10 Unsolved Martian MysteriesS

For decades, humanity has been launching missions to Mars, in hopes of making sense of the enigma that is our solar system's most Earth-like planet. And as with all good mysteries, many of our investigations have simply led to more questions.

Among the most pressing of these questions is whether Mars is, or ever has been, an environment hospitable to life — and on Saturday, NASA's Curiosity rover embarked for the Red Planet to determine precisely that. But the question of whether life ever thrived on Mars is just one of the Red Planet's many mysteries. Here are ten more that we look forward to finding answers to in the years ahead.

10 Unsolved Martian MysteriesS

10. The Martian Dichotomy
Also known as the hemispheric dichotomy, the Martian dichotomy is the conspicuous geological contrast between the planet's northern and southern hemispheres. Simply put: the planet's Southern hemisphere has a rugged, crusty surface, while much of the planet's Northern hemisphere remains relatively smooth. The Northern hemisphere is also, on average, about 1-3 kilometers lower in elevation than the Southern hemisphere. The planet's smooth Northern lowlands and craggy Southern highlands are illustrated in the image featured here.

Scientists are unsure what gave rise to the hemispheric dichotomy, though the relative lack of impact craters to the North has led many of them to conclude that the largely unblemished terrain of the Northern hemisphere is significantly younger than that of the pocked Southern highlands. Possible explanations for a young-looking Northern hemisphere include plate tectonic processes and impact — either in the form of multiple, smaller meteoric bumps, or a single "mega" collision with a pluto-sized body that would have effectively turned Mars' Northern hemisphere into a giant impact crater.

10 Unsolved Martian MysteriesS

9. What is the source of Mars' methane?
Back in 2005, the European Space Agency's Mars Express spacecraft detected methane in Mars' atmosphere. (The image featured here shows concentrations of Methane discovered on Mars.) Here on Earth, much of our atmospheric methane is believed to be produced by life. But scientists don't think methane could have stuck around in Mars' atmosphere for very long; whatever produced (or is still producing) the gas probably did so within the last 300 years. Subterranean methane-producing microorganisms? It's one of the leading hypotheses, one we'd be happy to see substantiated, either by Curiosity when it starts roving in August, or by ESA's ExoMars spacecraft, designed specifically to analyze Mars' methane, and scheduled to launch in 2016.

10 Unsolved Martian MysteriesS

8. What's up with Phobos and Deimos?
Phobos and Deimos are Mars' two lumpy little potato-shaped moons, and how they came to be is a common subject of debate among astronomers and astrophysicists (pictured here is Phobos). The two most common hypotheses are that the two formed either by capture (that is to say they were once asteroids and were wrangled into Mars' orbit by its gravity) or by accretion (i.e. formed in the planet's orbit following a massive collision, much like our own moon may have formed.)

Russian scientists had hoped that soil samples recovered by its Phobos-Grunt probe might help shed light on this debate, but the spacecraft, unfortunately, failed to leave Earth orbit.

10 Unsolved Martian Mysteries

7. So, NASA, are there phyllosilicates or not?
Earlier this month, NASA announced that their Opportunity rover had discovered something entirely new on the Red Planet's surface: a line of light-colored rocks referred to by agency scientists as "Homestake." At the time, some scientists speculated that Homestake could provide hard evidence of the existence of phyllosilicates — a class of minerals that form in the presence of a watery (and presumably life-friendly) environment.

It's been a little over three weeks since the discovery of Homestake was announced, and, as these images show, NASA has done quite a number on the peculiar outcropping in the process of analyzing it. But the agency has yet to announce the results of its investigations; did they come up empty handed, are they still waiting for more conclusive evidence, or are NASA scientists holding onto some game-changing news?

10 Unsolved Martian MysteriesS

6. What lines the walls of Mars' dry ice pits?
Mars' poles are permanently covered by large slabs of frozen carbon dioxide (which you may know better as dry ice), but near the end of every Martian summer, some of the dry ice at the planet's South Pole actually defrosts, sublimating from its solid form directly into gas. When it does, pits begin to form, and gradually expand in diameter (as a point of reference, the pit featured in the middle of this image is about 200 feet across). The edges of these pits appear at first glance to be lined with gold — but the true composition of the dust comprising the walls of these pits remains a mystery.

10 Unsolved Martian MysteriesS

5. Was Mars ever home to liquid oceans?
Mars is currently colder and dryer than Antarctica, but several previous Mars missions have returned evidence that the Red Planet may have once been warm enough to host expansive oceans and winding streams. The question, then, is what happened?

NASA's latest Mars rover, Curiosity, is the size of a Mini Cooper and has the wheelbase of a Hummer, making it big enough to lug around the most sophisticated scientific payload of any rover we've ever sent to space. Using its suite of scientific instruments, Curiosity will be able to analyze Mars' geology and send high resolution pictures back to Earth that will help us make sense of its shifting climate conditions.

10 Unsolved Martian MysteriesS

4. Valles Marineris: The Grand Canyon of Mars
At over 3,000 kilometers long, up to 600 kilometers across, and as much as 8 kilometers deep, Valles Marineris is the largest canyon in the entire Solar System. (The canyon is clearly visible slicing across the face of Mars in the image featured here. Just imagine: you could fit over three Grand Canyons end-to-end across Valles Merineris' length).

How Valles Marineris formed has been a point of contention among scientists for decades, though one of today's leading hypotheses holds that the geological gash originated as a crack billions of years ago as the planet cooled, and has formed over the ages by rift faults. Whether or not the erosive force of flowing liquid water played a role in the canyon's formation early in Mars' history is a theory that stands to be substantiated by investigations by NASA's Curiosity rover.

10 Unsolved Martian MysteriesS

3. Is the Martian surface home to liquid water, even to this day?
Mars' atmospheric pressure is too low and its climate far too dry to host liquid water on its surface for any extended period of time, but there is evidence that streams (at least little ones) could very well flow on the face of the Red Planet even today.

The image featured here was released by NASA back in August, and reveals clusters of dark, narrow lines that appeared on the sloping surfaces of Mars' Newton crater in the planet's spring and summer months. Analyses conducted by planetary scientists at the University of Arizona have led researchers to conclude that these geological formations could be forming every year due to ephemeral streams of liquid salt water.

10 Unsolved Martian MysteriesS

2. The Solar System's Largest Dust Storms
Mars is effectively a global desert. That means there's plenty of dust to kick up when air gets to whipping around the planet's surface. In fact, Mars is home to what are widely regarded as the biggest dust storms in the solar system. (The series of images featured here shows a dust storm quickly enveloping the entire planet.)

Scientists believe all Martian dust storms to be driven by sunshine; how they grow to be so large, however, is something of a mystery. According to Phil Christensen, a geophysicist who studies the Red Planet's bizarre storms, the answer could lie in planetary-scale, atmospheric feedback loops:

One theory holds that airborne dust particles absorb sunlight and warm the Martian atmosphere in their vicinity. Warm pockets of air rush toward colder regions and generate winds. Strong winds lift more dust off the ground, which further heats the atmosphere.

1. Did life on Earth come from Mars?
It sounds like an awfully big jump, especially when we've yet to show that life ever existed on Mars in the first place, but it's an intriguing theory. The young Mars was likely the most Earth-like of all our solar system's planets, and recent experiments have shown that primitive microbes may have actually been capable of enduring the interplanetary trip from Mars to Earth (following, say, a major impact event). Could it be that the last universal common ancestor originated not on Earth, but in outer space?

Top image via; Valles Marineris via Viking Project/USGS/NASA; Martian dry ice pits via HiRISE/MRO/LPL/NASA; Homestake before/after by NASA/JPL via; Martian dust storm via NASA; Phobos via NASA/JPL/University of Arizona; Martian dichotomy via NASA/JPL; Martian methane via NASA; Martian ocean by Michael Carroll via; spring/summer geology via NASA/JPL/University of Arizona