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On the 6th of Feb 2004, the Spirit Rover would make history…
And change our understanding of Mars forever.
So far Spirit had journeyed across the gulf of space to Mars, endured a 6-minute descent
through hell onto the planet’s surface, and overcame a malfunction that threatened
to doom its mission from the start.
But its trials were only just beginning.
Now that it was on Mars, its 90-sol mission to find evidence of water was paramount.
To find that evidence, Spirit would have to explore the planet’s craters and use its
tailored tools to uncover the secrets contained within the Martian rocks.
What did Spirit find?
I’m Alex McColgan and you’re watching Astrum.
This series will take you on Spirit’s journey across Mars, the challenges it faced, and
the remarkable discoveries it made.
In this episode, the second in the series, we will follow Spirit as it attempts to fulfil
its primary objective and find evidence of water.
For Spirit and its instruments, Martian rocks were more than just targets to grind, they
were time capsules that offered a glimpse into the planet’s environmental past.
The question for Spirit then was which capsule to open.
One rock, named Sashimi, was close by and a straightforward drive.
A tempting target, but there was a hitch.
The surface of Sashimi was dusty, which would lead to Spirit’s observations becoming obscured.
The rock was also pitted, making it a poor match for the rover’s grinding tools.
Luckily, there was another option.
Named Adirondack, this rock was roughly the size of a football, had a dust-free, flat
surface, and a high level of cleanliness.
It would be a trickier drive to get to than Sashimi but such a good match for Spirit’s
tools, that it presented an irresistible target.
Venturing out into the vast Gusev crater, Spirit turned 40 degrees in short arcs for
just under 1 meter, then turned to face the rock, finishing its journey with four short
straightforward moves of just under 2 meters.
This might sound specific, but for Spirit, navigating through the rocky terrain of the
Gusev crater, this is how careful it had to be.
The enormous Gusev crater, 166km in diameter is roughly 3-4 billion years old.
One theory, based on imagery and mapping , suggested that Gusev was an ancient lakebed sourced
from the Ma’adim Vallis - a 700km long Martian outflow channel, much larger than Earth’s
Grand Canyon.
This theory was one important possibility that Spirit had come to investigate.
The Rock Abrasion Tool (RAT) was Spirit’s grinding and brushing tool.
It was 10cm long, and, to take rock samples it used a wheel coated in diamond dust and
resin, which spun at 3000 rpm - giving the RAT the ability to drill a 45 by 5mm deep
borehole into rocks.
Alongside this, the RAT had two brushes that removed dust from the boreholes and gave the
scientists the ability to make closer inspections.
Astonishingly, for a machine that performed rock grindings on the surface of another planet,
the RAT only consumed about 30 watts of electricity - which is crucial given how conservative
Spirit has to be when rationing its power consumption.
Once the machine had cut the Martian rock, it provided Spirit’s other tools with a
smooth, clean surface that could be used for precise observations - such as the rover’s
microscopic imager that helped determine a rock’s composition.
Before performing any grinding, Spirit returned microscopic images, as well as Mossbauer spectrometer
readings of Adirondack - these were unprecedented readings, as no investigation like this had
ever occurred on Mars - or on any other planet!
These readings suggested that Adirondack contained minerals composed of magnetite, olivine, and
pyroxene - not dissimilar from Earth’s volcanic basalt rocks.
But to confirm this, Spirit had to cut into the rock.
On February 6, its 34th sol on the planet, Spirit used the RAT on Adirondack.
Simply breaking open a rock might sound mundane - but remember how far from Earth Spirit had
travelled to be able to do this!
Not only that, but this was the first time humanity ever performed a rock abrasion on
another planet to more easily identify its composition.
Using the instruments on the freshly exposed material, it helped confirm the suspicion
that the rock was volcanic basalt, and provided intriguing insights into the makeup of Mars.
So what did Spirit learn?
Well, Adirondack was a remarkably well-chosen target, because it turned out to represent
a typical rock found on the plains of the Gusev crater.
Spirit’s instruments showed that Adirondack, and later other Gusev rocks were filled with
pyroxene, plagioclase, olivine, and magnetite minerals.
These made the rocks picritic basalts, similar to earthbound rocks called basaltic komalities.
Adirondack in particular has been ever so slightly altered, possibly as a result of
thin films of water interacting with the rock.
These small quantities of water may have induced mineralisation.
One key giveaway is that Gusev rocks such as Adirondack were easier to grind compared
to similar rocks on earth.
On March 5th, Spirit trekked to Humphrey Rock - where it found some striking hints of water
history on Mars.
Humphrey was a dark volcanic rock, about 60 centimetres tall, with bright material in
its interior crevices, alongside cracks that appeared to be minerals crystallised from
water.
These were revealed when Spirit used its RAT to scrape away the rock’s surface by 2 millimetres.
These materials were another signpost that water did indeed exist on Mars - and either
interacted with Humphrey after it formed, or was contained within it.
This was a huge step towards potentially discovering the answer to one of the Mars Exploration
Program’s key questions: did life ever arise on Mars?
If we are ever to colonize Mars, answering this question is crucial.
Spirit’s investigation wasn’t destined to end at Humphrey rock.
Our rover had more work to do.
On Sol 65, March 11, 2004, Spirit journeyed to an impact crater known as Bonneville crater
- home to Spirit’s heat shield, which was jettisoned during landing.
Bonneville crater was located within Gusev and was not particularly large.
It had a diameter of roughly 210 meters, a depth of 14 meters, with a rim that rose 6.4
meters above its surroundings.
Given this low depth relative to its diameter, Bonneville was likely a secondary crater,
formed from the ejecta of a much larger impact elsewhere.
It was also relatively pristine and not subject to any water erosion.
However, as interesting as the crater might have been in theory, once Spirit reached the
crater’s edge, no interesting target rocks could be found, so the rover avoided descending
into the crater’s belly.
Instead, Spirit headed towards the Columbia hills - which were briefly glimpsed from Spirit’s
landing site in the last episode.
This journey brought Spirit into contact with another crater.
On sol 118, already outlasting its original mission target, it reached Lahontan crater,
which was even smaller, at 55m across and 9.1 meters deep - but it posed a serious risk
to Spirit.
On the southwestern side of Lahontan was a long sand dune -a threat to the stability
of the rover’s wheels.
Spirit couldn’t risk coming into contact with this and toppling over, so the dune was
avoided.
Even with obstacles like Lahontan’s dunes, Spirit’s path to the Columbia Hills was
a relatively straight line.
The rover was directly controlled by engineers when the terrain became more complex, but
otherwise, was largely safe to travel autonomously for its lonely journey across the dunes of
Mars.
The Columbia hills, as their name suggests, were a range of hills within the larger Gusev
crater, where, until now Spirit had spent the entirety of its journey.
Lying roughly 3 kilometres from Spirit’s landing point, they were far and away the
most distinctive landmark visible when the rover first looked out on the planet’s surface.
The hills were named in memorial of the Space Shuttle Columbia, which tragically disintegrated
upon reentry to the Earth’s atmosphere.
This happened because, during Columbia’s launch, insulative foam from its external
tank broke off and damaged thermal protection system tiles on the space shuttle’s left
wing.
When Columbia reentered Earth’s atmosphere, hot gases perforated the heat shield, destroying
the internal wing structure and causing the entire shuttle to fall apart.
Each hill was named after one of the seven astronauts who lost their lives in the disaster.
Anderson Hill - Michael P. Anderson Brown Hill - David M. Brown Chawla Hill - Kalpana
Chawla Clark Hill - Laurel Clark McCool Hill - William C. McCool Ramon Hill - Ilan Ramon
And Husband Hill - Rick Husband, the Commander of the shuttle.
By Sol 159, Spirit had reached the foot of the Columbia Hills, at a location called West
Spur.
As it got closer to the hills, foreboding signs indicated that the rover was beginning
to age.
Spirit’s right front wheel began drawing up to three times as much current as the other
wheels.
This meant that the wheel could be stuck, and moving it required more effort - draining
Spirit’s limited energy and heating the motor to an inoperable degree.
What was the cause of this?
One possibility was that debris could have lodged into the wheel’s mechanism, preventing
rotation.
Another was that congealed lubricant might no longer be spreading over the wheels’
moving parts.
Ingeniously, Spirit had heaters in the wheel to solve this very problem.
But, without knowing the true cause of the issue Spirit needed to minimize damage in
the short term.
Moving forward, Spirit started using the front wheel sparingly, leaving the brunt of the
effort to the other five.
That said, drag from the front wheel would have to be accounted for, and in instances
where climbing steep slopes or difficult terrains became necessary, Spirit would have no choice
but to use all six wheels.
On Sol 174, Spirit began a “3,000-meter tune-up” to help itself get in better shape
for its oncoming missions, designed to help it get through the next 3000 metres.
One fix was a front hazard-avoidance camera calibration.
This was necessary as Spirit had begun to experience a 2 to 3 centimeter inaccuracy
in predicting the actual location of targets.
To help fix this, Spirit used various arm poses and images to refine its ability to
target objects with its stereo hazard avoidance camera images.
This was followed by a front wheel diagnostic and lubrication to help fix its rotation - as
well as visual odometry tests to help Spirit navigate with greater speed.
The tune-up was also where Spirit had its first, and well-deserved, deep sleep.
This mode left the rover de-powered overnight, allowing it to save energy spent on survival
heaters and electronics usually present when it was napping.
This was essential if Spirit was to save energy over its coming sols on Mars.
But deep sleep wasn’t without its dangers.
A s the survival heater was powered down, there was a risk Spirit’s mini thermal emission
spectrometer instrument would not turn back on.
So, Spirit undertook two potentially final observations with the instrument, before finally
powering down.
Throughout this, Spirit’s solar panels were tilted toward the winter sun, conserving as
much energy as possible - and helping them overcome interference from the setting sun
and increased levels of dust.
Once its tune-up had ended , Spirit spent the following sols studying many rocks across
the Columbia hills - such as rocks named Clovis, Ebenezer, and Lutefisk.
On Sol 188, Spirit found a solution to its issues with the front wheel.
A solution more creative than diagnostics and lubrication…
It started driving backwards.
This helped Spirit improve performance and extend the lifetime of the front right wheel’s
motor.
An example of the simple changes and clear thinking that kept the rover operational for
so long.
A solar conjunction delayed Spirit, but once it was over, the rover journeyed to the summit
of Husband Hill, which sat around 107m higher than the surrounding plains of the Gusev crater.
Aside from being home to views of Gusev, it was flanked by a beautiful albedo feature
on its south side - rich with dark rolling sands, not unlike a dune on Earth - and which
gives Mars its distinctive texture when viewed from space.
And now by Sol 344, Spirit was in a good place to continue its mission on Mars.
And maybe you’ve noticed something.
Spirit’s mission was only meant to last 90 sols… and yet h ere it was at Sol 344.
Still operating.
Still mission ready.
Still uncovering the secrets of Mars.
Check in on the next episode, as we follow Spirit to Cumberland Ridge, Tennessee Valley,
and encounters with dust devils and global storms!
The way NASA works means it doesn’t have an unlimited budget and has to pick and choose
between potential missions by carefully evaluating the potential returns of each one.
Fortunately, Spirit and Opportunity were missions that got the needed investment because of
that analysis, and it proved to be a good decision.
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