At an average of 140 million miles away, Mars is pretty much in Earth’s planetary backyard. With a well-timed launch, a rocket can reach it in less than a year. Since 1965, humankind has successfully sent over 20 missions—flybys, orbiters, landers, rovers—to survey Mars’s dusty terrains. And we’ve learned a lot about our neighbor, from its subterranean secrets to meteorological phenomena.
“It’s so close,” says Shannon Curry, a planetary physicist at the University of California, Berkeley, and the principal investigator of the Mars Atmosphere and Volatile Evolution (MAVEN) mission, launched in 2013. “We’re lucky that we get to explore it the way we can.”
Humankind’s instruments have mapped the corners of the planet, poked at the gravel underfoot and sampled the air that wafts across the dusty plains. Scientists have learned much from the data collected and beamed back to Earth. We spoke to four experts and dug through reports to figure out which otherworldly discoveries have made the biggest splash.
1. Mars is a cold, dry place
Before our first probes swung by Mars, scientists thought that the planet could be lush and teeming with life. With their telescopes, early scientists made out canal-like features on the surface that they mistook as artificial structures built by intelligent species. But after the Mariner 4 probe flew by the Red Planet in 1965 during the first Mars mission and snapped pictures of a barren surface, these interpretations were proved wrong.
Thanks to that and other missions, we’ve learned that Mars is devoid of liquid water. Its rocky surface is pockmarked by impact craters, scraped by strong winds, brushed by slow-moving sand dunes, carved by deep valleys and punctuated by impressive volcanoes. Mars is also home to the tallest and largest volcano in the solar system: Olympus Mons, which rises two and a half times as high as Mount Everest at 13.6 miles. The most precise measurement of the volcano’s dimensions so far comes from the Mars Global Surveyor in 2004. Although Mars’s surface is bone dry, as far as scientists can tell, it has water—just not in liquid form. H2O is locked away as ice, atmospheric vapor and atoms embedded in the molecular structure of its minerals.
Mars is a still and quiet place compared to Earth, and it doesn’t harbor life as we know it. The planet is like a “freeze-dried environment,” says Kirsten Siebach, a Martian geologist at Rice University. The conditions allowed for Mars’s geologic record to be preserved for billions of years. “We can see this beautiful history recorded in the rocks,” she says. “There’s a bunch of stories that we’ll be able to tell as we understand and represent Mars better.”
2. The atmosphere is wispy and hosts dust storms
As Mariner 4 flew by Mars, it peered through the gassy sheath and picked up all kinds of information. For one, the atmospheric pressure is only 0.7 percent of Earth’s. The planet’s smaller gravity and lack of a magnetic field prevent it from retaining a thick gaseous shell. The atmosphere is so wispy that it doesn’t do a good job trapping heat from the sun to keep the surface warm throughout the day. Mars has an average temperature of minus 80 degrees Fahrenheit.
Given these frigid temperatures and low atmospheric pressures, Mars can’t support liquid water. If you were to place a tub of liquid water on the planet, most of it would quickly vaporize into gas; the rest would succumb to Mars’s bitter chill and freeze into ice.
Nevertheless, Mars’s atmosphere is thick enough to sustain dust storms, and they stir up so frequently that they’re a perennial feature of the Red Planet. In 1971, the orbiter Mariner 9 found an entire planet shrouded in dust after it first arrived. “The dust storms are wild,” Curry says, because they can encircle the whole globe and last for months. “They’re so large. It would frankly be terrifying to be there.” Inside these blizzards, dust particles whip along at speeds of up to 60 miles per hour. The dust particles consist of iron oxide, the same chemical as rust. These grains give Mars its signature red hue.
Since the atmosphere is so thin, collectively the winds can’t sweep a person off their feet. That’s one detail the movie The Martian got wrong: astronaut Mark Watney, played by Matt Damon, couldn’t have been stranded on Mars by dust storms for this reason. But these grimy gusts are a nightmare for engineers, as the particulates can stick to solar panels and reduce their efficiency. After 14.5 productive years, the NASA rover Opportunity went quiet in 2018 in a global dust storm that blotted out the sun and prevented solar power generation.
3. Mars once sustained liquid water
Starting with Mariner 9’s visit in 1971, scientists have collected ample evidence that liquid water once flowed on the planet. Mars’s topographical features—river runoff channels, carved gullies, eroded impact craters and mud cracks baked into the ground—hint at a watery past. Moreover, in 2005, the European Space Agency’s Mars Express orbiter detected hydrated minerals—rocks containing a telltale mix of hydrogen and oxygen atoms in their crystal structure—that could have only formed in the presence of liquid water.
Ever since the rover Curiosity landed on Mars in 2012, it has been wandering inside Gale Crater, which was formerly a lake bed. Curiosity has found proof that liquid water once pooled here. In 2014, it photographed sedimentary outcrops with a layered structure; scientists think they’re silty pileups that gushed from a river onto a delta.
Although Martian liquid water likely lasted for millions of years, it all but vanished two billion to three billion years ago. But its presence and persistence imply that long ago Mars experienced a thicker atmosphere and warmer temperatures. All this points toward an ancient environment suitable for harboring life.
4. Shifting polar icecaps tell of Mars’s climatic past
In the 17th century, scientists made telescopic observations that Mars’s poles appeared as bright spots set against a darker planet, but it was not until 1969 using Mariner 7 that humankind was able to take close-up images of the poles for the first time. Young icecaps bookend the Red Planet, and they are estimated to have formed only within the last five million years of Mars’s lifetime. The Mars Reconnaissance Orbiter and the Mars Express orbiter revealed that the polar ice is made of layers of solidified CO2, frozen water and windswept dust, and it grows and shrinks during the annual seasons. These layers are like tree rings—geological records of Mars’s seasonal climate that scientists can study.
The icecaps may disappear altogether and form anew over cycles that span hundreds of thousands of years. Scientists think this happens because Mars experiences episodes of planetary warming and cooling that stretch far beyond one loop around the sun. Modeling studies suggest that Jupiter’s not-so-gentle gravitational tug causes Mars’s axis to wobble between a 15-degree tilt and a more extreme 35 degrees. (By contrast, Earth’s axial tilt is 23 degrees and stays within 3 degrees of the average over millions of years.) At a larger slant, the smaller planet experiences more heating at the poles that can melt its icecaps entirely.
5. Mars has a mosaic of local magnetic fields on its crust
From their high perches above Martian ground, the MAVEN and Mars Global Surveyor satellites confirmed that Mars has no planetary magnetic dipole like Earth. However, mapping revealed pockets of regional magnetic fields stamped all over the crust. The magnetic mosaic comes from volcanic rocks at the surface that carry traces of Mars’s remnant magnetic field.
A planet’s magnetic shield comes from the churning of the sphere’s interior. Melt in the core consisting of swirling iron and nickel generates an electric current, which in turn powers a planet-sized electromagnet. Invisible magnetic field lines envelop most planets in the solar system, including Earth. Being a smaller planet, Mars cooled off much sooner than Earth did, and its roiling innards quietened. With that went Mars’s global magnetosphere. But metals near the planet’s surface captured a snapshot of the field as they solidified, and they have retained their magnetic imprint.
These scattered magnetic fields on the crust lead to spectacular auroras that look nothing like Earth’s. Auroras are caused by charged particles that rain down from the sun into the atmosphere. On Earth, these ghostly glows congregate at the poles, as the magnetic field is strongest there. But the auroras on Mars can shimmy close to the equator, since no planetary field confines them. Photographs taken by the United Arab Emirates’ Hope orbiter in 2021 show that the heavenly displays can take on all kinds of serpentine shapes.
6. Martian visits confirmed the origin of mystery meteorites on Earth
Among the thousands of meteorites in the scientific cache on Earth, a tiny fraction of them stand out from the crowd. Scientists didn’t know where they came from, but after the Viking landers arrived on Mars in 1976 and sampled the environment, compositional analysis of Mars’s atmosphere confirmed that the oddball rocks were Martian meteorites.
Around 300 such meteorites sit in scientists’ collections on Earth. These rocks were exiled from Mars after impactors struck the planet’s surface and kicked debris into space. The resulting fragments floated in the void for millennia until they made landfall on Earth. These rocks have led to thousands of publications since and given scientists a close-up peek at the rusty planet.
7. Mars was once a lot like Earth
“If I were to boil it down to just a few highlights, the discovery that Mars really was a lot more like the Earth is probably No. 1,” says Jim Bell, a planetary scientist at Arizona State University. We know that early Mars was cozy, wet, lusciously gassy and protected by a magnetosphere. We know that it was once a tectonically happening place, though seismic movement has abated. For some reason, Mars hopped onto a different trajectory and transformed into the barren world it is today.
But during that idyllic episode, Mars could have harbored life. In 2013, Curiosity identified in the rocks all the key chemical ingredients needed to support life, and around 2013 and in 2018, it even detected organic matter in the rocks. Most scientists think that Mars is lifeless now—although a few argue that life could be sheltering underground—but no one can say for sure whether the Red Planet could have also been a teeming one. “‘Habitable’ means [life] could have existed. It doesn’t mean ‘inhabited,'” says Bruce Jakosky, a planetary scientist at the University of Colorado Boulder and MAVEN’s former principal investigator. “But it justifies searching for evidence.”
Whether Mars has or has had life is the biggest mystery fueling much of the excitement about our ruddy neighbor. Whichever way the answers swing will illuminate Earth’s place in the universe. But for now, researchers don’t have the means to answer this question. Humankind’s limited meteorite samples are too weathered. Martian probes don’t have apparatus sensitive enough to say. The best way to tell is to collect rock samples from Mars and subject them to the scrutiny of the most advanced instruments on Earth. “That’s the only way we’re going to answer those questions,” Jakosky says.
The Perseverance rover is busy squirreling away Martian samples from Jezero Crater, where liquid water once existed. A return mission to retrieve these samples is slated for 2027. If all goes well, these specimens are due to arrive on Earth in 2033, when they may open up a whole new era of discovery.