After Earth, Mars is the planet with the most hospitable environment in the Solar System. So hospitable that it may once, well over 3.5 billion years ago when liquid water was present, have harboured primitive microbial-like life. Channels, gullies and other geological features provide plenty of evidence that liquid water once flowed on the Martian surface, but where did the water go? What caused the change in the Martian climate? Could microbial life be present in the subsurface today?

Unfolding the story of water on Mars will be crucial not only to reveal its past climate history, but also to study its geology and possible planetary habitability. Water is a fundamental ingredient to life and could hold clues to past or even present life on Mars. If humans are ever to travel to Mars, the availability of resources such as water will also play a crucial part in the planning of such missions.

Mars, the closest planet to Earth, was once very Earth-like. Its surface has been altered over the years by impacts from other planetary bodies, volcanism, atmospheric effects such as dust storms, or movements of its crust. Mars has some remarkable geological characteristics: it is home to both the deepest valley in the solar system, Valles Marineris, and the highest mountain, Olympus Mons, which is about three times as tall as Mount Everest. It also has the largest volcanoes in our solar system, about 370 miles in diameter, and many other types of volcanic landforms, from enormous plains coated in lava to small steep-sided cones.

NASA/JPL-Caltech/Malin Space Science Systems

Gullies, channels and valleys are found all over the planet’s surface, suggesting that liquid water was present in recent times, and may still lie in pores and cracks in the subsurface. While it may not have been so earlier in the history of the solar system, Mars is nowadays much colder than Earth, with an average temperature of -60 °C. This is not only due to its greater distance from the Sun, about 130 million miles away at the closest point in its orbit, but also because Mars has lost most of its atmosphere.

With a gravity that is only 38% that of Earth, many of the gases needed to retain heat close to the surface have escaped into space. Mars also lacks a spinning, molten core, precluding the generation of a magnetic field, without which Mars is constantly exposed to solar winds and cosmic and galactic radiation. These bombard the atmosphere, blowing away more gases needed to heat the surface.

Mars’ current atmosphere, mainly formed of carbon dioxide, is too thin to allow liquid water to remain in the surface for long. However, despite being roughly 100 times less dense than Earth´s, it remains thick enough to support winds, weather and clouds.

Why has Mars changed so dramatically? This is the question that leads us to explore Mars today. By studying the reasons for climate change, we may begin to understand the geological and biological processes that have shaped Mars. As we begin to explore the planet, we wonder: did Mars once exhibit the minimal conditions necessary for the formation of life?

Mars Exploration

Robotic spacecraft were first sent to Mars by NASA in the 1960s, with Mariner 4, followed by Mariner 6 and 7 a few years later. They all revealed a desolate world, without any signs of the life or even civilizations that had been imagined there. Public attention focussed on Mars again in 1997, when Mars Pathfinder touched down on the Martian surface. Two months later, Mars Global Surveyor was inserted into orbit, sending back pictures of volcanoes and chasms at resolutions never seen before. In 1998 and 1999 another lander and orbiter were launched, and every 26 months over the following decade, when the alignment of Mars and Earth was suitable for launches, more robotic spacecraft ventured to Mars.

In 2003, when Mars was the closest to the Earth in nearly 60,000 years, NASA landed two rovers, Spirit and Opportunity, at two very different sites on opposite sides of Mars. Spirit landed in the crater Gusev, a massive lava plain. The rover sent home a frustrating panorama of dry rock, evaporating the hopes of many water hunters. However, Opportunity detected sediments formed by the presence of water all around its landing site at the equator, in Meridiani Planum. It found abundant evidence of past water, and scientists believe that perhaps Meridiani Planum once held an actual sea, most likely during Mars´ earliest geological stages well over three billion years ago.

Since its landing, Curiosity has sent to Earth more than 190 gigabits of data, fired its laser more than 75,000 times and returned more than 70,000 pictures

Most recently, in 2012, NASA´s Mars Science Laboratory’s rover Curiosity successfully landed in Gale Crater, near the equator. Gale Crater, a fascinating 96 mile-wide crater (the remnant of a very old impact which occurred three billion years ago), is considered to be the thickest pile of sediments yet identified on Mars. It was the chosen landing location for Curiosity as these sediments record a very long history of rock erosion and deposition, representing a direct view into the processes that have occurred on Mars over a huge amount of time. This choice of location also reflects the `follow the water´ strategy in the effort to find life on Mars.

Signs of Life?

Curiosity was designed to identify organic compounds, determine the isotope ratios of key elements, investigate the building blocks of life, identify possible biosignatures, determine planetary processes such as atmospheric processes or cycling of water and characterize the surface radiation on Mars.

Organic (carbon-based) molecules are considered the crucial ingredients of life. Finding organic molecules is a real challenge however, as they easily break down when exposed to harsh environments such as extreme radiation or chemical oxidants from dust storms. A good place to find ancient organic molecules nowadays is in rock layers, which can preserve organic molecules that were suddenly trapped and buried in layers of sediment or mud.

Scientists believe that the Gale Crater contains rocky layers that formed in ancient times, when liquid water was present. Even though the water has since dried up, the layers could still preserve some organic compounds. If Curiosity finds organic compounds in ancient rocks, it would not prove that life existed on Mars, but it will surely prove that Mars once had the right ingredients for life to exist.

As part of its exploration, the rover has also measured the radiation exposure inside the spacecraft, both on its way to Mars and on the surface of the planet. These data, never measured before, are crucial for planning future manned missions to the Red Planet.

A Monster Truck of Science

Curiosity, with its six-wheel drive, suspension system and mounted cameras, is a nuclear-powered monster truck of science, worth 2.5 billion dollars. Its 20-inch aluminium wheels tear over different obstacles and explore over 660 feet per day on Martian terrain. The rover is equipped with the most advanced tools available for drilling and analysing rocky Martian samples. A truly sophisticated mobile laboratory, it has the most advanced instruments ever built and sent to Mars. It is able to drill holes, collect rock powders and analyse them.

Unlike its predecessors, Curiosity uses onboard test instruments. One of them, using fluorescence and X-ray diffraction, can identify minerals in the rocks and soil samples. Other sets of instruments, such as a gas chromatograph, a laser spectrometer and a mass spectrometer, can identify organic compounds, including carbon and oxygen. Scientists are therefore not only receiving images of rocks that could represent an ancient water-rich environment, they are also able to test these hypotheses directly in the field.

The NASA Deep Space Network (DSN), an international network of antennae, provides communication links between spacecrafts and rovers on Mars and the scientists on Earth. This network of antennae consists of three deep-space communication facilities spread around the world, which allow constant observation of spacecrafts as the Earth rotates.

Curiosity also uses the DSN to communicate to Earth, but messages from the rover are first sent to the spacecrafts orbiting Mars, rather than directly to Earth. This is for several reasons: the orbiters are closer to the rover (250 miles above the surface) than the DSN antennae on Earth and have Earth in their field of view for much longer periods of time than Curiosity on the ground. They also have bigger antennae and a lot more power than the rover. The data rate from Curiosity to the orbiter can be as high as 2 million bits per second. When an orbiter passes over the rover and is able to communicate with it, it can collect between 100 and 250 megabits of data and transfer them to Earth within a few hours. That same 250 megabits would take up to 20 hours to arrive to Earth directly from Curiosity.

Evidence of Water

Curiosity has found evidence that a lake once existed, which contained fresh water and other chemical ingredients suitable for the existence of life. A big surprise was to find clay minerals near the landing site, since there were no signs of them from orbit. The presence of minerals means that water was surely flowing at some point in the Martian history.

The presence of minerals means that water was surely flowing at some point in the Martian history

Curiosity also found conglomerates. Conglomerates are rocks made of little round pebbles encrusted in a finer sandy or muddy component called the matrix. On earth these rocks are formed in riverbeds. The key to pebbles in a river conglomerate on Earth is that they are round. These round edges are the result of the pebbles being lifted and transported by flowing water. As the water moves the rocks around, they collide with one another, slowly causing the sharp edges of the transported clasts to be chipped off, eventually becoming smooth. Finding a conglomerate on Mars with features identical to those we find here on Earth proved once again that, for an extended time, water must have flowed on the surface of the planet.

Curiosity also measured the natural cosmic and solar radiation on its way to Mars. This is extremely important since Curiosity came into contact with the same environment that human explorers are expected to experience one day. As it turns out, the radiation arriving from both the sun and interstellar space will pose a significant challenge for future Mars astronauts: Curiosity absorbed much more radiation than astronauts are allowed to over their entire career.

In the past few years, both Mars orbiters and terrestrial telescopes detected methane on Mars; a gas that could be the result of present biological activity. However, Curiosity has so far detected very little methane in the Gale Crater. So while it appears that the conditions of Mars in the past were at some stage conducive to life, today´s environment seems to be too harsh for life to exist.

What Next?

Since its landing, Curiosity has sent to Earth more than 190 gigabits of data, fired its laser more than 75,000 times and returned more than 70,000 pictures. Just a year into its mission, Curiosity has already achieved its main goal: to further our understanding of the Martian environment.

The remnants of the ancient Mars environment found by Curiosity confirm that fast-moving, deep water existed on the planet’s surface at some stage in its history. It has also determined that Mars could have hosted living oxygen-producing organisms. The rover found traces of oxygen, hydrogen, sulphur, nitrogen, carbon and phosphorus. It also found clay minerals and calcium sulphate, suggesting not only that water was present, but also that it must have been freshwater, favourable for living organisms to thrive in.

Future missions are already being planned to look for evidence of microbial life. The ExoMars mission, due to launch in a few years, will have similar onboard instruments in order to visually, mineralogically and chemically analyse the environment right down to the microscopic level. It will look for biosignatures left in rocks that could only be explained by the presence of ancient life. In addition, the rover will collect samples and store them for a later mission to bring them back to Earth.

Knowing as much as we can about Mars and its past will help us verify whether life was ever present on our planetary neighbour. Each technological advance in space exploration provides new opportunities to take us further and further along the way. But regardless of how far we get with these incredible rovers, one problem remains to be solved: to determine how humans can live in space and function in these harsh environments. The next 30-40 years will see a lot of activity in space exploration, with the hope that man will keep on pushing the boundaries of our human ability. All of this, together with previous missions and space projects, sum up how we truly explore space – one step at a time.