91̽��

Abstract:

The Lucy spacecraft flew past the ∼738 m diameter, S-class main-belt asteroid (152830) Dinkinesh on 2023 November 1, revealing a satellite named Selam. We used images acquired during the flyby to evaluate surface features on both Dinkinesh and Selam. We find a shallow crater size–frequency distribution (SFD) for Dinkinesh, consistent with crater SFDs observed on other subkilometer asteroids. We derive crater depth-to-diameter ratios near 0.1, also consistent with typical values seen on other asteroids. We calculate a cumulative boulder SFD for Dinkinesh with power-law index 3.93 ± 0.15 slightly steeper though in the range of other S-class asteroids. We find growing evidence that boulder SFDs are, on average, steeper for S-class than C-complex asteroids. Two major surface features on Dinkinesh, Sumak Fossa (a large trough) and Fab Dorsum (an equatorial ridge), are likely an outcome of YORP spinning up Dinkinesh fast enough to produce failure. A self-consistent structure for Dinkinesh that complies with the global shape, feature morphologies, and the estimated 10–20 Myr YORP spin-up timescale is a rubble-pile object with a nearly strengthless surface and an interior strength that is less than tens of Pa. Selam could have formed via YORP-driven mass shedding from Dinkinesh, though other formation mechanisms are possible. Combining a low-strength surface with the crater population and an impact model, we estimate a ∼1 Myr surface age for Dinkinesh. The presence of mass wasting and young troughs indicates that stress accumulation and release continue on Dinkinesh to the present day.

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:

In 2018, the European Space Agency (ESA) asked the scientific community for proposals for a new ‘Fast class’ of missions: faster, lower cost, and allowing more experimentation than flagship programs. The selected mission would be a payload of opportunity sharing a launch with the medium class ARIEL exoplanet telescope to the Earth-Sun L2 Lagrange point, around1.5m kilometers from the Earth.

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.