India’s Chandrayaan-3 Moon mission has
already proven historic. It put India in the history books for being the 4th nation to
ever successfully land a spacecraft on the Moon, and the very first to ever land at its south
pole. The lander Vikram and rover Pragyan already made discoveries that could profoundly
impact our understanding of the Moon’s chemical composition and geological history, and has given
the world vital data that will aid future return missions to our lunar neighbour. And yet, for
all the praise – which is well deserved – it’s what Chandrayaan-3 did not discover, but
should have, that I find the most intriguing. Have you noticed it too? India’s
NASA equivalent ISRO – the Indian Space Research Organisation – made
press releases before the launch, and there was always one thing they claimed
they were primarily there to find. One reason the South Pole was picked out over all other
locations. One mystery about the Moon that is deepening the more we investigate, and yet
needs to be solved before we can expect to start setting up permanent bases up there.
Simply put: where is all the water? I’m Alex McColgan, and you’re watching Astrum.
And today we’ll take a look at Chandrayaan-3, explore its successes, and attempt to use its
discoveries to answer that one important question. It was India’s Chandrayaan-1 mission that helped
provide some of the clearest proof that the Moon’s polar craters might hold water ice. Water on
the Moon had been hypothesised since the 1960’s, and in 1971 Apollo 14 found some traces of water
vapor at the lunar surface. But water ice itself proved difficult to pin down. Scientists thought
that if water ice was anywhere, it would be in the craters at the North and South Lunar poles; these
craters were angled such that they never received direct sunlight, and thus were very cold – some of
the coldest places in our entire solar system. The perfect forming ground for ice. Sadly, images of
these crater cold-traps were too low resolution, or simply too dark for us to know for sure, and
while there were numerous detections of hydrogen on the Moon’s surface, it was unclear whether
this took the form of actual water ice or not. However, in 2009, signs of hydration began
to emerge. Chandrayaan-1 (carrying a NASA Moon Minerology Mapper) found the first
definitive spectroscopic signatures of water ice in dozens of craters congregated
around the Moon’s poles. This water map was later confirmed by Chandrayaan-2, leaving
scientists increasingly confident that there was potentially 600 million metric tons of water
ice to be found in the Moon’s darkened craters. Water on the Moon is a big deal. More and more
nations have goals of setting up bases on the Moon, and being able to source your water
from the Moon’s surface saves you from having to spend huge amounts of resources getting
it up there. Water is vital for human life, but also could be broken down for hydrogen and
oxygen – useful ingredients for rocket fuel, or a breathable atmosphere in your Moon base. So, it should come as no surprise that
when Chandrayaan-3 began to make its way towards the Moon, one of the things the media
reported it was hoping to find was water ice. Chandrayaan-3 was launched on the 14th July 2023.
It was made up of a lander module called Vikram, a small rover called Pragyan, and an orbiter module
that carried the other two components across the gulf of space. On the 23rd of August Vikram, with
little Pragyan tucked inside, touched down on the Moon’s surface at the beginning of a lunar day.
But time was not on their side. Chandrayaan-3 was a surprisingly cost-effective
mission. While NASA’s Artemis mission launches will each cost on average $4.1 billion, the entire
Chanrayaan-3 mission only came to 6.15 billion rupees, or about $75 million – ironically, less
than what many modern blockbuster space films take to produce. Perhaps Hollywood should consider
filming their next Moon film on site. However, with this lower budget came technological
limitations. When the lunar night fell, Vikram and Pragyan would be subjected
to temperatures of -120°C – temperatures they were not designed to survive. A lunar day
lasts 14 Earth days. The Chandrayaan-3 mission would need to complete its major objectives
in that time, as their odds of surviving to the day after that were slim.
And so, Vikram lowered its ramp, and Pragyaan the rover powered up and
headed out down onto the Moon’s surface. Pragyaan is a 27kg 6 wheeled rover that
came equipped with an Alpha Particle X-ray Spectrometer (APXS), for analysing the
chemical composition of the moon by firing radiation at it and seeing what wavelengths
bounced back, and a Laser-Induced Breakdown Spectroscopy (LIBS) instrument, that does
a similar thing but this time by firing a laser at the target of interest, and analysing
the light wavelengths that are released by the resulting plasma. These two tools together would
be enough for Pragyaan to attempt to find water, or any other interesting substances, confirming
their composition for scientists once and for all. And so, it set to work deploying
both instruments on the ground next to it. Within days, the results started coming
in. Aluminium, Calcium, Iron, Chromium, and Titanium were all found on the Moon’s surface,
along with other interesting elements like oxygen. Indian scientists were most excited at the first
ever in-situ measurement of Sulphur at the Moon’s pole. Sulphur is an exciting element to find,
as it helps us understand the evolution of the Moon over time, and indicates there used
to be volcanic activity in the region. But in spite of all these discoveries, there was one
element that was not showing up in the analysis: the all-important hydrogen was notably absent.
Pragyaan set off to explore further afield. Guiding the rover was all done manually
by scientists back on Earth, looking through Pragyaan’s onboard navigation
camera. This had to be carefully done, as the signal delay between Earth and the Moon
meant that orders for the rover to halt lagged by a little under 3 seconds – time that might
make all the difference, if the little 6-wheeled rover was to avoid overturning. And indeed, this
nearly happened. Early on in Pragyan’s journey, the rover had to speedily stop to avoid falling
into a large 4m crater scientists hadn’t initially realised was there. I say speedily. Pragyan’s move
speed was 1 cm per second – hardly the fastest of sprinters. Over the course of its two-week life,
Pragyan travelled no more than 100m from Vikram. Fortunately, the crater was detected
in time, and scientists were able to turn around and choose another route.
However, when you look at Pragyan’s route, you notice that there was a second moment
where Pragyan did not travel down into a crater it came across – instead electing to
go around. No photos of this second crater are currently available, so we are left
to conclude that no ice was spotted there. Vikram itself did not remain idle during this
time. It performed temperature readings of the Moon’s surface, digging 10cm deep to measure the
Moon’s warmth at different depths. It measured the plasma content of the atmosphere –
good news, there’s not much up there, so radio communication to the Moon likely
won’t get much interference. It detected a possible moon-quake which, given the small
two-week window, was some excellent timing. At the very end of its journey,
in a moment of final enthusiasm, the Vikram lander even successfully performed
a 40cm high “hop”, firing its boosters to lift itself off the ground - moving 30-40cm
along from its previous destination. Indian scientists had wanted to test how
easy it would be for future landers to one day propel themselves back into orbit from
the Moon, and this was a useful practice run. But none of this helped the
Chandrayaan-3 mission to find water ice. By the 4th of September, time was up. Vikram
and Pragyan were both ordered to power down. ISRO scientists had hoped to be able to
wake them up again once the night ended, but this hope proved to be fruitless. The two
lunar explorers had communicated with the Earth for the last time. ISRO and the scientific
community at large lauded their efforts, and called the mission a success. And indeed it
was, as India had gained first-hand data from the Moon that would be extremely helpful in building
a picture of conditions at its poles, along with furthering our understanding of the Moon’s
history. However, it definitely raises a mystery. When I first heard that water ice
had been detected on the Moon, I envisioned in my mind frozen ice-lakes, or
possibly tall penitentes. Perhaps a light frost, as vapor from the Moon’s atmosphere ended
up trapped in these darkened craters, freezing over the surface and building
up over time. We know from orbiters like Chandrayaan-1 that water ice is
indeed in these craters. And yet, Chandrayaan-3 has joined other missions in
failing to actually see this ice for themselves. I remember feeling similarly disappointed when
I first saw the images captured by Shadowcam, a NASA camera carried on the South Korean Danuri
Moon orbiter. Shadowcam was so good at detecting light, it could see into the polar craters that
hadn’t seen direct sunlight in millions of years without issue. And yet, once again there
was nothing there. Nothing but arid dust. Given that Pragyan’s analysis of
the lunar regolith revealed no signs of water molecules, where is the
water ice that Chandrayaan-1 detected? While this mystery is confusing, Chandrayaan-3
offers us a possible answer. Not through Pragyan’s explorations. It’s actually Vikram,
that possibly hinted at the solution. When Vikram used its ChaSTE temperature sensor, it was able to take 10 different readings of the
Moon’s temperature – starting at the surface, and working its way down in 1cm increments.
What it found in the space above the Moon’s surface was a temperature a little under 60°C
– definitely too hot for you to walk around in, if you happened to be on the Moon and somehow
didn’t care about the lack of air. But curiously, as ChaSTE measured deeper and deeper
beneath the surface, this sweltering temperature dropped off fast. By 8cm deep, the
new temperature Vikram was detecting was -10°C. That’s a big drop. From this we see that lunar
regolith is a really poor heat conductor. But that also indicates quite clearly that
the best place we’re likely to see ice is not resting on the Moon’s surface,
but we actually need to look beneath it. There’s much we don’t understand about the
Moon and its water cycles. There’s growing evidence that the Moon contains quite a
lot of water – and yet extracting it will take understanding where that water can be
found, and how it moves throughout the long lunar days and nights. Is it affected by solar
radiation? Is it trapped in hidden deposits? Although Chandrayaan-3 only lasted 2 weeks – which
I’m sure is less time than ISRO scientists would have liked – it has offered us vital insights
into conditions on the Moon. As far as water is concerned, at the very least, it has given
future astronauts this one piece of advice: If you want to find a drink of water on the Moon,
you might want to start by bringing a shovel.
