A recent analysis of clay pebbles from Mars' Jezero crater suggests the planet experienced a warm and wet climate during the Noachian epoch billions of years ago. This finding challenges the prevailing view of a cold and icy environment at that time. The evidence comes from NASA's Perseverance rover and points to conditions potentially suitable for life.
The Noachian epoch on Mars, spanning from about 4.1 to 3.7 billion years ago, coincided with the Late Heavy Bombardment, a period of intense meteorite impacts across the solar system. Prominent features from this era include the vast Hellas and Argyre impact basins, each over a thousand miles across and capable of holding volumes of water comparable to the Mediterranean Sea.
Despite the cataclysmic conditions, geological evidence such as dried-up river valleys, lake beds, ancient coastlines, and river deltas indicates water once shaped the Martian landscape extensively. Scientists debate the climate during this time: one scenario posits a cold and icy world with occasional melting from impacts or volcanism, while the other suggests a warm, wet, and largely ice-free environment.
The Sun was approximately 30% dimmer then, requiring a thick Martian atmosphere rich in greenhouse gases like CO2 to maintain warmth. However, high CO2 levels could lead to cloud formation, potentially countering the greenhouse effect.
NASA's Perseverance rover, which landed in Jezero crater in February 2021, targeted the site due to its ancient lake history and visible water-carved channels with clay deposits. A new study examines aluminum-rich kaolinite pebbles from these channels, which show depletion in iron and magnesium but enrichment in titanium and aluminum.
These compositions indicate alteration under modest temperatures and persistent heavy rainfall, rather than brief hydrothermal events from melted ice. The researchers compare the pebbles to Earth clays from past greenhouse climates, concluding they formed under high rainfall conditions similar to those on Earth. The paper states these intervals, lasting thousands to millions of years, likely represented some of the wettest and most habitable periods in Mars' history.
Perseverance also collected samples last year from Jezero crater containing possible biosignatures, now cached for a future sample return mission. However, NASA recently cancelled that mission, delaying Earth-based analysis. Astrobiologist Andrew Knoll's criterion emphasizes that potential life evidence must be inexplicable without biology.