It should have been, as we say in Glasgow, a doddle.
Create and design a commemorative plaque that could
get sent out into space to convey the essential aspects
of Rough Science to any passing alien. What could be
simpler? So why am I still sitting here at dusk scratching
my head about what to put on it and, more importantly,
where on Earth to get the materials to make it. As a
geologist, the second bit shouldn’t be a problem.
But I was on my own with this one. Kathy and Jonathan
were making a light-communication device, and Mike and
Ellen were to knock up an upside-down pen.
Things had started OK. It was clear right away that
it would take more than three days to make the plaque
out of metal or rock. For me, the obvious choice was
something very familiar to us all - plaster of Paris.
Plaster of Paris is that white powder that forms a paste
when it is mixed with water and then hardens into a
solid, used in making casts, moulds, and sculpture.
Chemically it is calcium sulphate, but geologically
it is found naturally as the rock ‘gypsum’.
And the reason I was so confident about making the plaque
out of this was that in our desert environment there
was likely to be tons of gypsum around. That’s
because most gypsum ‘grows’ in the dried-out
interiors of salt lakes, or playas, and there were plenty
of them close by. In fact, we’d gone to the closest,
Owens Lake, in our search for water in programme one.
What happens is that these playas are vast evaporating
dishes that crust over with a collection of salt minerals.
Gradual evaporation produces an orderly precipitation
of salt crystals out of the shrinking pool of water.
First to grow out are the most readily soluble –
carbonates of calcium and magnesium which are deposited
around the edges of the lake. As evaporation continues,
sulphates of calcium and sodium are deposited, and if
there is enough calcium left over from the first carbonate
stage then we get calcium sulphate, or gypsum. If evaporation
keeps going, then the water (or brine) becomes incredibly
salty and ordinary table salt (sodium chloride) gets
The trouble is – that’s the theory. The
reality is that playas have their own unique chemical
environments, since the surrounding geology determines
the chemical mixture being washed in. So different playas
often have distinctive assemblages of salts. Owens Lake,
for example, is mainly covered by a crust of halite
(sodium chloride). A hot, sweaty walk across the dry
lake bed failed to find anything that looked much like
gypsum (if only I had chips to go with all that salt).
That’s because there was very little calcium in
the surrounding rocks – what we really needed
was some limestone hills, since these would be made
from calcium carbonate. As for the other dry lakes where
limestone rocks were nearby, well they were off limits
to us. They were either fenced off as US airforce bombing
ranges or they were protected scientific sites. At Searles
Lake, for example, where there ought to have been really
thick deposits of almost pure gypsum, collection of
rock samples is only permitted to authorised geologists
during one week of the year. It was amazing to think
that these barren, inhospitable and even poisonous deserts
were so protected (if at least from hammers, not missiles).
With no chance getting to big deposits of gypsum, the
alternative was to find places where it might crop up
in smaller quantities. A common way that gypsum forms
naturally is where water has moved along cracks in the
rock, allowing calcium sulphate in the water to form
crystals on the walls of the crack, sealing it into
what is known as a vein. So the hunt was on to find
rocks that had loads of veining in them, to see if some
of those veins were gypsum.
With the other four scientists busy pushing on with
things, Kate was more than happy to escape out into
the wild. With her old Top Gear camera and sound crew
in the back, out came Off-road Kate - driving down dry
river beds in a 4-wheel landrover while I nervously
peered up at the ravine walls for my white gypsum. Around
5 o’clock we found it – a network of white
stripes cutting dark-grey limestone rocks, sometimes
injected along the rock layers, other times cutting
right across them. But as we hammered out what seemed
like tons of the rock, I had a nagging doubt; the gypsum
I knew from the UK was often soft enough to dig out
by hand, whereas this stuff was hard. With the sun setting
quickly, we grabbed what we could and headed for home,
the dusk concealing the worry on my face that, despite
20 years of geological experience, we’d just collected
a pile of ….