The fourth rock from the sun, Mars has long captured both popular imagination and scientific interest. For decades, robots exploring the red planet have been beaming back pictures of a strange world full of breathtaking beauty.

With mountains three times higher than Everest and canyons five times longer than the Grand Canyon, Mars is an adventure traveller’s paradise. And with its dusty atmosphere, polar caps that change with the seasons, and roughly 24-hour days, Mars is Earthlike enough to beckon human visitors.

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As NASA’s next big mission, the InSight lander, prepares to touch down in late November, take a look at some of the biggest mysteries about Mars yet to be solved—including some things we may never know until humans set foot on Martian soil.

Does liquid water flow on Mars today?

The Martian atmosphere today is so cold and thin that liquid water on the surface should either evaporate or freeze into the soil. For over four decades, though, Mars spacecraft have snapped photos of what look like hundreds of dried-up river channels and canyons that may have been carved by fast-flowing water in the distant past.

So where did all the water go? Scientists think these eroded features could be left over from a time when Mars was warmer and wetter, and that some of it may still be locked underground as ice or even deep liquid reservoirs.

Orbiters looking down on Mars have shown large amounts of water ice frozen at the planet’s poles. In 2015, images from NASA’s Mars Reconnaissance Orbiter showed strong evidence that liquid water may flow intermittently on the modern Martian surface. Based on the orbiter’s data, researchers identified the chemical fingerprints of hydrated minerals on many steep crater slopes where mysterious darkish streaks have been previously spotted.

It’s possible that briny Mars water flows on these hills during warm seasons and fades away when it gets cold. But without closer examination, it’s been hard to say for sure whether these recurring features are indeed being made by water or by simple flows of dry dirt.

Meanwhile, Europe’s Mars Express Orbiter used ground-penetrating radar to discover signs of a 12-mile-long lake under the planet’s south polar ice cap. Scientists believe that the underground lake can stay liquid because of its concentrated briny nature. Mars may have many such large water reservoirs scattered across its polar regions, the scientists suggest. Finding them and figuring out how to access their bounty could be critical for potential human explorers visiting Mars in the future.

Why is the northern hemisphere smooth and the southern hemisphere heavily cratered?

In the 1970s, NASA’s Viking missions made the first complete survey of the topography of Mars. Since then, scientists have been puzzling over why the planet has two faces: The northern hemisphere is much flatter and lies lower than the heavily cratered highlands of the southern hemisphere, with a difference in elevation of between three and five miles.

Theories have suggested that internal geological process, like heat convection in the mantle, could have formed Mars’ present-day features. It’s also possible that the planet’s northern half was worn down over time thanks to a vast ocean filling this global basin.

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Other studies, however, have come up with a more violent hypothesis for this bizarre dichotomy: Perhaps a large asteroid the size of Earth’s moon smashed into the planet’s south pole 3.9 billion years ago. Such a devastating impact would have been a defining moment on Mars, churning up a magma ocean that gave rise to the red planet’s volcanism, which in turn might have spewed the material that created the southern highlands.

Figuring out this aspect of the red planet’s past could help scientists better understand where future explorers may want to land to find the best resources for establishing a sustained human presence.

What is generating methane in Mars’ atmosphere?

In the last few years, both Earth-based telescopes and Mars orbiters have detected traces of methane on Mars—a gas that could be the result of present-day biological activity or that could signify other geological processes at play.

Recently, findings from NASA’s Curiosity rover suggested that low levels of methane on Mars skyrocket ten-fold over the course of months. This indicates that there is ongoing production of methane, which is perhaps being vented and quickly dispersed around the rover’s Gale Crater landing site. While the same gas in Earth’s atmosphere is mostly the result of biological activity, scientists say that these Martian observations are not necessarily hard-core evidence of microbial life.

NASA believes that the source of this methane is north of Curiosity, but it is nearly impossible to define its precise location. The methane source may remain a mystery for the time being, since the rover is not headed in that direction and instead has its sights set on investigating the layered rocks of the crater’s central mountain.

Is there life on Mars?

The main ingredient for life as we know it is liquid water, and signs of its presence on Mars have kept hopes alive of finding past or present signatures of life. But the Martian surface is a harsh place, with wild temperature swings and little protection from harmful ultraviolet radiation.

Many scientists believe that dried-up lake beds like Gale Crater could perhaps harbor fossils or other traces of past organic life near the surface, and NASA’s upcoming mega-mission, known for now as the 2020 Mars rover, will look for these kinds of traces. Meanwhile, extreme life-forms on Earth—including signs of microbes living deep in the planet’s interior—offer hope that something could be alive on Mars today. (However, some experts argue that sending humans to Mars will mess up the hunt for alien life.)

Could humans live on Mars?

The race is on to send humans to Mars, with NASA aiming for a Mars mission perhaps by the mid-2030s, and public and private ventures around the world developing the necessary technology.

But if humans are to survive at all on Mars, they will have to live and work independently of Earth, and carve out a living from the red planet’s natural resources. Habitats will likely need to be built underground, to protect people from dangerous cosmic radiation. Growing food on Mars will also be a challenge, as rovers have shown that the surface soil is sterile and full of toxic compounds called perchlorates.

Ambitious space engineers are even now drawing up plans for the next generations of nuclear, chemical, and solar-powered technologies that will not only be able to advance science on Mars, but may also provide the foundation for self-sufficient human habitats. Building more efficient fuel cells and batteries will be necessary for surviving weeks of darkness during regional or global dust storms. Mining the dirt and rocks under their boots will be critical for making air to breathe, clean drinking water, rocket fuel, and basic building materials.