Last updated January 12, 2018 at 1:55 pm
Imagine a wall of ice 30 stories high, rising above barren dessicated red soils. This is no Game of Thrones tale but rather what you would see today on Mars according to the latest research published in Nature.
Satellites images from the Mars Reconnaissance Orbiter have revealed scarps up to six kilometres long with sharp edges that in places glow brightly at wavelengths of light associated with ice. These edges extend 100m in height, suggesting that the ice is at least this high and likely formed from snowfall on Mars compacted into a glacier-like state.
That bare ice can exist on the surface of Mars is surprising as the low air pressure (less than a percent of what you are experiencing right now) allows solid ice to directly sublimate into the Martian atmosphere. Typically above the ice are one to two metres of dirt forming a protective layer. It is only at recently exposed surfaces where the dirt has been removed that we see the ice which is then constantly being eroded. As only a hair-thick layer of dust would blind our satellites to the water-ice these surfaces must be amazing dust-free. When exposed to the atmosphere the erosion process is fast. Over observations spanning several years we can see the ice retreating, and boulders which have having tumbled downhill as the ice sublimates away. Near these scarps are large, kilometre scale pits, which are all thought to be all that remains of vast ice deposits having sublimated away in less than a million of years.
We have known for years that ice existed on Mars, in the polar ice-caps mixed with carbon dioxide (dry ice). What makes this finding extraordinary is that the ice is at mid-latitudes, far from the Poles. On Earth these regions correspond to the base of Canada, the upper states of the continental USA, cutting through the UK and northern regions of China. On Earth we might expect to see ice. Not on bone dry Mars.
Large deposits of nearly pure ice close to the surface that are easily accessible to regions closer to the equator are a boon to future missions where in situ resource utilisation (ISRU) is seen as a critical step for human exploration. Simply put if we need to take everything with us the costs will be truly astronomical.
Water water everywhere?
This latest discovery of water ice on Mars might sound familiar, indeed it seems like water on Mars has been ‘discovered’ several times in the last few years alone. The main discoveries have been:
- Polar ice caps, similar to Earth, but with a mix of frozen carbon dioxide (dry ice) and water ice. The former sublimates in summer and the caps visible shrink in extent.
- The newly uncovered ice reserves at mid-latitudes, close to the surface.
- Light frosting on our landers that form at night which sublimate away in the day.
- Vast oceans of water locked deep in the rocks of Mars which differ to Earth where water is stored to the surface and hence can be recycled back into our hydrological cycle:
- The potential for even occasional snow-fall on the Red Planet
But what about the huge announcement in 2015 about water flowing on Mars still to this day?
Visible from space are recurring slope lineae (RSL), which appeared to flow in the high noon sun of summer on Mars (when temperatures can approach 25C) and resemble water-melt darkened tributaries. Spectral analysis from space even appeared to show that these regions contained hydrated salts, or in other words, these features contained salty water / brine. This salt would then prevent it from freezing or evaporating on the surface.
The latest results from NASA on the cause of the RSL tend to throw cold water on that, and instead find that dry sand flows are a better explanation of these features.
In particular NASA has reported that the RSL only occur on slopes that are steep enough (at least 27 degrees) for dry, granular flows to be possible indicating it’s sand sliding not water.
How then can we explain the fact that the features do appear to have hydrated salts visible in the spectrum of their reflected light? One thought is that the salts absorb water from the atmosphere, dry as that may be, and that this could lubricate the slide, but at such trace levels as to be too little to support life or even trickle.
Future water restrictions
The history of water on Mars is a complex one. Billions of years ago Mars had seas and oceans, and may well have been more suited to life than Earth was at the time. Slowly over time that water was locked away in rocks and trapped many kilometres underground. The moisture in the air was lost to space after the Sun’s ultraviolet light split water molecules into hydrogen and oxygen. The molecules, and indeed the atmosphere itself, was then stripped away by enormous Solar storms known as Coronal Mass Ejections slamming into the planet. A weak Martian magnetic field gave little protection to this atmosphere from these storms.
The water that remained froze as ice within the dust forming a dirty but tough ice-concrete mix. Apparently a significant amount fell as snow. These ever-deepening snow falls compacted and squeezed the bottom layers to form ice-reserves in a similar way to glaciers of Earth. Those ice-rich regions that were then covered by just 10cm of dust were afforded some protected from sublimation and could survive for millions of years. Outside of these ice-deposits the air and dirt are bone dry, no flowing streams it now seems.
To access water on Mars, future astronauts will be tied to these ice-sheets like an oasis in the Sahara. Water is critical both for life and rocket fuel and without using in situ reserves the costs to visit, much less live, on Mars are prohibitive. The dream of dipping your hands into Martian soil and it feel wet will have to await our exploration of the Red Planet; melting of its ice reserves and intentional watering of soils under pressurised greenhouse domes.
There may well be more water accessible on Mars than we once thought, shared across the surface from the Poles, but it is still a dry world and every drop will have to be conserved. Lessons that Martians may learn from water-conscious Australians.
The research has been published in Science.