MARS has received its fair share of visitors over the years, each one arriving with ever more sophisticated instruments and promising to uncover ever more of the Red Planet’s secrets. Right now, spacecraft are circling it and rovers are trundling across its surface – the most recent being NASA’s Curiosity. Yet, when it comes to the question of Mars’s ancient climate, they only seem to be muddying the waters.
For over a century, we have blown hot and cold, or perhaps wet and dry, on the Martian climate. Then after decades of observing geological features, such as sinuous networks reminiscent of river valleys, one view came to prevail. It goes something like this… Way back when, just as life was getting started on Earth, Mars, too, was relatively wet and hospitable. It had a thick atmosphere, an active rainfall cycle and a system of rivers and lakes. The Red Planet may even have had a huge and long-lasting ocean.
But what if we’ve got the story all wrong? Several strands of evidence seem to be pointing in a different direction. If they are correct, Mars has never had a lasting hydrological cycle of open water, evaporation and rainfall, similar to Earth’s. Instead, it has always been a cold, arid desert, punctuated by a smattering of brief episodes of warmer, wetter conditions. It’s a radical view that – if it holds up – will re-write our understanding of Mars’s climate and cause us to reassess the types of life that may have once flourished there.
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Today Mars is too cold and its atmosphere too thin to support liquid water on the surface. All of its water is instead locked up at the poles or under the surface in rock-hard ice that reaches down for tens or hundreds of metres. Pockets of liquid might exist at such depths, warmed by geothermal activity.
To best understand Mars’s remote past, it makes sense to start from what we see now and work our way back. That’s the approach taken by , a planetary geologist at Brown University in Providence, Rhode Island, who has been a leading investigator of early geological processes on Mars.
He became fascinated decades ago when orbiting spacecraft starting beaming back images showing networks of valleys. “I’ve been hypnotised by them, trying to figure out what they’re really about,” he says. He’s especially excited about new images of beds of rounded pebbles sent back last month by Curiosity, which seem to indicate that the rover is exploring what was once a river.
It’s clear that such features represent a significantly different Mars than the one we see now. But just how different?
Planetary researchers work out the age of features on Mars based on how densely packed the impact craters are, with older regions more pockmarked than younger ones. With the help of spacecraft images and modelling, Head looked for evidence of changing climate embedded in the rocks through the ages. Three billion years ago, at the end of a geological period called the Hesperian, the surface geology “still looks cold and dry,” much like today. Wind back half a billion years to the early Hesperian – when Curiosity’s landing site at Gale crater was forming – and a whole panoply of features show up that perhaps paint a different picture. They are suggestive of an essentially Earth-like climate, with temperatures and atmospheric pressures so much higher than present-day Mars that rivers and lakes could form, water could evaporate to form clouds, and rainfall could keep the cycle going.
“There are a whole bunch of things that indicate warm and wet,” Head says, “but our strategy is to see if these things actually required warm and wet conditions.”
“There are a whole bunch of features on Mars that indicate warm and wet. Our strategy is to see whether they actually required warm and wet conditions”
Deluge, deluge, done
There are a few things missing, Head says. While the individual networks in a given location look terrestrial and river-like, closer examination tells a different story… they lack the large-scale integration that a long-lived drainage system should exhibit. Instead of mature, widely connected systems, these disconnected patches suggest shorter episodes in which large volumes of water may have flowed on a local scale, but not for very long or very far. Even the rounded-pebble beds seen by Curiosity cannot yet tell us if the river that produced them flowed for thousands of years – a short timescale by geological standards that would be consistent with the revisionist view of a cold, dry Mars – or for millions of years, as a more stable feature of a warm, wet Mars. Head concludes that the view of early Mars as a cold, arid desert with just a few short-lived episodes of surface water is a “plausible interpretation”.
Another line of evidence comes from mineralogy. The presence of clay, detected by landers and orbiters, is usually interpreted as strong evidence for surface water. But work by , a geologist at the California Institute of Technology in Pasadena, is challenging that view. In a published in November last year, Ehlman analysed the distribution of different minerals detected in thousands of surface locations by the spacecraft and . She discovered that most of the clays are in fact a type that forms in the hot, oxygen-starved conditions found underground. They are a far cry from the mix of mostly aluminium-rich clays that form in exposed surface waters and are seen in only a few patches on Mars.
Interestingly, the few places with clay and other minerals that clearly did form in flowing water are virtually all from that same period, roughly 3.6 billion years ago. That’s around the transition into the Hesperian era. This happens to coincide with the time when most of the surface channels seem to have formed. That fits in with the picture of an overall cold Mars with just brief, wet episodes during that one era. “It’s very clear that early Mars was warmer and wetter than today,” she says, “but that may not imply a sustained warm climate like we have on Earth.”
A more recent report finds even stronger evidence that Martian clays may have a volcanic origin, and may have had no long-term contact with surface water at all. , a team led by of the University of Poitiers in France argues that clays found in Mururoa atoll in French Polynesia formed by precipitating out of water-rich magma, rather than forming in a river or a lake. That’s relevant because the spectral signature of the Mururoa clays closely matches many of the clays on the Red Planet’s surface, as well as in some Martian meteorites. According to Meunier, this might mean that there was no need for liquid water to have persisted for millions of years on, or even near, the Martian surface.
It’s a claim that may be tested by Curiosity and the Opportunity rover, which has been exploring the surface of Mars since 2004. Both are poking around inside craters that feature abundant deposits of clay minerals. A close examination of these clays with the arsenal of instruments they carry – especially the sophisticated chemical and mineralogical instruments aboard Curiosity – could soon pin down whether those clays originated in lakes or volcanoes.
Head and others also believe volcanic activity has a role to play. Their argument is based on climate models. Billions of years ago, the young sun was much fainter than it is now. This together with what we know about the composition of the early Martian atmosphere makes it virtually impossible for early Mars to provide the sustained warm, wet conditions required to create an Earth-like scenario. Rather, he says, the climate may have been cold and dry for the vast majority of the time, but with brief, episodic bursts of warming induced by explosive volcanism.
There is abundant evidence all over the Martian surface for brief bursts of warmer climate brought on by volcanoes injecting water vapour and carbon dioxide into the atmosphere, says Head. “If you get enough of these eruptions, you can raise the temperature sufficiently to get sustained melting of ice sheets and permafrost,” he says. But these episodes could have lasted just a few hundred or a few thousand years, rather than the hundreds of millions of years envisioned by the conventional warm, wet model.
Uphill flows
The increasingly popular “top down” model, in which transient melting, evaporation and rainfall was induced by volcanoes or impacts rather than by an overall warmer crust and atmosphere, is supported by other recent evidence. , a glaciologist at the University of Maine in Orono, has shown that long, sinuous ridges on plains near the Martian poles are in fact eskers – deposits of gravel carried by rivers of water flowing beneath thick sheets of ice, rather than on the surface.
Fastook claims there are features on Mars that are impossible to explain any other way: eskers can flow up and over ridges and up slopes because the water is squeezed along channels by the pressure of the overlying ice, rather than just flowing downhill like an exposed river. These uphill flows are there for all to see in high-resolution images, he says.
What kind of climate would sustain ice sheets long enough for eskers to form? The climate modelling is clear, Fastook says; in regions near the poles, only a temperature range of -75 °C to -50 °C will work. What’s more, under such conditions even the equatorial regions would never reach a mean annual temperature above freezing. In the Martian tropics, back in what was perhaps the warmest sustained climate the planet ever had, he says, “It was not even as warm as Greenland.”
But what if instead of warm and wet or cold and dry Mars was mostly cold, but fairly wet?
“People don’t seem to be able to put the words cold and wet together,” says , a planetary scientist at NASA’s Ames Research Center in Mountain View, California. But cold and wet is exactly what he and many others argue is the likeliest scenario for the earliest epoch of Mars’s history, based on the best evidence we have today. McKay has had quite a bit of experience with such environments, on his many trips to places such as the so-called dry valleys of Antarctica. In these micro-environments, despite temperatures that rarely reach as high as freezing, water flows occasionally in the summer, lakes remain liquid year round under a cover of insulating ice, and ecosystems of microbes and algae thrive below that ice.
There are clues to this scenario on Mars. One example is the enigmatic pedestal craters visible in high-resolution images (see). Unlike craters anywhere else, they may be the result of major impacts when the surface was covered with an ice sheet hundreds of metres thick. The blankets of rocky ejecta extending for tens of metres around these craters’ edges would have insulated the ice underneath, leaving behind unique table-like plateaus while the ice all around it melted or sublimed away.
But how do you get those hundreds of metres of ice in the first place? Head and others say that it implies a sustained period of precipitation, not necessarily rain, but perhaps the kind of long-term snowfall that has built up the kilometre-thick ice sheet in Antarctica. It may have been cold, but there must have been an active water cycle going on for long periods, he says.
Still, many researchers find it hard to accept the dry, cold picture, or even the wet cold alternative, insisting that they simply cannot account for the large-scale, fluvial-like features that cross the Martian surface. at Penn State University in University Park, who specialises in planetary climates, says it’s hard to see how the giant Martian canyons, for example, could have formed without a long period of Earth-like climate.
A back-of-the-envelope calculation illustrates his point. Several valleys on Mars are similar in size to the Grand Canyon, which took 17 million years to form and collected most of the run-off from the vast Colorado plateau. He estimates that this would have amounted to about 5 million metres of rain in total. Others have argued that such features on Mars could have formed from just 500 metres of rainfall from brief periods of warming in an otherwise cold and arid climate. But Kasting says that just can’t realistically account for the vast size of these features. “I think they miss by a factor of 10,000,” he says.
Just about everyone agrees that we need to know more. “I thought I had it all figured out 15 years ago,” Head says, but the vast wealth of new images has confused things further. For now, he says “call me agnostic” as to how warm or cold or wet or dry Mars used to be.
Curiosity may be able to tell us how long the wet conditions lasted. Its landing spot is near hundreds of metres of layered deposits that span an era thought to have seen wet conditions change to much drier ones. “We may be able to walk right through the transition,” says Head.
Morphological, chemical and mineralogical evidence found there could help prove the case one way or another. For example, if the thin beds of rounded pebbles like the ones Curiosity has already seen are interspersed with layers of windblown material and volcanic ash then that would support the view that Mars’s climate was cold and dry, punctuated by short bursts of rainfall brought on by volcanic activity. However thick beds of pebbles suggestive of water erosion might push towards the warmer, wetter interpretation.
Definitive answers may have to wait for future missions that can drill deep into the Martian soil or return samples to Earth. However the new results flowing back from Curiosity finally raise the possibility that we’ll resolve the mystery one way or the other. Head says he doesn’t care which way the evidence falls. “Frankly, I just want to know what the heck went on there.”
