here is a very basic overview of the very basics.

One of many possible links

We have been measuring our planet from orbit from about the 1970’s. We have about 50 years of experience and we still have a lot of trouble with it. Equipment is expensive, and as soon as you get it upstairs it is most likely out-of-date. We end up having to rely on out-of-date tech that is in need of constant repair and if we had our way, replacement. Add to that, the atmosphere keeps reflecting, scattering and absorbing most of our satellite signals making it hard to focus on the ‘smaller’ planet surface area.

Remote sensing applications use the interaction of the atmosphere and earth systems (in your case planetZ systems) with the electromagnetic (EM) spectrum. Our satellites measure the EM responses in the microwave, infra-red, visible light, and UV spectrum. E.g. Snow scatters visible light, water vapour absorbs Infra-red and hail scatters microwave radiation and we have that pesky ozone hole letting in all the cancerous UV.

Our satellites have passive sensors, which measures using the EM from solar radiation, and we have active sensors, which sends an EM signal of its own and measures the response.

Data received is also reliant on what orbit your satellite is in. We have many satellites in orbit around the equator that are great at capturing images for large areas at an almost constant and regular rate but because they are quite far from the planet the resolution scale is quite large and we can’t see much below 1km. If you have your satellite orbit over the poles, you will get a much finer resolution image, but the temporal coverage is not great. Each location will only get a measurement once a day or even every 3-7 days! We use a combination of several orbits, and sensors.

Just some of the many satellites upstairs. satellites

Plants use incoming radiation to photosynthesis and also reflect the radiation back. We use satellites to measure the reflected light wavelength from plants in very narrow wavelength ‘windows’. While green plants do reflect slightly differently than other pigmented plants, all plants reflect differently to other features. These other features, the ground, rocks, metals, sand, freshly-tilled earth, artificial surfaces, buildings, animals are all reflecting light back into space, as well as the water bodies, atmosphere chemicals, aerosols and atmospheric moisture. So your dry green desert sand would have a different reflection wavelength than your ‘green’ plants.

What does this mean? It means you first have to get through the atmosphere and any cloud cover before you can view the surface features. Different atmosphere compositions absorbs, reflects, scatters incoming and outgoing radiation in different ways. This is one of the (if not the only) reasons why we can tell the composition of far-distant atmospheres other than Earth but why we needed a rover to explore Mar’s surface. We use the reflected light from these planets that we receive from various satellites and compare it to known reflection patterns on Earth and make educated guesswork.

Temporal and seasonal changes need to be taken into account as well. Moist soil reflects differently than dry soil. Plants in dry soil reflect differently than plants in wet soils. Plants ‘breathe’ in Carbon Dioxide and ‘transpire’ Oxygen at different times of the day and night affecting the chemical composition of the nearby atmosphere and how it scatters/absorbs/reflects any light. Any Instruments in orbit would receive it all. It would have to be collaborated to remove the noise (the unwanted reflection signatures). That is why we currently rely so heavily on ground-truthing.

We have had to use ground-truthing over the last 50 years to figure out what each signal means, what is the best narrow wavelength ‘window’ and what is the best temporal ‘window’ to measure each object. For your planet colonisation, along with a massive multi-satellite project, your colonisation planning should involve a large amount of pre-arrival planetary exploration.

I would suggest a large portion of your planning funding be allocated to landing an armada of rovers full to the brim with equipment all over your planet. The rover option has a large unplanned failure potential. I think some manned missions to PlanetZ would be in order too. If this was an emergency situation, as in my related question, you wouldn’t have the ground-truthing data available to filter out the noise. You can possibly get this after you crash-land/evacuate your ship but that will be a bit late which is why I was focussed more on the larger atmospheric and oceanic features which my survivors can use to work out just what is in store for them and try and prepare.


I could go on and on about remote sensing but let me try focus on your title question, with some focus on your inbody questions - based on orbit only (no ground truthing).

Let’s assume you have found another goldilocks type PlanetZ with water, plants and animals (land is optional). Lucky you!

You will hopefully have more than enough time/just enough time to figure out the orbit of your planet, angle of inclination, distance to the sun/s etc. From this you will be able to tell the length of the days for any particular time of the year. From that you should be able to deduce your basic season characteristics (for ease I’m just using earthlike but it could be anything…). Colonisation Scenario could provide accurate temperature maps of the entire planet for the entire year (this can be measured by the amount of IR, near IR (NIR) and thermal IR (heat) each location reflects along with other sensors). Emergency scenario would be trickier. Based on current temperature readings you are able to determine the temperature at both the poles and equator and make deductions about possible temperature ranges of said seasons and latitudes.

From this initial detailed/basic planetary system survey combined with your temperature maps you will have your preferred ‘goldilocks’ latitude to search for a location of your colony/emergency settlements.

Next you look for water. You can’t have biomass without water (well, any known biomass. Maybe your PlanetZ has developed an entire plant and animal system that draws water straight from the atmosphere. Weird!).

If your planet is a water world, great! It should be fairly simple to spot the water!

If has got a reasonable amount of land, and you don’t want to live on the ocean, you have to determine where your world’s water actually flows from and to. You have 3 main areas to look at. High Atmosphere, near surface atmosphere, and surface water. The surface water will flow from higher mountains areas, through your midlands and out the lowlands into your oceans (with the odd stopover in a lake or glacier). I don’t think you could work out groundwater flow just from orbit but you might be able to calculate it based on surface features (if there is an oasis in the middle of a desert, it’s a sure sign of groundwater).

High up in the Atmosphere: Water Vapour reflects visible light in the 6.7 μm IR wavelength. The more water vapour in the upper atmosphere, the less surface can be seen. But that is not too bad, as you can track the direction your water vapour is flowing and determine upper atmosphere circulation patterns (wind can also be measured with satellite based doppler. Colonisation Scenario – detailed upper atmosphere patterns e.g. Jet streams, warm gulf stream, Emergency Scenario – snapshot. Deductions made off guesswork e.g. local weather report.

Low near the surface:

Now estimates of specific humidity are made directly from the satellite microwave sensors such as the Defense Meteorological Satellite Program (DMSP) Special Sensor Microwave Imager (SSM/I) observations and the NOAA Advanced Microwave Sounding Unit B (AMSU-B).

Colonisation Scenario – You get near surface humidity reading and possibly can track low lying clouds and windflow. Coupled with your high atmosphere patterns you can deduce your complete atmospheric circulation system. Emergency Scenario – snapshot. Deductions made off guesswork.

Multi-angle Imaging Spectro-Radiometer (MISR), which was launched on the NASA >Terra satellite in 1999, views clouds from nine different angles and locates >them in three dimensions. It scans at blue, green, red, and near-IR >wavelengths (0.446, 0.558, 0.672, 0.866 µm, respectively), has spatial >resolution of 275 m across a 360-km swath, and achieves global coverage about >once every 9 days at the equator.

If you can track cloud movements over a long time, an entire year or several years you can deduce cloud forms and wind circulation. Are those thunderheads you are tracking or just dreary old clouds that never let down the rain? Is that the Hadley, Ferrel, and Polar cells forming that air convection and torrential downpour? Colonisation Scenario – you can track long term cloud patterns and calculate where the majority of rain falls and any seasonal patterns like monsoons as well as climate regions which would influence your biomass. Emergency Scenario – I’m still waiting for more answers.

Hadley cell, climate, and environment

Surface Water: You have two features of interest here, Sea-level, which is the ‘same’ throughout the world (It is not even close but a range of ~20m is fairly small considering you measuring an entire planet) and surface water on land. Surfacewind patterns over waterbodies can be worked out by measuring surface wave height with Doppler, Lidar or Radar. Surface water is largely dependent on your topography. You can measure both sea-level and topography with various satellite based sensors using lidar, radar, etc. Once you have your topographic measurements you can create a detailed/basic Digital Elevation Model (DEM).

On your DEM you can work out what altitudes are out of bounds. Try not set up your colony in the high reaches of mountain ranges (these tend to be slightly cold and have less breathable oxygen content) or in the sunken depressions below sea level (these tend to be arid, salty regions with possibility of severe flooding). Unless of course this region has some feature of extreme interest and is why you came to this planet in the first place (such as mining) or you had no choice in the matter, and are just glad to be alive.

Surface water has its own spectral reflection signature. However be aware that sediment and organics will alter the range of the signals. E.g. you can see areas of upwelling in the ocean not just from the cold sea surface temperatures but also from the chlorophyll NIR reflection signature. You will be looking for an area to settle preferably close to a water source but not at risk of flooding. If there is flooding risk, a semi-predictable event calendar would be nice; like the annual Nile flood in Egypt before they built Aswan Dam. You can figure out where surface water flows by looking at your DEM and then focussing on any likely water signatures. If it is particularly detailed you would be able to see erosion features, like V shaped valleys (U shaped are Glaciers), gully’s, etc etc. If it is a basic DEM, you can use rough guestimates on valley shapes, and water flowing from highland to lowlands.

Here is where your biomass comes into play.

What techniques could be used to identify large amounts of biomass from orbit?

Read this journal paper. It’s old, but it explains measuring plants reflection signatures in a lot more detail.

Currently we measure this in the visible and Near Infra-red light spectrums. We measure very narrow wavelength ‘windows’ and analyse the spectral signature of the biomass in question – normally coupled with groundtruthing data and inaccurate algorithms. These narrow ‘windows’ remove noise from neighbouring features and the atmosphere.

A Colonisation project would have a lot of planning time so you would have ‘enough’ time to figure out the atmospheric content and reflective, absorption and scattering properties. You could then try filter this out and focus on the ground. An emergency situation would have no forward panning time, so the best choice is to look at your spaceship viewscreen. Where you see green, dive dive dive! You can position your equipment (if you have time) to focus on a particular area that appears feasible from space. Focus in on the ‘green plantlife’ wavelength window rather than the ‘green colour’ wavelength window to pick a particularly vibrant spot.

Obviously without going down to the planet surface you can’t tell how useful said biomass is going to be, but generally biomass grows near water and in fertile soils. Fertile soils are transported by river (see watersource!) from the eroded mountains down to the lowlands (see topography!). An earth-like world would have a lot more rainfall in the equatorial region and in the higher latitudes as snowfall, than the mid – latitudes (see AtmosphericCirculation!). When the snow and ice melt the water normally flows down into the mid latitudes and lower altitudes (see DEM!), as seasonal spring melt allowing seasonal savannahs and swamps to form such as the Okavango delta.

What I believe you are looking for is a river delta. This is where the river deposits the majority of its fine sediment load which is excellent for farming. Plenty of ‘fresh water’ and it is also abundant in animal life as a result, so you can hunt for meat …and be hunted as meat.

If you are not wanting the flood risk of a delta, you are looking for a midland area still in the tropics. Slightly elevated altitude but not high mountains. This would give you either rolling hills (still good for farming) or possibly Mesas and buttes which would provide excellent positions for hillforts. These don’t necessarily have to be in dry, arid regions such as the Grand Canyon but can be covered in plantlife such as the, um. It can exist! I just drawing a blank right now. Again all this can be found using your DEM, water locations, atmospheric circulation models. You just have to decide which features are of interest to you.

Is there any way to identify metals or metal ores near the surface?

Simple answer yes. There would be some ground-truthing needed for alien compositions but any known earth-like absorption signatures should be fine.

geology satellite image

The Morenci satellite image above is an open-pit copper mine in southeast Arizona is North America's leading producer of copper. This processed and interpreted ASTER image used short wavelength infrared bands to highlight in bright pink the altered rocks in the Morenci pit associated with copper mineralization.

Satellite images can also benefit geologists, scientists, and exploration managers due to the multiple bands that the satellites carry which allow them to interpret wavelengths that cannot be seen by the human eye. Near infrared, short wave infrared, and thermal infrared can be used to identify the difference in structural features of the earth's surface.

Multispectral imaging and thematic mapping allows researchers to collect reflection data and absorption properties of soils, rock, and vegetation. This data could be utilized by trained photogeologists to interpret surface lithologies, identify clays, oxides, and soil types from satellite imagery.

Colonisation Scenario – plan to settle in location with all desired features, water, biomass, and minerals. Or bring along technology to provide all the water conversion and biomass replicators you could need for your inhospitable mining site. Emergency Scenario – you’d be lucky if you got all one, let alone all three, water, food, and metal, in one go. But you’ll be able to gather plantstuffs (hunter -gatherer style), prospect for metals, follow hints to water, on the ground. Depending on your emergency situation you could have some basic to detailed geological maps along with your DEM. You could have launched small satellites in orbit providing you with basic real time data which could point you in the right direction. Etc etc. It all depends on your story requirements.


HOPE I COVERED EVERYTHING!


References

(1) All quotes (except mineral related) and First Image Some Satellites- http://www.goes-r.gov/users/comet/tropical/textbook_2nd_edition/print_2.htm#page_3.0.0

(2) Second Image Hadley Cell - https://askabiologist.asu.edu/explore/desert

(3) Mineral related Quote and Third Image - http://www.satimagingcorp.com/applications/energy/mining/

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    $\begingroup$ Shortened version, flash the planet and see how it reacts. $\endgroup$ – Xandar The Zenon Aug 2 '16 at 18:48
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Water

Use infra-red to check the heat of the water as hot water in cold land can suggest contamination. Also check for algae on top of water and check water flow to make sure the water is not stagnant.

Animals and Plants

Use infra-red to check the planet. Anywhere with a lot of heat in one place suggests plants. Animal shapes can often be seen by infra-red.

Ores

There are already satellite techniques looking at the emitted wavelengths of the ground to give a rough idea of any minerals. Surface features may be able to be seen from orbit giving more evidence for mineral locations.

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  • $\begingroup$ Neat! Do you have any links for where I could read about some of these projects? Or an example of where I could read more about this $\endgroup$ – Jon Takagi Jul 14 '16 at 7:56
  • $\begingroup$ @Jon Takagi Sattelie imaging for minerals is being developed. See m.earthobservatory.nasa.gov/Features/ASTERProspecting for some details. $\endgroup$ – Bellerophon Jul 14 '16 at 21:10
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Short answer: You wouldn't. You could get close, and it's much better to make estimates than to go in blind. But ultimately it is too big a risk to land and expose your colonists to the unknown. This process would be gradual and controlled with detailed scientific studies.

I think you are underestimating the complexity of colonization. This isn't a camping trip where you can just settle down any old place and be assured to have reasonably safe water and perhaps some food. This is something that will be planned decades if not centuries before the colonists arrive, and even then there are many uncertainties. Some points to this effect:

  1. You're going to know almost everything about the nature of this planet before your colonists get there, with the exception of its exact effects on humans. From your question, it sounds like this planet is a "New Earth" blooming with plant-like creatures and possibly animals too. On top of that, it happens to be habitable for humans, presumably without terraforming. This is an absurdly rare planet (the "win the lottery 100 days in a row" analogy comes to mind). Such a unique planet would be intensely studied, especially with the expectation that it would be colonized. It would be identified and studied remotely first, and then probed and explored by robots. You're going to know the soil and water content, some basics about the existing lifeforms, general terrain and so on.

  2. The existing life will probably kill you. That pretty opalescent flower actually releases a pollen which paralyzes your lungs. That cute furry creature actually carries a microorganism which eats your skin. These are things that you will have no idea about until you land and expose yourself to the environment. Even if you survive long enough to establish a colony, there's no guarantee that the existing life can support a human diet. There is simply too much uncertainty involved to risk this. The closest historical example would be the Columbian Exchange, which, while ultimately beneficial, was initially disastrous for both sides due to the diseases traded

  3. There is no guarantee modern materials would be available from nature. Take something like steel. If there was very little mine-able iron on your planet, you're completely without a resource. This is even more true with rarer materials essential for scientific development.

Based on these factors, I would think you would start with a "colony in a box", then explore later. Take everything along that you don't know you can find and easily access there. Crop seeds, animals, medicines, fuel, and other essentials. Start with as little from the environment as possible. Once this is established, then you can start exploring and testing the use of new materials and creatures. Initially, the colony is closed off to the environment, but if it proves safe to both sides, those barriers could relax. That said, you want the initial location to be close to promising sites. A body of flowing water would be ideal, both for purification and rudimentary hydro-power. Flat land would be easiest to build upon and least hazardous to explore, but you might also want a nearby mountain range for easier geologic surveys and eventual mining. You also would want to be close to the equator to make any launches from the planet a bit easier. If your robot studies indicated a soil content promising to agriculture in an area, try there. This may go along with the river valley idea, but it may not; the planet is an unknown. The initial years of the colony is going to be an intense study of the surrounding environment, so it should be someplace biologically diverse and geologically interesting. Using these factors, scientists could determine the best sites for a successful colony.

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