In a fanfare of publicity, the long awaited Mars Odyssey results on permafrost on Mars were recently published, complete with detailed images. The results are at the upper end of the numbers that have been circulating for some time now, and it is a huge step forward to have proven how much ice there is on Mars. The numbers are quite amazing. There is so much ice stuffed into the top metre of the soil on Mars that there isn't any room left for the rock grains to touch. What has been found is not so much frozen soil, as dirty ice. Based on this discovery, a number of popular articles were written saying that "Mars' missing water has been found" or that ""An ocean of water has been found on Mars". But ice is not water, and there is a problem or two hidden away in this discovery.

Jim Bell of Cornell University published a perspective in ScienceExpress, summarising and discussing the discovery. He dubbed the discovery "the tip of an iceberg". The reason for this that  Odyssey can only sense the top metre of the surface of Mars, and it is possible that all we have seen is a thin icy crust a few metres thick above a deep dry heap of rocks and dust, but it is far more likely that the ice just keeps on going to great depths, until it is finally warm enough to melt the ice. In this case, we are looking at the top of a column that could equal as much as 500m of ice, spread uniformly over the entire planet. Put another way, this is equivalent to an ocean 500m deep over the entire planet, if we could melt the ice.

Now the ocean wouldn't cover the whole planet. It would run downhill, like water does on Earth, and pool in the lowland basins like Hellas, Argyre, and the northern plains, to form an ocean as described by Tim Parker and others. And that is why everyone is jumping with excitement, because Odyssey has discovered the "missing" water on Mars, so there was an ocean, so there was life, so we can go pick up fossils, and make Mars colonies and everything will be right, OK?
 

Wrong.
 

Trouble is, there is too much ice on modern Mars. Sit with me for a minute while I explain why.

In the atmosphere of Mars we can measure the isotopes of Hydrogen - normal hydrogen (H) and heavy hydrogen or deuterium (D). Deuterium is the stuff needed to moderate nuclear fission in a heavy water reactor. It occurs naturally in Earths oceans, rivers, and air, and in the ice and water vapour on Mars, and in comets. In fact it occurs everywhere that normal hydrogen does, and most of it is tied up in water (or ice, or water vapour) as is normal hydrogen. The D/H ratio measures the history of water on a planet, or wherever. Earth and comets have a "normal" D/H ratio because the water (or ice) has sat there for the entire life of the Solar System in a big lump, not going anywhere (except round in circles). Mars and Venus both have a strong Deuterium enrichment, as measured by a higher than normal D/H ratio. On Mars D/H is about 5 times the normal value.

What does Deuterium "enrichment" mean?

In fact, it means the opposite - loss of normal hydrogen. For both Venus and Mars, the planets lack a strong magnetic field and energetic solar wind protons smash into the upper atmosphere all the time. On Earth, these protons get deflected by the magnetic field and end up as colourful Aurorae. For Mars and Venus, things are less good. If a proton hits a heavy atom, it bounces off and the heavy atom takes a kick, but settles back into the atmosphere, even if the molecule it used to be part of got broken up (ionised) by the impact. For light atoms - especially hydrogen, the kick is enough to eject the atom to escape velocity.

Over geologic time, Mars and Venus have lost large amounts of hydrogen from their atmosphere in this way, but relatively little deuterium, so D/H measures the proportion of lost hydrogen. And the hydrogen comes from water vapour, so D/H measures loss of planetary water. (The oxygen liberated from these collisions is retained by the planet and over time goes to oxidise surface rocks and help make Mars a rusty red planet). Using the best measurements available (D/H 5 times normal), Mars has lost 60% to 90% of its available water.

So if there was an ocean on early Mars (or a more recent ocean, as some authors claim), then the D/H ratio tells us that the size of that original ocean was between 3 and 10 times as large as the amount of ice on Mars today (best guess, 5 times, like the D/H ratio). SO the 500m of ice we see today should be the survivor of a 1500m to 5000m deep ocean on early Mars.
 

"Yippee!", I hear you say, "Lots of water on early Mars!".

Trouble is, where are we going to put it?

Here's a map of Mars with the minimum amount of ocean added - 1500m. Because it all runs downhill, it fills the northern lowlands to an average of ~3 km. You can see that the "high tide" mark has already drowned hundreds of old impact craters around the edges of the supposed ocean basins. These impact craters have been preserved on Mars since the Noachian epoch - the very earliest moment of geologic time. There can never have been an ocean this deep on Mars, and the best guess for the amount of water is almost twice this, making things just plain impossible, so what are we to do?

The obvious answer is that not all of the ice on Mars was ever melted. In fact probably only a little bit of it. The rest has been deep-frozen ice since Mars was first formed. It never made an ocean, never formed rain, or clouds, never flowed in rivers and never got hit by solar wind protons to lose its hydrogen.

In fact, why don't we just adopt a model where none of the ice melted, ever?

I seem to recall that a chap called Nick Hoffman has been suggesting this for a few years.

Unless we can lose a lot of the Odyssey ice, in yet another paradox of Mars surface, we probably can't ever have had an ocean.
 

      Created: June 2002
      Last modified: June 2002
      Authorised by:  Head, Earth Sciences

      Maintained by: Nick Hoffman
      Email: nhoffman@unimelb.edu.au