The Economist - USA (2020-11-13)

70 Science & technology The EconomistNovember 14th 2020

2 surely absent from Venus, so the question

became whether there was a plausible, nat-

ural, but non-biological explanation for

the gas being there. Neither Dr Greaves nor

anyone else has yet come up with one, so

that leaves open the tantalising possibility

that it is a sign of life on the planet.

But there is another possibility. This is

that the signal Dr Greaves and her team

suggest is phosphine isn’t. And, in the

weeks since the results were published,

other groups have been busy poring over

them, conducting their own analyses and

attempting to poke holes in the original

claims. Their concerns are twofold. One is

an inability to find evidence for phosphine

in independent observations of Venus’s at-

mosphere. The other is whether Dr Greaves

and her colleagues have processed their

data correctly.

Crucial gaps

Those data came from the Atacama Large

Millimetre Array (alma), a set of radio-

telescope dishes that sit at an altitude of

5,000 metres in the mountains of Chile.

The solar radio spectrum reflected from Ve-

nus has, according to Dr Greaves, a gap

known as an absorption line in it at a wave-

length of around 1.1 millimetres. Phos-

phine molecules are known to absorb radi-

ation of this wavelength.

But phosphine also absorbs other wave-

lengths. A robust way to verify Dr Greaves’s

findings, therefore, would be to find simi-

lar characteristic gaps in other parts of Ve-

nus’s reflected solar spectrum. Therese En-

crenaz of the Paris Observatory set herself

this task, and went hunting for appropriate

gaps in the infrared region of that spec-

trum. She combed through data collected

using texes, a spectrograph at the Gemini

Observatory in Hawaii, between 2014 and

2016. But she drew a blank. That result,
      published in the November issue of Astron-
      omy & Astrophysics, seems to be a contra-
      diction to the original claim of phosphine
      on Venus.
      The second possible contradiction, of
      Dr Greaves’s data-processing methods,
      comes from Ignas Snellen of Leiden Uni-
      versity in the Netherlands. Any work of this
      sort requires the data to be passed through
      a software noise-filter in order to subtract
      the effects of both Earth’s atmosphere and
      the telescope array itself. Dr Snellen and
      his colleagues have reprocessed the origi-
      nal alma data using a different noise-filter,
      to see if similar results emerge.
      In a paper posted on arXiv (a website for
      so-called preprints, which have not yet
      been peer-reviewed but which their au-
      thors wish nevertheless to put into the
      public domain), they found some evidence
      for phosphine, but not enough to claim a
      confident discovery. More troubling, per-
      haps, was that when they used Dr Greaves’s
      noise-filter on a wider portion of the Venu-

sianspectrumtheyfoundfiveotherstrong

signalsformoleculesnotactuallybelieved

tobepresentintheplanet’satmosphere.

DrGreaves’sclaiminSeptemberwas,

then,justthestartinggun.Investigations

aboutphosphinewillcontinue,probably

foryearsandperhapsfordecades,asas-

tronomersspiralinonthetruth.Indeed,as

iftohighlightboththemessinessofthe

currentuncertaintyandthedesireofmost

scientificresearcherstogetatthetruthre-

gardless,DrGreavesherselfisoneoftheco-

authorsofthephosphine-dissentingpaper

publishedbyDrEncrenaz.

Onewaytosettlethematterwouldbeto

send a spacecraft to Venus and take

close-upmeasurementsofitsatmosphere.

Therearehopeshere.India’sspaceagency

planstolaunchShukrayaan-1, whichisin-

tendedtoorbittheplanet,in2025.Mean-

while,nasa, America’sspaceagency,has

two Venus probes—veritas and da-

vinci+—inthefinalselectionstageforits

nextprogrammeofmissions.RocketLab,a

privatespacecompanywitha launchsite

inNewZealand,isalsoconsideringdis-

patchinga missionassoonas2023.Per-

hapsit won’ttakedecadesafterall.^7

S

ugar canecontains around 10% sugar.

But that means it contains around 90%

non-sugar—the material known as bagasse

(pictured) which remains once the cane

has been pulverised and the sugar-bearing

juice squeezed out of it. World production

of cane sugar was 185m tonnes in 2017. That

results in a lot of bagasse.

At the moment, most of this is burned.

Often, it fuels local generators that power

the mills, so it is not wasted. But Zhu Hon-

gli, a mechanical engineer at Northeastern

University in Boston, thinks it can be put to

better use. As she and her colleagues de-

scribe in Matterthis week, with a bit of

tweaking bagasse makes an excellent—and

biodegradable—replacement for the plas-

tic used for disposable food containers

such as coffee cups.

Dr Zhu is not the first person to have this

idea. But previous attempts tended not to

survive contact with liquids. She thought

she could overcome that by spiking the

sugar cane pulp with another biodegrad-

able material. She knew from previous re-

search that the main reason past efforts fell

to pieces when wet is that bagasse is com-

posed of short fibres which are unable to

overlap sufficiently to confer resilience on

the finished product. She therefore sought

to insert a suitably long-fibred substance.

Bamboo seemed to fit the bill. It grows

quickly, degrades readily and has appropri-

ately long fibres. And it worked. When the

researchers blended a small amount of

bamboo pulp into bagasse, they found that

the result had a strong interweaving of

short and long fibres. As a bonus, they also

discovered that the hot pressing used as

part of the process had mobilised some of

the lignin in the fibres, and that this stiff,

water-repelling material was now acting as

an adhesive that bound the fibres together.

To put their new material through its

paces, Dr Zhu and her colleagues first

poured hot oil onto it and found that, rath-

er than penetrating the material, as it

would have with previous bagasse pro-

ducts, the oil was repelled by their inven-

tion. They also found that when they made

a cup out of the stuff and filled it with water

heated almost to boiling point, the cup re-

mained intact for more than two hours.

Though this is not as long as a plastic cup

would last (it would survive indefinitely) it

is long enough for all practical purposes.

Moreover, the new material is twice as

strong as the plastic used to make cups, and

is definitely biodegradable. When Dr Zhu

buried a cup made out of it in the ground,

half of it rotted away within two months,

and she reckons six months would have

seen it gone completely.

Last, but by no means least, she esti-

mates that cups made from the new mate-

rial would cost $2,333 a tonne. That is half

the $4,750 a tonne cost of biodegradable

cups made from polylactic acid (fermented

plant starch), and only slightly more than

the $2,177 a tonne that it takes to make plas-

tic cups. Overall, then, Dr Zhu argues that

bagasse is an obvious choice for making

coffee cups, straws, disposable plates,

lightweight cutlery and so on. Once used,

these could be dumped in landfills with a

clear conscience. 7

How to kill two environmental birds

with one stone

Materials science

Would you like

sugar cane in that?

Waste not, want not