Science News - December 3, 2011

conditions under which ozone can exist. Ozone often flags a planet as potentially life-bearing, but on Venus the molecule’s concentration is only one-thousandth the amount on Earth — far too low to tantalize alien hunters.

Striking signal

While scientists are coming to accept that Venus and Earth share similar volcanic and atmospheric processes, the presence of Earthlike lightning in the Venusian clouds remains disputed. Christopher Russell of the University of California, Los Angeles is convinced that Venus has stormy skies, complete with electric flashes similar to those seen on his home planet.

Russell studies lightning using a magnetometer aboard Venus Express that detects low-frequency electromagnetic waves in the Venusian ionosphere, the charged region of the upper atmosphere. “Some days, there are a lot of signals coming into the ionosphere,” Russell said in October at a planetary sciences meeting in Nantes, France. “Basically, those are our stormy days.” The signals observed by Venus Express depend on the density of charged particles in the ionosphere, as a scientist studying Earth would expect for electrical activity, Russell says. The flashes, which resemble those seen by previous probes, also have an Earthlike frequency and intensity.

On Earth, lightning is presumed to have helped life form from the mix of molecules swimming in the primordial stew. Seeing lightning on Venus suggests that electrical discharges might be common wherever the right atmospheric ingredients are found. Planets elsewhere in the cosmos, then, might possess a potential catalyst for life, too.

But skeptics point out that the
Cassini spacecraft, which swung by
Venus in the late 1990s, failed to detect
anything resembling the flashes that
occur on Earth. “Terrestrial lightning is
always occurring somewhere on Earth,”
says planetary physicist Don Gurnett of
the University of Iowa in Iowa City. “If
it were terrestrial-like, we would have
detected it.”
Russell and Gurnett
would both like evidence
of lightning in the form
of an optical flash within
the planet’s acid storm
clouds — something Venus
Express can’t image. But
the Japanese Akatsuki
spacecraft, launched in
May 2010, might have
made such an observation
this year. Unfortunately,
in December of 2010 the
spacecraft’s engine failed
during orbit insertion.
Akatsuki is still wandering
the inner solar system, with
a second attempted rendez-
vous planned for 2015.

Swirling shroud

If Akatsuki does enter Venus orbit, it will begin imaging the Venusian clouds to produce data that will help scientists peer more closely at a thick shroud that reaches 70 kilometers above the planet.

At the planet’s poles, that shroud’s
height shrinks to 65 kilometers, and
it is embellished by mysterious vorti-
ces — enormous, shape-shifting masses
of swirling clouds. Whipped into a frenzy
by some unknown mechanism, the
vortices swirl around a central 2- to
3-kilometer-wide hole, a tunnel that
plunges through the atmosphere. And
they look like Earth’s hurricanes, except
that they are about the size of Europe, says
Dmitry Titov of the European Space
Agency and the Max Planck Institute for
Solar System Research in Katlenburg-
Lindau, Germany. “The morphological
similarity is really striking,” he says. “But
the physical mechanisms, I think they
should be different.”
Earth’s hurricanes are maintained
by moist air condensing and releasing
energy. On Venus, Titov says, the vor-
tices appear to be the result of a global
pattern of atmospheric circulation
known as “superrotation.” The clouds on
Venus fly around the planet at speeds up
to 150 meters per second,
completing a round trip in
just four Earth days, even
though a single Venusian
day lasts for 243 Earth
days. Whatever is crank-
ing up the clouds might
also drive the vortices.