By David Warner 
When we look at Mars today, we see a cold, dry, and dusty red desert. Its atmosphere is incredibly thin, and any water on the surface is frozen solid as ice. It is a harsh and empty world. But science tells us that this was not always the case. Billions of years ago, the Mars we see today was a completely different planet. It was a dynamic, active world with a thick atmosphere, clouds, and vast amounts of liquid water.
Thanks to the hard work of robotic explorers like the Perseverance and Curiosity rovers, as well as orbiters like MAVEN, scientists are piecing together the story of Mars’s ancient past. Every new piece of evidence, from the rocks in a crater bed to the minerals in the dust, is a clue. And recent discoveries, even those made in 2025, are painting an increasingly clear and surprising picture. This ancient Mars was not just a little different; in some ways, it would have been almost as recognizable as a young Earth.
This leads to the most exciting questions of all: What did this younger version of Mars actually look like? If you could stand on its surface three or four billion years ago, what would you have seen, felt, and heard? How do we even know what it was like so long ago?
This is one of the most common questions, and the answer is surprisingly complex. Today, Mars is called “The Red Planet” for a very obvious reason. Its surface is covered in a fine, rusty dust. But it probably was not always that way. In fact, its red color is one of the biggest clues to its watery past.
Recent research published in 2025 has helped solve this mystery. The red color comes from iron oxides. For a long time, scientists thought this “rust” formed slowly over billions of years as the surface was exposed to the air, like a piece of metal left out in the yard. But new studies show the main mineral responsible for the red color is something called ferrihydrite. This specific type of iron mineral is not just dry rust. It is a nanoparticle that, scientists have found, requires liquid water to form.
This means Mars was not just a dry planet that later rusted. Mars turned red because it was wet. The water on the surface reacted with the iron-rich volcanic rocks, creating this rusty mineral. So, what color was it before it rusted? The rock on Mars is mostly basalt, which is a dark, greyish-black color, similar to the volcanic rock you see in Hawaii. Therefore, ancient Mars was likely a planet of dark, grey-black landscapes, not red ones. It would have been a world of dark rock, blue water, and white clouds, which was only slowly turning a rusty, muddy red in the places where water and rock met.
If you stood on Mars today, you would see a thin, pale, pinkish-red sky. This is because the atmosphere is very thin (less than one percent of Earth’s) and filled with the fine red dust that gets kicked up from the surface. But the sky on ancient Mars would have looked completely different.
Four billion years ago, Mars had a much, much thicker atmosphere. This thick blanket of air was the key to its entire climate. With a denser atmosphere, the air pressure would have been far higher. While you still could not breathe the air, the pressure might have been high enough that liquid water could easily exist on the surface without boiling away. This thick atmosphere would have also scattered sunlight differently.
On Earth, our thick atmosphere scatters the blue light from the sun more than other colors, which is why our sky looks blue. This is called Rayleigh scattering. With its own thick atmosphere, ancient Mars would not have had a red sky. It likely had a pale, possibly whitish-blue sky. It might not have been the deep, rich blue of Earth, but it would have been familiar. This thicker air would also have carried sounds, meaning you would have heard the wind, the splash of water, and the rumble of distant volcanoes.
This question has been one of the biggest puzzles for scientists, known as the “faint young sun paradox.” Four billion years ago, our sun was much younger and about thirty percent dimmer and weaker than it is today. With so much less sunlight, Earth itself should have been a frozen snowball, and Mars, which is even farther from the sun, should have been frozen solid. Yet, all the evidence clearly shows that Mars had liquid water.
The answer is that ancient Mars had a powerful greenhouse effect. Its thick atmosphere acted like a blanket, trapping the sun’s heat. For decades, scientists assumed this blanket was made almost entirely of carbon dioxide (CO2), which is a well-known greenhouse gas. However, modern computer models showed that even a very thick CO2 atmosphere was not enough to keep the planet warm; much of the CO2 would have frozen and fallen as dry ice.
More recent theories, supported by new models, suggest a different key ingredient: molecular hydrogen (H2). In addition to CO2 and water vapor, Mars’s many active volcanoes would have pumped massive amounts of hydrogen gas into the atmosphere. Hydrogen is a very light gas, but it is an incredibly powerful greenhouse gas. Even a small percentage of hydrogen mixed with the carbon dioxide would have been super-effective at trapping heat, allowing the planet’s temperature to rise above the freezing point of water. This is what made the “cold and wet” model of ancient Mars possible. It was not a tropical paradise, but it was warm enough for water to flow and pool.
Yes, the evidence is now extremely strong that Mars not only had water, but it had a massive ocean. This is known as the “Mars Ocean Hypothesis.” This ancient ocean, sometimes called Oceanus Borealis, would have covered almost the entire northern hemisphere of the planet.
The northern third of Mars is very different from the southern two-thirds. The south is high, rocky, and heavily cratered. The north is a vast, smooth, low-lying plain, almost like a giant, empty basin. This basin is, on average, several miles lower in elevation than the rest of the planet, making it the perfect place for water to collect. Scientists estimate that this northern ocean may have covered up to one-third of the entire planet and held more water than Earth’s Arctic Ocean. In some places, it could have been over a mile deep.
We have several lines of evidence for this ocean. First, all the giant river valleys and outflow channels we see on Mars flow downhill, and they all flow toward this northern basin. Second, scientists have used orbiters to trace what appear to be ancient shorelines, named the Arabia and Deuteronilus shorelines. These features wrap around the northern basin at a consistent elevation for thousands of kilometers, just like a bathtub ring. More recent radar data has even found what looks like a buried, sloping beach, made of sand and pebbles, hidden just beneath the surface.
The giant northern ocean was not the only water on the planet. The ancient Martian landscape was shaped by water in many forms. The most direct evidence we have comes from the rovers, which are driving directly on top of ancient riverbeds and lakebeds.
The Perseverance rover is currently exploring Jezero Crater. This crater is not just any crater; it is a confirmed ancient lake delta. This means a powerful river once flowed over the crater rim and into a large, standing lake that filled the crater. As the river entered the lake, it slowed down and dropped all the sand and mud it was carrying, building up a classic fan-shaped delta, just like the Mississippi River delta in Louisiana. The Curiosity rover, in Gale Crater, also found clear evidence of a large, long-lasting lake.
Beyond these calm lakes, Mars also had catastrophic megafloods. We see enormous “outflow channels” carved into the surface. These are not like normal river valleys. They are massive, scoured-out canyons, miles wide, that were likely carved in a very short amount of time. Scientists believe these were formed when massive amounts of water, perhaps from melting ice sheets or giant underground reservoirs, were suddenly released, flooding the landscape with more water than all of Earth’s rivers combined.
There is even evidence for tsunamis. Some scientists were puzzled why the ancient ocean shorelines were not always perfectly clear. The answer could be giant waves. If a large asteroid or comet slammed into the northern ocean, it would have created a mega-tsunami with waves hundreds of feet high. These waves would have raced across the ocean, crashing into the coastlines and completely washing away the evidence, which helps explain the jumbled, messy appearance of the landscape at the ocean’s edge.
If ancient Mars was so wet and had such a thick atmosphere, where did it all go? To understand this great disappearance, we have to look deep inside the planet. Earth is protected from the sun’s harsh radiation by a strong, global magnetic field. This field is generated by the movement of liquid metal in our planet’s outer core, a process called a dynamo. This invisible shield, called the magnetosphere, deflects the “solar wind,” a constant stream of high-energy particles from the sun, and keeps it from stripping our atmosphere away.
Mars used to have one of these, too. We know this from “paleomagnetism.” In the oldest parts of Mars, in the southern highlands, are rocks that are over four billion years old. When these volcanic rocks first cooled and solidified, tiny magnetic minerals inside them (like pyrrhotite) aligned themselves with the planet’s magnetic field, just like billions of tiny compass needles. These rocks are still “frozen” in that alignment today, acting as a permanent record that Mars once had a strong magnetic field, just like Earth’s.
But Mars is much smaller than Earth. Because it is smaller, its internal core cooled down and solidified much faster. As the core “froze,” the dynamo stopped. The magnetic field-generating engine shut down. By about 4 billion years ago, Mars’s protective shield was gone.
Once the magnetic shield was down, Mars was left completely defenseless against the sun. At that time, the sun was young, but it was also more active and violent, blasting out a much stronger solar wind than it does today.
This solar wind slammed directly into the top of Mars’s thick atmosphere. This process, which NASA’s MAVEN orbiter is still studying today, is called “atmospheric sputtering.” The high-energy particles from the sun acted like tiny billiard balls, hitting the gas molecules in Mars’s upper atmosphere and knocking them away, one by one, into deep space. The lightest gas, hydrogen (the key greenhouse gas), was lost first and fastest.
Over millions of years, this sputtering process stripped the Martian atmosphere away, molecule by molecule. As the atmosphere thinned, the air pressure dropped. This caused a chain reaction. First, with the greenhouse gases gone, the planet began to cool rapidly. Second, as the pressure fell, it reached a point where liquid water was no longer stable on the surface. It could not exist as a liquid anymore. The great northern ocean, the lakes, and the rivers began to boil away into vapor in the thin air, a process called sublimation.
Once in the air as water vapor, those water molecules were also broken apart by solar radiation and stripped away into space. The rest of the water froze solid, becoming the polar ice caps we see today, or was locked away as permafrost, a massive layer of ice buried just beneath the planet’s surface. Mars transformed from a wet, blue-and-grey world into the frozen, red desert we see today.
This is the biggest question of all, and it is the primary reason we send rovers to Mars. Based on everything we know, ancient Mars had all the ingredients necessary for life as we know it. It had liquid water in oceans and lakes for long periods. It had energy sources, like heat from volcanoes and light from the sun. And it had the key chemical building blocks, including organic molecules, which our rovers have confirmed are present in ancient Martian rocks.
This “habitable” period on Mars, known as the Noachian period, was happening at the exact same time that life was first emerging on Earth. If life could start here, could it have started there, too? We do not have a final answer, but in 2025, scientists announced the most exciting clue yet.
The Perseverance rover, driving through the ancient river channel in Jezero Crater, analyzed rocks in an area called “Bright Angel.” Inside these rocks, it found “potential biosignatures.” A biosignature is not a fossil of a creature, but rather a chemical or mineral trace that is very difficult to explain without a biological process. In this case, the rover found minerals like vivianite and greigite. On Earth, these specific minerals are often created as a byproduct of microbes. Essentially, they are the chemical “leftovers” from tiny organisms “eating” organic matter in the mud.
This is not yet proof of life. Scientists are careful to say that there might be a non-biological, purely chemical way for these minerals to form. But it is the “strongest hint yet” and the “closest we’ve ever come” to finding evidence of past life on Mars. The only way to know for sure is to get those rock samples back to Earth, a mission that is now being planned.
The story of Mars is a dramatic one. It is not just a red rock; it is a “fossil planet.” It began as a dynamic, watery world with a thick, blue-ish sky, a huge northern ocean, and massive volcanoes. It was a world that was actively “rusting” as its water reacted with its dark volcanic rock. And it was a world that may have even hosted the first sparks of simple, microbial life.
But its small size doomed it. Its core cooled, its magnetic shield died, and the sun slowly sandblasted its atmosphere and water away into space. This transformed it into the frozen, preserved desert we see today. Mars is a postcard from the distant past, showing us what a young, rocky planet can look like and what can happen when its protective systems fail.
As our rovers continue to explore and as we prepare to one day bring pieces of Mars back to Earth, we get closer to answering the final, thrilling question. If life did start in those ancient Martian oceans, could it still be hiding somewhere deep underground, where liquid water might still exist?
Ancient Mars was likely not red. Its surface is made of dark, grey-black volcanic rock (basalt). The planet only turned red as its vast amounts of liquid water reacted with the iron in the rocks to create rusty minerals like ferrihydrite. So, it was a dark grey planet that was actively “rusting.”
New research from 2025 shows Mars turned red because of a specific iron mineral called ferrihydrite. This mineral can only form in the presence of liquid water. This means the planet’s red color is not from being dry, but is instead direct evidence of its ancient, “cold and wet” past.
Ancient Mars was not a tropical paradise. Scientists now believe it had a “cold and wet” climate, similar to Earth’s arctic regions. It was just warm enough for liquid water to exist, thanks to a thick atmosphere of carbon dioxide and powerful greenhouse gases like hydrogen.
The main evidence is the northern hemisphere of Mars, which is a giant, low-lying, smooth basin. We see huge river valleys that all flow downhill into this basin. Scientists have also mapped what appear to be ancient shorelines at a consistent elevation around its edge.
Mars lost its magnetic field, which allowed the solar wind to strip away its thick atmosphere. As the atmosphere thinned, the air pressure dropped so low that liquid water was no longer stable. It boiled away (sublimated) and was lost to space, or it froze solid into the polar ice caps and underground ice (permafrost).
A potential biosignature is not a fossil, but rather a chemical, mineral, or structure found in rock that could have been created by life. It is a “hint” of life, but it could also potentially be made by non-living, geological processes, so it is not considered final proof.
In 2025, NASA announced that the Perseverance rover found potential biosignatures in Jezero Crater. It discovered minerals like vivianite and greigite in ancient lakebed rocks. On Earth, these specific minerals are often produced by microbes as a waste product, making this the strongest hint of past life on Mars found so far.
Mars is smaller than Earth, so its internal core cooled down and solidified much faster. Earth’s liquid outer core is what generates its magnetic field. When the Martian core solidified, this “dynamo” stopped, and the protective magnetic field shut down.
The Noachian period is the name for Mars’s most ancient geological era, from about 4.1 to 3.7 billion years ago. This is the period when Mars had a thick atmosphere, liquid water, oceans, lakes, and active volcanoes. It is the time when the planet was most “Earth-like” and when life may have had a chance to start.
While the surface of Mars is too harsh for life as we know it (extreme cold, high radiation, low pressure), some scientists believe life could persist. If life ever started, it may have retreated underground. Deep beneath the surface, it could be warm enough for liquid water to exist, potentially supporting microbial life.