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How Curiosity's groundbreaking discoveries have unveiled an Earthlike, habitable past for Mars

It's been four years since NASA's Curiosity Rover touched down on the Martian surface to begin an exciting new step in the agency's ongoing study of the red planet.

Launched from Cape Canaveral in November 2011, the 2,000 pound, car-sized rover set off toward Mars' Gale Crater to determine if conditions were favorable for life, both past and present.

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Since then, the technological eyes, nose and ears of Curiosity have provided researchers with astonishing discoveries and insights into present conditions, and even past conditions of the Red Planet, making it one of the most accomplished of NASA's exploratory Martian missions.

Take a look at a few of the major discoveries Curiosity has provided over the past four years as the anniversary of its voyage commences.

This low-angle self-portrait of NASA's Curiosity Mars rover shows the vehicle at the site from which it reached down to drill into a rock target called "Buckskin" on lower Mount Sharp. (Photo/NASA)

Conditions for life and ancient lakes

While no evidence has ever been shed on the existence of other intelligent life in the universe, the prospect for past microbial life on the nearby planet was more than enough to stir excitement among researchers during Curiosity's analysis of Martian geology.

Chemicals discovered in the Martian rocks indicate the possibility of a much higher oxygen content in its atmosphere in the past. In addition, evidence regarding ancient lakes that once adorned the planet's surface have been revealed, according to a NASA report.

Curiosity has found evidence of fresh water on Mars as well as the presence of carbon, hydrogen, oxygen, phosphorus and sulfur -- all key ingredients in supporting life. While microbes playing a part might be speculative at this point, the recent discovery has raised more questions.

"The only ways on Earth that we know how to make these manganese materials involve atmospheric oxygen or microbes," Los Alamos National Laboratory Planetary Scientist Nina Lanza said in the report. "Now we're seeing manganese oxides on Mars, and we're wondering how the heck these could have formed?"

Ancient steam beds

A lot of information rests in the rocks that litter the Gale Crater, helping to unearth further clues into the planet's past.

Photographs taken by Curiosity show pieces of rock called clasts that were discovered in the Gale Crater. Some of these small fragments appear to have been formed by ancient steam beds and flowing water, according to NASA.

"Some of the clasts are round in shape, leading the science team to conclude they were transported by a vigorous flow of water. The grains are too large to have been moved by wind," NASA said in a 2012 press release shortly after Curiosity landed.

Martian methane

Just like a human explorer, Curiosity has been equipped with a wide array of senses in order to gain a better understanding of the planet. In its pursuit, Curiosity has been actively sniffing the Martian air in an attempt to find traces of methane.

Methane is an organic chemical, and the most abundant hydrocarbon in the entire solar system, according to NASA. Its presence on Mars could be organic or inorganic, but Curiosity's initial findings surprised scientists.

In what seemed like a blow to those seeking evidence of life, Curiosity was unable to find any significant trace of methane during its first several months of exploration.

However, this changed when there was a sudden surge, a tenfold spike, which Curiosity recorded in late 2014.

"This temporary increase in methane, sharply up and then back down, tells us there must be some relatively localized source," said Curiosity science team member Sushil Atreya said in a NASA report. "There are many possible sources, biological or non-biological, such as interaction of water and rock."

While this methane surge was a pleasant surprise to scientists, Curiosity also detected different organic chemicals in powder drilled from a rock, leading to the first detection of organics in surface materials of Mars. It is unknown whether these organics formed on Mars or were a result of a meteorite impact.

High radiation levels

A mission to Mars with astronauts might be decades away, but Curiosity's initial findings have discovered another obstacle scientists will have to overcome if they want people to set foot on the rusty, Red Planet.

Either directed radiation from the sun in the form of solar energy particles, or from galactic cosmic rays formed by the explosive death of distant stars, the radiation levels on Mars may pose higher risks to human health for space exploration.

The radiation levels Curiosity has endured are much higher than the career limits that NASA allots astronauts, according to NASA.

By studying these radiation levels, Curiosity has provided insight that will continue to play a role in any future explorations of Mars in the decades to follow.