Planetary surfaces

Abstract:

The long-term survival of Enceladus' ocean depends on the balance between heat production and heat loss. To date, the only place where a direct measurement of Enceladus's heat loss has been made is at the south pole. Here, we show that the north pole also emits heat at a greater rate than can be explained by purely passive models. By comparing winter and summer observations taken with the Cassini Composite InfraRed Spectrometer, we find a winter temperature ~7 kelvin warmer than passive modeling predicts, accounting for uncertainties in emissivity and thermal inertia. An additional endogenic heat flux of 46 ± 4 milliwatts per square meter is required to match the observed radiance. The implied local shell thickness is 20 to 23 kilometers-consistent with the higher end of thickness models based on gravity, topography, and libration measurements. This work provides a previously unidentified constraint for models of tidal heat production, shell thickness, and the long-term evolution of Enceladus' ocean.

Abstract:

Substellar companions such as exoplanets and brown dwarfs exhibit changes in brightness arising from top-of-atmosphere inhomogeneities, providing insights into their atmospheric structure and dynamics. This variability can be measured in the light curves of high-contrast companions from the ground by combining differential spectrophotometric monitoring techniques with high-contrast imaging. However, ground-based observations are sensitive to the effects of turbulence in Earth’s atmosphere, and while adaptive optics (AO) systems and bespoke data processing techniques help to mitigate these, residual systematics can limit photometric precision. Here, we inject artificial companions to data obtained with an AO system and a vector Apodizing Phase Plate coronagraph to test the level to which telluric and other systematics contaminate such light curves, and thus how well their known variability signals can be recovered. We find that varying companions are distinguishable from non-varying companions, but that variability amplitudes and periods cannot be accurately recovered when observations cover only a small number of periods. Residual systematics remain above the photon noise in the light curves but have not yet reached a noise floor. We also simulate observations to assess how specific systematic sources, such as non-common path aberrations and AO residuals, can impact aperture photometry as a companion moves through pupil-stabilized data. We show that only the lowest order aberrations are likely to affect flux measurements, but that thermal background noise is the dominant source of scatter in raw companion photometry. Predictive control and focal-plane wavefront sensing techniques will help to further reduce systematics in data of this type.

Abstract:

The Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft has detected as many as 1,125 plasma depletion events (PDEs) in the Martian ionosphere from October 2014 to May 2021. PDEs, characterized by significantly reduced plasma density, elevated electron temperatures, and increased electrostatic fluctuations, remain poorly understood in terms of their formation and spatiotemporal characteristics. This study combines MAVEN data with concurrent observations from Mars Express (MEX) to investigate these aspects. The analysis of PDE recurrence rates across subsequent MAVEN orbits reveals 80 recurring events. These events are formed at the same locations within 18–30 hr. Additionally, we identified two conjugate PDEs observed by both MAVEN and MEX. These observations suggest that PDEs can extend spatially up to 750 km and last for a couple of hours. Our findings suggest that PDEs are large-scale and possibly recurring phenomena, potentially important for ion loss, and that understanding them is important for accurately characterizing the Martian ionosphere.