Page 37 - BBC Sky at Night - September 2017 UK

CASSINI AT SATURN SEPTEMBER 37

AS ABOVE, SO BELOW?

Observations of Enceladus have revealed much about what lies below the moon’s icy crust

Reflecting 95 per cent of the sunlight that strikes How water could react
it and brighter than a field of freshly fallen with rocks on the bottom
snow, Enceladus’s youthful façade was of an ocean on Enceladus
first observed by the twin Voyager
spacecraft, which raised the
possibilities of ‘cryovolcanism’ and
perhaps the existence of a life-
sustaining ocean beneath its frozen
crust. A quarter of a century later,
in 2005, Cassini turned its gaze to
Enceladus and validated long-held
theories that water-ice geysers in Ice shell
the polar region – gushing from ˜ 5km
deep fissures at hundreds of metres per
second – were responsible for supplying
many microscopic particles to Saturn’s E ring. Ocean
Later that year, Cassini passed directly through one of the plumes,
revealing the presence not only of water vapour, but also volatiles such ˜ 65km
as nitrogen, methane and carbon dioxide. This enhanced the possibility
that some form of hydrothermal activity could be at work deep within
Enceladus and strengthened theories that the tiny moon could harbour Rocky core Hydrothermal
circulation
a salty subsurface ocean ripe for nurturing microbial life. Plumes of
salty particles from so-called ‘tiger-stripe’ fissures at the south pole
and gravitational field data from Cassini strongly inferred the existence
of a salty ocean, perhaps up to 10km deep.
“Cassini’s revolutionary findings at tiny Enceladus include a
subsurface global, salty ocean containing organics, ammonia,
hydrogen and silicates, with hydrothermal vents on its seafloor,”
says Spilker. “These discoveries have fundamentally altered many
of our concepts of where life may be found in our Solar System.” Water-rock reactions Hydrothermal vents Surface jets

Cassini is only expected to adjusting the spacecraft’s course and causing it to
send back data from the plunge deep into Saturn’s atmosphere. The final,
upper reaches of Saturn’s partial orbit will end at 10:44 UT on 15 September,
atmosphere during its and Cassini will begin its descent just south of the
downward plunge
equator, in the dead of the Saturnian winter.
With thrusters pulsing as it struggles to keep its
high-gain antenna locked on Earth, Cassini will
continue to gather new data, transmitting until the
very end. Its demise will be swift. “Cassini’s tiny
thrusters will not be able to keep the spacecraft
pointed at Earth very deep into the atmosphere,”
explains Spilker. “The flight team is not sure when
the spacecraft will begin to tumble, but it will
happen quickly. Cassini will only send back data on
the uppermost portion of Saturn’s atmosphere.”
Back on Earth, we will know nothing of the
speccraft’s end for over an hour. Travelling at the
speed of light, across the 1.2 billion km gulf
between Saturn and Earth, its final data will reach
the electronic ears of NASA’s Deep Space Network at
12:07 UT. Confirmation of the final loss of signal is
anticipated only a minute or so later; a bittersweet
end to one of our longest space missions. Yet as
Cassini vanishes from existence and becomes part
of the planet it has spent its life exploring, we can
modify a line from the famous poet Rupert Brooke,
and be certain that some tiny corner of a foreign gas
giant will remain forever Earth. S

skyatnightmagazine.com 2017