The Economist - USA (2020-07-25)

62 Science & technology The EconomistJuly 25th 2020

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within it seems to have been a river leading

to a well-preserved delta. The layers of sed-

iment in this feature (colour-coded in the

picture on the previous page according to

their mineral composition) are prime tar-

gets in the search for Martian biology.

On Earth, some of the oldest evidence

for life comes in the form of stromatolites.

These stratified structures form in shallow

water when colonies of microbes grow lay-

er upon layer, trapping sediment as they do

so. The most ancient examples are thought

to be those found in Greenland in 2016,

which have been dated to 3.7bn years be-

fore the present day. If there was sufficient

time for stromatolite-forming organisms

to evolve on Earth by this date then there is

no obvious reason why they might not also

have evolved on Mars.

Spotting stromatolite-like layers in

rocks will not, though, be enough on its

own. Researchers will also need to consid-

er the textures of the rocks concerned and

the distribution within them of potentially

telltale minerals and organic molecules.

Confusingly, in chemistry-speak, an or-

ganic molecule is not necessarily of biolog-

ical origin. The term just means that it is

built around carbon atoms, so organic mol-

ecules can also originate inorganically, as

it were. The biological nature of an organic

molecule has thus to be justified by other

evidence. As Kathryn Stack Morgan, a geol-

ogist who is the Perseverance mission’s

deputy project scientist, observes, “This is

exactly the type of thing that we do here on

Earth to make a case for biosignatures in

our own rock record, and for the very first

time using our instruments we can do that

on the surface of Mars.”

Rocks and hard places

Perseverance carries two instruments in

particular that are intended to examine the

surfaces of rocks which the rover encoun-

ters. Both will look for pertinent minerals

and organic molecules. sherloc, situated

at the end of the rover’s robotic arm, will

shine a laser onto tiny grains in rocks it

comes across. By analysing the spectrum of

the light that is scattered back, this instru-

ment will be able to identify molecules in

the grains under scrutiny. watson, a cam-

era, will then take close-ups of rocks that

sherlocdeems worthy of further study.

Mapping sherloc’s chemical analyses

onto watson’s high-resolution images will

show how different mineral layers are ar-

ranged and textured. That will be a big im-

provement over the instruments on board

nasa’s current operational Mars rover, Cu-

riosity, which arrived in 2012. These are ca-

pable only of grinding up rocks to work out

whether or not organic molecules are pre-

sent in the bulk material. If there are stro-

matolites (or even fossils of more complex

creatures) Perseverancewill be able to see

them, both chemically and optically.

As did Curiosity, Perseverancewill rely

on an autopilot to guide it through the at-

mosphere to the planet’s surface, after ar-

riving at a velocity, relative to its target, of

19,500km per hour. “We refer to it as the

seven minutes of terror,” says Matt Wal-

lace, an engineer who is the mission’s dep-

uty project manager. The rover’s autonomy

will then carry over to its everyday opera-

tions. Because of the time it takes radio

waves to travel from Earth to Mars, Perse-

verancewill receive instructions only once

a day. On the ground on Mars it will need to

find and avoid awkwardly placed rocks,

and also more serious hazards, such as

cliffs, by processing, in real time, pictures

coming from its 23 cameras. This autono-

my, nasais confident, will permit the new

rover to cross the Martian surface routinely

and safely at a speed of around 150 metres

per hour, double that at which Curiosityis

usually allowed to travel.

As well as eyes, Perseverancehas ears. A

pair of microphones on board will permit

people to hear the winds of Mars for the

first time. They will also be able listen to

the whirr of the rover’s gears, the crunch of

its wheels as it moves across the regolith

(the crushed rock that passes for soil on

Mars) and the percussive sounds of the

drill at the end of its arm as it chips out

samples of rocks to study.

Not all of those samples will be discard-

ed after investigation. Some will be packed

for eventual dispatch to Earth by a project

called the Mars Sample Return mission.

This is a collaboration between nasaand

the European Space Agency, esa, that in-

volves launching five separate spacecraft

over the course of a decade. Perseveranceis

the first, and its collaboration-related job is

to seal samples of Martian rock that its op-

erators think worthy of further investiga-

tion into one of around 30 titanium tubes

which it carries. As the illustration overleaf

presages, it will leave these on the surface

to be picked up by an esa-designed “fetch

rover” that could arrive as early as 2028.

Once collected, the tubes will be brought

back to Earth by a system of relay craft, and

their contents analysed.

Perhaps most intriguingly of all, Perse-

verancewill also carry a 1.8kg helicopter,

called Ingenuity. If this manages to fly in

Mars’s thin atmosphere (which has about

1% of the density of Earth’s at the surface), it

will represent the first controlled flight, be-

yond the landing and lift-off of a space-

craft, to take place on another heavenly

body. And if that happens, it will pave the

way for more sophisticated drones on fu-

ture missions to act as scouts.

Every contact leaves a trace

The life-seeking instruments on Persever-

anceare more advanced than anything that

has come before them, but this was not the

original plan for the next phase, after Curi-

osity, of nasa’s attempt to find life on Mars.

In February 2012, while Curiositywas still

making its way there, Barack Obama’s ad-

ministration slashed nasa’s planet-explo-

ration budget by a fifth. At the time, Ameri-

can scientists had been developing a

programme called ExoMars, in collabora-

tion with esa. This was to involve an orbi-

ter and several rovers being launched from

2016 onwards, with a combination of tools

intended to look for signs of life.

Mr Obama’s cuts killed American in-

volvement in ExoMars and, by the time Cu-

riosityreached Mars in August 2012, nasa

had no plans to send any future rovers. The

overwhelmingly positive public reaction

to Curiosity’s nail-biting landing, however,

helped persuade the agency’s chiefs to re-

consider their priorities and put together a

scaled-back version of previous plans that

required no increase in the budget. The re-

sult, the mission now known as Persever-

ance, was announced a few months later.

Meanwhile, esahad kept its part of the

ExoMars programme alive, turning to Rus-

sia for help with launching and hardware.

In 2016 the agency delivered the first part of

the programme, the Trace Gas Orbiter. Its

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