Titan's lower atmosphere has long been known to harbor organic aerosols (tholins) presumed to have been formed from simple molecules, such as methane and nitrogen (CH₄ and N₂). Up to now, it has been ...assumed that tholins were formed at altitudes of several hundred kilometers by processes as yet unobserved. Using measurements from a combination of mass/charge and energy/charge spectrometers on the Cassini spacecraft, we have obtained evidence for tholin formation at high altitudes (~1000 kilometers) in Titan's atmosphere. The observed chemical mix strongly implies a series of chemical reactions and physical processes that lead from simple molecules (CH₄ and N₂) to larger, more complex molecules (80 to 350 daltons) to negatively charged massive molecules (~8000 daltons), which we identify as tholins. That the process involves massive negatively charged molecules and aerosols is completely unexpected.
Soon after the MAVEN (Mars Atmosphere and Volatile EvolutioN) spacecraft started orbiting Mars, the SEP (Solar Energetic Particle), SWIA (Solar Wind Ion Analyzer), and STATIC (Supra‐Thermal and ...Thermal Ion Composition) instruments on board the spacecraft detected planetary pickup ions. SEP can measure energetic (>60 keV) oxygen pickup ions, the source of which is the extended hot oxygen exosphere of Mars. Model results show that these pickup ions originate from tens of Martian radii upstream of Mars and are energized by the solar wind motional electric field as they gyrate back toward Mars. SWIA and STATIC can detect both pickup oxygen and pickup hydrogen with energies below ~30 keV and created closer to Mars. In this study, data from the SEP, SWIA, and STATIC instruments containing pickup ion signatures are provided and model‐data comparisons are shown. During the times when MAVEN is outside the Martian bow shock and in the upstream undisturbed solar wind, the solar wind velocity measured by SWIA and the solar wind (or interplanetary) magnetic field measured by the MAG (magnetometer) instrument can be used to model pickup oxygen and hydrogen fluxes. By comparing measured pickup ion fluxes with model results, the Martian thermal hydrogen and hot oxygen neutral densities can be probed outside the bow shock, providing a helpful tool in constraining estimates of neutral oxygen and hydrogen escape rates. Our analysis reveals an order of magnitude density change with Mars season in the hydrogen exosphere, whereas the hot oxygen exosphere was found to remain steadier.
Key Points
Model‐data comparisons of MAVEN SEP, SWIA, and STATIC measured oxygen and hydrogen pickup ions are presented
Three case studies demonstrate how pickup ion model‐data comparisons can constrain Mars exospheric neutral densities
Factor of 10 change with Mars season in the hydrogen exosphere is observed, whereas the oxygen exosphere remained steadier
Mars Atmosphere and Volatile EvolutioN mission (MAVEN) observes a tenuous but ubiquitous flux of protons with the same energy as the solar wind in the Martian atmosphere. During high flux intervals, ...we observe a corresponding negative hydrogen population. The correlation between penetrating and solar wind fluxes, the constant energy, and the lack of a corresponding charged population at intermediate altitudes implicate products of hydrogen energetic neutral atoms from charge exchange between the upstream solar wind and the exosphere. These atoms, previously observed in neutral form, penetrate the magnetosphere unaffected by electromagnetic fields (retaining the solar wind velocity), and some fraction reconvert to charged form through collisions with the atmosphere. MAVEN characterizes the energy and angular distributions of both penetrating and backscattered particles, potentially providing information about the solar wind, the hydrogen corona, and collisional interactions in the atmosphere. The accretion of solar wind hydrogen may provide an important source term to the Martian atmosphere over the planet's history.
Key Points
We observe H+ and H− in the atmosphere of Mars, at the solar wind energy
Solar wind protons charge exchange and penetrate as ENAs then reconvert
MAVEN can monitor hydrogen deposition and backscatter in the atmosphere
The Mars thermosphere-ionosphere-exosphere (TIE) system constitutes the atmospheric reservoir (i.e. available cold and hot planetary neutral and thermal ion species) that regulates present day escape ...processes from the planet. The characterization of this TIE system, including its spatial and temporal (e.g., solar cycle, seasonal, diurnal, episodic) variability is needed to determine present day escape rates. Without knowledge of the physics and chemistry creating this TIE region and driving its variations, it is not possible to constrain either the short term or long term histories of atmosphere escape from Mars. MAVEN (Mars Atmosphere and Volatile Evolution Mission) will make both in-situ and remote measurements of the state variables of the Martian TIE system. A full characterization of the thermosphere (∼100–250 km) and ionosphere (∼100–400 km) structure (and its variability) will be conducted with the collection of spacecraft in-situ measurements that systematically span most local times and latitudes, over a regular sampling of Mars seasons, and throughout the bottom half of the solar cycle. Such sampling will far surpass that available from existing spacecraft and ground-based datasets. In addition, remote measurements will provide a systematic mapping of the composition and structure of Mars neutral upper atmosphere and coronae (e.g. H, C, N, O), as well as probe lower altitudes. Such a detailed characterization is a necessary first step toward answering MAVEN’s three main science questions (see Jakosky et al.
2014
, this issue). This information will be used to determine present day escape rates from Mars, and provide an estimate of integrated loss to space throughout Mars history.
Crustal magnetic fields on Mars were first found by the Mars Global Surveyor and reported in 1998. One interesting aspect of the interaction between crustal fields and the Martian ionosphere is the ...impact on total ion loss over time, which is a central question addressed by the Mars Atmospheric and Volatile EvolutioN (MAVEN) spacecraft currently orbiting Mars. In this work, we seek to improve understanding of the impact of crustal fields on plasma energetics, and consequent ion loss, by examining the effects of ionospheric current sheets that may form at the boundary between crustal and induced magnetic fields. These current sheets will not just affect the external induced field regions but will also affect the observed magnetic fields in the crustal field regions. Variations in field strength should be evident by comparing crustal field models and measured magnetic fields collected by the MAVEN magnetometer instrument (MAG). One way to confirm such variation is to compare dayside and nightside orbit pairs that occur over the same crustal field regions. The induced magnetic field exists mostly on the dayside and is largely absent on the nightside. We examine these orbit pairs and discuss the perturbation of magnetic fields in the dayside ionosphere. We confirm that deviations on the dayside are larger than on the nightside, and that current sheets in the ionosphere could be responsible for these deviations, particularly in the strong crustal field regions.
Plain Language Summary
Crustal fields are magnetic fields that are produced in the Martian crust and extend into and above the atmosphere. These fields are thought to play a role in atmosphere loss from Mars. The crustal fields change over billions of years but are unchanged over the course of this work. There have been many models created to try and map out these fields on Mars. Alternatively, induced magnetic fields are generated from the interaction between Mars and the solar wind. The induced fields do vary substantially over local time (particularly daytime vs. nighttime). We look at the boundary between these two types of magnetic fields, using data from the Mars Atmospheric and Volatile EvolutioN (MAVEN) spacecraft. We compare differences between MAVEN data and predicted crustal field model values, then compare these differences between two orbits—one dayside and one nightside—that pass over the same region of Mars. We also compare dayside cases, for different local times. We find that dayside orbits have larger differences between model and data, and the closer to noon, the larger the difference. We aim to understand the interaction between induced and crustal fields and speculate how this could affect atmospheric loss from Mars.
Key Points
Mars Atmospheric and Volatile EvolutioN magnetometer data shows deviation from crustal models. Commonly, larger deviations are seen on the dayside rather than on the nightside
Typical dayside deviations are about 20 nT, but vary with solar zenith angle
Ionospheric currents induced by the solar wind interaction with Mars can in part explain the deviations seen
The spatially localized and highly variable polar cap emissions at Jupiter are part of a poorly understood current system linking the ionosphere and the magnetopause region. Strong X‐ray emission has ...been observed from the polar caps and has been explained by the precipitation of oxygen and sulfur ions of several MeV energy. The present paper presents results of an extended model of the ion precipitation process at Jupiter. Specifically, we add to a previous model a more complete treatment of ionization of the atmosphere, generation of secondary electron fluxes and their escape from the atmosphere, and generation of downward field‐aligned currents. Predictions relevant to observations by the upcoming NASA Juno mission are made, namely the existence of escaping electrons with energies from a few eV up to 10 keV, auroral H2 band emission rates of 80 kR, and downward field‐aligned currents of at least 2 MA.
Key Points
We make predictions relevant to observations by the upcoming NASA Juno mission
We predict escaping electrons with energies from a few eV to 10 keV
We estimate downward field‐aligned currents of about 2 MA as a lower limit
The Rosetta spacecraft has escorted comet 67P/Churyumov-Gerasimenko since 6 August 2014 and has offered an unprecedented opportunity to study plasma physics in the coma. We have used this opportunity ...to make the first characterization of cometary electrons with kappa distributions. Two three-dimensional kappa functions were fit to the observations, which we interpret as two populations of dense and warm (density 10 cubic centimeters, temperature 2 times 10 (sup 5) degrees Kelvin, invariant kappa index 10 to 1000), and rarefied and hot (density equals 0.005 cubic centimeters, temperature 5 times 10 (sup 5) degrees Kelvin, invariant kappa index equals 1 to 10) electrons. We fit the observations on 30 October 2014 when Rosetta was 20 kilometers from 67P, and 3 Astronomical Units from the Sun. We repeated the analysis on 15 August 2015 when Rosetta was 300 kilometers from the comet and 1.3 Astronomical Units from the Sun. Comparing the measurements on both days gives the first comparison of the cometary electron environment between a nearly inactive comet far from the Sun and an active comet near perihelion. We find that the warm population density increased by a factor of 3, while the temperature cooled by a factor of 2, and the invariant kappa index was unaffected. We find that the hot population density increased by a factor of 10, while the temperature and invariant kappa index were unchanged. We conclude that the hot population is likely the solar wind halo electrons in the coma. The warm population is likely of cometary origin, but its mechanism for production is not known.
Since Mars Atmosphere and Volatile EvolutioN (MAVEN)'s arrival at Mars on 21 September 2014, the SEP (Solar Energetic Particle) instrument on board the MAVEN spacecraft has been detecting oxygen ...pickup ions with energies of a few tens of keV up to ~200 keV. These ions are created in the distant upstream part of the hot atomic oxygen exosphere of Mars, via photoionization, charge exchange with solar wind protons, and electron impact. Once ionized, atomic oxygen ions are picked up by the solar wind and accelerated downstream, reaching energies high enough for SEP to detect them. We model the flux of oxygen pickup ions observed by MAVEN SEP in the undisturbed upstream solar wind and compare our results with SEP's measurements. Model‐data comparisons of SEP fluxes confirm that pickup oxygen associated with the Martian exospheric hot oxygen is indeed responsible for the MAVEN SEP observations.
Key Points
Oxygen pickup ions are detected by MAVEN SEP in the undisturbed solar wind at Mars
Model‐data comparisons indicate pickup oxygen associated with the Martian exospheric hot oxygen
Statistical analysis of SEP data constrains exospheric hot oxygen densities and escape at Mars
Between 26 April and 15 September 2017, Cassini executed 23 highly inclined Grand Finale orbits through a new frontier for space exploration, the narrow region between Saturn and the D Ring, ...providing the first opportunity for obtaining in situ ionospheric measurements. During the Grand Finale orbits, the Radio and Plasma Wave Science instrument observed broadband whistler mode emissions and narrowband upper hybrid frequency emissions. Using known wave propagation characteristics of these two plasma wave modes, the electron density is derived over a broad range of ionospheric latitudes and altitudes. A two‐part exponential scale height model is fitted to the electron density measurements. The model yields a double‐layered ionosphere with plasma scale heights of 545/575 km for the northern/southern hemispheres below 4,500 km and plasma scale heights of 4,780/2,360 km for the northern/southern hemispheres above 4,500 km. The interpretation of these layers involves the interaction between the rings and the ionosphere.
Plain Language Summary
For the final 5 months of the Cassini mission in 2017, the spacecraft executed 23 orbits through a new frontier for space exploration, the narrow region between Saturn and the innermost of Saturn's main rings, the D Ring. For the first time in the history of space exploration, the Cassini instruments were able to take measurements inside Saturn's ionosphere. This paper provides the density distribution of Saturn's ionospheric electrons, derived from plasma waves detected by the Radio and Plasma Wave Science instrument. The electron density distributions with altitude and latitude show that the ionospheric electron densities peak at 10,000 particles per cubic centimeter at low altitudes in the equatorial region and drop below 100 particles per cubic centimeter at higher altitudes and latitudes. Two simple ionospheric scale height density models for the northern and southern hemispheres are presented.
Key Points
We present the first in situ measurements of the electron density in the low to middle latitudes of Saturn's ionosphere
The distribution of electron density measurements with altitude shows evidence of a two‐layered ionospheric electron density distribution up to an altitude of 15,000 km
We present a scale height electron density model for a double‐layered ionosphere for both the northern and southern hemispheres
Small‐scale magnetic structures have been observed in the induced Martian ionosphere by magnetometers onboard the Mars Global Surveyor (MGS) and the Mars Atmosphere and Volatile EvolutioN (MAVEN) ...spacecraft. The origin and evolution of these structures remain poorly understood and the goal of the current paper is to better characterize them and their distribution in the dayside ionosphere using Mars Atmosphere and Volatile EvolutioN (MAVEN) data. Mars Atmosphere and Volatile EvolutioN (MAVEN) Langmuir probe data is used to find thermal pressures in the ionosphere. The structures studied range in size from about 20 km up to a couple of hundred km. We constrain our investigation to the northern hemisphere, dayside Martian ionosphere in order to minimize crustal magnetic field interference. Magnetic pressure spikes (and/or field component variations), along with thermal pressure behavior can also sometimes characterize small‐scale structures. In addition to pressures and field components, minimum variance analyses are carried out for each structure in order to help classify them (e.g., horizontal slabs, ionopause‐like structures, flux tube and flux ropes). A “statistical” catalog of properties (pressure, field amplitude, ellipticity, width, etc.) is generated for about 1,000 structures. One conclusion we reached from this survey is that slab‐like features are more likely to be found than rope‐like features for altitudes above 250 km.
Key Points
The magnetic barrier connection to the lower ionosphere is explored
We characterize and examine the evolution of small‐scale magnetic structure
Mars Atmospheric and Volatile EvolutionN (MAVEN) data is analyzed for magnetic and thermal structure
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