78 research outputs found
Seeing Double at Neptune's South Pole
Keck near-infrared images of Neptune from UT 26 July 2007 show that the cloud
feature typically observed within a few degrees of Neptune's south pole had
split into a pair of bright spots. A careful determination of disk center
places the cloud centers at -89.07 +/- 0 .06 and -87.84 +/- 0.06 degrees
planetocentric latitude. If modeled as optically thick, perfectly reflecting
layers, we find the pair of features to be constrained to the troposphere, at
pressures greater than 0.4 bar. By UT 28 July 2007, images with comparable
resolution reveal only a single feature near the south pole. The changing
morphology of these circumpolar clouds suggests they may form in a region of
strong convection surrounding a Neptunian south polar vortex.Comment: 10 pages, 7 figures; accepted to Icaru
Retrieving Neptune's aerosol properties from Keck OSIRIS observations. I. Dark regions
We present and analyze three-dimensional data cubes of Neptune from the
OSIRIS integral-field spectrograph on the 10-m Keck telescope, from July 2009.
These data have a spatial resolution of 0.035"/pixel and spectral resolution of
R~3800 in the H and K broad bands. We focus our analysis on regions of
Neptune's atmosphere that are near-infrared dark- that is, free of discrete
bright cloud features. We use a forward model coupled to a Markov chain Monte
Carlo algorithm to retrieve properties of Neptune's aerosol structure and
methane profile above ~4 bar in these near-infrared dark regions.
Using a set of high signal-to-noise spectra in a cloud-free band from 2-12N,
we find that Neptune's cloud opacity is dominated by a compact, optically thick
cloud layer with a base near 3 bar and composed of low albedo, forward
scattering particles, with an assumed characteristic size of ~1m. Above
this cloud, we require a vertically extended haze of smaller (~0.1 m)
particles, which reaches from the upper troposphere (~0.6 bar) into the
stratosphere. The particles in this haze are brighter and more isotropically
scattering than those in the deep cloud. When we extend our analysis to 18
cloud-free locations from 20N to 87S, we observe that the optical depth in
aerosols above 0.5 bar decreases by a factor of 2-3 or more at mid- and
high-southern latitudes relative to low latitudes.
We also consider Neptune's methane (CH) profile, and find that our
retrievals indicate a strong preference for a low methane relative humidity at
pressures where methane is expected to condense. Our preferred solution at most
locations is for a methane relative humidity below 10% near the tropopause in
addition to methane depletion down to 2.0-2.5 bar. We tentatively identify a
trend of lower CH columns above 2.5 bar at mid- and high-southern latitudes
over low latitudes.Comment: Published in Icarus: 15 September 201
ALMA Observations of Io Going into and Coming out of Eclipse
We present 1-mm observations constructed from ALMA [Atacama Large (sub)Millimeter Array] data of SO₂, SO and KCl when Io went from sunlight into eclipse (20 March 2018), and vice versa (2 and 11 September 2018). There is clear evidence of volcanic plumes on 20 March and 2 September. The plumes distort the line profiles, causing high-velocity (≳500 m/s) wings, and red/blue-shifted shoulders in the line profiles. During eclipse ingress, the SO₂ flux density dropped exponentially, and the atmosphere reformed in a linear fashion when re-emerging in sunlight, with a "post-eclipse brightening" after ∼10 minutes. While both the in-eclipse decrease and in-sunlight increase in SO was more gradual than for SO₂, the fact that SO decreased at all is evidence that self-reactions at the surface are important and fast, and that in-sunlight photolysis of SO₂ is the dominant source of SO. Disk-integrated SO₂ in-sunlight flux densities are ∼2--3 times higher than in-eclipse, indicative of a roughly 30--50% contribution from volcanic sources to the atmosphere. Typical column densities and temperatures are N ≈ (1.5±0.3)×10¹⁶ cm⁻² and T ≈ 220−320 K both in-sunlight and in-eclipse, while the fractional coverage of the gas is 2--3 times lower in-eclipse than in-sunlight. The low level SO₂ emissions present during eclipse may be sourced by stealth volcanism or be evidence of a layer of non-condensible gases preventing complete collapse of the SO₂ atmosphere. The melt in magma chambers at different volcanoes must differ in composition to explain the absence of SO and SO₂, but simultaneous presence of KCl over Ulgen Patera
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