"Mars once was wet and fertile. It's now bone dry. Something bad happened on Mars. I want to know what happened on Mars so that we may prevent it from happening here on Earth." -Neil deGrasse Tyson

Oh, it's true alright, something bad did once happen on Mars. And although there isn't any real danger of that happening to Earth, a little conversation I had earlier this week made me think that it's time to tell all of you a story about our red neighbor, and why it is the way it is today.

Image credit: NASA / Viking Orbiter.

You see, when we think about Mars, we think about the smallish, red, desolate world that fascinates us today. We mostly think about the tremendous impact craters, the large mountains and the dry, rocky terrain.

But Mars wasn't always this way, and we know it. There are clues to that, even today. Aerial views from orbiting spacecraft show us phenomena like dried up riverbeds with oxbow bends in them,

Image credit: ESA / Mars Express; yes, that's false color!

polar ice caps and occasionally atmospheric clouds,

Image credit: NASA, ESA and the Hubble Heritage Team (STScI / AURA).

and recently, landers and rovers have found sedimentary structures in the terrain, silica-rich layers beneath the surface, and even hematite spheres, all evidence that water once flowed freely across the Martian surface.

Image credit: NASA / JPL / Cornell / Mars Opportunity Rover.

But no longer, and not for more than a billion years, to the best of our knowledge. In fact, liquid water is pretty much an impossibility on Mars today! You probably remember that like all substances, water exists in the solid, liquid or gaseous phases dependent on the pressure and temperature of its environment. While all three phases are common on Earth, only two of those phases -- the solid and gaseous ones -- are possible if the pressure is below a certain threshold, occurring at the triple point of water.

Image credit: Science Education Resource Center @ Carleton College, via http://serc.carleton.edu/

Unfortunately, the average atmospheric pressure on Mars is just 0.6% of what it is on Earth, placing it below the triple point of water and making the liquid phase an impossibility at all locations, excepting the depths of the deepest Martian trenches.

But the overwhelming evidence for a watery past tells us it wasn't always this way. In fact, if we go back to the early Solar System, Mars and Earth likely weren't all that different.

We know that our home world -- in its infancy -- was different from Earth today in a number of important ways. The atmosphere was rich with hydrogen, as the most common gases were water, ammonia, methane and hydrogen gas, all excellent at trapping heat. Even though the Sun was only about 80% as luminous as it is today, Earth still had vast oceans and liquid water throughout its surface, bombardment by asteroids and comets was many orders of magnitude more common than it is today, and all the organics necessary for complex life -- the building blocks of everything in our biosphere today -- was in place.

And to the best of our knowledge, aside from being a bit smaller and farther from the Sun, so was Mars.

We know that life took hold on Earth relatively quickly, within the first few hundred million years. We also know that life was able to sustain itself on Earth; Mars, to the best of our knowledge, was not so lucky. At some point -- perhaps after the first billion or two years -- those similar conditions on Earth and Mars, those conditions that were so conducive to life, became very different. We're not certain what happened, but we have a leading (and compelling) hypothesis.

Image credit: NASA.

We don't think about it on a day-to-day basis, but the Sun is constantly spewing out a stream of ionized, high-energy particles in all directions. If all we had, instead of Earth, was a big rock covered in gas, that stream of particles -- the solar wind -- would strip that gas away in short order. But that doesn't happen! The main reason that the Earth still has an atmosphere that's as thick as it is is because we've got a powerful magnetic field generated in our planet's core. We typically think of the magnetic field as it is on the Earth's surface, deflecting compass needles and aiding in navigation, but the reality is that it extends far into space! As high-energy, ionized particles stream towards us, the magnetic field deflects them, and protects us mightily from the solar wind, keeping our atmosphere intact.

But the Solar System is 4.5 billion years old now, and Mars is much, much smaller than Earth.

Image credit: NASA/JPL; Galileo orbiter / Mars Global Surveyor Orbiter.

Planets radiate heat away according to their surface area, and with a radius of just 3,390 km (or just 53% of Earth's), Mars has just 28% of Earth's surface area. But it's much lower in mass; Earth is approximately ten times as massive as Mars! Due to its much smaller mass-to-surface-area ratio, Mars has cooled much, much more quickly than Earth, all the way down to its core. At some point, the dynamic magnetic field that Mars once had (and it had one; we've already seen the relic magnetic field imprinted in Martian rock) ceased to be, and when that happened, the atmosphere of Mars was no longer protected from the solar wind.

Image credit: Lundin et al., Science, Vol. 305. no. 5692, pp. 1933 - 1936, doi:10.1126/science.1101860, 2004.

On a world where a thick atmosphere and liquid water ruled for maybe a billion years or more, the death of Mars' magnetic field meant that these high-energy, ionized particles would begin colliding with the particles in Mars' upper atmosphere, giving many of them enough kinetic energy to escape from the gravitational pull of the red planet! In the span of just a few million years, Mars went from a world teeming with organics, liquid water and all the building blocks of life to the desolate, barren and mostly frozen world we see today.

Image credit: John Raoux / AP.

At least, that's the leading hypothesis. The big news from earlier this week is that the mission that's about to test that hypothesis, Mars MAVEN, was just successfully launched!

We've already got rovers on the ground, digging into the soil, returning photographs, performing analysis and exploring the terrain in unprecedented detail. But if we want to know the story of what happened to Mars' surface, that means learning what happened to Mars' atmosphere, and MAVEN is going to be the first spacecraft to attempt to figure that out by measuring what's happening to Mars' atmosphere right now.

Image credit: NASA / LASP.

When it reaches Mars in September of next year, MAVEN will enter an eccentric orbit around the red planet, spending most of its time a significant distance -- thousands of miles (kilometers) -- above the top of the Martian atmosphere, but once-per-orbit, dipping down into the upper atmosphere and taking data. By measuring the solar wind and the flux of atmospheric particles escaping from Mars today, it's going to give us the first hard data that will allow us to extrapolate how Mars lost its atmosphere in the first place!

Image credit: Mark Thiessen / National Geographic.

One of the great things is that Mars Curiosity has similar instruments on board to the ones on MAVEN, so while MAVEN determines the atmospheric composition, elemental isotope ratios and presence-and-formation of volatiles (such as methane) in the upper atmosphere, Curiosity can do the same thing all the way down at the bottom of Mars' atmosphere. With data combined from both missions, this incredible hypothesis will be scrutinized and possibly either validated or falsified by the forthcoming data!

Image credit: Brendan Alexander of Donegal Skies, via flickr.

So when you see this cold, dry, red planet hovering in the night skies, know that it not only wasn't always that way, but that we just might be on the cusp of finding out how it came to be this way! And that's a little bit of insight into the story of Mars!