rover touched down on Mars in one of the most complicated space landings ever.

But in August, the rover celebrated its fifth birthday.

Our little robot's growing up so fast!

And for a kindergartener, it's made some really impressive discoveries.

Curiosity's main mission is to find out if Mars is or ever was habitable.

Because for a planet to support life, all of its systems have to be just right.

Among other things, the soil has to contain enough nutrients; the atmosphere has to have

the right composition and thickness; and, as far as we know, there needs to be at least

some amount of water.

Over the last five years, Curiosity has taught us about all of those things from its home

in Mars' Gale Crater.

For one, it recently confirmed the crater used to have at least one habitable lake,

based on sedimentary rocks on Mount Sharp, a mountain in the crater.

The structure and chemistry of the rocks point to a lake-y environment.

Which is awesome!

But Curiosity has also been finding the remains of a watery environment for years.

In 2014, it found deposits made of mudstone, a kind of rock made up of tiny clay particles,

which were very similar to the ones it later found on Mount Sharp.

And on Earth, mudstone is found in former lake beds and riverbeds.

At the time, that wasn't enough evidence to definitively say, “Yes!

There was totally a big old lake here!”

But we did think water was probably flowing all throughout that area -- and now, thanks

to some more investigating, we're pretty confident that big old lake was actually there.

And because of Curiosity, we now know Mars used to have rivers, too.

Back in 2013, Curiosity found a bunch of conglomerates, which are basically some pebbles cemented

together by sandstone, mudstone, or something similar.

What was really neat about these conglomerates was the shapes of the pebbles: They were smooth on all sides.

That happens when pebbles are subjected to eroding forces from every angle — like if

they're suspended in a river.

So these rocks were direct evidence for rivers on ancient Mars!

Curiosity also looked for the kinds of molecules that life needs.

Life as we know it requires six basic elements: sulfur, phosphorus, oxygen, nitrogen, carbon, and hydrogen.

Carbon is at the heart of organic chemistry, but alone, it can't do much.

When you add in the other elements, though, you get the molecules that work together to

make life -- DNA, proteins, sugars, you name it.

In 2014, a set of papers was published in the journal Science that showed Curiosity

found all of those elements, as well as more complex molecules, when it drilled into the

rock in Gale Crater.

And in 2015, another paper was published showing even more organics in the same area.

That doesn't mean there actually was life on Mars.

We don't have an answer on that yet.

But we do know that the raw materials are there, which is still kind of a big deal.

And then there's the atmosphere.

Right now, Mars' atmosphere is pretty unimpressive, except for when it whips up big dust storms.

Mostly, it's just thin and cold.

But Mars might have once had a thicker atmosphere — the kind that could have supported life.

So Curiosity looked into when that atmosphere thinned out.

In 2013, the rover took samples of the atmosphere, measuring both the types of elements and the

abundance of each element's isotopes.

Isotopes are different versions of the same element — they just have different masses,

which corresponds to the number of neutrons they have.

When scientists analyzed the data, they found that, for each element, there were a lot more

heavy isotopes than they expected.

Which was not good news when it comes to whether there was once life on Mars.

Heavier isotopes are usually less common than lighter ones, so researchers don't think

there just happen to be a bunch of heavy isotopes on Mars.

The lighter isotopes had to go somewhere.

When there aren't a lot of chemical reactions going on, heavy isotopes tend to hang out

below the lighter ones, which happens because an isotope's mass affects its velocity and thermodynamics.

That's probably how Mars lost its lighter isotopes.

Billions of years ago, the lighter isotopes moved upward in the atmosphere, closer to

space, and were stripped away by solar winds, which left behind a thin atmosphere with a

higher proportion of heavy isotopes.

When scientists compare these measurements from Curiosity to really, really old Martian

meteorites -- like, almost 4 billion years old -- the isotope ratios are still very similar.

That suggests that most of this atmospheric loss took place sometime within the first

500 million years or so of Mars' history.

So there was a relatively thick atmosphere, but only for a little while.

That doesn't necessarily mean Mars isn't or never was habitable, just that it's been

home to some extreme, low-pressure environments for a very long time.

From water to complex molecules to the atmosphere, Curiosity has made some amazing discoveries

in five years, and it's still looking for more.

So here's to the next five years, li'l buddy!

You're doing such a good job up there!

Thanks for watching this episode of SciShow Space.

For more on Curiosity's past and future, you can check out our video on how long the rover will last.