The red and blue world
MANY years ago, the Mars Odyssey spacecraft detected the presence of water ice just below the surface of the planet Mars, in some places that were well away from the Martian polar regions.
It wasn’t the only time that ice on Mars has been found: the Phoenix lander, which touched down on the Martian surface in 2008, sent back pictures of slabs of ice at its landing site.
Now, the Mars Reconnaissance Orbiter (MRO) spacecraft has shown us thick layers of exposed ice near the tops of cliff faces in locations spaced widely apart. It is an exciting discovery in itself, but amazingly, this ice has been shown to be about 100m thick.
Mars has been the target of more spacecraft visits than any other planet. There has been a mix of orbiting and landing spacecraft, not all of which have been successful. MRO is one of several craft currently operating at Mars, and it has been in orbit around the Red Planet since 2005. Its main aim is to image the surface of Mars in greater detail than ever before, and it has been highly successful.
Cliff faces and scarps provide wonderful opportunities for studying the history of Mars, as they do on Earth. This is because the gradual erosion of cliff faces exposes material, and the greater the depth measured from the top of the cliff, the older the material. There are many examples of this around the world.
Another famous example of being able to study the history of a location through exposed rock faces was that experienced by the Opportunity lander on Mars. When it arrived in 2004, mission scientists were at first rather dismayed to discover that the craft had landed in a small crater. However, this meant the exposed crater walls could be examined closely to determine conditions on Mars at various times in the past. Its measurements showed that region of Mars once had running water — which cannot exist on Mars today, because of the combination of temperature and pressure at the surface.
In the case of the recently discovered Martian ice, the MRO imaged layers in the cliff faces that had a different colour than the normal reddish surroundings. The first thoughts were that they may be just coatings of frost that appeared and disappeared with the seasons, but it is now clear that they are far more substantial, permanent deposits that start just a few metres below the surface. The exposed ice is relatively fresh, as it is gradually sublimating (turning directly from solid to gaseous form) and exposing new icy faces. In the process, it dislodges darker, rocky material.
The instrument that has provided the detailed information is called CRISM, which stands for Compact Reconnaissance Imaging Spectrometer for Mars. A spectrometer, when applied to the measurement of electromagnetic radiation (for example, visible light) is a device that measures the intensity of a large range of wavelengths of light. Where a particular wavelength of light reflected from a surface has a high intensity, for example, it can tell researchers much about the type and amount of a particular substance. CRISM has shown that without a doubt there is a large amount of water ice there.
Although in our everyday lives we think of “ice” as being water ice, it is important to make this distinction in planetary science, because there are other kinds of ices. In particular, Mars also contains a good deal of carbon dioxide ice, also known as “dry ice”, which is the solid form of carbon dioxide; in our everyday experiences it is a well-known gas.
The more water we find on Mars, the more exciting it becomes to think of the possibility of life there, at least having existed in a primitive form long ago when there was actually running water.
However, there is also the benefit to humans who may one day go to Mars. Last week, as part of the 50th anniversary celebrations at the Launceston Planetarium, Mars One candidate Josh Richards spoke to an enthralled audience about his shortlisting to go to Mars in a decade and a half. He and his colleagues would never come back, and I am sure that this recent finding has heartened him and all those who aspire to make such a trip!