Solar system

Abstract:

Planets smaller than Neptune and larger than Earth make up the majority of the discovered exoplanets. Those with H2-rich atmospheres are prime targets for atmospheric characterization. The transition between the two main classes, super-Earths and sub-Neptunes, is not clearly understood as the rocky surface is likely not accessible to observations. Tracking several trace gases (specifically the loss of ammonia (NH3) and hydrogen cyanide (HCN)) has been proposed as a proxy for the presence of a shallow surface. In this work, we revisit the proposed mechanism of nitrogen conversion in detail and find its timescale on the order of a million years. NH3 exhibits dual paths converting to N2 or HCN, depending on the UV radiation of the star and the stage of the system. In addition, methanol (CH3OH) is identified as a robust and complementary proxy for a shallow surface. We follow the fiducial example of K2-18b with a 2D photochemical model on an equatorial plane. We find a fairly uniform composition distribution below 0.1 mbar controlled by the dayside, as a result of slow chemical evolution. NH3 and CH3OH are concluded to be the most unambiguous proxies to infer surfaces on sub-Neptunes in the era of the James Webb Space Telescope.

Abstract:

NASA’s InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) mission has operated a sophisticated suite of seismology and geophysics instruments on the surface of Mars since its arrival in 2018. On 18 February 2021, we attempted to detect the seismic and acoustic waves produced by the entry, descent and landing of the Perseverance rover using the sensors onboard the InSight lander. Similar observations have been made on Earth using data from both crewed1,2 and uncrewed3,4 spacecraft, and on the Moon during the Apollo era5, but never before on Mars or another planet. This was the only seismic event to occur on Mars since InSight began operations that had an a priori known and independently constrained timing and location. It therefore had the potential to be used as a calibration for other marsquakes recorded by InSight. Here we report that no signal from Perseverance’s entry, descent and landing is identifiable in the InSight data. Nonetheless, measurements made during the landing window enable us to place constraints on the distance–amplitude relationships used to predict the amplitude of seismic waves produced by planetary impacts and place in situ constraints on Martian impact seismic efficiency (the fraction of the impactor kinetic energy converted into seismic energy).

Abstract:

In this work, we have analyzed images provided by the Hubble Space Telescope's Wide Field Camera 3 (HST/WFC3) in December 2016, with a spectral coverage from the ultraviolet to the near infrared. We have obtained the spectral reflectivity of the GRS and its surroundings, with particular emphasis on selected, dynamically interesting regions. A spectral characterization of the GRS area is performed following two different procedures: (a) in terms of Altitude/Opacity and Color Indices (AOI and CI); (b) by means of automatic spectral classification. We used the NEMESIS radiative transfer suite to retrieve the main atmospheric parameters (e.g., particle vertical and size distributions, refractive indices) that are able to explain the observed spectral reflectivity. The optimal a priori model atmosphere used for the retrievals is obtained from a grid of about 12,000 different atmospheric models, and choosing the one that best fits South Tropical Zone (STrZ) spectra and its observed limb-darkening. We conclude that the spectral reflectivity of the GRS area is well reproduced with the following layout: (a) a stratospheric haze with its base near the 100 mbar level, with optical depths at 900 nm of the order of unity and particles with a size of 0.3 μm; (b) a more vertically extended tropospheric haze, with τ (900 nm) ∼10 down to 500 mbar and micron sized particles. Both haze layers show a stronger short wavelength absorption, and thus both act as chromophores. The altitude difference between clouds tops in the GRS and surrounding areas is ∼10 km.