Mars – wet or dry?
This is an exciting time for Mars exploration. Orbiters are circling the planet, sending back optical, spectral, radar and altimetry data which is expanding our knowledge base by leaps and bounds. Meanwhile, the rover Opportunity is still active on the surface and will hopefully soon by joined by the Mars Science Laboratory rover, Curiosity. The quality of the data we’re getting is unparalleled – the best images from Mars Reconnaisance Orbiter have a resolution of up to 0.3m, better than satellites around Earth give us of our own planet. Some of these are in stereo, so by simply donning some 3D glasses, you can feel like you’ve stepped right into the picture.
There’s so much I could say about Mars and you’ll doubtless be hearing a lot about it in future posts. I’ll start by bringing you up to speed on where we are with the Red Planet. The most common question I get asked by people of my own age (educated in the 80′s-90′s) is ‘so IS there water on Mars, then?’ We were taught to disdain the over-imaginative observers of the 19th century who pointed out ‘canals’ and dreamed of the civilization that must have dug them. To us, Mars was a very dry planet, with the dust storms to prove it.
True enough, the ‘canals’ were very much in the eye of the beholder, but the more evidence we gather the more we realise that water has played an major role in the formation of the Martian surface. It may be drier than the Earth, but it’s nothing like the volatile-depleted Moon.
1. An early wet Mars
There’s now no doubt that large amounts of liquid water have flowed over the surface of Mars, particularly at the end of its earliest period, the Noachian. The strongest evidence is the many branching valley networks of that age in the southern highlands. These look very similar to river networks on Earth and often form alluvial deltas as they drain into basins the size of large lakes or small seas. Another possible sign of the early climate is the southern uplands-northern lowlands dichotomy of Mars: it’s suggested that the northern lowlands were a huge sea. Time will tell whether this theory has legs, but for now the valley networks are the best climate indicator.
As usual, there’s disagreement on how they formed: some say by snow melt, while others think it was by rainwater runoff or groundwater sapping to different degrees. Whichever of these is correct, they all require some kind of precipitation, whether rain or snow. That’s not possible in today’s Martian climate, so Early Mars certainly would have felt a lot more like home then than it does today.
This early warmer, wetter Mars transitioned to drier conditions around 3.5 billion years ago, with deposition of evaporites in dried-up lake beds and an end to activity in the southern uplands valley networks. This may have happened simply due to the gradual cooling of the planet, but the suddenness of the change may have more to do with the end of heavy bombardment by meteorites at that time. All those impacts contributed a lot of heat to the planet’s surface and would have put a lot of steam into the atmosphere, an effect which ceased when meteorite impacts settled down to the lower levels we’ve seen since.
So what happened to the water? Did the meteorite impacts drive it all off and leave the planet dry as a bone? Certainly some water is likely to have been lost that way, but new evidence increasingly suggests the continuing presence of substantial volumes of water on Mars.
2. Mars may still be wet – or at least icy
In the ensuing periods, you only see a few branching river networks, and those are on the flanks of volcanoes where internal heat could be expected to have provided the meltwater to form them.
More characteristic of this period are huge, shallow outflow channels. These can be hundreds of kilometers long and up to a kilometer wide and often start in areas of rubbly surface collapse called chaotic terrain. While there are some who claim these channels were formed by lava or a CO2 fluid, they’re generally accepted to be formed by catastrophic outflows of water.
So where does this water come from? The most popular explanation is that there is water near the Martian surface now, but it’s frozen solid due to the low temperature and atmospheric pressure. At depth, though, where pressures and temperatures are higher, water is liquid. Occasionally, when for instance a dyke of hot magma breaks the icy crust (the cryosphere) and releases pressure on the liquid aquifer, water shoots up and flows onto the surface. Though liquid water is unstable at the Martian surface under current conditions and would tend to boil away, it’s thought that it can remain liquid long enough to make these channels, possibly by forming a crust of dusty ice which protects the liquid water beneath.
There’s supporting evidence for the presence of this icy cryosphere. In the area I’ve studied myself, the Cerberus Plains near the equator, there are a lot of landforms which are seen in periglacial environments on Earth: places where there’s permafrost and erosion of the surface by cycles of freeze and thaw. These include polygonal ground, sorted stone circles, closed depressions made by melting and mounds called pingos. Since we’ve only viewed these from a distance, we can’t be sure they’re really caused by ice – other explanations can be found for many of them. But in combination and with their close proximity to outflow channels such as Athabasca Valles, the evidence that this area is ice-rich is looking stronger and stronger.
While low latitude ice is still contentious, we know for sure that there is water ice at higher latitudes. Both poles have a water ice layer and the plains around them show a wide variety of periglacial landforms. Craters here too suggest an icy composition: the ejecta thrown out by meteorite strikes forms flowing lobes rather than the dust and rubble sprays you’d expect from a dry surface. With the aid of a telescope, you may also see water vapour in the atmosphere, forming icy clouds over the highest volcanic peaks.
All of this is as we’d expect because this is a larger planet than Mercury and so has been internally hot and volcanically active through much of its history. With volcanoes come volatiles: gases including H2O which were dissolved in the magma in the mantle and explode out when it’s erupted at the surface. In fact volcanism and water flow are connected in many ways on Mars and, like water outflows, we’re finding that volcanic eruptions have been occurring much more recently than we used to think – probably up to the present day.
That’ll be my topic for next time, so please come back to hear all about the ongoing rumblings of Mars.