On Monday 23 September 2019, NASA Chief Scientist Dr James Green
knocked our socks off at the Royal Aeronautical Society with his
presentation of the agency’s Strategy for Lunar Exploration over
the next 5 or so years.
[You can listen to the AUDIO from the RAeS lecture by following the link on this page:
https://www.aerosociety.com/news/audio-forward-to-the-moon-nasa-s-strategic-plan-for-lunar-exploration/ ]
I can’t do Dr Green’s talk justice here; I was mesmerised by what he had to say and forgot to take any notes at the time, so all this is from a (poor) memory, scribbled on the back of the Evening Standard on the tube journey home. Notes below are my summary and generally don’t use his words – and of course, I may have inadvertently scrambled some of his messages. I’ve also concentrated on the “origins” section below, much more than on the “futures”.
https://www.aerosociety.com/news/audio-forward-to-the-moon-nasa-s-strategic-plan-for-lunar-exploration/ ]
I can’t do Dr Green’s talk justice here; I was mesmerised by what he had to say and forgot to take any notes at the time, so all this is from a (poor) memory, scribbled on the back of the Evening Standard on the tube journey home. Notes below are my summary and generally don’t use his words – and of course, I may have inadvertently scrambled some of his messages. I’ve also concentrated on the “origins” section below, much more than on the “futures”.
Looking back on
Lunar exploration and the formation of the Solar System.
NASA has sent
astronauts to 6 sites on the Moon in different terrains and orbited
many more times. As part of the Apollo mission series, 850lb [c 385kg] of Lunar
material has been returned to Earth and stored securely and cleanly
in the Lunar Archive at the Johnson Space Flight Centre. Scientists
around the world can get access to study it by submitting a research
proposal; currently teams from the USA, UK, France, Germany and Japan
are still working on those rocks, 50 years after they were collected.
Apollo is the programme that keeps on giving.
These results,
combined with analysis of orbiter data and what we know of the
geology of the Earth, have helped to constrain current models of the
formation of the Earth-Moon system and the Solar system itself, as
well as shaped scientific objectives for future Lunar exploration.
[There is a very
readable summary of US Lunar exploration here:
The Moon wasn’t a
roaming body captured by the Earth’s gravity during the formation
of the Solar system. From the chemical (whole rock), mineralogical
(rock constituent) and isotope analyses, it’s now clear that the
Earth and the Moon are made of the same stuff. The gross structure
is similar – crust, mantle and core but different in detail –
thickness/proportions, magnetic field and seismic/thermal regimes.
So what happened ?
The model currently in favour with the bookies (other models are
available, but not many) accounts for the distribution of crust,
mantle and core, gravity field, the overall angular momentum, orbits
and spin of both bodies. A Mars-sized body of the proto-Moon (known
as Thea) condensed from material in the same initial orbit as the
proto-Earth in their paths around the early Sun. Thea and the
proto-Earth came together in a massive collision > 4000 M years
ago. [Context: geologists generally agree the age of the Earth to be c 4567 M years].
Thea was blown apart
and its debris field enveloped the proto-Earth. The majority of
material forming Thea’s core orbiting the proto-Earth within c 3
Earth-radii (the Roche Limit) fell back to a now molten proto-Earth.
Material outside the Roche Limit coalesced to form the Moon but much
closer to the Earth than it is today. Heat from the extremely hot
post-collision Earth ablated the Moon’s nearside surface causing it
to fall back onto its farside, thickening the crust there. With the
Moon reconstituted, it seemed much larger in the early sky, tidally
locked to the Earth’s rotation but orbiting more closely and quickly around a
faster spinning Earth.
But today, the Moon
is at c 60 Earth-radii distant – can it have moved away ? The
Apollo 11 laser ranging experiment placed on the Sea of Tranquillity
by Neil Armstrong has confirmed that the Moon is currently retreating
from the Earth by c 4 cm a year. About the same as one’s
fingernails grow, but over geological time, enough to get the Moon
where it is today. The Moon’s orbital period around the Earth
would now take longer and the Earth’s spin on its axis would now be
slower (ie fewer days in an Earth year). This is in agreement with
daily growth increments on rugose corals from the Earth’s Devonian
period (c 400 M years ago) indicating 420 days in an Earth year at
the time. [Context: the Cambrian explosion of hard bodied life was c 540 M year ago].
We are lucky to be living now; in a few more Million years, the Moon’s disc will no longer exactly obscure the Sun’s face and there will be no more spectacular total Solar eclipses.
We are lucky to be living now; in a few more Million years, the Moon’s disc will no longer exactly obscure the Sun’s face and there will be no more spectacular total Solar eclipses.
So, we now have a
Moon with:
* a tidally locked,
spin-orbit coupling keeping one face (the nearside) towards the Earth
* a very small core
(most of Thea’s core material was incorporated back into the Earth)
* a thick gabbroic
mantle (volcanic outbursts of which formed much of the Lunar maria)
* a dry, thinnish
crust, thicker on the farside than the nearside, of mainly
Anorthosite (a rock made predominantly of Anorthite, a
calcium-aluminium plagioclase feldspar, with almost no water or
hydroxyl molecules bound within).
And yet….
When the LCROSS
spacecraft was deliberately crashed onto the Lunar surface near its
South Pole, an orbiter detected a decaying plume of water vapour
whenever it passed over the crash site. The crash site was in the
South Pole – Aitken (SPA) Basin, the largest and oldest clearly
recognisable crater, possibly produced by a 250km scale impactor,
arriving at c 15km/sec. The craters within it are in almost
permanent shadow, no matter what aspect the Moon presents to the Sun,
so it’s been very cold, for a very long time.
More intense study
of the orbiter’s detection of water vapour revealed that peaks were
also produced as the Earth-Moon system passed through debris in their
orbit around the Sun; these are the meteor showers we see from Earth.
So impacts were releasing water held in the Lunar regolith at the
SPA. As a by-product from this analysis, peaks in the SPA water
vapour detection at times when there weren’t known meteor showers
on Earth revealed that the meteor showers were there but we just
hadn’t realised it !
The rest of the Moon
is bone dry; so how did the water get into the SPA ? Crater impact
studies, begun by the late Gene Shoemaker in the 1960s and backed up
by radiometric dates from Apollo rocks have revealed several phases
of meteorite bombardment. Our best theory at the moment is that the
Late Heavy Bombardment, a 200 M year impacting episode either side of
c 3900 M years ago, was responsible. [Context: 200 M years is c 3 times as long as the dinosaurs have been gone].
But where did all
this impacting material come from ? The inner planets were thought
to be fairly well formed and pretty dry by this time. For the outer
planets (the gas giants), it was another story. Despite their shiny
and powerful new computers, planetary scientists had been unable to
agree on a plausible model for their formation. In frustration, some
tried to model a system that would form the observed number of gas
giants at all, and worry about the constraints (mass, distance,
separation, angular momentum, composition etc) on the solution
afterwards. As it turned out, making adjustments to the model
parameters produced 4 gas giants of approximately the right relative
masses; just not at the right distances.
Letting that model
run with the outer planets much closer together than they are today,
an instability developed and tidal (gravitational) forces eventually
tore material from all 4 of them and in releasing it towards the Sun,
the planets also shifted their orbits; Jupiter inwards; Saturn,
Uranus and Neptune outwards, to their present configuration. This
material formed the stuff of the Late Heavy Bombardment (LHB) and the
inner planets and their moons all got it with both barrels for c 200
M years, bringing water molecules with it. Mercury and Venus
couldn’t retain the water; the Earth, our Moon and Mars held on to
more or less of it.
While the Earth was
getting pummelled and hydrated during the LHB, so was the Moon. It
has been calculated that together with mantle outgassing from Maria
eruptions, the Moon received enough material in this 200 M year span
that it was able to form an atmosphere with about 12-15 hPa (mBar) pressure [Context: the Earth's present day atmosphere is around 1010 hPa] which could be sustained as long as the LHB continued and
before its initial liquid core cooled and the magnetic dynamo ran
out. That fleeting atmosphere allowed Lunar weather and the rain,
snow, CO and CO2 accumulated in deep, dark places like the
SPA and froze solid. When the LHB ended, water that was in sunlit
areas melted and evaporated away into space.
Looking forward to
further exploration.
There is good
evidence for much of this theory but we haven’t visited the SPA and
subjected the rocks there to the closest scrutiny to see if they can
support this.
Turns out, from
infrared remote sensing and gravimetric measurements, the SPA is also
rich in Platinoid group elements – thought to have been brought
there by a large impact after Lunar formation but before the LHB, forming the SPA itself –
and slamming the Moon hard enough to readjust its internal mass and
possibly alter its rotational axis by up to 5 degrees. This is just
the sort of thing to interest mining companies, as those materials
are quite rare at reachable depths on the Earth.
These and the desire
to get to Mars has propelled NASA to produce a near-term Lunar
exploration strategy.
The Artemis
programme has 3 phases and aims to establish a habitation near the
Lunar South Pole by 2024.
Its objectives are
to:
* create a Lunar
orbiting Gateway spacestation in a highly elliptical orbit over the
SPA
* establish a series
of remote telescopes in different parts of the spectrum, on the Lunar
farside away from radio interference from Earth
* send the first
woman and the next man to a sustainable Lunar habitation near the
South Pole
* offer mining of
the SPA resources
* enable
manufacturing in low gravity from local materials and those brought
in via Gateway
* provide a fuel
depot for topping up the Gateway for spacecraft in transit to Mars
This ambitious
programme will collaborate with industrial partners and other
nations’ Space Agencies.
Technologies are
already available to process available water to provide Hydrogen
(fuel) and Oxygen (to breathe) and to house humans sustainably. One
of the decisions involves where to place the habitat. Putting it on
the surface could subject its exterior to temperature variations of
as much as 200 deg C over a Lunar day / Earth month; that could make
it hard to sustain a manageable temperature range inside the
structure. Putting it inside a Skylight – something that looks
like an elliptical crater but is actually a lava tube with part of
the roof collapsed – is more attractive as the temperature extremes
are much less.
[A case is made for
visiting the South Pole – Aitken Crater area, here:
https://lunar-landing.arc.nasa.gov/sites/default/files/presentations/06_Jolliff_SPA_landing_sites_0.pdf ]
= = =
[Of course, in terms of lava tubes and habitation technology, us Brits know who really got there first:
https://www.youtube.com/watch?v=Ok6CoIwcJ-E ]
= = =
[Of course, in terms of lava tubes and habitation technology, us Brits know who really got there first:
https://www.youtube.com/watch?v=Ok6CoIwcJ-E ]
