Our tale of two planets begins four billion years ago. One planet was Earth, and the other planet was Mars, and the two had much in common in their infancy. Rivers and lakes etched their surfaces, craters pockmarked their faces, and volcanoes rose from their plains. But something seems to have changed on one and not the other.

In Earth’s burbling warm water, fate and chemistry combined amino acids into complex molecules, and in a process we still don’t understand, these gave rise to single cells that figured out how to make copies of themselves. Tiny mistakes in those copies eventually turned them into oxygen-exhaling organisms we call algae. Endless forms flowed from these humble ancestors, and after eons, there we were: All of human culture and hope and possibility arising within a tiny slice of time.

Mars was not so lucky. Mars dried up. Mars is small, about half Earth’s diameter, so it cooled off faster than Earth did after their birth in the cloud of dust left over from the sun’s creation. Compared to its overall volume, more of Mars’ mass is exposed to the icy blackness of space. As it cooled, its iron-nickel core solidified. When this happened, we think, the Martian magnetic field shut down, robbing Mars of its protective shield, of the sort that still safeguards Earth from solar and cosmic rays. Time and the brightening sun stripped away the Martian atmosphere before the planet’s algae, if it existed, had a chance to make the air thick and warm. Mars turned to rust before any skeletons could adorn its deserts, before any creatures could look up and contemplate their place among the other dots in the night sky. While Earth is fecund and bursting with life, Mars is, and may have always been, barren.

To me, this is why Mars is the best planet. A few simple changes turn its history into our history, and vice versa. That’s the key thing; that could have been us.

Ashwin Vasavada has a similar view. He is the project scientist for the Mars Science Laboratory, the six-wheeled robot car we know and love as Curiosity. He can quote the easy answer for why we go to Mars, the assumption most scientists and science writers make: Mars is close. It’s practically right next door, and you can fling a robot there in half a year.

“That’s the NASA answer. It’s the most accessible place for life other than on Earth. But I have my own answer,” he says. “It’s a place that you can go today that’s like going to early Earth. You remove that dusty exterior of Mars, and you have this planet that is just so reminiscent of Earth. It’s like finding a dusty Earth in your attic. Shake off the dust a little bit, and it’s this amazing place that you can recognize. That’s why I like it.”

(NASA / JPL-Caltech / MSSS)

Mars would seem familiar to anyone who has seen the national parks of the American West, especially the ones full of wind-whipped rock formations and surprising color. The terrain at Gale Crater’s Mount Sharp, where Curiosity has been trundling along since 2012, might as well be Utah or Colorado. The rocks are reddish brown, sun-baked, and partly blanketed in sand dunes. Their carved-away hillsides are jagged, however — no rivers or softening rains have given them Earth’s gentle countenance.

Of course, Mars was familiar to us long before we sent robots there. With Mercury, Venus, Jupiter, and Saturn, Mars is one of the night sky’s unblinking wanderers, visible with eyes alone. The planets have accompanied human culture since we started writing stories. The next planet over has been a fixture in myths dating to the Babylonians, who called it Nergal, after the god of destruction. In Roman mythology, Mars was the god of war and destabilization. This figure also appears in various forms in Greek, Norse, and Hindu mythologies. To ancient Chinese, it was Ying-huo, the Shimmering Planet.

Even now, we acknowledge Mars on the third day of every week. In romance languages, the name comes from the Latin day of Mars, or “Dies Martis” — that became martes in Spanish, mardi in French. In Norse mythology, Mars is associated with the god Tyr, so our Tuesday comes from the Old English word Tiwesdæg. We also honor Mars on the third month of every year. Bellicose Mars was also a protector of the Roman people and a patron of agriculture, so the month named for this god marked the beginning of the growing season.

Mars has always stood out from the other wanderers, in part because it is so obviously red, a ruddy, unblinking dot hanging with an air of menace. Its hue — which comes from oxidized iron, in the same chemical reaction that turns blood red — linked Mars to war, and to death, long before we knew it was dead in a literal sense. What’s more, it moves backward, or seems to. The sun, moon and stars rise in the east and fall in the west because of Earth’s rotation. But the planets orbit the sun at different rates, and so sometimes, Earth will lap one of them, like a runner on an inside track lane. From our perspective on Earth, the other planet seems to be moving west to east. This aberrant behavior has long been associated with omens or astrological predictions.

Its omnipresence in our sky made Mars a prime target as soon as we figured out how to use glass to make the night sky’s features appear larger. By the 17th century, astronomers resolved its polar ice caps through telescopes — perhaps one of the earliest discoveries that the fourth planet shared something in common with ours. And the more we looked, the more we found these similarities. Mars is the best planet because Mars and Earth have more in common than any other worlds in the solar system. It cowers next to humongous Jupiter, but unlike that gas giant, its hard surface beckons visitors. Mars lacks our dewy, oxygen-rich atmosphere, but neither is it shrouded in a poisonous, bone-crushingly dense one like Venus. Its day (called a sol) is just 40 minutes longer than our own. Its axis is tilted slightly more than ours, at 25 degrees, unlike weirdly slanted Uranus. And anyone who argues it is ugly, especially compared to the art deco elegance of Saturn, is simply mistaken. Mars is lovely to behold. Mars has snow. It has mountains, and lake beds, and recognizable landscapes. Earth and Mars are the same in so many ways. And yet. The biggest difference is the only one that really matters. The only life on Mars is the kind we imagine.

Martian fantasies grew in complexity alongside Martian observations. Astronomers frequently turned to the red planet when Mars was opposed to the sun and close to Earth, making it appear larger and brighter. The most famous of these was the opposition of 1877, in which the Italian astronomer Giovanni Schiaparelli observed networks of lines on Mars, which later turned out to be optical illusions (though Mars does have streaks of water lining its slopes today). He called them canali, which was translated into English as “canals.”

This momentous finding brought Mars closer to Earth than ever. It became even easier to imagine Mars as a place just like Earth, positively crawling with life. “The present inhabitation of Mars by a race superior to ours is very probable,” the French astronomer Camille Flammarion wrote in 1892. Around the same time, the American astronomer Percival Lowell scrutinized Mars extensively. He believed he saw “non-natural features,” including canals, which he imagined were devised to transport water from the drying planet’s ice caps. Mars soon loomed even larger in science fiction and pop culture. By 1897, H.G. Wells’ “War of the Worlds” imagined Mars as a slowly drying planet full of desperate beings who launch rockets to Earth, where they feast on human blood.

If Mars was arguably the first place storytellers imagined we would find aliens, it was indeed the first place we went looking for them. Project Ozma, in which astronomer Frank Drake pointed a radio telescope at the stars Tau Ceti and Epsilon Eridani, is the traditional SETI origin story. But we listened to Mars first. In Aug. 22, 1924, the head of the U.S. Navy, Edward Eberle, instructed all naval stations to turn their receivers toward Mars. The red planet was at its closest approach to Earth in 120 years, and some astronomers thought Martians might use the opportunity to make contact over the airwaves. Onshore stations were advised to listen to as many frequencies as possible, and to “report any electrical phenomenon [of] unusual character,” according to a telegram from Eberle. If someone wanted to talk, the Navy was ready to listen.

No Martians made contact that day, because there are no Martians, as far as any of our satellites and robots can tell. But this has not stopped our storytellers, and it certainly has not stopped our scientists. We have been attempting to land spacecraft on Mars for nearly 50 years, and almost all of them have been looking for life, in one way or another.

The history of Mars landings has shown that the planet is anything but hospitable now, however. And that goes for machines just as much as microbes. More than half of the robots sent to Mars have been destroyed in the process, most recently last fall. The European Space Agency’s Schiaparelli lander plummeted through the atmosphere Oct. 19, but crashed after it cut its parachute too soon and its hovercraft-like retrorockets didn’t fire long enough.

Still, those lucky few that have made it, most notably Curiosity, have shown us Mars was habitable in its past. The fleet of orbiters looping around the planet have sent back data that suggests there is some water there today, mostly at the poles. But we are still not sure if Mars had lasting oceans, or just big lakes and rivers, says Ray Arvidson, a renowned planetary scientist who has directed or taken part in every NASA Mars mission since Viking. And we certainly don’t know whether it had life.

“It’s not a given. It’s not a shoo-in, by any means, that Mars developed life, and if it did, that the evidence is still there,” Arvidson says. “Whether or not you get organic molecules that move to prebiotic compounds, that then move to replicating systems, that’s a big leap.”

Even assuming that leap happened, Vasavada and others say we are more likely to find evidence of ancient life than modern life. Extant or extinct Martians would probably be bacteria or some other simple cells, definitely not limbed beings that communicate using light or language. In this way, Mars may seem disappointing to some. But it is the best planet precisely because a null result, as scientists would call it, would raise an even bigger question: Why here? Why us?

“I think it just becomes a real scientific mystery if we don’t find life. That tells us we don’t quite understand how unique life is on Earth,” Vasavada says. “If you don’t find it, that almost becomes more interesting, and makes you get more existential about life on Earth. That’s where I am now.”

For those who take an even longer existential view, Mars is important — vital, even. When Elon Musk unveiled his plans for giant rockets and spaceships to transport human settlers to Mars, he invoked not only exploration, but our shared future. Mars is an opportunity for humans to carry forward the light of consciousness, letting it propagate alongside us, and to survive after we are gone, much like the first cells on Earth found a way to send copies of themselves into the future. If Musk and other dreamers have their way, the first life forms on Mars might be us.

But make no mistake: It will be a horrible, destructive journey. Unlike the territories of colonial history — the West Indies, the American West and other frontiers — Mars does not tickle the mind with dreams of untold riches. Mars is no El Dorado. Its atmosphere does not hold any heat. It has no pressure to prevent your blood from vaporizing. Without a spacesuit, you would literally boil and freeze to death, simultaneously. Mars travelers would be consigned forever to pressurized domes or, more likely, radiation-shielding caves. They would never again see waves lapping at a shoreline. They would never again hear the wind singing through pine trees. They would never again be surprised by the sight of a silvery crescent moon.

(NASA / JPL-Caltech / MSSS / Texas A&M Univ.)

Why go, then?

I was at JPL when Curiosity landed in August 2012, and I couldn’t help clapping along with the NASA engineers and scientists who whooped and hollered at the news. The dust had barely settled after Curiosity’s audacious sky crane landing when the rover sent back its first postcard, the grainy image you see here.

“It’s the wheel! It’s the wheel!” someone in the control room shouted. Squinting at the black-and-white image of a wheel on a rocky plain, I felt a rush of emotion. The rover’s belly cast a shadow in the afternoon sun. The scene looked so familiar, but felt so wrong. It could have been the mountain West where I grew up, only it was empty, lifeless but for the robot now perched on the sand.

If Saturn is a spur to the scientific imagination, letting us glimpse how far our minds must go to meet what the cosmos has in store, Mars does the opposite. It is a sign of how unlikely we are. It makes us confront how fragile the Earth is. It’s a reminder of how lonely we remain on our pale blue dot, the only home we have ever known. Mars is the best planet because Mars is a mirror. We look to it and we see ourselves — our past and possibility, and, with some imagination, our future.