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.
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| 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.
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| 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.
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| 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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