A
new study has constrained the Enceladus's global conductive heat flow by
studying its seasonal temperature variations at its north pole (yellow). These
results, when combined with existing ones of its highly active south polar
region (red) provide the first observational constraint of Enceladus's energy
loss budget (<54 GW) – which is consistent with the predicted energy input
(50 to 55 GW) from tidal heating. This implies that Enceladus's current
activity is sustainable in the long term – an important prerequisite for the
evolution of life, which is thought possible to exist in its global sub-surface
ocean. Credit: University of Oxford / NASA / JPL-CalTech / Space Science
Institute (PIA19656 and PIA11141)
A new study led by researchers from
Oxford University, Southwest Research Institute and the Planetary Science
Institute in Tucson, Arizona has provided the first evidence of significant
heat flow at Enceladus's north pole, overturning previous assumptions that heat
loss was confined to its active south pole.
This finding confirms that the icy
moon is emitting far more heat than would be expected if it were simply a
passive body, strengthening the case that it could support life.
The research is published in the journal Science Advances.
Enceladus is a highly active world,
with a global, salty subsurface ocean, believed to be the source of its heat.
The presence of liquid water, heat and the right chemicals (such as phosphorus
and complex hydrocarbons) means that its subsurface ocean is believed to be one
of the best places in our solar system for life to have evolved outside Earth.
But this subsurface ocean can only
support life if it has a stable environment, with its energy losses and gains in balance. This balance is maintained
by tidal heating: Saturn's gravity stretches and squeezes the moon as it
orbits, generating heat inside. If Enceladus doesn't gain enough energy, its
surface activity will slow down or stop, and the ocean could eventually freeze.
Too much energy, on the other hand, could cause ocean activity to increase,
altering its environment.
"Enceladus is a key target in
the search for life outside Earth, and understanding the long-term availability
of its energy is key to determining whether it can support life," said Dr.
Georgina Miles (Southwest Research Institute and Visiting Scientist at the
Department of Physics, University of Oxford), lead author of the paper.
Until now, direct measurements of
heat loss from Enceladus had only been made at the south pole, where dramatic
plumes of water ice and vapor erupt from deep fissures in the surface. In
contrast, the north pole was thought to be geologically inactive.
Using data from NASA's Cassini
spacecraft, the researchers compared observations of the north polar region in
deep winter (2005) and summer (2015). These were used to measure how much
energy Enceladus loses from its "warm" (0°C, 32°F) subsurface ocean as heat travels through its icy shell to the
moon's frigid surface (–223°C, –370°F) and is then radiated into space.
By modeling the expected surface
temperatures during the polar night and comparing them with infrared
observations from the Cassini Composite InfraRed Spectrometer (CIRS), the team
found that the surface at the north pole was around 7 K warmer than predicted.
This discrepancy could only be explained by heat leaking out from the ocean
below.
The measured heat flow (46 ± 4
milliwatts per square meter) may sound small, but this is about two-thirds of
the heat loss (per unit area) through Earth's continental crust. Across the
whole of Enceladus, this conductive heat loss totals around 35 gigawatts:
roughly equivalent to the output of over 66 million solar panels (output of 530
W) or 10,500 wind turbines (output of 3.4 MW).
When combined with the previously
estimated heat escaping from Enceladus's active south pole, the moon's total heat loss rises to 54 gigawatts, a
figure that closely matches predicted heat input from tidal forces. This
balance between heat production and loss strongly suggests that Enceladus's
ocean can remain liquid over geological timescales, offering a stable
environment where life could potentially emerge.
"Understanding how much heat
Enceladus is losing on a global level is crucial to knowing whether it can
support life," said Dr. Carly Howett (Department of Physics, University of
Oxford and Planetary Science Institute in Tucson, Arizona), corresponding
author of the paper. "It is really exciting that this new result supports
Enceladus's long-term sustainability, a crucial component for life to
develop."
According to the researchers, the
next key step will be to determine whether Enceladus's ocean has existed long
enough for life to develop. At the moment, its age is still uncertain.
The study also demonstrated that
thermal data can be used to independently estimate ice shell thickness, an
important metric for future missions planning to probe Enceladus's ocean, for
instance using robotic landers or submersibles. The findings suggest that the
ice is 20 and 23 km deep at the north pole with an average of 25 to 28 km globally—slightly
deeper than previous estimates obtained using other remote sensing and modeling
techniques.
"Eking out the subtle surface temperature variations caused by Enceladus's conductive heat flow from its daily and seasonal temperature changes was a challenge, and was only made possible by Cassini's extended missions," added Dr. Miles. "Our study highlights the need for long-term missions to ocean worlds that may harbor life, and the fact the data might not reveal all its secrets until decades after it has been obtained."
Provided by University
of Oxford
edited by Stephanie Baum, reviewed by Robert Egan
Source: Saturn's icy moon may host a stable ocean fit for life
