Sky & Telescope - March 2025 - 63

TRUE BLUE The intensely
blue image released
shortly after Voyager 2's
flyby is a byproduct of
efforts to enhance the
visibility of atmospheric
features (left). It bears
little resemblance to a
recently reprocessed
version of Voyager data,
calibrated to estimate
Neptune's true color
(right).
Early Voyager 2
image
Reprocessed image
punch upwards like thunderheads and then subside, dragging
the compounds downward as they do. Hydrogen sulfide
clouds would be consistent with radio-wavelength measurements
that probe to these depths, which suggest the presence
of sulfur-bearing compounds.
Giant Dark Storms
Neptune's dim color belies its remarkably dynamic atmosphere.
The Voyager view in 1989 was dominated by a huge
dark storm we unimaginatively called the Great Dark Spot.
Voyager also revealed that the bright white clouds I had seen
in the 1980s were companion clouds skirting the edges of this
giant dark storm. My very first Neptune observations with
Hubble's refurbished optics in 1994 were most exciting for
what was missing: There was no hint at all of the huge dark
spot that dominated the Voyager images! In just five short
years, it had completely vanished.
Subsequent Hubble observations over the next 30 years
have revealed many more dark spots that appear and then
vanish. The lifetimes range from one to six years. Small,
bright white clouds often accompany the dark spots, floating
at higher altitudes above the dark spots' edges. These clouds
may form when atmospheric gases flow up and over the dark
spot, where the methane in the gas can cool and condense
into clouds.
If the white clouds are methane clouds, then what are the
dark spots? Our highest-resolution data come from Voyager's
observations of the Great Dark Spot. That dark vortex seemed
to be created by an absence of clouds: a wobbly gap in the
2-bar cloud deck that revealed a deeper, darker layer. New
models of more recent dark spots, however, link their appearance
not to holes but to variations in the aerosols in that
deeper 5-bar layer.
We won't solve the mystery with current equipment.
Hubble and the 10-meter Keck Telescopes, along with NASA's
Infrared Telescope Facility and large 8-meter telescopes such
as those at Gemini Observatory's two sites and the European
Southern Observatory's Paranal Observatory, have all served
as key tools for studying the ever-changing atmosphere of
Neptune. Yet Neptune's disk is so small as seen from Earth
that we are hard-pressed to even resolve these dark vortices,
let alone to truly understand their details or dynamics. Thus,
we are still unsure what is driving their remarkably changeable
behavior. Finding the answers will have to wait for the
upcoming generation of 30-meter telescopes (S&T: Nov. 2018,
p. 14), or a large space telescope that senses blue wavelengths,
or perhaps even a future spacecraft mission to Neptune.
In the meantime, observations with the James Webb Space
Telescope (JWST) will give us new insights into Neptune's
atmosphere. JWST's first images of Neptune are spectacular
and whetted our appetites, but the very best results - the
spectroscopy - are yet to come.
A primary objective of our JWST studies is to untangle the
complex interplay among different layers in Neptune's atmosphere.
The layers interact both chemically and dynamically:
Compounds move up and down, mixing and creating new
molecules or being destroyed, all while wind currents shuttle
gas to and fro and giant air cells rise and fall. In particular,
JWST's ability to capture exquisite infrared spectra will enable
us to study the composition and temperature variations of
Neptune's atmosphere in unprecedented detail.
Neptune . . . and the Sun?
Until today's modern telescopes began to probe more deeply,
Neptune's tiny disk as seen from Earth prevented nearly all
atmospheric details from being seen. Nevertheless, beginning
in the 1950s, astronomers at Lowell Observatory dutifully
recorded the overall brightness of the planet, year after year,
decade after decade. In recent years, amateur astronomers
have also recorded these brightness changes.
The Lowell team noticed subtle changes in Neptune's
brightness, which showed hints of being related to the
sk yand tele scope .o r g * MARCH 2025 63
PATRICK IRWIN
http://skyandtelescope.org
Sky & Telescope - March 2025

Table of Contents for the Digital Edition of Sky & Telescope - March 2025
