Mars, a cold and arid planet today, presents a desolate landscape. Its surface is marked by craters, canyons, and vast plains, with a thin atmosphere offering little protection from harsh radiation. This contrasts sharply with evidence that Mars was once a different world. The planet underwent a profound transformation over billions of years, transitioning from a potentially water-rich environment to the dry, dusty sphere we observe.
Unveiling Mars’ Past
Scientists reconstruct Mars’ ancient environment using data from orbiters and rovers like Curiosity and Perseverance, which have provided direct evidence through geological analysis. They have identified features such as ancient riverbeds, deltas, and lakebeds, with Jezero Crater being a notable example of a former lake and river delta system.
The discovery of hydrated minerals like clays and sulfates further supports the presence of past water. These minerals indicate extensive water-rock interactions on the Martian surface. Analysis of Martian meteorites that have fallen to Earth also offers clues, with some containing evidence of exposure to liquid water. Observations of the polar ice caps and subsurface ice also suggest significant water reservoirs, though mostly frozen today.
A Warmer, Wetter World
Billions of years ago, Mars likely harbored vast amounts of liquid water, featuring expansive oceans, large lakes, and numerous rivers across its landscape. Geological formations like wave ripples found in rocks further confirm the existence of shallow, open bodies of water approximately 3.7 billion years ago. These conditions point to a different planet than its current state.
A thicker atmosphere, primarily composed of carbon dioxide, would have enveloped ancient Mars. This denser atmosphere would have created a greenhouse effect, trapping heat and maintaining temperatures above freezing, allowing liquid water to persist. Volcanism also contributed gases and water vapor to the atmosphere and shaped the terrain. Such an environment would have resembled early Earth, with clouds, rain, and active erosion carving the landscape.
How Mars Transformed
The transformation of Mars from a warmer, wetter world to its current cold and dry state began with the loss of its global magnetic field. This protective field, generated by a dynamo in the planet’s core, shut down billions of years ago, around 3.9 billion years ago. Without this magnetic shield, the Martian atmosphere became vulnerable to the solar wind, a stream of charged particles from the Sun.
The solar wind stripped away much of Mars’ atmosphere, leading to a drop in atmospheric pressure. This atmospheric thinning caused any remaining surface water to either evaporate directly into space or freeze into subsurface ice. Concurrently, the planet’s interior cooled, leading to a decrease in volcanic activity, which further limited the replenishment of atmospheric gases and water vapor. These combined processes altered Mars’ climate and landscape, leading to the desiccation observed today.
Searching for Life’s Echoes
The ancient conditions on Mars, characterized by liquid water, energy sources, and a substantial atmosphere, would have been conducive to the emergence of microbial life. Scientists are actively searching for biosignatures, or evidence of ancient life, within the Martian rock record. Missions like Perseverance are collecting rock and soil samples from locations such as Jezero Crater, which was once a lakebed and river delta. These samples are being sealed for a potential future return to Earth, where they can be analyzed with more sophisticated laboratory equipment.
The search also extends to the subsurface, as current conditions on Mars’ surface are harsh due to radiation and toxic perchlorates. Studies of Martian meteorites suggest that the subsurface may still hold conditions suitable for microbial life, similar to deep biospheres found on Earth. If life ever existed on Mars, the planet’s interior might offer a refuge, preserving clues to its past biological activity.