One of the scientists’ main interests in Mars research is water. Is there water on Mars?
At the expected time, the orbiter, flying at about 300 km in altitude and around 3.5 km s-1, focused toward the surface of the arid, cold and reddish planet and opened the shutter of the complex and precise camera. A new image from the Martian surface, and gigabytes of data, were then recorded at the same time. This process has been repeated many times in recent decades by the different orbiters, landers and roversw1 that we have sent to our nearest planet. Every new image and data set increases the incredibly huge databasew2,w3 of information on Mars. Scientists from all around the world use these resources to study the planet’s chemical, physical, climatic and geological environment, with the aim of understanding more about Mars and how it – and even Earth – has evolved.
This widespread availability of imagesw4 and data is one of the most amazing phenomena in the history of science never before have so many experts shared so much information and produced so many models and results.
The northern polar cap of Mars is mainly H2O ice. However, the southern polar cap comprises H2O ice and CO2 ice.
One of the scientists’ main interests in Mars research is water. Is there water on Mars? Where is it? Is it liquid or frozen? In the past, were there oceans, seas, lakes and rivers on Mars? How did they disappear? Is their disappearance related to the past and present climate of the planet? But also: Is there, or was there, life on Mars? And could humans live there one day?
Thanks to the data acquired by orbiters, landers and roversw5, we know that Mars had liquid water millions of years ago – flowing on its surface, filling the basins of impact craters, forming lakes, flowing downslope of some volcanoes’ flanks, from the Martian highlands to the lowlands, where an extensive ocean could have existed (referred to as Oceanus Borealis).
Solid water is a different matter. Ice polar caps on Mars were identified from telescopes and clearly recognisable on orbiters’ imagesw6, and frozen soils have been confirmed in several places on the planet, mostly thanks to the images reported by NASA’s Phoenix mission at high latitudes. The low temperatures of the planet throughout the year, measured by different landers and rovers (such as Viking I and II, Mars Pathfinder, Spirit, Opportunity and Curiosity, which arrived on Mars in August 2012), confirm that ice is stable at all latitudes. In fact, the mean temperature of Mars is about -80ºC during daytime; at the Equator, the sunny slopes in the summer at noon hours can reach a surface temperature of 15 ºC.
Today, research on Martian icew7 focuses on finding evidence of the presence of ice and glacial-related features using new high-resolution (as high as 35 cm/pixel) images acquired by the active missions of NASA and the European Space Agency (ESA). Our research group focused on the northwest flank of the Hecates Tholus volcano in the Elysium region of Mars, at tropical latitudes of the northern hemisphere. We analysed all the available images from different orbiters covering this region (at different spectral, temporal and spatial resolutions), and observed features that we interpreted to be caused by glacial erosion or sedimentation: moraines, crevasses, roches moutonnées, glacial cirques, hanging valleys, eskers, drumlins or arêtes, among othersw9.
Our interpretations were based on the comparison between the marsforms (the reliefs observed on Mars) and the terrestrial landforms in the Alps, Iceland or Antarctica, where we conducted fieldwork looking for terrestrial analogues. We also used the ‘multiple working hypotheses’ scientific method to discard other processes that are able to produce similar features as the origin of the marsforms we observe. Then, after months of work in front of the computerw8 and on different field trips, and thanks to the satellite images and topographic, spectrometric and thermal data, we carried out a detailed description, mapping and age determination of the features observed on the flank of the Martian volcano. Our first conclusion, based on the long list of glacial-related features on the Hecates Tholus volcano, is that an important amount of ice existed there for a long time, forming glaciers that flowed downslope, sculpting the flanks of the edifice.
Many Martian volcanoes show reliefs on their flanks that are caused by glacial ice flows – just as we see on Earth. Those volcanoes are located not at polar but at tropical latitudes. Olympus Mons, Ascraeus Mons and Hecates Tholus are examples of volcanoes with glaciers, similar to Mount Kilimanjaro (Tanzania) and Cotopaxi (Ecuador) on Earth.
Crater counting has provided evidence of glacial activity, both ancient (more than 1000 million years ago) and recent (less than 2 million years ago). The cold periods in Mars history are related to orbital changes (mainly changes to spin axis angle) – just like Earth, where the orbital cycles control most of the Quaternary climate change, as discovered by Milutin Milankovic in 1922!
The problem is… we couldn’t find ice anywhere! However, we could see some glacial features that we know can’t survive for long after ice melts. This is the case for crevasses: fractures in the glacier disappear when ice melts or sublimates. We didn’t see the ice on this part of Mars, but we could recognise the crevasses sculpted in the dust layer that covers the ice. For that reason, our second conclusion is that the ice causing the extensive fields of glacial marsforms must still be below the surface – or it melted very, very recently.
Through crater counting (see box below), we also calculated the age of the different glacial deposits that we observed on the images. We found a wide range of ages – from 1000 million years to 350 000 years – which means that the Hecates Tholus volcano had a long history in which the glaciers slowly sculpted its northwestern flank. In fact, we proposed the existence of cold periods, in which the ice tongues covered an important part of the volcano and its surroundings, and also warmer periods, in which the glaciers were smaller and covered only parts of the flank, such as in the present time.
It is simple and efficient: they count the craters left by meteorites after their impacts on the surface of Mars – and any other planet or moon. High crater density corresponds to old surfaces and low crater density is linked to young surfaces.
We plan to repeat our observational study in other volcanic regions on Mars to see if the same pattern exists and to get a better idea of the global distribution of glaciers and ice on Mars. These studies will further our understanding of the climate, its evolution and its characteristics on our neighbouring planet. The presence of glacial features on volcanic edifices could also mark the location of sites where life, if it existed, could have found water and heat to survive, even in the cold and dry environment on Mars.
Upcoming studies will use a new kind of tool: penetration radar. This technique allows us to measure the properties of materials below the surface and to investigate their variations: if there is ice below the surface, it should be distinguishable in the radar data, just as we observed in the Martian polar caps. Radar data from NASA and ESA will allow us to corroborate our interpretations of observations from the Hecates Tholus glaciers and other areas on the red planet, thanks to a modern, collaborative and interplanetary effort to further science.
All the water that has been found on Mars exists in a gas or solid state. Mars’ atmospheric pressure is very low (around 6 mbar, compared with Earth’s average of 1013 mbar) and below the minimum required for liquid water stability.
Forget C, Costard F, Lognonné P (2006) Planet Mars: story of another world. Chichester, UK: Springer-Verlag/Praxis. ISBN: 978-0387489254
Hartmann WK (2003) A travelers’ guide to Mars. USA:Workman Publishing Company. ISBN: 978-0761126065
Carr MH (1996) The water on Mars. Oxford, UK: Oxford University Press. ISBN: 978-0195099386
Carr MH (2006) The surface of Mars. Cambridge, UK: Cambridge University Press. ISBN: 978-0521872010
Kargel JS (2004) Mars: a warmer and wetter planet. Chichester, UK:Springer-Verlag/Praxis. ISBN: 978-1852335687