Page 96 - Latinaero_magazine_issue_05_oct_nov_dec-2012

96 Latinaero magazine
SPACE & SCI ENCE
Numero 5 - 2012
nity for interesting chemistry along the boundary between liquid
and solid, a boundary that may have been important in the ori-
gin of terrestrial life.”
Being the only other body besides Earth in our solar sys­
tem with stable bodies of liquid on its surface, Titan is now
even worth of further close studies. But while our planet's
cycle of precipitation and evaporation involves water, Titan's
cycle involves hydrocarbons in the form of organic molecules
of ethane and methane, which scientists think can be buil­
ding blocks for the more complex chemistry from which life
arose. Cassini has seen a vast network of these hydrocarbon
seas cover Titan's northern hemisphere, while a more spora­
dic set of lakes bejewels the southern hemisphere. Let's recall
that Titan has two major components of Earth’s atmosphere
– nitrogen and oxygen – but the oxygen is likely frozen as
water ice within the body of the moon. If Titan received more
sunlight, its atmosphere might more nearly resemble that of
a primitive Earth. That atmosphere also carries a little bit of
methane and photochemical smog. That smog is made up of
organic molecules created through the reaction of sunlight
on methane [see illustration on page 91].
Up to this point, Cassini scientists assumed that Titan lakes
would not have foating ice, because solid methane is den­
ser than liquid methane and would sink. But the new model
considers the interaction between the lakes and the atmos­
phere, resulting in diferent mixtures of compositions, poc­
kets of trapped nitrogen gas, and changes in temperature.
The result, scientists found, is that winter ice will foat in Ti­
tan's methane-and-ethane-rich lakes and seas if the tempera­
ture is below the freezing point of methane (-183°C or 90.4 K).
They also realized all the varieties of ice they considered
would foat if they were composed of at least 5% “air”, which
is an average composition for young sea ice on Earth. As a
predominant gas, “air” on Titan has signifcantly more nitro­
gen than Earth’s air and almost no oxygen. Then, if the tem­
perature drops by just a few degrees, the ice will sink because
of the relative proportions of nitrogen gas in the liquid ver­
sus the solid. Temperatures close to the freezing point of
methane could lead to both foating and sinking ice – that is,
a hydrocarbon ice crust above the liquid and blocks of hydro­
carbon ice on the bottom of the lake bed. Experts haven't en­
tirely fgured out what colour the ice would be, though they
suspect it would be colourless, as it is on Earth, perhaps tinted
reddish-brown from Titan's atmosphere.
“We now know it's possible to get methane-and-ethane-rich
ice freezing over on Titan in thin blocks that congeal together as
it gets colder – similar to what we see with Arctic sea ice at the
onset of winter,” said Jason Hofgartner, the paper’s author and
a Natural Sciences and Engineering Research Council of Ca­
nada scholar at Cornell. “We'll want to take these conditions into
consideration if we ever decide to explore Titan’s surface some
day.”
Cassini's radar will be able to test this model by watching
what happens to the refectivity of these lakes and seas. The
beam of the radar is quite wide – about a third of a degree
by six degrees, so it covers a large area on Titan's surface. A
tiva. Essa atmosfera carrega também um pouco de metano
e nevoa fotoquímica. Essa fumaça é formada por moléculas
orgânicas criadas através da reação da luz solar no metano
[Veja ilustração na página 91].
Até este ponto, os cientistas da Cassini assumiram que
os lagos de Titã não poderiam ter gelo futuante, porque o
metano sólido é mais denso do que o metano líquido e afun­
daria. Porém o novo modelo considera a interação entre os
lagos e a atmosfera, resultando em diferentes misturas de
composições, bolsas de gás de nitrogênio preso, e alterações
na temperatura. O resultado, que os cientistas descobriram, é
que o gelo do inverno vai futuar nos mares e nos lagos ricos
emmetano e etano de Titã se a temperatura estiver abaixo do
ponto de congelamento do gás metano (-183 °C ou 90.4 K).
Eles também perceberam que todas as variedades de
gelo que foram consideradas futuariam se fossem compos­
tas de pelo menos 5% de “ar”, que é uma composição média
dos gelos mais jovens na Terra. Como um gás predominante,
o ”ar” em Titã tem signifcativamente mais nitrogênio do que
o ar na Terra e é quase sem oxigênio. Então, se a temperatura
cair apenas alguns graus, o gelo irá afundar por causa das
proporções relativas do gás nitrogênio no processo líquido
versus sólido. Temperaturas próximas do ponto de congela­
mento de metano podem levar o gelo tanto a futuar quanto
afundar – isto é, a crosta de gelo de hidrocarbonetos acima
do líquido e os blocos de gelo de hidrocarbonetos na parte
inferior do leito do lago. Os especialistas ainda não descobri­
ram inteiramente qual seria a cor do gelo, embora suspeitem
que seria incolor, como é na Terra, e talvez da cor marrom-
avermelhada da atmosfera de Titã.
“Nós agora sabemos que é possível obter gelo rico em me-
tano e etano em Titã em fnos blocos que se solidifcam juntos
 Colinas suaves e lagos em Titã… Acredita-se que as colinas sejam de
metano congelado e os lagos formados por hidrocarbonetos líquidos.
Soft hills and lakes on Titan… But the hills are thought to be of iced
methane and the lakes made of liquid hydrocarbon. © NASA