Imagine a time when Mars, the Red Planet, was not the cold, barren desert we know today, but a warm and wet world, teeming with potential for life. This mind-boggling concept is at the heart of a recent study that challenges our understanding of Mars' ancient past.
For decades, scientists have been captivated by the question of whether Mars was ever habitable. Mars, like Earth, is an ancient world, dating back around 4.5 billion years. Its geological history is divided into epochs, and the focus of this study is the Noachian epoch, a period from 4.1 to 3.7 billion years ago. This era, known as the Late Heavy Bombardment, saw catastrophic meteorite impacts across the solar system, leaving scars on many celestial bodies, including Mars.
Two massive impact basins, Hellas and Argyre, each over a thousand miles wide, bear witness to this tumultuous time. Despite their size, these basins could easily accommodate all the water in the Mediterranean Sea, suggesting a time when Mars was not just warm and wet, but also potentially habitable.
The climatic conditions of the Noachian epoch are a subject of intense debate. Two contrasting theories have emerged: one suggesting a cold, icy Mars with occasional melting due to meteorite impacts and volcanic activity, and the other painting a picture of a warm, wet, and largely ice-free planet.
Here's where it gets intriguing: our Sun, like all stars, brightens with age. During the Noachian epoch, the Sun was about 30% dimmer, meaning Mars received less heat. To maintain a warm climate, Mars' atmosphere would have needed to be significantly thicker and rich in greenhouse gases like CO2. However, at high atmospheric pressures, CO2 tends to condense, forming clouds and reducing the greenhouse effect. This leads many to favor the cold, icy scenario.
Enter the Mars 2020 Perseverance Rover, which landed on Mars in February 2021. One of its primary objectives is to gather evidence to support either of these theories. And it seems the rover has delivered. Perseverance touched down at Jezero crater, a site once home to a lake. Orbital views of the crater reveal fan-shaped deposits, evidence of water flow through channels carved into the crater walls. Within these channels are abundant clay minerals.
A recent study analyzed aluminum-rich clay pebbles, known as kaolinite, found within one of these ancient flow channels. These pebbles show signs of intense weathering and chemical alteration by water during the Noachian epoch. While this is expected in a known watery environment, the interesting part is their chemical composition: depleted in iron and magnesium, and enriched in titanium and aluminum. This suggests they were not altered in a hydrothermal environment, but under modest temperatures and heavy rainfall.
The authors of the study found similarities between these clay pebbles and clays from warmer, wetter periods in Earth's history. They conclude that these kaolinite pebbles were altered under high rainfall conditions comparable to Earth's past greenhouse climates, and that they represent some of the wettest and most habitable periods in Mars' history. Furthermore, these conditions may have persisted for thousands to millions of years.
Perseverance also made headlines for discovering possible biosignatures in samples collected from within Jezero crater. These samples, now cached in special containers on the rover, were intended for collection by a Mars sample return mission. Unfortunately, NASA recently canceled this mission, meaning these samples may not be examined in an Earth-based laboratory for many years.
The key to understanding these samples lies in the "Knoll criterion," formulated by astrobiologist Andrew Knoll. This criterion states that for something to be considered evidence of life, it must not only be explicable by biology, but also inexplicable without it. Whether these samples meet the Knoll criterion remains to be seen, and depends on their eventual arrival on Earth.
Regardless, it's astonishing to consider a time on Mars, billions of years before humans walked the Earth, when a tropical climate, possibly supporting a living ecosystem, existed in the now desolate Jezero crater.
