The Venus
Express spacecraft has peered through the crushing and poisonous atmosphere of
the planet, and found that it's spinning slower than expected.
Using the
orbiter's VIRTIS spectrograph at infrared wavelengths, to penetrate the thick
cloud cover, scientists studied surface features and discovered that some were
more than 10 miles from where they should be.
The length of Venus's day is 6.5 minutes shorter than previously thought. The discovery could help scientists understand the forces at work on the hostile planet, and plan missions to its surface.
The Venus Express orbiter used its VIRTIS spectrograph at infrared frequencies to measure the position of features on the surface - and found that the planet was spinning more slowly than expected |
The Venus Express orbiter's measurements found that the planet's day is just over six minutes longer than thought |
These
detailed measurements from orbit are helping scientists determine whether Venus
has a solid or liquid core, which will help our understanding of the planet’s
creation and how it evolved.
The orbit
could be being affected by Venus's dense atmosphere – more than 90 times the
pressure of Earth’s and high-speed weather systems, which are believed to
change the planet’s rotation rate through friction with the surface.
More...
Earth
experiences a similar effect, where it is largely caused by wind and tides. The
length of an Earth day can change by roughly a millisecond and depends
seasonally with wind patterns and temperatures over the course of a year.
In the
1980s and 1990s, the Venera and Magellan orbiters made radar maps of the
surface of Venus, long shrouded in mystery as well as a dense, crushing and
poisonous atmosphere. These maps gave us our first detailed global view of this
unique and hostile world.
Topographic maps from the Magellan and Venus Express spacecraft clearly show that features on the surface have 'shifted' |
Over its
four-year mission, Magellan was able to watch features rotate under the
spacecraft, allowing scientists to determine the length of the day on Venus as
being equal to 243.0185 Earth days.
However,
surface features seen by Venus Express some 16 years later could only be lined
up with those observed by Magellan if the length of the Venus day is on average
6.5 minutes longer than Magellan measured.
This also
agrees with the most recent long-duration radar measurements from Earth.
Then the two
maps did not align, I first thought there was a mistake in my calculations as
Magellan measured the value very accurately, but we have checked every possible
error we could think of, said Nils Müller, a planetary scientist at the DLR
German Aerospace Centre, lead author of a research paper investigating the
rotation.
Scientists,
including Özgur Karatekin of the Royal Observatory of Belgium, looked at the
possibility of short-term random variations in the length of a Venus day, but
concluded these should average themselves out over longer timescales.
On the
other hand, other recent atmospheric models have shown that the planet could
have weather cycles stretching over decades, which could lead to equally
long-term changes in the rotation period. Other effects could also be at work,
including exchanges of angular momentum between Venus and the Earth when the
two planets are relatively close to each other.
An accurate
value for Venus’ rotation rate will help in planning future missions, because
precise information will be needed to select potential landing sites, noted
Håkan Svedhem, ESA’s Venus Express project scientist.
While
further study is needed, it’s clear that Venus Express is penetrating far
deeper into the mysteries of this enigmatic planet then anyone dreamed.
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