Publications
107 results found
Moses JI, Brown ZL, Koskinen TT, et al., 2023, Saturn’s atmospheric response to the large influx of ring material inferred from Cassini INMS measurements, Icarus, Vol: 391, Pages: 1-40, ISSN: 0019-1035
During the Grand Finale stage of the Cassini mission, organic-rich ring material was discovered to be flowing into Saturn’s equatorial upper atmosphere at a surprisingly large rate. Through a series of photochemical models, we have examined the consequences of this ring material on the chemistry of Saturn’s neutral and ionized atmosphere. We find that if a substantial fraction of this material enters the atmosphere as vapor or becomes vaporized as the solid ring particles ablate upon atmospheric entry, then the ring-derived vapor would strongly affect the composition of Saturn’s ionosphere and neutral stratosphere. Our surveys of Cassini infrared and ultraviolet remote-sensing data from the final few years of the mission, however, reveal none of these predicted chemical consequences. We therefore conclude that either (1) the inferred ring influx represents an anomalous, transient situation that was triggered by some recent dynamical event in the ring system that occurred a few months to a few tens of years before the 2017 end of the Cassini mission, or (2) a large fraction of the incoming material must have been entering the atmosphere as small dust particles less than 100 nm in radius, rather than as vapor or as large particles that are likely to ablate. Future observations or upper limits for stratospheric neutral species such as HCN, HCN, and CO at infrared wavelengths could shed light on the origin, timing, magnitude, and nature of a possible vapor-rich ring-inflow event.
Agiwal O, Moore L, Martinis C, et al., 2022, First Steps Towards a New Saturn Ionosphere Model Including Ring-Planet Coupling and Electrodynamics
<jats:p>&lt;div&gt;&lt;div&gt;&lt;div&gt;&lt;div&gt;The Cassini Grand Finale revealed that there is still much that we do not understand about Saturn&amp;#8217;s upper atmosphere. In-situ observations reveal highly complex coupling between the planetary atmosphere and rings and inter-hemispheric electrodynamic coupling at latitudes that are magnetically connected to the intra D-ring region in the magnetosphere. Current Saturn models are ill-suited to treating electrodynamics and ring-planet interactions at Saturn. Thus, we adapt SAMI, a well-known terrestrial ionosphere model that is flux-tube based and already includes electrodynamics, to Saturn, with the aim of using it in conjunction with existing Saturn models such as the STIM-GCM (Saturn Thermosphere Ionosphere Model) to decipher the long-standing unexplained morphologies in Saturn&amp;#8217;s ionosphere and investigate the ring-atmosphere coupling and electrodynamics revealed by the Cassini end-of-mission data. We will present initial results having adapted SAMI to Saturn, showing the full extent of the atmospheric chemistry and model capabilities at present. We will discuss future directions of development towards the construction of the new model capable of resolving the complex ring-atmosphere coupling and electrodynamics, and the possibility of adapting this model to other planets.&lt;/div&gt;&lt;/div&gt;&lt;/div&gt;&lt;/div&gt;</jats:p>
Brown ZL, Medvedev AS, Starichenko ED, et al., 2022, Evidence for Gravity Waves in the Thermosphere of Saturn and Implications for Global Circulation, GEOPHYSICAL RESEARCH LETTERS, Vol: 49, ISSN: 0094-8276
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- Citations: 4
Greaves JS, Richards AMS, Bains W, et al., 2021, Phosphine gas in the cloud deck of Venus (vol 5, pg 655, 2021), Nature Astronomy, Vol: 5, Pages: 726-728, ISSN: 2397-3366
Siddle AG, Mueller-Wodarg ICF, Bruinsma S, et al., 2021, Density structures in the martian lower thermosphere as inferred by Trace Gas Orbiter accelerometer measurements, ICARUS, Vol: 357, ISSN: 0019-1035
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- Citations: 5
Shebanits O, Hadid L, Cao H, et al., 2021, The conductive dusty ionosphere of Saturn
<jats:p>&lt;p&gt;Cassini&amp;#8217;s Grand Finale orbits brought us historical first in-situ measurements of Saturn&amp;#8217;s ionosphere, showing that it contains dusty plasma in the equatorial region. We present the Pedersen and Hall conductivities of the top ionosphere (10:50 &amp;#8211; 12:17 Saturn Local Time, 10N &amp;#8211; 20S planetocentric latitude), derived from particle and magnetometer data. We constrain the Pedersen conductivities to be at least 10&lt;sup&gt;-5&lt;/sup&gt; &amp;#8211; 10&lt;sup&gt;-4&lt;/sup&gt; S/m at ionospheric peak, a factor 10-100 higher than estimated previously by remote measurements, while the Hall conductivities are very close to 0 or in fact negative. We show that this is an effect of dusty plasma. Another effect is that ionospheric dynamo region thickness is increased to 300-800 km. Furthermore, our results suggest a temporal variation (decrease) of the plasma densities, mean ion masses and consequently the conductivities over the period of one month.&lt;/p&gt;</jats:p>
Agiwal O, Cao H, Cowley SWH, et al., 2021, Constraining the Temporal Variability of Neutral Winds in Saturn's Low-Latitude Ionosphere Using Magnetic Field Measurements, JOURNAL OF GEOPHYSICAL RESEARCH-PLANETS, Vol: 126, ISSN: 2169-9097
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- Citations: 5
Greaves J, Richards A, Bains W, et al., 2020, Phosphine gas in the cloud decks of Venus, Nature Astronomy, Vol: 5, Pages: 655-664, ISSN: 2397-3366
Measurements of trace gases in planetary atmospheres help us explore chemical conditions different to those on Earth. Our nearest neighbour, Venus, has cloud decks that are temperate but hyperacidic. Here we report the apparent presence of phosphine (PH3) gas in Venus’s atmosphere, where any phosphorus should be in oxidized forms. Single-line millimetre-waveband spectral detections (quality up to ~15σ) from the JCMT and ALMA telescopes have no other plausible identification. Atmospheric PH3 at ~20 ppb abundance is inferred. The presence of PH3 is unexplained after exhaustive study of steady-state chemistry and photochemical pathways, with no currently known abiotic production routes in Venus’s atmosphere, clouds, surface and subsurface, or from lightning, volcanic or meteoritic delivery. PH3 could originate from unknown photochemistry or geochemistry, or, by analogy with biological production of PH3 on Earth, from the presence of life. Other PH3 spectral features should be sought, while in situ cloud and surface sampling could examine sources of this gas.
Brown Z, Koskinen T, Muller-Wodarg I, et al., 2020, A pole-to-pole pressure-temperature map of Saturn's thermosphere from Cassini Grand Finale data, Nature Astronomy, Vol: 4, Pages: 872-879, ISSN: 2397-3366
Temperatures of the outer planet thermospheres exceed those predicted by solar heating alone by several hundred degrees. Enough energy is deposited at auroral regions to heat the entire thermosphere, but models predict that equatorward distribution is inhibited by strong Coriolis forces and ion drag1,2. A better understanding of auroral energy deposition and circulation are critical to solving this so-called energy crisis. Stellar occultations observed by the Ultraviolet Imaging Spectrograph instrument during the Cassini Grand Finale were designed to map the thermosphere from pole to pole. We analyse these observations, together with earlier observations from 2016 and 2017, to create a two-dimensional map of densities and temperatures in Saturn’s thermosphere as a function of latitude and depth. The observed temperatures at auroral latitudes are cooler and peak at higher altitudes and lower latitudes than predicted by models, leading to a shallower meridional pressure gradient. Under modified geostrophy3, we infer slower westward zonal winds that extend to lower latitudes than predicted, supporting equatorward flow from approximately 70° to 30° latitude in both hemispheres. We also show evidence of atmospheric waves in the data that can contribute to equatorward redistribution of energy through zonal drag.
Shebanits O, Hadid LZ, Cao H, et al., 2020, Saturn’s near-equatorial ionospheric conductivities from in situ measurements, Scientific Reports, Vol: 10, ISSN: 2045-2322
Cassini’s Grand Finale orbits provided for the first time in-situ measurements of Saturn’s topside ionosphere. We present the Pedersen and Hall conductivities of the top near-equatorial dayside ionosphere, derived from the in-situ measurements by the Cassini Radio and Wave Plasma Science Langmuir Probe, the Ion and Neutral Mass Spectrometer and the fluxgate magnetometer. The Pedersen and Hall conductivities are constrained to at least 10−5–10−4 S/m at (or close to) the ionospheric peak, a factor 10–100 higher than estimated previously. We show that this is due to the presence of dusty plasma in the near-equatorial ionosphere. We also show the conductive ionospheric region to be extensive, with thickness of 300–800 km. Furthermore, our results suggest a temporal variation (decrease) of the plasma densities, mean ion masses and consequently the conductivities from orbit 288 to 292.
Siddle AG, Mueller-Wodarg ICF, Stone SW, et al., 2019, Global characteristics of gravity waves in the upper atmosphere of Mars as measured by MAVEN/NGIMS, Icarus, Vol: 333, Pages: 12-21, ISSN: 0019-1035
We present an analysis of gravity waves in Mars' upper atmosphere above 120 km. Using in-situ data from NGIMS onboard MAVEN we have been able to characterise waves from nearly 4000 orbits. We have used temperature and density profiles to extract perturbations and interpret these as vertically propagating gravity waves which we characterise by their amplitude and wavelength. In this region of the atmosphere gravity waves have amplitudes of around 10%. Vertical wavelengths are found to be around 10–30 km. We observe an increase in gravity wave amplitudes with increasing solar zenith angle. Gravity wave amplitudes appear invariant in altitude on the dayside, however increase with altitude on the nightside.
Cravens TE, Moore L, Waite JH, et al., 2019, The Ion Composition of Saturn's Equatorial Ionosphere as Observed by Cassini, GEOPHYSICAL RESEARCH LETTERS, Vol: 46, Pages: 6315-6321, ISSN: 0094-8276
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- Citations: 21
Müller-Wodarg ICF, Koskinen TT, Moore L, et al., 2019, Atmospheric waves and their possible effect on the thermal structure of Saturn's thermosphere, Geophysical Research Letters, Vol: 46, Pages: 2372-2380, ISSN: 0094-8276
Atmospheric waves have been discovered for the first time in Saturn's neutral upper atmosphere (thermosphere). Waves may be generated from instabilities, convective storms or other atmospheric phenomena. The inferred wave amplitudes change little with height within the sampled region, raising the possibility of the waves being damped, which in turn may enhance the eddy friction within the thermosphere. Using our Saturn Thermosphere Ionosphere General Circulation Model, we explore the parameter space of how an enhanced Rayleigh drag in different latitude regimes would affect the global circulation pattern within the thermosphere and, in turn, its global thermal structure. We find that Rayleigh drag of sufficient magnitude at midlatitudes may reduce the otherwise dominant Coriolis forces and enhance equatorward winds to transport energy from poles toward the equator, raising the temperatures there to observed values. Without this Rayleigh drag, energy supplied into the polar upper atmosphere by magnetosphere‐atmosphere coupling processes remains trapped at high latitudes and causes low‐latitude thermosphere temperatures to remain well below the observed levels. Our simulations thus suggest that giant planet upper atmosphere global circulation models need to include additional Rayleigh drag in order to capture the effects of physical processes otherwise not resolved by the codes.
Ferri F, Karatekin O, Lewis SR, et al., 2019, ExoMars Atmospheric Mars Entry and Landing Investigations and Analysis (AMELIA), SPACE SCIENCE REVIEWS, Vol: 215, ISSN: 0038-6308
Strobel DF, Koskinen TT, Mueller-Wodarg I, 2019, Saturn's Variable Thermosphere, SATURN IN THE 21ST CENTURY, Editors: Baines, Flasar, Krupp, Stallard, Publisher: CAMBRIDGE UNIV PRESS, Pages: 224-250
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- Citations: 1
Moore L, Cravens TE, Mueller-Wodarg I, et al., 2018, Models of Saturn's equatorial ionosphere based on in situ data from Cassini's grand finale, Geophysical Research Letters, Vol: 45, Pages: 9398-9407, ISSN: 0094-8276
We present new models of Saturn's equatorial ionosphere based on the first in situ measurements of its upper atmosphere. The neutral spectrum measured by Cassini's Ion and Neutral Mass Spectrometer, which includes substantial methane, ammonia, and organics in addition to the anticipated molecular hydrogen, helium, and water, serves as input for unexpectedly complex ionospheric chemistry. Heavy molecular ions are found to dominate Saturn's equatorial low‐altitude ionosphere, with a mean ion mass of 11 Da. Key molecular ions include H3O+ and HCO+; other abundant heavy ions depend upon the makeup of the mass 28 neutral species, which cannot be uniquely determined. Ion and Neutral Mass Spectrometer neutral species lead to generally good agreement between modeled and observed plasma densities, though poor reproduction of measured H+ and H3+ variability and an overabundance of modeled H3+ potentially hint at missing physical processes in the model, including a loss process that affects H3+ but not H+.
Coustenis A, Atreya S, Castillo-Rogez J, et al., 2018, Preface to the special issue of PSS on "Surfaces, atmospheres and magnetospheres of the outer planets, their satellites and ring systems: Part XII", PLANETARY AND SPACE SCIENCE, Vol: 155, Pages: 1-1, ISSN: 0032-0633
Mendillo M, Trovato J, Moore L, et al., 2018, Comparative ionospheres: terrestrial and giant planets, Icarus, Vol: 303, Pages: 34-46, ISSN: 0019-1035
The study of planetary ionospheres within our solar system offers a variety of settings to probe mechanisms of photo-ionization, chemical loss, and plasma transport. Ionospheres are a minor component of upper atmospheres, and thus their mix of ions observed depends on the neutral gas composition of their parent atmospheres. The same solar irradiance (x-rays and extreme-ultra-violet vs. wavelength) impinges upon each of these atmospheres, with solar flux magnitudes changed only by the inverse square of distance from the Sun. If all planets had the same neutral atmosphere—with ionospheres governed by photochemical equilibrium (production = loss)—their peak electron densities would decrease as the inverse of distance from the Sun, and any changes in solar output would exhibit coherent effects throughout the solar system.Here we examine the outer planet with the most observations of its ionosphere (Saturn) and compare its patterns of electron density with those at Earth under the same-day solar conditions. We show that, while the average magnitudes of the major layers of molecular ions at Earth and Saturn are approximately in accord with distance effects, only minor correlations exist between solar effects and day-to-day electron densities. This is in marked contrast to the strong correlations found between the ionospheres of Earth and Mars. Moreover, the variability observed for Saturn's ionosphere (maximum electron density and total electron content) is much larger than found at Earth and Mars. With solar irradiance changes far too small to cause such effects, we use model results to explore the roles of other agents. We find that water sources from Enceladus at low latitudes, and ‘ring rain’ at middle latitudes, contribute substantially to variability via water ion chemistry. Thermospheric winds and electrodynamics generated at auroral latitudes are suggested causes of high latitude ionospheric variability, but remain inconclusive due to the l
Limaye SS, Lebonnois S, Mahieux A, et al., 2017, The thermal structure of the Venus atmosphere: Intercomparison of Venus Express and ground based observations of vertical temperature and density profiles, ICARUS, Vol: 294, Pages: 124-155, ISSN: 0019-1035
Coustenis A, Atreya S, Castillo J, et al., 2016, Preface to the special issue of PSS on "Surfaces, atmospheres and magnetospheres of the outer planets and their satellites and ring systems: Part XI", PLANETARY AND SPACE SCIENCE, Vol: 130, Pages: 1-2, ISSN: 0032-0633
Müller-Wodarg ICF, Bruinsma S, Marty J-C, et al., 2016, In situ observations of waves in Venus’s polar lower thermosphere with Venus Express aerobraking, Nature Physics, Vol: 12, Pages: 767-771, ISSN: 1745-2481
Waves are ubiquitous phenomena found in oceans and atmospheres alike. From the earliest formal studies of waves in the Earth’s atmosphere to more recent studies on other planets, waves have been shown to play a key role in shaping atmospheric bulk structure, dynamics and variability1, 2, 3, 4. Yet, waves are difficult to characterize as they ideally require in situ measurements of atmospheric properties that are difficult to obtain away from Earth. Thus, we have incomplete knowledge of atmospheric waves on planets other than our own, and we are thereby limited in our ability to understand and predict planetary atmospheres. Here we report the first ever in situ observations of atmospheric waves in Venus’s thermosphere (130–140 km) at high latitudes (71.5°–79.0°). These measurements were made by the Venus Express Atmospheric Drag Experiment (VExADE)5 during aerobraking from 24 June to 11 July 2014. As the spacecraft flew through Venus’s atmosphere, deceleration by atmospheric drag was sufficient to obtain from accelerometer readings a total of 18 vertical density profiles. We infer an average temperature of T = 114 ± 23 K and find horizontal wave-like density perturbations and mean temperatures being modulated at a quasi-5-day period.
Tinetti G, Drossart P, Eccleston P, et al., 2015, The EChO science case, Experimental Astronomy, Vol: 40, Pages: 329-391, ISSN: 1572-9508
The discovery of almost two thousand exoplanets has revealed an unexpectedlydiverse planet population. We see gas giants in few-day orbits, whole multi-planet systemswithin the orbit of Mercury, and new populations of planets with masses between that of theEarth and Neptune—all unknown in the Solar System. Observations to date have shown thatour Solar System is certainly not representative of the general population of planets in ourMilky Way. The key science questions that urgently need addressing are therefore: What areexoplanets made of? Why are planets as they are? How do planetary systems work and whatcauses the exceptional diversity observed as compared to the Solar System? The EChO(Exoplanet Characterisation Observatory) space mission was conceived to take up thechallenge to explain this diversity in terms of formation, evolution, internal structure andplanet and atmospheric composition. This requires in-depth spectroscopic knowledge of theatmospheres of a large and well-defined planet sample for which precise physical, chemicaland dynamical information can be obtained. In order to fulfil this ambitious scientificprogram, EChO was designed as a dedicated survey mission for transit and eclipsespectroscopy capable of observing a large, diverse and well-defined planet sample withinits 4-year mission lifetime. The transit and eclipse spectroscopy method, whereby the signalfrom the star and planet are differentiated using knowledge of the planetary ephemerides,allows us to measure atmospheric signals from the planet at levels of at least 10−4 relative tothe star. This can only be achieved in conjunction with a carefully designed stable payloadand satellite platform. It is also necessary to provide broad instantaneous wavelengthcoverage to detect as many molecular species as possible, to probe the thermal structureof the planetary atmospheres and to correct for the contaminating effects of the stellarphotosphere. This requires wavelength coverage of at l
Koskinen TT, Sandel BR, Yelle RV, et al., 2015, Saturn's variable thermosphere from Cassini/UVIS occultations, Icarus, Vol: 260, Pages: 174-189, ISSN: 0019-1035
We retrieved the density and temperature profiles in Saturn’s thermosphere from 26 stellar occultations observed by the Cassini/UVIS instrument. These results expand upon and complement the previous analysis of 15 Cassini/UVIS solar occultations by Saturn’s upper thermosphere. We find that the exospheric temperatures based on the stellar occultations agree with the solar occultations and range from 380 K to 590 K. These temperatures are also consistent with the recent re-analysis of the Voyager/UVS occultations. The retrieved density profiles support our earlier inference that the shape of the atmosphere at low pressures is consistent with a meridional trend of increasing temperatures with absolute latitude. This implies a high-latitude heat source, such as auroral heating, although the existing circulation models that include auroral heating still underestimate the equatorial temperatures by overestimating the meridional temperature gradient. This suggests either that the circulation models are somehow incomplete or there is some other heat source at low to mid latitudes that is relatively less efficient than high-latitude heating. We also find evidence for the expansion of the exobase by about 500 km between 2006 and 2011 near the equator, followed by possible contraction after 2011. The expansion appears to be caused by significant warming of the lower thermosphere that anti-correlates with solar activity and may be connected to changes in global circulation. Lastly, we note that our density profiles are in good general agreement with the Voyager/UVS data. In particular, the Voyager density profiles are most consistent with the Cassini/UVIS stellar occultations from late 2008 and early 2009 that roughly coincide in season with the Voyager flybys.
Coustenis A, Atreya S, Castillo J, et al., 2014, Surfaces, atmospheres and magnetospheres of the outer planets and their satellites and ring systems: Part X Preface, PLANETARY AND SPACE SCIENCE, Vol: 104, Pages: 1-2, ISSN: 0032-0633
Moore L, O'Donoghue J, Mueller-Wodarg I, et al., 2014, Saturn ring rain: Model estimates of water influx into Saturn's atmosphere, Icarus, Vol: 245, Pages: 355-366, ISSN: 0019-1035
Cui J, Yelle RV, Li T, et al., 2014, Density waves in Titan's upper atmosphere, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 119, Pages: 490-518, ISSN: 2169-9380
Mueller-Wodarg ICF, 2014, Titan's upper atmosphere: thermal structure, dynamics, and energetics, Titan: Interior, Surface, Atmosphere, and Space Environment, Editors: Müller-Wodarg, Griffith, Lellouch, Cravens, Publisher: Cambridge University Press, ISBN: 9780521199926
Titan, the largest of Saturn's moons, shares remarkable similarities with Earth. Its thick atmosphere is composed primarily of nitrogen; it features the most complex organic chemistry known outside of Earth and, uniquely, hosts an analog to Earth's hydrological cycle, with methane forming clouds, rain and seas. Using the latest data from the ongoing Cassini–Huygens missions, laboratory measurements and numerical simulations, this comprehensive reference examines the physical processes that shape Titan's fascinating atmospheric structure and chemistry, weather, climate, circulation and surface geology. The text also surveys leading theories about Titan's origin and evolution, and assesses their implications for understanding the formation of other complex planetary bodies. Written by an international team of specialists, chapters offer detailed, comparative treatments of Titan's known properties and discuss the latest frontiers in the Cassini–Huygens mission, offering students and researchers of planetary science, geology, astronomy and space physics an insightful reference and guide.
Mueller-Wodarg ICF, Griffith CA, Lellouch E, et al., 2014, TITAN Interior, Surface, Atmosphere, and Space Environment Introduction, TITAN: INTERIOR, SURFACE, ATMOSPHERE, AND SPACE ENVIRONMENT, Editors: MullerWodarg, Griffith, Lellouch, Cravens, Publisher: CAMBRIDGE UNIV PRESS, Pages: 1-9
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- Citations: 7
Coustenis A, Atreya S, Castillo J, et al., 2013, Surfaces, atmospheres and magnetospheres of the outer planets and their satellites and ring systems: Part IX, PLANETARY AND SPACE SCIENCE, Vol: 88, Pages: 1-2, ISSN: 0032-0633
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- Citations: 1
Sornig M, Sonnabend G, Stupar D, et al., 2013, Venus' upper atmospheric dynamical structure from ground-based observations shortly before and after Venus' inferior conjunction 2009, ICARUS, Vol: 225, Pages: 828-839, ISSN: 0019-1035
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- Citations: 11
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