New Scientist - USA (2019-07-13)

13 July 2019 | New Scientist | 41

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instrumentation has allowed us to make

ever more sensitive measurements. “Each

one of those opens up a new avenue of study

about the moon,” says Zeigler.

It also allows us to re-examine old questions

in light of more precise information. Of these,

the biggest question is how the moon formed.

“We look up in the sky and see the moon and

we want to know why it’s there, and how it got

there,” says Zoe Leinhardt, an astrophysicist at

the University of Bristol, UK. But researchers

keep revisiting this question and changing the

answer bit by bit.

Astronomers have toyed with many ideas

about the moon’s origin. Perhaps Earth was

spinning very fast and a piece broke off?

Or maybe the moon was wandering through

space and was captured by our gravity? In 1946,

Canadian geologist Reginald Aldworth Daly

proposed what we now think is the right idea:

that a smaller planet hit Earth, kicking out a

ring of debris from which the moon formed.

In their first investigations of the Apollo moon

rocks, geologists found good evidence that

this was the case. The moon rocks looked

sufficiently similar to Earth rocks to suggest

that the pulverised impactor had been mixed

with a large portion of Earth debris.

Modern reanalysis shows that the moon

rocks are in fact almost identical to Earth’s.

In terms of the giant impact formation

heavy bombardment never happened – we

were fooled into believing it did by a single

huge event that scattered rocks across the

lunar nearside.

“This could be a massive bias in how

we’ve been interpreting these samples,”

says Joy. “For the next generation of

spacecraft, actually picking places where we

can go to sample bedrock is going to be really,

really important.”

Migrating giants

In this way, the true ages of other basins can

be determined. And it will show whether there

was a short, sharp late heavy bombardment

or a continual rain over a longer period.

It isn’t just about the Earth and the moon,

either. If moon rocks have taught us anything,

it is that the entire solar system is connected.

Indeed, knowledge about the surfaces of

Mercury, Venus and Mars has come from

counting craters on the moon and relating

that to the ages of the moon rocks. The late

heavy bombardment, however it unfolded,

was probably caused by gas giant planets

including Jupiter migrating through the

solar system, knocking asteroids out of their

way, some of which sailed towards Earth.

Because our solar system is thought to

form in essentially the same way as other

planetary systems, we are now realising that

the moon can teach us about things beyond

the reaches of our star’s influence. “There are

lots of giant impacts that happen at the end

of a solar system’s formation. That would

happen in extrasolar systems as well,” says

Leinhardt. She says that understanding giant

impacts is the key to grasping the diversity

of those planetary systems, and comparing

them with our own. It may even help tell us

which exoplanets are likely to be habitable,

because in our solar system the moon’s

gravitational pull stops Earth toppling over,

keeping its climate stable.

“Moon rocks have told us about so many

other places than just the moon,” says Zeigler.

And with a return on the cards in the near

future, who knows what family secrets our

sibling still has to share. ❚

model, that means just one thing: “They

had such a big impact that they were totally,

intimately mixed up,” says Leinhardt.

Her colleagues Simon Lock and Sarah

Stewart developed a new model in which

Earth was hit so hard that it melted, absorbing

the impactor and surrounding itself with a

doughnut shaped cloud of vaporised rock.

They think the moon formed out of this,

explaining the similarity of the rocks.

“The moon rocks have given us a huge

amount of information. What we need is to

be able to make an entire story,” says Leinhardt.

This involves using computers to simulate

this cataclysmic event from the moment of

impact to the birth of the moon.

At present, the simulations can’t follow

the process from beginning to end. They

can simulate the formation of the debris

doughnut, called a synestia, but can’t follow

its condensation into the moon. And while

they can keep track of the temperature and

pressure of the synestia, they don’t include

any chemistry. To make progress, Leinhardt

says better computing rather than more

exploration is necessary.

Although Leinhardt doesn’t think new

samples from the moon will be helpful for her

investigations, what happened to the moon

after it formed has got other researchers

itching for a return mission.

Even a casual glance at the moon reveals

dark markings across its surface. They are

thought to have begun forming during a

relatively short period called the late heavy

bombardment. Evidence came from the

Apollo samples, many of which are about

3.9 billion years old. These suggest a period

lasting somewhere between about 20 million

and 100 million years in which the moon

and the rest of the solar system were heavily

pummelled by asteroids, creating large

impact basins we see on the moon’s surface.

That may not be entirely accurate. “Our

ideas are changing as we are reanalysing those

samples,” says Joy. Part of that reanalysis is

intended to help understand where the moon

rocks came from. None of the Apollo samples

were bedrock – rocks sampled in the place

where they formed – and this has robbed

geologists of the geographical context needed

to fully interpret their results.

It is now thought that most of the Apollo

samples could be the debris ejected during the

formation of the Imbrium basin, a vast crater

formed 3.9 billion years ago. If so, the late

These are the smallest particles

brought back from the moon,

magnified many times. With no wind

to smooth them off, moon dust tends

to stay sharp and spiky

The moon experience

Be entranced by a 7-metre model

of the moon. Touch moon rocks.

Explore what the moon smells like.

http://www.newscientistlive.com/moon-

experience

Maggie Aderin-Pocock

Hear her talk about what the moon

has done for us.

http://www.newscientistlive.com/speakers/

maggie-aderin-pocock

Welcome to Lunarville

Architect Daniel Inocente will speak

about how he designed a moon village

http://www.newscientistlive.com/speakers/

daniel-inocente

4.5 billion years

The age of the moon

Stuart Clark is an astronomy writer

based in Hertfordshire, UK. His

latest book is The Unknown

Universe (Head of Zeus)

CELEBRATING

THE MOON

10-13 OCTOBER