- Press Release
- Feb 6, 2023
Salt on Io
A team of French and American astronomers, led by E. Lellouch,
from Paris-Meudon Observatory, has observed the presence of
salt (NaCl) in Jupiter’s moon Io’s tenuous atmosphere. This
species is most probably produced through volcanic emission.
Its presence provides an explanation to the “clouds” of atomic
sodium that have been observed to surround Io for almost 30
The atmosphere of Jupiter’s moon Io is one of the most
peculiar of the Solar System. In 1979, the Voyager spacecraft
revealed active volcanism (Figure 1, left) at the surface of
the satellite and discovered a local, tenuous SO2 atmosphere.
Since 1990, millimeter-wave observations acquired at IRAM
(French-German-Spanish telescope) and UV observations with
HST provided a somewhat more detailed description of this
atmosphere. The typical surface pressure is about 1 nanobar,
and, in a unique fashion in the Solar System, the atmosphere
exhibits strong horizontal variations, being apparently
concentrated in an equatorial band. The main atmospheric
compounds are SO2, SO and S2. The atmosphere is probably
produced, on the one hand by direct volcanic output, and on
the other hand by the sublimation of SO2 ices that cover
However, it has been long suspected than Io’s atmosphere
must contain other chemical species. As early as 1974,
visible imaging and spectroscopy revealed a “cloud” of
atomic sodium (Figure 1, right), roughly centered about
Io’s orbit. Detailed subsequent studies of this cloud
indicated a complex structure, including notably “fast
sodium” features, for the production of which the role of
molecular ions (NaX+) was evidenced. These discoveries
naturally raised the question of the origin of sodium in
Io’s environment. From the brightness of the optical
emissions of Na, one can estimate that about 10**26-10**27
sodium atoms leave Io each second.
In 1999, chlorine in atomic and ionized form was discovered
around Io, with an abundance comparable to that of sodium
(while the cosmochemical abundance of Na is about 15 times
that of Cl). This suggests a common origin, NaCl being a
natural plausible parent of both. At the same time, on the
basis of thermochemical equilibrium calculations, NaCl was
proposed to be an important compound of Io’s volcanic
magmas, with an abudance relative to SO2 as high as several
Based on these discoveries and predictions, an observing
campaign was conducted by E. Lellouch, from Paris
Observatory, and several French and American colleagues
at the IRAM 30-m radiotelescope in January 2002. Two
rotational lines of NaCl at 143 and 234 GHz were
unambiguously detected (Figure 2.). Because the vapor
pressure of this salt is entirely negligible, NaCl cannot
be in sublimation equilibrium with Io’s surface and its
presence must directly result from continuous volcanic
output. It appears to be a minor armospheric species.
The most plausible physical model depicts the NaCl
atmosphere as more localized than SO2, due to its very
short lifetime (a few hours at most), and probably
restricted to the volcanic centers. The local NaCl
abundance in this model is 0.3-1.3% of SO2, significantly
lower than predicted.
From the line strengths, volcanic emission rates of
(2-8) x 10**28 NaCl molecules per second can be derived.
According to photochemical and escape models, only a small
fraction of these molecules escape from Io (about 0.1 %).
A somewhat larger amount (1-2 %) leaves Io in atomic form
after being photolyzed to Na and Cl. The vast majority of
the volcanically-emitted NaCl molecules fall back to the
surface where they condense out, potentially contributing
to the white color of some of Io’s terrains.
In conclusion, it appears that NaCl provides an important
source of sodium and chlorine in Io’s environment; however
the precise chemical nature of the NaX+ molecular ions
remains to be elucidated.
E. Lellouch, G. Paubert, J.I. Moses, N.M. Schneider, and
D.F. Strobel. “Volcanically-emitted sodium chloride as a
source for Io’s neutral clouds and plasma torus” Nature,
2 January 2003, 421, p. 45-47
Figure 1. Two views of Jupiter’s satellite Io and its
environment by the Galileo spacecraft.
Color mosaic showing the global aspect of Io’s surface and
several manifestations of its volcanic activity. Note the
presence of two volcanic plumes, one at the limb (Pillan,
140 km altitude), and the other near the center (Prometheus,
seen from above).
A view of the atomic sodium cloud scattering solar light at
589 nm. The very bright spot on Io is the Prometheus plume,
also seen is scattered light.
The spectral lines of the salt molecule NaCl at frequencies
of 143 and 234 GHz detected with the 30m telescope of IRAM
on 15-17 January 2002. The spectral resolution is 40 kHz.