Saturday, August 4, 2012

Choosing a Landing site Part 2: Choose only one...


In part 1 I talked broadly about the history of choosing landing sites on Mars and what factors influence the selection process regardless of spacecraft type. I encourage you to read it all here.

In part 2 of our examination of the landing site selection process of the Curiosity mission to Mars, we focus on the scientific efforts that went in to trimming the list of candidates for examination by the new rover mission to just one choice that would just about cover everyone’s experiments’ goals.
Where do we go?

To be denied candy is punishment enough for a child but to be given a choice of only one out of a whole shop full of candies is next to torture! That was the situation that faced the 1st Mars landing site workshop held in 2006 in Pasadena, California. More than sixty choices were presented to them and the scientists and the engineers had to come up, by 2011, with a choice that promised to satisfy all the scientific objectives and all the engineering constraints the teams had in mind. As I stated before it was mostly down to the scientists’ decisions. All the sites looked interesting but going through each site would be too cumbersome for me. So let me focus on the mission’s objectives (we too are now trying to come up with exactly that at our university for our coming week at the field) and we will use those to see which landing site satisfies most. 
Four options, which is the best? (Wikimedia Commons)

In science when you want to start an investigation into something interesting we have to have objectives for your study to have focus and clearly defined organisation; what do you plan to achieve? We can divide the mission’s objectives in to 2 parts; a broad and specific objectives (which may be more than 1 as they focus on specifically 1 or 2 factors.)

The mission’s broad objective states: explore and quantitatively assess a local region on Mars’ surface as a potential habitat for life, past or present. The reason it is broad, I believe, is because the phrase ‘quantitative assessment’ implies thorough and rigorous experiments measuring certain quantifiable factors. But the experiments aboard Curiosity are based on different scientific disciplines and will examine the central question with different factors in mind. For example, habitability implies biological factors, chemical factors, physical factors, environmental factors, past factors, geological factors and a whole lot of other factors.

The specific objective are:                                                             


1) Assess the biological potential of at least one target environment.

    a) Determine the nature and inventory of organic carbon compounds.

    b) Inventory the chemical building blocks of life (C, H, N, O, P, and S).

    c) Identify features that may represent the effects of biological processes.

2) Characterise the geology and geochemistry of the landing region at all the appropriate spatial scales.

    a) Investigate the chemical, isotopic, and mineralogical composition of Martian surface and near-surface geological materials.    

    b) Interpret the processes that have formed and modified rocks and regolith.

3) Investigate planetary processes of relevance to past habitability, including the role of water.

    a) Assess long-timescale (i.e., 4-billion-year) atmospheric evolution processes.

    b) Determine present state, distribution, and cycling of water and CO2.

4) Characterise the broad spectrum of surface radiation, including galactic cosmic radiation, solar proton events, and secondary neutrons. 

As you can see dear reader, the specific objectives are more focused and they actually show you how doable the whole mission is. Bullet 4 will use the RAD (radiation assessment detection) instrument aboard the rover itself and has actually been returning data during the cruise until recently when it was switched off ahead of landing. Bullet 4 is quite important in the near term because it is providing invaluable information as to what happens to the radiation levels INSIDE the rovers cruise capsule while the space craft is bombarded by energetic particles from the sun and from the galactic environment, simulating a human inside a space capsule.

Coming back to bullets 1, 2 and 3, we see the systematic reasoning of the team. First we are interested in habitability so we must touch biology first. Of course we can’t DETECT life with the rover per se but we can indirectly investigate it by searching for organic carbon compounds (the building blocks of life), the essential elements of life as we know it (carbon, hydrogen, nitrogen, oxygen, phosphorus and sulphur) and traces of life processes such as methane gas detection which Curiosity can do. There is talk about potentially finding ‘fossils’ of past biota but this depends highly on the quality of sediments preserving the past life from and is not heavily listed in the scientific agenda.

Secondly, we examine the geology of the surface. This is a geological survey mission and it is only necessary that the team concentrates efforts in this category. Even if the landing site turns out to be devoid of evidence for past life (a bad public relation issue) investigating a geologically amazing area is still good science and is actually what’s driving Mars exploration.

Third, it was partially through the past research in the second objective that brings us to the third objective; investigating past hydrology. If you look at Mars today it has features that indicate a wet past. Dried up river beds, evidence of floods, frozen aquifers or permafrost that extend so far to the tropics and poles with water ice all show evidence for Mars’ hydrological dynamics past and present underground, on the surface and in the atmosphere. So whatever the site chosen, it must have some history connected to liquid water.

Over the years it has become clear that Mars has gone 3 main stages of evolution as far as water is concerned. An early Mars featured liquid water maybe with a thicker atmosphere than it has today. Slowly this changed into an acidic picture with deposition of sulphates and apparent disappearance of carbonates and clays which require neutral pH water to form. Finally with the loss of its atmosphere and the decay of its planetary magnetic field Mars became the dry and desiccated world we’ve all know and love (or abhor depending on who you ask).

So the aim of the choosing game becomes clearer! I would like a site that can preferably show me if life could have existed in the past. Life as we know it is supported by water with neutral pH (although some forms prefer acidic conditions) so we would go to a place that shows plenty of sulphates, carbonates and clays. I would also love to explore the geological history of the area and see how it fits with the rest of the planet (in short it must be an INTERESTING place geologically). And the geology must show some history of water. The terms are steep but Mars has plenty of such places. But in the end the scientists had to make choices. Everyone got to make their case for each landing site they adored. By 2010 it all came down to 4 choices (phew!).

The options were Holden crater, Eberswalde crater, Galecrater and Mawrth vallis (valley in Latin). They all looked so good (I personally was hoping for Mawrth). Holden is a crater about 140km in diameter in the southern regions with evidence for past flowing water and even sports a valley called Uzboi entering it to which presumably once brought flowing liquid water in to it. It has one of the best exposed lake deposits and exhibits orbital signature for clays.

Just north east of Holden lies Eberswalde crater, 65.3km in diameter and also exhibits geomorphologies connected with water and has the most impressive form of river deltas preserved.
Mars Global Surveyor image of Eberswalde's delta (NASA)
Impressive landforms but not very good with the mineralogy, this little crater still looked like a good choice   at the time.

Mawrth vallis is certainly spectacular in terms of landforms and potential surface vistas. It’s an area rich in clay minerals in the northern regions though its geological history is somewhat sketchy and confusing. It is I think the most ancient form in the group so it promised to open up the exploration of Mars’ early geological
and hydrological histories. But the complex landforms around it I think spooked the engineers in to dropping it!

Which then left Gale crater, a 154km hole with 5.5km high mountain at the centre ( informally named ‘Mount Sharp’), making it higher than the walls of the crater itself. This implies that the crater has been exhumed after being buried by sediments in the remote past (around 3 billion years ago) at least as high as the mountain. Scientists suspect therefore that the mound at the centre could have the history of not only the crater itself but potentially the whole region or the all of Mars extending to the early days of hydrological activity! This is was a bonus that no other landing site could match. The site features clays, sulphates and landforms indicating past water activity.

So in 2011, the workshop group recommended Gale as their priority to NASA which had the final say in the matter. And they agreed! Gale crater here we come! Thanks to the systematic (though tedious) and exciting scientific process.

If you would like to see all the landing workshop's publications you can visit their resource rich website here.
In the next part I'll post details about the landing ellipse which I remember promising about long ago somewhere. Then it will soon be time to move this blog to the next stage (i.e. dropping the COSPAR designation 2011-070A in the banner above) and enter surface operations!

We're almost there folks. We're talking 56 HOURS away now!

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