Watching Venus
Glow In The Dark
European Space Agency's (ESA) Venus Express spacecraft has
observed an eerie glow in the nighttime atmosphere of Venus. This
infrared light comes from nitric oxide, and it is showing scientists
that the atmosphere of Earth's nearest neighbor is a temperamental
place of high winds and turbulence.
Unfortunately, the glow on Venus cannot be seen with the naked eye
because it occurs at the invisible wavelengths of infrared. Venus
Express, however, is equipped with the Visible and Infrared Thermal
Imaging Spectrometer (VIRTIS) instrument that can see these
wavelengths.
VIRTIS has made two detections of the so-called nightglow for nitric
oxide at Venus. This is the first time such infrared detections have
been made for any planet, and they provide a new insight into Venus'
atmosphere.
"The nightglow can provide details about the temperature, wind
direction, composition and chemistry of an atmosphere," said Antonio
Garcia Munoz, who was at the Australian National University when the
research was carried out; he is now located at the Instituto de
Astrofisica de Canarias, Tenerife, Spain.
The nightglow is ultimately caused by the Sun's ultraviolet light,
which streams into a planet's atmosphere and breaks up the molecules
into atoms and other simpler molecules. The free atoms may recombine
again and, in specific cases, the resulting molecule is endowed with
extra energy that is subsequently lost in the form of light. On the
day-side of the planet, any atoms that find their way back together
are outshone by the sunlight falling into the atmosphere.
But on the night-side, where a vigorous diurnal circulation
transports atoms, the glow can be seen with appropriate instruments,
such as VIRTIS.
A nitric oxide nightglow in the infrared has never been observed in
the atmospheres of Mars or Earth, although we know that the
necessary nitric oxide molecules are present because they have been
observed in ultraviolet.
The nightglow on Venus has been seen at infrared wavelengths before,
betraying oxygen molecules and the hydroxyl radical, but this is the
first detection of nitric oxide at those wavelengths. It offers data
about the atmosphere of Venus that lies above the cloud tops at
around 43 miles (70 kilometers). The oxygen and hydroxyl emissions
come from 56-62 miles (90-100 kilometers), whereas the nitric oxide
comes from 68-75 miles (110-120 kilometers) altitude.
Yet, even VIRTIS cannot see the nitric oxide nightglow all the time
because it is often just too faint. "Luckily for us, Venus has a
temperamental atmosphere," said Garcia Munoz. "Packets of oxygen and
nitrogen atoms are blown around." Sometimes these become dense
enough to boost the brightness of the nightglow, making it visible
to VIRTIS.
Venus Express can observe the three-nightglow emissions
simultaneously, and this gives rise to a mystery. The nightglows
from the different molecules do not necessarily happen together.
"Perhaps when we have more observations, we will understand the
correlation between them," said Garcia Munoz.
In order to do that, the VIRTIS team plans to continue monitoring
the planet, building up a database of this fascinating phenomenon.
This false-color composite image of Venus' atmosphere was obtained
by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on
board ESA’s Venus Express, from a limb (or profile) perspective.
The top panel shows the oxygen nightglow of Venus at an altitude of
approximately 60 miles (96 kilometers) over the surface of the
planet, seen at a wavelength of 1.27 microns.
The bottom panel shows the same portion of the atmosphere observed
at the same time, but at a different wavelength (around 1.22
microns). Here it is possible to see the nightglow of nitric oxide,
which is much weaker than that of oxygen and comes from an higher
altitude — around 68 miles (110 kilometers) above the surface.
In red is the thermal emission of Venus at 1.74 microns — one of the
atmospheric windows of Venus exploited by VIRTIS. ESA/VIRTIS/INAF-IASF/Obs.
de Paris-LESIA.
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