We present spectroscopy of nine planetary nebulae (PNe) in the outskirts of M31, all but one obtained with the 10.4 m GTC telescope. These sources extend our previous study of the oxygen abundance gradient of M31 to galactocentric radii as large as 100 kpc. None of the targets are bona fide members of a classical, metal-poor and ancient halo. Two of the outermost PNe have solar oxygen abundances, as well as radial velocities consistent with the kinematics of the extended disk of M31. The other PNe have a slightly lower oxygen content ([O/H]∼−0.4) and in some cases large deviations from the disk kinematics. These PNe support the current view that the external regions of M31 are the result of a complex interaction and merger process, with evidence for a widespread population of solar-metallicity stars produced in a starburst that occurred ∼2 Gyr ago.

The response of the Mars ionosphere to changes in solar irradiance is an important aspect of how conditions on Mars are shaped by our dynamic Sun. Changes in the composition of ionospheric plasma with changes in solar irradiance will affect how the ionosphere mediates the interaction of the neutral atmosphere with the surrounding space environment. Here we use MAVEN ion and neutral density measurements acquired at low (Deep Dip 8) and high (Deep Dip 2) solar irradiance conditions to determine how ion and neutral densities change when the ionizing solar irradiance doubles. We find that the neutral composition does not change significantly when examined at fixed total neutral number density. Furthermore, we find that normalCnormalO2+ ${\mathrm{C}\mathrm{O}}_{2}^{+}$ and O+ densities increase by a factor of 2, but O2+ ${\mathrm{O}}_{2}^{+}$ densities increase by a factor of 1.5. The relative abundance of O2+ ${\mathrm{O}}_{2}^{+}$ decreases as solar irradiance increases. These results are explained by straightforward theoretical considerations of changes in ion production and loss rates. However, a photochemical model fails to reproduce these results with its default set of inputs. Consistent with previous modeling efforts, normalCnormalO2+ ${\mathrm{C}\mathrm{O}}_{2}^{+}$ densities are over‐predicted. Acceptable model–data agreement requires significant adjustments to important model inputs, such as reduction in irradiance by 15%, reduction in CO2 density by a factor of 2, and increase in O density by a factor of 2. These large adjustments are suggestive of the need for improvements to the state of Mars ionosphere models based on MAVEN inputs rather than issues with the underlying data.

The ionosphere of Venus is a weakly ionized plasma layer embedded in the planet's upper atmosphere. Planetary ionospheres provide an excellent opportunity to study how our variable Sun affects the planets in our solar system. Because ionospheres are reservoirs from which atmospheric species can be lost to space, studying how ionospheres respond to changes in solar activity can help us understand how planetary atmospheres have evolved since their formation. While variations of the main and lower ionospheric peaks of Venus have been well studied, the behavior of the ionosphere above the altitude of the greatest electron density has not been fully constrained. To investigate the behavior of this region, we use electron density profiles obtained by the Venus Radio Science experiment aboard Venus Express. An increase in the response of the electron density to increasing solar irradiance with increasing altitude above the peak is readily apparent in these data. By using a one‐dimensional photochemical equilibrium model to investigate the factors that drive the variations of the ionosphere of Venus, we find that changes in the composition of the underlying neutral atmosphere are responsible for the observed increase in ionospheric response with altitude.

Earth's nearest neighbors, Mars and Venus, have CO 2-dominated atmospheres and lack global magnetic fields, but differ greatly in the magnitude of their surface gravity, atmospheric density, and incident solar radiation. These similarities and differences are reflected in their ionospheres; in their densest regions, their ionospheres are dominated by  2 O , which results from the rapid charge-exchange reaction between photoproduced  2 CO with the small fraction of available O. However, their atmospheric scale heights differ by a factor of two and their peak electron densities differ by an order of magnitude and show different dependencies on solar zenith angle (SZA) (e.g.

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