Astronomy – September 2019

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52 ASTRONOMY • SEPTEMBER 2019

But was it really an under-

ground ocean, or more of a local

southern sea? Thankfully,

Cassini could answer this ques-

tion, too. By verifying excess

wobble over Enceladus’ orbital

period, the imaging cameras

confirmed that the icy crust is

not connected to the world’s

rocky core. This could only be

possible if the crust is f loating

on a global, subsurface, liquid-

water ocean.

And Cassini didn’t stop there.

Mass spectrometers aboard the

spacecraft analyzed the gas

and grains during multiple f ly-

throughs of the plume. These

instruments, the Ion and

Neutral Mass Spectrometer

(INMS) and Cosmic Dust

Analyzer (CDA), found the

plume contains mostly water,

but also salts, ammonia, carbon

dioxide, and small and large

organic molecules. These

findings help

us paint a pic-

ture of the world

underneath the ice: a pos-

sibly habitable ocean that’s

slightly alkaline, with access to

chemical energy in the water

and geothermal energy at the

rocky seaf loor.

Possible

energy

sources

One of the great-

est legacies of the

Cassini mission is

that it established

Enceladus as pos-

sessing all three

ingredients for

life as we know it:

water, chemistry,

and energy. Water

in the ocean —

check. Chemistry in

the simple and complex organics

detected in the plume — check.

These could be utilized to form

the molecular machinery of life.

Energy takes a bit more

explaining.

It is likely that hydrothermal

vents are present at the seaf loor

of Enceladus. We know this

because of three lines of evi-

dence. First, INMS detected

methane in the plume, at higher

concentrations than would exist

if sourced from clathrates

(water-ice cages at high pressure

with methane trapped inside) or

other reservoirs in the ice.

Methane is a key product of

hydrothermal systems.

Second, CDA discovered sil-

ica nanograins of a particular

size and oxidation state traced to

the ocean. These only could

have formed where liquid water

is touching rock at temperatures

of at least 194 degrees Fahrenheit

(90 degrees Celsius), in the range

of hydrothermal vents like

“white smokers” here on Earth.

And third, the recent confir-

mation of molecular hydrogen

in the plume by the INMS team

strongly suggests interaction of

liquid water with a rocky core.

On Earth, hydrothermal

vents at the base of the Mid-

Atlantic Ridge host teeming

ecosystems, living as far

removed as one can imagine

from photosynthesis. These hab-

itats survive off of geothermal

and chemical energy. A similar

community

might exist near

a hydrothermal

vent at the

seaf loor of

Enceladus.

So, we have

water, chemistry,

and energy. Let’s

say they have

mixed together

long enough for

life to form.

(Your guess is as

good as any-

body’s here —

estimates range from 100,000 to

25 million years.) How might we

detect it?

Assuming an energy-limited

scenario (a good analog is Lake

Vostok, a body of water in

CASSINI
FOUND
100 geysers —
most along the four
tiger stripe fractures —
while conducting an
imaging survey of
Enceladus’ south polar
region. Researchers
plotted them on this
polar stereographic map
to compare geyser
activity with enhanced
thermal emission
observed by Cassini’s
heat-measuring
instruments and with
the distribution of tidal
stresses across the
region. Those
comparisons produced
clues that helped
explain how the geysers
work. In this image, the
more precise a geyser
location is known, the
smaller the circle
representing it. These
four sulci (the large
fractures) are named for
ancient cities on Earth.
NASA/JPL-CALTECH/SPACE SCIENCE
INSTITUTE

IT IS LIKELY

THAT HYDRO-

THERMAL

VENTS ARE

PRESENT

AT THE

SEAFLOOR OF

ENCELADUS.