For decades we've been sending probes and landers and rovers, and we're finally planning to send crewed missions, too.

We've learned a lot about the planet over the years,

but as we find out more, we keep uncovering new mysteries, important, fundamental aspects of Mars that we just can't explain.

Here are three of them.

When it comes to the search for life, methane is pretty important.

On Earth, almost all the methane in the atmosphere comes from life forms: whether termites or microbes or cow farts.

So when the Curiosity rover detected a spike in methane in Mars' atmosphere, it was an exciting moment.

The spike was announced in late 2014, and stretched across a two-month period a year earlier.

It's surprising there's any methane on Mars at all, much less a big spike.

The sun's radiation breaks it down so quickly that it shouldn't last more than a few hundred years.

But scientists have come up with a few possible explanations.

If this was a one-time thing, it could have come from an underground lake suddenly releasing a bunch of old, trapped methane.

If the spike is seasonal, though, it's possible that compounds in the Martian soil release methane during certain times of year.

Or, that life on Mars releases methane during certain seasons.

Methane production on Earth is the same way, increasing at times of year when plants and bacteria are more active.

Any sign of Martian life is kind of a huge deal, so researchers started looking for another methane increase to see if there's a seasonal pattern.

So far, Curiosity hasn't found another example of a methane increase, so maybe it really was just a subsurface lake.

For now, we don't know for sure.

Even if it was a lake, life could still have produced that methane in the first place.

It's just more likely that it was produced through active geology instead.

Here's the thing, though: only 5% of the methane on Earth is produced by geologic processes,

so Mars would have to be pretty darn active to explain away the methane.

So, no matter what the answer to this mystery is, it'll be an important discovery.

We also found gullies, areas of the Martian surface that look like old rivers, and in one case, a huge ocean.

If liquid water once did carve out those features, there doesn't seem to be much of it left now. That wouldn't be surprising.

Water on Mars would probably have been lost over time because of its weaker gravity and thinner atmosphere.

But water isn't necessarily what created the gullies and lakes.

It's actually pretty hard for scientists to come up with a scenario where Mars would be warm enough to support a huge ocean full of water.

Which is why they've been trying to figure out what else could have caused those dark markings on the surface.

Some of them could have been created by a process that doesn't exist on Earth at all: the sublimation of natural dry ice.

Dry ice is frozen carbon dioxide, and there's a lot of CO2 on Mars.

Seasonally, CO2 forms frost on the ground in certain places.

Sunlight could shine through the dry ice, heating the base and vaporizing it into CO2 gas.

That process would release trapped martian soil with enough force to creat gullies.

But dry ice can't explain all the gullies on Mars'surface.

There are some in places where dry ice wouldn't have vaporized that way.

Some scientists argue that this process wouldn't produce the gullies from scratch, it would just make them bigger.

So there could be a few different processes at work.

Another possibility is that the gullies are formed by gases or winds that blow dust and rocks down steep hills.

Some of the gullies near the equator even look like they could be volcanic.

But recent evidence from a 4.5 billion year-old Martian meteorite seems to suggest that Mars has indeed lost a lot of water.

Plus, we've seen some meandering gullies that really look like they could only have been formed by water.

Those two observations have reignited the idea that there was a lot of water on Mars in the past, including a whole ocean.

But for now, it's still controversial.

There's a lot of conflicting evidence, so we're a long way from knowing for sure.

Speaking of conflicts: the northern and southern halves of Mars are totally different, and we have no idea why.

The northern half of Mars is super flat, while the south has a bunch of huge mountains and volcanoes.

The most popular explanation is that a huge impact 4 billion years ago caused the split, around the same time as the impact that created our moon.

Back then, the solar system was a chaotic mess, with regular, almost constant collisions.

An impact in the northern hemisphere would have caused the volcanic activity that created the mountains in the southern hemisphere, sending magma shooting to the surface.

It is possible that that's what happened, but if it did, it would be the biggest known impact in the history of the solar system.

Plus, if a giant rock smashed into Mars, it should have left a pretty clear mark on its surface.

But there's no obvious crater rim in the northern hemisphere.

Some researchers have proposed that a series of smaller impacts caused the divide instead,

but one enormous impact seems to fit our simulations better.

Another problem is that if early Mars was that volcanically violent,

it would be even less likely to have a liquid water ocean, since the planet would have lost much of its water very early in life.

So it might turn out that the leading explanations for these last two Mars mysteries can't both be right.

But one thing's for sure: there's still a lot we have left to learn about our next-door planet.

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