New Scientist - USA (2019-06-15)

12 | New Scientist | 15 June 2019

ON THE moon is a giant crater

called the South Pole-Aitken basin,

measuring about 2500 kilometres

across. It is thought to have been

created by a space rock striking

the moon 4 billion years ago and

is among the largest craters in

the solar system. The remains of

that asteroid may now have been

found under the lunar surface.

Using gravity data from NASA’s

GRAIL spacecraft, which orbited

the moon from 2011 to 2012,

Peter James at Baylor University

in Texas and his colleagues

spotted a huge mass sitting about

300 kilometres beneath the basin.

They calculate its mass to be

2 × 1015 tonnes, spread across an

area hundreds of kilometres wide.

The team believes the mass is

the core of an asteroid that shaped

much of the moon we know today,

including by forming mountains

called massifs nearby. “It

decimated the whole surface

of the moon,” says James.

The asteroid would have

blasted a huge hole, with material

then mixing and sinking into it.

This included the iron-nickel core

of the asteroid itself, which is

thought to have been about

95 kilometres across. As the moon

cooled, this core would have been

suspended underground, a large

chunk of iron within the upper

lunar mantle (Geophysical

Research Letters, doi.org/c6x3).

“The deep density anomaly

is fascinating,” says Linda Elkins-

Tanton at Arizona State University.

“It’s such a large observation.”

Another possible explanation

for the large mass is that it is a

remnant of the moon’s early

magma ocean, formed not long

after the satellite’s birth about

4.5 billion years ago.

But the fact that it is below

such a large crater favours the

asteroid explanation over the

magma ocean one. “I am not

aware of a reason why we would

expect that to happen under the

crater specifically, as opposed to

somewhere else on the moon,”

says James.

There are still some mysteries

we need to resolve to figure out

if this truly is the asteroid’s core.

“The location of the mass is not

quite in the middle of the crater,

but also not near the edge,” says

Gareth Collins at Imperial College

London. “To me, that suggests that

the impact was at a relatively steep

angle, and that might not be

compatible with the fact that the

basin is clearly quite elliptical.”

The mass is too deep to access

directly, but if a network of seismic

instruments were placed around

the moon, it could reveal more

details. With the US aiming to send

humans to the moon’s south pole

in 2024, we could get a closer look

at the material ejected by the

asteroid onto the lunar surface

and learn about its composition

and origin.  ❚

BEING a female tree is hard. A study

of a species that can switch sex has

revealed that most striped maple

trees die while female.

“We had a suspicion they were

changing sex, which is relatively

rare among plants,” says Jennifer

Blake-Mahmud at Princeton

University. Between 2014 and

2017, she and Lena Struwe at

Rutgers University in New Jersey

tracked the life cycles of 457 striped

maples (Acer pensylvanicum) in

New Jersey, recording their health,

diameter and whether they had

female or male flowers each spring.

They found that 54 per cent of

the trees changed sex during that

time, and a quarter of those did so

at least twice (Annals of Botany, doi.

org/c6x6). A model based on these

findings showed that, contrary to

previous suggestions, healthy trees

were more likely to be male and a

tree’s size doesn’t influence its sex.

The growth rate of trees that

remained female for multiple years

deteriorated. Of the trees that died,

75 per cent had recently produced

female flowers. “It’s remarkable,”

says Blake-Mahmud. “When I see

a tree that’s dead and I look back

in my data sheet, it was almost

always female the year before.”

It isn’t clear why this is the case.

It could be that female trees need

more nutrients because they

produce seeds, and this is so

taxing the trees die, she says.

But it could also be that dying

trees become female as a last effort

to create offspring and pass their

genes on to the next generation.

“These populations have a lot

more male trees than female

trees in general, so just by luck

of the draw, a female has a better

chance of being a parent of the

next generation than a male,” says

Blake-Mahmud. “If you’re going

to die anyway, then being female

is the way to go. That would make

more evolutionary sense.”  ❚

Chelsea Whyte

Botany

Striped maples

can switch to

making flowers

of a different

sex throughout

their lives

Lunar science

Jonathan O’Callaghan

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The moon’s South Pole-

Aitken basin (blue and

green) is one of the solar

system’s largest craters

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Mass of suspected asteroid

remnant in tonnes

Preserved asteroid on moon

The space rock seemingly responsible for a giant crater may still be there

Female trees

more likely to die

than male ones