University of Michigan - Department of Astronomy

Version: intro

Searching For Life

Life. Don't talk to me about life.
--Marvin the Paranoid Android (As recorded by Douglas Adams)


When we begin to search for life elsewhere, we can either search for the things life needs to exist, or the markers produced by metabolism. What we search for is influenced by what resources we want to put into the search and what we want to find.  For example, the markers of metabolism include atmospheric gases produced by respiration, which can be observed from Earth, but won’t give you much information.  If instead, we want to search for the things life needs, we usually need to go there and get a sample.

Atmospheric Markers

The easiest markers of extant life are atmospheric gases that usually don’t last long in the atmosphere, like oxygen. Oxygen quickly reacts with other materials, so a high level of oxygen, especially O2, indicates that something is replenishing the supply. Methane is another marker, although it can also be produced by geologic processes and doesn't react much at lower temperatures (it is one of the most common ices in comets). 

We can search for atmospheric markers by observing the absorption spectrum of a planet as it passes in front of a star. As you saw in the spectroscopy lab, each element (and molecule) produces a unique spectrum. Astronomers use this to determine the composition of the atmosphere. We do this now with known planets that transit their parent star dozens of light years away.  However, the information we get from this is very limited. For example, there may be some chemistry going on that we've never seen before that generates oxygen. Additionally, the atmospheric composition now doesn't tell us anything about past life, life underground, or life on a body that can’t retain an atmosphere. It also won’t tell us anything about things we might call life but that aren't "life as we know it."

Searching by Needs

Although it is harder, we can also search for life by looking for the things life needs.  The main advantage of this method is that it can also tell us if life was present in the past, if it exists in extreme environments or if it is or was beneath the surface.

The requirements for life (as we know it) are:


It is difficult to detect a “food” source.  Different types of life consume different materials, and most of the common materials, like carbon and oxygen, are common throughout the universe.  Nearly every body we search will therefore have some materials that some form of life can use for metabolism. In this case it is actually much easier to search for the products rather than the raw materials, but we usually need very close observation to tell if it is the result of life, or geology.


Water is the best transport medium for life as we know it.  While other molecules may work, they tend to either break down the nutrients, the nutrients don’t dissolve easily in them, or they have a very limited temperature range where they are liquid.

In order to be useful for transporting nutrients, the water must be in liquid form, which is usually highly visible if it is on the surface. For example, from space, the most obvious feature of Earth are its oceans and clouds, showing Earth has an abundance of water. Liquid water also does a lot of other things, like eroding rock, transporting silt and assisting in the formation and deposition of salts and minerals. This makes it easy to tell even from space if there was ever water present on the surface, but it is harder to tell how much, how long, or if it is below the surface.

Photographs and measurements of the height and structure of surface features can tell us if there is liquid on the surface, or if erosion and transportation of surface materials ever occurred. However, it can't really tell us if the liquid is water, and it can be hard to determine how much liquid was involved over how long a time. To really get a good idea of the history of water on a body, we need some different kinds of information.

Orbiting spacecraft can make many observations we can't make from Earth, especially if we put more specialized instruments like spectrometers on board. Spectrometers can determine what materials are present on the surface (for more on spectra and identification of materials, see the Spectography activity.) Scientists can then make maps showing the locations of the things that would indicate the presence (current or past) of water.

Hydrogen ions are liberated from water ice when UV light from the sun hits is, so hydrogen ion maps indicate where water ice exists on or just under the surface.

Hematite is a mineral that usually forms when it precipitates out of water, especially in hot or mineral springs. However, it may also form from volcanic activity. The shape of the grains and other minerals present tell scientists which way it formed, but a sample is needed for that analysis. The landing sites for the Mars Exploration Rovers were chosen partly for the high abundance of hematite, and the analysis done by the rovers indicate both of those sites were wet, probably for a very long time.

In addition to providing the transport medium for food, areas that were wet for an extended period have a better change of having fossil remnants. Microorganisms in lakes and hot springs can become trapped in iron oxides, phyllosilicates, zeolites, carbonates, and sulfates, so these minerals are particularly good to look for.


Energy can come from many sources.  The Sun or other parent star is a good source for any body close enough to get sufficient energy. Most life on Earth ultimately derives its energy from the Sun. Mars is at about the limit of the useful distance from the Sun. Geothermal energy is also a good source, and is the source of energy for many forms of life on earth that live along deep ocean vents.  Geothermal energy can be heat left over from a planet’s formation, heat produced by radioactive decay, or tidal heating.  The first two provide the geothermal heat on Earth.  Jupiter’s moon Europa is tidally heated, possibly enough to support life. However, sudden and drastic changes in energy are very bad for most organisms. It is very difficult for life to develop on a planet in a highly elliptical orbit, because neither the light from the star nor tidal heating will provide a consistent energy source. Even of Earth, areas with highly variable temperatures have less biodiversity. For example, only a few organisms are adapted for living in or near a large active volcano where they may be cooked by an eruption.

Summary and Final Notes

From this, you can see that looking for atmospheric markers lets us look the farthest, even into other solar systems, but doesn't actually give us very much information. Go to another world tells us a lot more, but is a lot harder and more expensive.

When we search for a suitable place for life (as we know it), we really need to find liquid water and an energy source. Once we find that, we can look for specific things, such as mineral deposits, ongoing chemical reactions, or even cells. However, to find any these things, we have to actually have a sample to analyze. We need to either bring samples back to Earth (expensive and potentially risky), or send a laboratory there.

When we send a spacecraft to another world, whether to bring samples back or to analyze them there, we have to make sure we don’t contaminate the planet.  Not only do we run the risk of destroying any life that may already be present, we also run the risk of ruining our experiment so we’d never even know what we’d done. Because of this, NASA has a Planetary Protection policy, requiring that spacecraft that land (or even crash) on potentially habitable worlds be clean and that they avoid landing in any area that looks like it could harbor life. You can find out more at

One other thing scientist have to consider is whether or not it is safe to put a spacecraft down in any particular spot. No scientist wants to spend 15 years of their life planning and organizing a mission, just to have it get stuck in a field of boulders on landing.

There are two parts to this activity. Part 1 uses the information in the introduction and a set of maps to pick some good potential landing sites. Part 2 uses a detailed map browser called JMars to determine which of the sites in part 1 are safe to land on. Check with your GSI for which part(s) you need. JMars might be worth checking out even if you don't do part 2.


Landing Site on Mars Part 1: Where to Search (List of maps: ../life/maps.html)

Landing Site on Mars Part 2: Safe Place to Land (JMars:

updated: 3/19/10 by SAM

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