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.