By: Adeene Denton
Was early Mars warm and wet, or cold and icy?
For planetary scientists studying our sister planet this question plagues our research, because finding an answer has direct ramifications for society’s near future in space exploration and our understanding of the evolution of habitable worlds elsewhere in the galaxy. Mars today is a hyperarid, hypothermal desert with mean annual temperatures of ~ 218 Kelvin (-55° C/-67°F, also known as really really cold), where liquid water cannot survive on the surface and a flimsy atmosphere continues to gradually disappear into the vacuum of space. It’s a harsh place where most life, including us, can’t survive without the aid of advanced insulating technology. However, the valley networks and basin lakes that crisscross the southern highlands suggest that at some point in the past the Martian climate permitted abundant liquid water to remain present on the surface over long timescales. Mars may be a cold, windy desert now, but it probably looked very different 3 billion years ago when these features formed.
…and that’s what scientists agree on.
What we disagree on is basically everything else about Mars’s early history. Such vigorous disagreement within the field comes from the lack of real information we can glean from the surface about what Mars may have been like 3 billion years ago, combined with the burning need to answer such a fundamental question. When we, dreaming of our future as a multiplanetary species, look at Mars today we think: sure, Mars is inhospitable to life now, but could it have been habitable in the past? Could it be again? Big questions like these are what drive NASA and other space agencies to shoot for manned missions to Mars in the 2030s and 2040s. And yet, there’s so much we don’t know about a planet that’s a year’s travel away. We can’t afford to be unsure when our astronauts’ lives are on the line.
When planetary scientists try to decode the geologic and climate history of Mars, we have several basic tools at our disposal. First, the geologic record itself – we can look at Mars through our increasingly high-resolution images from decades of Martian spacecraft, observe the way different layers of material interact, and date the surface based on relative crater densities as well as speculate on formation mechanisms for various morphological features. Second, other datasets such as mineralogy from both spacecraft and the three rovers on the surface can tell us about the composition of these features. NASA’s rovers, the now-defunct Spirit as well as the still-operating Opportunity and Curiosity, are remotely operated science labs on wheels, doing their best to get information on the mineralogy of the rocks over which they drive. The minerals involved can indicate the presence or absence of water during their formation, and leave some hints about how long the water was there. And third, we can use our understanding of the physics of terrestrial planets and our knowledge of Earth to extrapolate to Mars using numerical modeling. Climate modelers can incorporate atmospheric compositions, wind patterns, changing amounts of solar luminosity, and many other factors to reconstruct the evolution of the Martian atmosphere, while dynamicists look at the death of the Martian magnetic field and the growth of massive volcanic fields that occurred at the same time as valley networks formed on the surface.
The current problem faced by the field is that a) these methods are yielding a variety of information about the early Martian climate, and b) that information is being interpreted in a myriad of contradictory ways. The two ends of the early Martian climate hypothesis spectrum, the “warm and wet” and “cold and icy” climate scenarios, hinge on vastly different interpretations of the geology, mineralogy, and climate models. This is possible because the overland flow of water (one of the few things we know occurred) can be produced through both regional precipitation in a warm and wet scenario and meltwater off a large ice sheet in a cold and icy one. The same mineralogies, including hydrated clays, can be formed through relatively short periods of water involvement and tell us less about their climatic source than we would like. Additionally, the climate models span a broad parameter space and have many unknown variables – when we estimate the axial tilt of Mars 3 billion years ago, it is most definitely a guess.
As such, in many cases we’re right back where we started. There was water on Mars – but where did it come from? How long was it there? What mean annual temperatures are needed to produce liquid water? Was there an ice sheet in the south, or an ocean in the north? We have so much data to look at, and somehow it’s not quite enough.
The growth of two robust, yet contradictory hypotheses is the direct result of this conundrum. The warm and wet climate hypothesis posits that early Mars contained a sizeable insulating atmosphere and had mean annual temperatures above 273 K, which in to permits the consistent present of regional precipitation across the southern highlands to create the valley networks. Some adherents of this scenario believe that a global ocean existed in the smooth, younger northern lowlands at multiple points in Mars’ history, despite the cold conditions. While putative shorelines of an ocean have been mapped, the presence or absence of an ocean has yet to be definitively proved. And Curiosity itself, our newest rover, may be traversing what was once a lake-filled crater, the amount of water that lake contained and the timescales over which that water was present remain unknown.
Meanwhile, recent climate scientists have predicted that the dimmer light of a younger sun as well as an atmosphere that may have been thinner than we would like actually prevent warmer temperatures from persisting even on an early Mars. In this cold and icy scenario, temperatures remain below freezing most of the time, with surface water locked up in an ice sheet covering the higher altitudes of the southern highlands. Geologically brief periods of warming enable melting at the margins of this ice sheet, which can in turn also create the broad patterns of valley networks and pool in the basin lakes. In this scenario there can be no ocean, and the presence of liquid water is transient at best.
For those of us working in the field of Martian planetary science today, our job is to continue to test these models, canvassing the surface of Mars for clues and making our models as realistic as possible. While we hope to solve the problem of early Martian climate history sooner rather than later, we also look forward to the possibilities that human exploratory missions can offer for better sample collection and observation of the planet with human eyes. It’s an exciting frontier of planetary science, and we continue to boldly go!