Solar system

Abstract:

Abstract:

Human activity causes vibrations that propagate into the ground as high-frequency seismic waves. Measures to mitigate the coronavirus disease 2019 (COVID-19) pandemic caused widespread changes in human activity, leading to a months-long reduction in seismic noise of up to 50%. The 2020 seismic noise quiet period is the longest and most prominent global anthropogenic seismic noise reduction on record. Although the reduction is strongest at surface seismometers in populated areas, this seismic quiescence extends for many kilometers radially and hundreds of meters in depth. This quiet period provides an opportunity to detect subtle signals from subsurface seismic sources that would have been concealed in noisier times and to benchmark sources of anthropogenic noise. A strong correlation between seismic noise and independent measurements of human mobility suggests that seismology provides an absolute, real-time estimate of human activities.

Abstract:

Here we search for hydrogen chloride (HCl) in Neptune’s stratosphere using observations of the 1876.22 GHz J=3–2 transition from the Heterodyne Instrument for the Far-Infrared (HIFI) on Herschel. Observations comprise a 7.2 hr disc-averaged integration, originally designed to investigate stratospheric methane. Significant HCl emission was not detected. Instead, we determine upper limits using step-type abundance profiles, defined by zero deep abundance and uniform volume mixing ratio for pressures less than a transition pressure (assumed to be 0.1 or 1 mbar). These profiles are a reasonable first-order approximation for an externally sourced species; at higher pressures HCl is expected to be removed by aerosol scavenging and reactions with ammonia. The 3 upper limits are 0.70 parts per billion (ppb) for a 0.1 mbar transition pressure and 0.076 ppb for a 1 mbar transition pressure. These upper limits are the most stringent to date and are consistent with current estimates of interplanetary dust particle flux and the hypothesis that Neptune experienced a large comet impact in the past 1000 years.