XPost: alt.astronomy, alt.fan.heinlein

I'm caused to remember Arthur C. Clark's "Rendezvous with Rama"
where he predicts that Mercury has huge energy potential and
so colonies there can be productive.

from https://www.bbc.com/future/article/20240410-mercury-the-solar-systems-smallest-planet-may-once-have-been-as-large-as-earth

Mercury: The Solar System's smallest planet may once have been as large
as Earth
14 April 2024
By Alessia Franco and David Robson,
Features correspondent

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Nasa False Colour image of Mercury (Credit: Nasa/Johns Hopkins
University Applied Physics Laboratory/Carnegie Institution of
Washington)Nasa
(Credit: Nasa/Johns Hopkins University Applied Physics
Laboratory/Carnegie Institution of Washington)
From its "absurd" core to the baffling chemical composition of its
surface, Mercury is full of surprises – not least the planet's origins.
But some answers could be held in rocks found in Cyprus.

Curiosity has killed many an explorer, and Nicola Mari feared he was to
be the next.

Driving around Cyprus's remotest mountains, Mari had relied on his cell
phone for directions. But as the light of the day faded, so did his
phone battery – and he found himself stuck in the middle of nowhere with little idea of the way back to his lodgings. "I'd travelled for more
than 50km (31 miles) without seeing another vehicle," he says.

He thought he could remember the way to an inn, where he might refill
his stomach, engine, and phone battery – but having arrived there, he
found it was deserted. A lucky turn eventually led him to another establishment, but he admits to fearing for his life on those lonely
mountain roads. "I made some bad calculations," he says.

Fortunately, his mission was not in vain. Mari is a planetary geologist
at the University of Pavia in Italy who studies ways that our neighbours
in the Solar System formed and evolved. For his PhD, he had studied
Martian lava flows. This time, his sights were set on Mercury – by way
of Cyprus. His aim was to find a certain kind of rock, named "boninite",
that is thought to bear an uncanny similarity to the rocks found on
Mercury – a supposition which, if right, could be a clue to the planet's unique origins.

First rock from the sun
Mercury is a planet of extremes. With a total volume little more than
the Moon, it is the smallest planet in the Solar System and is situated
the closest to the Sun. Mercury has no atmosphere to retain heat,
meaning that the temperature on the surface varies from 400C during the
day to -170C (750F to -275F) at night. It also has the shortest orbit of
any planet in the Solar System; each year lasts just 88 Earth-days.

The Mercury we see today may be nothing more than the kernel of the
planet that was once there – Nicola Mari
Mercury's location has made it very difficult for scientists to study.
One reason is the heat. Spacecraft approaching the planet need to be
able to withstand scorching temperatures as they wander so close to the
Sun. The other is gravity. The nearer you get to the Sun, the stronger
its pull, accelerating the speed of the spacecraft. This makes delicate manoeuvres much harder to pull off. To avoid it travelling too fast, the spacecraft can take a convoluted route, with lots of detours around
other planets, which helps to slow its path – but the spacecraft still
need a lot of fuel to decelerate and gain control of its movements.

Nicola Mari The barren landscape found in parts of Cyprus, like this
ancient salt lake, could hold clues to the origin of the planet Mercury (Credit: Nicola Mari)Nicola Mari
The barren landscape found in parts of Cyprus, like this ancient salt
lake, could hold clues to the origin of the planet Mercury (Credit:
Nicola Mari)
"From the trajectory point of view, it's probably harder to reach than Jupiter," says Ignacio Clerigo, spacecraft operations manager of
BepiColombo, the European Space Agency's ongoing mission to Mercury, a
project Mari's work is contributing to.

These difficulties mean that Mercury has been less well-studied than our
other neighbours. Two previous missions – Mariner 10 and Messenger – had flown close enough to map its surface, which is pockmarked with craters
– and revealed some major surprises about its structure.

One surprise was the planet's core. The other rock-based planets –
Venus, Earth and Mars – all have a relatively tiny core, surrounded by a thick mantle made of magma, and a hard crust. Mercury's crust, however,
appears to be surprisingly thin, while its core is unexpectedly huge
compared to the mantle. "It's absurd," says Mari.

Even more unexpectedly, these missions revealed that Mercury is
surrounded by a magnetic field. This, combined with its density,
suggests that it has an iron core – and, like that of the Earth, the
core may be partly molten.

To add to the mystery, the ratio of chemicals on Mercury's surface is
highly unusual. By using a technique known as "spectrometry" to analyse
the chemical composition of the planet at a distance, scientists know
that Mercury has a much high concentration of thorium than its nearest neighbours. Thorium should have evaporated in the extreme heat of the
early Solar System. Instead, its thorium content is closer to that of
Mars – three planets away – which would have formed at cooler
temperatures due to its distance from the Sun.

Nicola Mari A sample of boninite with many green crystals, called
"olivines" (Credit: Nicola Mari)Nicola Mari
A sample of boninite with many green crystals, called "olivines"
(Credit: Nicola Mari)
Such anomalies have led some planetary scientists to hypothesise that
Mercury originally formed at a more distant point from the Sun, near to
Mars – and that it started out with a much bigger mass, around the size
of the Earth, that would befit its large core. At some point in its
history, however, it is hypothesised that Mercury collided with another planetary body that sent it spinning towards the Sun. Such a collision
could have blown away its crust and much of its mantle but left behind
the huge liquid core.

"The Mercury we see today may be nothing more than the kernel of the
planet that was once there," says Mari.

Alien rocks
The ideal way of investigating this theory would be to analyse samples
of rocks from the crust of Mercury, or to drill into its mantle – but no probe has been able to land on the surface, leading scientists on Earth
to look for other sources of information.

Some clues may come from a class of meteorites known as aubrites, which
are named after the French commune of Aubres where they were first
discovered. These rocks have a similar chemical composition to Mercury,
and some scientists have even hypothesised that they may be debris from
the interplanetary collision that knocked Mercury into its current position.

It is a tempting idea, but Mari is sceptical. The evidence so far, he
says, suggests the aubrites come from asteroids that formed in the same
part of the solar nebula as Mercury – but were never part of the planet itself.

An alternative line of evidence can come from "geochemical analogues" –
rocks formed on Earth that closely resemble the structures of those
found on other planets. We have a much better knowledge of the
geological processes closer to home, after all, and we can then use this understanding to guide theories about the formation of their
extrafterrestrial twins.

Nicola Mari An outcrop in Cyprus shows the trace of ancient lava that
erupted at the Tethys Ocean floor (Credit: Nicola Mari)Nicola Mari
An outcrop in Cyprus shows the trace of ancient lava that erupted at the
Tethys Ocean floor (Credit: Nicola Mari)
This was the aim of Mari's mission to Cyprus – which, according to the available geological data, was the most likely to contain the specific composition he was seeking. Embarking on his quest through these
deserted mountains, he felt like a "modern Indiana Jones", he says.

Cyprus is a piece of crust that formed under the Tethys Ocean more than
90 million years ago. With the collision of tectonic plates, it was
eventually thrust towards the surface, whether it became the island we
know today. The landscape still has an other-worldly feel, Mari says,
with greenish rocks rich in minerals. "In certain areas of Cyprus's
mountains, it's like you are still walking over an ancient ocean bed,"
he says. Eventually, he found the specific pieces of lava, known as
boninites, that he sought.

Mari returned home and, working with colleagues at Nasa and Italy's
Museum of Planetary Science, analysed the composition of the rocks and
compared them to measures taken from Mercury. When the results came in,
he was astonished. "They weren't just similar; they were identical." The
mix of elements such as magnesium, aluminium and iron was the same as
that seen on the mysterious planet with the huge core. The only
difference was that the rocks from Cyprus had been oxidised – which is inevitable given Earth's oxygen-rich atmosphere. This makes it the first
true terrestrial analogue for Mercury, says Mari – providing a precious
extra datapoint for our understanding of the planet.

Further study of these rocks might help reveal some clues about the
geological activity in Mercury's past. We know, after all, that the
Cypriot boninites were formed from lava that erupted through a shallow
point in the Earth's crust. Their almost perfect resemblance to rocks on Mercury therefore supports the idea that the mantle there – across the
planet – is unusually close to the surface, Mari says – which is
consistent with a violent origin that blew away much of the planet's
original crust.

Future voyages
Mari's findings are one piece in a very large puzzle, and many further
insights may come from the BepiColombo mission – a collaboration between
the European Space Agency and Japan – which launched in October 2018. It
is named after the mathematician and engineer Giuseppe (Bepi) Colombo,
who – among other things – helped to plan the complicated path of the Marine 10 spacecraft.

ESA/ATG medialab; Mercury: NASA (Credit: ESA/ATG medialab; Mercury:
NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie
Institution of Washington)ESA/ATG medialab; Mercury: NASA
(Credit: ESA/ATG medialab; Mercury: NASA/Johns Hopkins University
Applied Physics Laboratory/Carnegie Institution of Washington)
As part of its circuitous route, BepiColombo has already made three
flybys around Mercury as part of the strategy to reduce its velocity.
The spacecraft will make its final approach to the planet in 2025, where
it will split into two orbiters – one measuring the magnetic field, and
the other studying the surface and internal composition. Mari's research
on geochemical analogues may be relevant here, as they could be used as
a benchmark for some of these measurements, he says.

"Laboratory measurements of Mercury-like analogues help us to better
interpret results for measurements we obtained from our infrared and
thermal infrared) spectrometers and also to some kind of the X-ray spectrometer," explains Johannes Benkhoff, project scientist for
BepiColombo.

Over the following year, the orbiters will make more precise
measurements of Mercury's mineral compositions, its topography and its
inner structure. By comparing this data with those from past missions, scientists may even be able to determine whether the planet is still geologically "alive". There are hollows on the surface that appear to be
formed from the evaporation of material from inside Mercury – but it's
not clear whether this process is still active.

Together, these measurements may finally allow us to get to the bottom
of Mercury's mysterious origins – and, by extension, tell us much more
about our own place in the cosmos. "The question of why Mercury is so
dense and has such a large core are so important for understanding the formation and history of our Solar System," Benkhoff. "And the
spacecraft has a very comprehensive suite of payloads and instruments
that we hope we will really advance our scientific knowledge."

Already, so much has changed in how we view the first planet from the
Sun. "Fifteen years ago, Mercury was considered a boring planet,"
Benkhoff says. "But I expect to find many more surprises."

For Mari, Mercury is just the start. "In Lanzarote we have found lava
similar to the mantle on Mars. And to look for traces of Venus, we're investigating Sicily, Hawaii, Indonesia and Kamchatka in Russia."

With BepiColombo's full scientific operation set to commence in 2026, we
might soon have a better understanding of just how much these rocks on
Earth can tell us about our other neighbours in the Solar System.

--

* Alessia Franco is an author and a journalist focusing on history,
culture, society, storytelling and its effects on people.

** David Robson is an award-winning science writer and author. His next
book is The Laws of Connection: The Transformative Science of Being
Social, to be published by Canongate (UK) and Pegasus Books (USA &
Canada) in June 2024. He is @d_a_robson on Twitter, and @davidarobson on Instagram and Threads.

--

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