All_About_Space_-_Issue_94_2020

romthe4-billion-year-old,

230-kilometre(143-mile)wideClavius

impactcratertoNeilArmstrong’s

50-year-old,size-nine-and-a-half‘one

smallstep’,theMoonhasrecordedmomentous

episodesacrossmuchofourSolarSystem’shistory.

Forscientistsitssurfaceprovidesa 4-billion-year-

oldastronomicalarchiveofeventsthatshapedboth

itsownpepperedsurfaceandourplanettoo.A

passiveobserver,itsrecord-keepingvalueliesinthe

factthatthingstendtohappentoit,ratherthanit

showingsignsofitsownactivity.

Andyetitseemswemighthavemissed

something.Somethingsmallinsizebutlargerin

significance.Locatedrightbetweentheeyesofthe

‘ManintheMoon’,somescientistsbelievetheyhave

foundevidencethatlunarvolcanoeswereerupting

whendinosaursroamedtheEarth– andperhaps

theyarestilleruptingsomewhereonthesurface

rightnow.Ifconfirmedsuchactivitywouldtearup

currentmodelsoftheMoon’sinternalstructureand

howitevolved,aswellasrewritingassumptions

aboutothersupposedly‘lifeless’worldsbeyond.As

missionsareproposedtovisitthisintriguingsite,

weask,istheMoonstillalive?

Aquickglanceofitsancient,batteredsurface

offerslittlehope.TheMoon’scrateredappearance

isinstarkcontrasttonotonlytheplanetitorbits,

F

but also many local, smooth-surfaced planets, dwarf

planets and moons. In each case the main concealer

of these impact blemishes is volcanism. This

recycling of surface crust is familiar to us on Earth,

but also accounts for the lack of cratering on Venus,

Jupiter’s moon Io and even further out to Pluto.

But no such rejuvenation appears to have played

out on the lunar surface for a while. While the

large, dark mares covered some of the most ancient

cratered crust with wide-scale eruptions, billions of

years of exposure to Solar System violence has left

them sporting plenty of blemishes of their own. The

concentration of such cratering dates the mares to

between 3 and 4 billion years old, figures backed

up by analysis of recovered lunar rock samples.

And after that? Well, not much. “We thought that

volcanism had ended by 1 billion years ago,” says

Julie Stopar, a director at the Lunar and Planetary

Institute in Houston.

This made sense. The Moon, as a relatively small

body, shouldn't have ever had huge amounts of

internal heat. And what it had should have been lost

quickly due to the large surface area to mass ratio

that blights smaller worlds.

This heat might have another effect. Evidence

from thrust faults shows the lunar surface shrunk

as it cooled, contracting the crust up against itself.

This makes it even more difficult to imagine

volcanic activity piercing through. But then came a

photograph that put that all in doubt.

In 1971, while Apollo 15 astronauts David

Scott and James Irwin test-drove the first rover on

the lunar surface, Command Module pilot Alfred

Worden, orbiting above, took a picture of a strange

blotchy feature. Named Ina, it was an exposure

of a dynamic landscape – by lunar standards

anyway. The Moon’s surface is a collection of

homogeneously f lat vistas covered by broken-up

rock and dusty regolith. It's a landscape bombarded

into uniform consistency, where any notable

topography has been eroded through landslides and

impacts over vast eons of time.

“ It is hard to explain how you can have

young volcanism based on everything we

know about the Moon” JulieSStoppar

Right

(clockw ise):

In 1971, while

Apollo 15

astronauts David

Scott and James

Irwin test-drove

the first lunar

rover, Alfred

Worden , orbiting

above, took the

first pictures

of Ina

As the most

volcanically

active body

in the Solar

System, Jupiter’s

moon Io has

a very young,

uncratered

surface

Thrust up lunar

cliffs is evidence

of the Moon’s

contraction as

it cooled. Some

have questioned

where such

compression

forces would

allow lava to

reach the surface

All photos © NASA

The Moon is alive