Page 73 - The Economist20171214

The Economist December 16th 2017 Science and technology 73
2 have their contents extracted for human First contact?
use. This has caused the sea’s level to fall
and the pressure it exerts to diminish. That Greetings, Earthlings
drop in pressure is felt in the surrounding
rocks, which are often porous and water-
filled. The rocks are a battleground be- Agroup ofscientists wonderifan extrasolarvisitoris an alien spacecraft
tween salt water from the lake and fresher
water from elsewhere—a battle that, be- UMUAMUA, an object tumbling to huntingforalien life, plans to turn the
cause of the pressure drop, the fresher wa- ’Othrough space that was discovered world’s biggest steerable radio telescope,
teris winning. on October19th, has already made his- the Green Bankinstrument in Virginia,
When this incoming water makes con- tory. The speed at which it is moving towards ’Oumuamua to see ifit can hear
tact with the extensive underground rock- relative to the sun means that it cannot be anythinginteresting. ’Oumuamua is
salt formations that surround the Dead native to the solarsystem. Its official currently about twice as farfrom Earth as
Sea, it dissolves them. That creates cavities designation is thus1I/2017 U1, with the “I” Earth is from the sun. At that range, the
which, if they grow too large, eventually standingfor“interstellar”—the first time telescope should be sensitive enough to
fail to support the rock layers above. The this designation has everbeen used. pickup a transmitterabout as powerful
consequence is the sudden collapse of That is exciting. Some scientists, as a mobile phone afterjust a few sec-
those layers into a hole in the ground, tak- though, entertain an even more exciting onds-worth ofobservations.
inganythingon the surface with them. possibility: what if’Oumuamua is not an Will it find anything? Almost certainly
A technique called interferometric syn- asteroid, as most think, but an alien not. ’Oumuamua has the same reddish
thetic-aperture radarcan give a few weeks’ spacecraft? Asteroids come in all sorts of colouras many asteroids, so presumably
notice of such a collapse. It permits re- shapes and sizes, but ’Oumuamua seems has a similarcomposition. And, ifit really
searchers to spot subtle signs of subsi- particularly odd. As best as astronomers is a spaceship, it is odd that signs ofits
dence that often presage a sinkhole’s for- can tell, it is cigarlike, beingroughly 180 artificial origin have not been seen al-
mation. This grants enough time to metres longbut only about 30 metres ready—and also odd that it is tumbling. It
evacuate the site, but not enough to plan wide. That makes it more elongated than could, in theory, be a derelict. But in that
the relocation of an extensive spa facility. anythingknown ofin the solarsystem. case the telescope is unlikely to hear
Dr Abelson, though, speculated that the Such a shape would be a sensible choice anything. By farthe most likely option is
disintegration of the rock-salt formations fora spaceship, since it would minimise that it is exactly what it seems to be: an
which creates the holes might cause minor the scouringeffect ofinterstellar dust. itinerant hunkofspace rock, albeit one
tremors well in advance ofa collapse. With that in mind the Breakthrough that has come to the solarsystem from
To determine whether this was so, he Listen project, an organisation dedicated the vast voids between the stars.
and a team of colleagues drilled, in 2012,
five small boreholes around Mineral
Beach. They then placed sensors into each ground, of tectonic plates. Crucially, he The first such collapse was in 2014, a lit-
borehole, at depths ranging from 11 to 25 and his colleagues were able to use the tle more than two years after the team had
metres. These sensors, called geophones, slightly staggered arrival times at different detected a series of small tremors emanat-
recorded movements of the earth and geophones of these tiny tremors to locate ing from that very spot. Seven additional
transmitted theirdata regularlyto a nearby each tremor’s epicentre. This let them sinkholesformed in 2016. Again, theywere
monitoringstation. make a map ofMineral Beach that showed in places where the researchers had de-
During the two months in which they where the crust was beginning to buckle tected clusters of tremors during their two
operated Dr Abelson’s geophones de- because of cavity formation below. They months of monitoring. Not all of the trem-
tected 82 seismic events, of which 75 came then waited and watched, to see if the epi- ors they recorded have yet been followed
from the surface layers rather than being a centres were at places where sinkholes by the formation of sinkholes. But, cru-
result of the movement, deeper under- subsequently formed. cially, all the sinkholesthathave formed so
farhave been at the epicentres oftremors.
Thatobservation arguesforthe installa-
tion of permanent geophone networks in
tourist areas around the Dead Sea. Indeed,
trackingchanges in the rate and strength of
tremors, before any distortion of the
ground detectable by radar shows up, may
provide information about when a sink-
hole might be expected to form, as well as
where. That would give building owners
more notice of problems, permitting them
to move facilities in good time.
Nor is the Dead Sea the only place
where Dr Abelson’s system has the poten-
tial to do good. Several states in America,
including Florida and Missouri, have large
swathes of land sitting on top of rock salt
and other materials that easily wash away
and generate sinkholes as a result. Indeed,
southern Missouri has seen over a hun-
dred sinkhole collapses since 2007, and
would benefit greatly from having means
in place to detectwhen and where they are
Sunk costs likely to happen. 7