The year was 1996. Scientists announced the results of an investigation that led to claims of past life on Mars. But in the discussions following this exciting announcement researchers concluded that the evidence did not support the claim.
Nearly 20 years later the conversation continues with increased data gathered by instruments driving across the surface of the Red Planet. Given the difficulty of finding ancient microbial life here on Earth, scientists seeking evidence of life-friendly conditions on Mars look for signs of liquid water, the basic chemical components required for life (C, H, N, O, P, S), and chemical energy gradients necessary for metabolism. Definitive evidence for or against extraterrestrial life remains elusive—but a body of data affirms that liquid water (and other ingredients required for life) existed early in Mars’s history.
In January 2004, two rovers, Spirit and Opportunity, landed on Mars for a three-month exploration mission. Although communications with Spirit ceased in 2011, Opportunity continues exploring the Martian surface and reporting back to NASA to this day. Since August 2011, those explorations have focused on the Endeavour Crater, an impact basin 14 miles across. The impact that created it excavated Mars’s surface layers during the Noachian period approximately 3.7 to 4.1 billion years ago, which corresponds to the time of the Late Heavy Bombardment.
A recent paper published by the Spirit and Opportunity team shows evidence of liquid water existing on Mars during the Noachian period. The evidence includes fine-grained layered rock, calcium sulfate veins cutting through the rock layers, and aluminum-rich clay materials in rock fractures.1 All three observations indicate flowing, liquid water before and after the crater’s formation. Furthermore, in contrast to studies of younger rock formations that show much higher acidity, recent analysis of Mars’s past aqueous environment indicates only a slightly acidic to neutral pH—a more favorable environment for prebiotic chemistry.
The larger and more capable Curiosity rover (which landed on Mars in August 2012) found corroborating evidence from the other side of Mars in the Gale Crater (formed approximately 3.5 to 3.8 billion years ago). According to a paper by the Curiosity team, the Yellowknife Bay region of the Gale Crater consisted of a “fluvio-lacustrine environment.” “Lacustrine” describes a lake environment and the term “fluvial” refers to rivers and streams and the deposits they form. Curiosity found rock layering in the Gale Crater, similar to what Opportunity discovered in the Endeavour Crater but with layers extending to a depth of over five meters. The composition of sediment in the Yellowknife Bay layers matches that of the surrounding lava-based rock. Measurements revealed the presence of carbon, hydrogen, oxygen, sulfur, nitrogen, and phosphorus—all chemical elements required by life. The chemical composition also revealed the chemical gradients necessary for chemoautotroph metabolism.2
Potentially Habitable but Transient
These data imply the existence of a river-fed lake with depth ranging from meters to a few tens-of-meters. The thickness of the sedimentary layers means the lake persisted for hundreds to tens-of-thousands of years. Analysis also indicates a neutral pH and low salinity environment. All these conditions led the authors to argue for the plausibility of habitable environments early in Mars’s history (before 3.5 billion years ago).
These results highlight the strong (and growing) body of evidence that Mars hosted liquid water environments for sustained periods early in its history—but this conclusion is not very surprising to astronomers. Water is the third most abundant molecule in the universe (behind H2 and H3+), thus we expect rocky planets to start with plenty of water. Furthermore, Mars’s distant location from the Sun (compared to Earth’s) means that the active and violent conditions during its early history would provide transient, local environments where liquid water might flourish. However, the potentially habitable conditions on Mars clearly ended within a few hundred million years—a result that shows sustainable life requires more than just the basic chemical ingredients.
Studies of neighboring planets continue to demonstrate the uniqueness of Earth’s remarkable capacity to host a diverse, sustained, and thriving system of life. The types of life that have existed throughout Earth’s history played a major role in maintaining and advancing that habitable character—even in the midst of significant astronomical, geological, and atmospheric change. The complex interplay between such different influences to sustain Earth’s habitability across the planet for billions of years continues to point to the work of a divine Designer.