broad band in our Triton spectrum. The band
in our Triton spectrum coincides with the
2.239 m m (4466.5 cm -1 ) band in the labora-
tory spectrum.
The strength of absorption of Triton’s N2
and CO ice bands varies with longitude, by
roughly a factor of two, with the strongest
absorption being on the leading part of the
sub-Neptune hemisphere (longitude ~50˚
East; see Grundy et al., (2010). We observed
when Triton was at a sub-Earth longitude of
113˚ East, not far from the maximum in N2
and CO absorption.
Looking ahead
On distant Triton, carbon monoxide and ni-
trogen freeze as solid ices. They can form
their own independent ices, or condense to-
gether in the icy mix detected in the Gemini
data. Our discovery, for the first time beyond
the lab, of an extraordinary union between
carbon monoxide and nitrogen ices is impor-
tant, as it could be involved in Triton’s iconic
geysers — first seen in Voyager 2 spacecraft
images as dark, windblown streaks on the
moon’s south polar region back in 1989 (Fig-
ure 1).
Since Voyager 2’s discovery of the geysers,
theories have focused on an internal ocean
as one possible source of erupted material.
Or, the geysers may erupt when the sum-
mertime Sun heats this thin layer of volatile
ice on Triton’s surface, potentially involving
the mixed carbon monoxide and nitrogen
ice revealed by the Gemini observation. That
ice mixture could also migrate around the
surface of Triton in response to seasonally
varying patterns of sunlight.
Seasons progress slowly on Triton, as Neptune
takes 165-Earth years to orbit the Sun. A sea-
son on Triton lasts a little over 40 years; Triton
passed its southern summer solstice mark in
2000, leaving about 20 more years to conduct
further research before its autumn begins.
January 2020 / 2019 Year in Review
We expect that these findings will shed light
on the composition of ices and seasonal vari-
ations on other distant worlds beyond Nep-
tune. Astronomers have suspected that the
mixing of carbon monoxide and nitrogen ice
exists not only on Triton, but also on Pluto,
where the New Horizons spacecraft found
the two ices coexisting in Sputnik Planitia
(Protopapa et al., 2017) — an icy basin that
has apparently caused Pluto’s entire crust
to shift over time. The same may be true
for more recently discovered small planets
like Eris and Makemake, both of which host
volatile ices like those on Pluto and Triton.
This Gemini finding is the first direct spec-
troscopic evidence of these ices mixing and
absorbing this type of light on either world.
Jennifer Hanley is an astronomer at Lowell Obser-
vatory. She can be reached at:
[email protected]
References
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