ADVANCES
14 Scientific American, August 2020DAVIDE OPPOUnlike Earth, Venus does not rotate once
on its axis every 24 hours but instead does
so once every 243 Earth days. Given that it
orbits the sun on a similar timescale (once
every 225 Earth days), one side of the planet
typically basks in sunlight, while the other
faces a lengthy darkness. A thick atmo-
sphere could easily circulate heat from the
dayside to the nightside, keeping Venus hot.
But in Way and Del Genio’s model, a giant
cloud on the dayside would act as a bright
shield to reflect incoming sunlight and allow
temperatures cool enough for liquid water.
Many researchers have already consid-
ered the idea that Venus was once habit-
able, but the new model further shows
how the planet could have transformed
into today’s hothouse—and it tosses con-
ventional wisdom aside. “There’s a story
about Venus that we tell ourselves. We
teach it in introductory astronomy classes,
and we write about it in books,” says
David Grinspoon, an astrobiologist at the
Planetary Science Institute, who was not
involved in the study, although he was a
co-author on the 2016 paper. “And it turns
out that story is wrong.” The idea is that
the sun slowly increased in brightness,
causing the planet to grow so warm that it
could no longer maintain a stable ocean.
In other words, it pushed the inner edge of
the so-called habitable zone—the orbital
region where liquid water can create con-
ditions conducive to life—past the solar
system’s second planet. But Way and Del
Genio’s model suggests cloud cover would
have provided enough shade to keep liquid
water on the surface of Venus even until
today—had something not tipped the
planet into its current state.
The authors propose a violent mecha-
nism best understood by looking at the
young Earth. Roughly 250 million years ago
deep gashes opened in Earth’s crust, pour-
ing lava onto the surface and spewing
enough carbon dioxide into the atmosphere
to kill 96 percent of marine species and
70 percent of terrestrial species in the larg-est mass extinction in history. These volca-
nic events, which leave deposits called large
igneous provinces, produce at least 100,
cubic kilometers of lava over one million
years. “We’re talking about an affront to God
in terms of the amount of lava that comes
out per unit time,” says Paul Byrne, a plane-
tary geologist at North Carolina State Uni-
versity, who was not involved in the study.
Although these eruptions have rocked
Earth on several occasions, often resulting in
mass extinctions, multiple events have never
happened at once. “That’s fortunate for life
on Earth,” Way says, but scientists see no
reason why more than one event could not
happen simultaneously. And if such multiple
events did occur on Venus, they would have
dumped enough carbon dioxide into the
atmosphere to drive the planet into an apoc -
alyptic greenhouse state, researchers say.
The hypothesis is attractive: “There’s
something romantically tragic about a
world so like our own that was killed,”
Byrne says. “I want so much for it to bePALEOSEISMOLOGYTalking Bones
Buried remains suggest
an ancient, destructive
tsunami in East AfricaThe 2004 Indian Ocean earthquake
sent a tsunami surging outward from
Sumatra, devastating coastlines across
Southeast Asia but doing much less dam-
age by the time it reached Africa. Many
scientists have since considered the tsuna-
mi risk in parts of East Africa to be relative-
ly low. But new research conducted on a
1,000-year-old sand layer full of human
bones, in northeastern Tanzania’s Pangani
Bay, puts the threat of a monster wave
back in the spotlight.
Supplementing years of fieldwork done
by archaeologists at the University of Dar
es Salaam, an international group of geolo-
gists combined its own sand-layer analysis
with computer-simulated earthquake sce-
narios. The group found evidence that an
ancient Pangani Bay fishing settlement was
wrecked by a tsunami emanating from the
Sumatra-Andaman subduction zone—which was also the source of the powerful
2004 tsunami. Lead author Vittorio Maselli
of Dalhousie University in Nova Scotia was
a National Geographic Explorer for the
study, published online in May in Geology.
Within the sand layer the researchersfound evidence of tiny marine fauna swept
in from the sea and broken human bones
that could not be attributable to disease,
violence or traditional burials. “Taking
the analysis together, the tsunami was
the best interpretation,” Maselli says.Dig site near the Pangani
River in Tanzania© 2020 Scientific American