Persistent plasma waves in interstellar space detected by Voyager 1.

In 2012, Voyager 1 became the first in situ probe of the very local interstellar medium1. The Voyager 1 Plasma Wave System has given point estimates of the plasma density spanning about 30 au of interstellar space, revealing a large-scale density gradient2,3 and turbulence4 outside the heliopause. Previous studies of the plasma density relied on the detection of discrete plasma oscillation events triggered ahead of shocks propagating outwards from the Sun and used to infer the plasma frequency and hence density5,6. We present the detection of a class of very weak, narrowband plasma wave emission in the Voyager 1 data that persists from 2017 onwards and enables the first steadily sampled measurement of the interstellar plasma density over about 10 au with an average sampling distance of 0.03 au. We find au-scale density fluctuations that trace interstellar turbulence between episodes of previously detected plasma oscillations. Possible mechanisms for the narrowband emission include thermally excited plasma oscillations and quasi-thermal noise, and could be clarified by new findings from Voyager or a future interstellar mission. The emission's persistence suggests that Voyager 1 may be able to continue tracking the interstellar plasma density in the absence of shock-generated plasma oscillation events.

Evidence for low density holes in Jupiter's ionosphere.

Intense electromagnetic impulses induced by Jupiter's lightning have been recognised to produce both low-frequency dispersed whistler emissions and non-dispersed radio pulses. Here we report the discovery of electromagnetic pulses associated with Jovian lightning. Detected by the Juno Waves instrument during its polar perijove passes, the dispersed millisecond pulses called Jupiter dispersed pulses (JDPs) provide evidence of low density holes in Jupiter's ionosphere. 445 of these JDP emissions have been observed in snapshots of electric field waveforms. Assuming that the maximum delay occurs in the vicinity of the free space ordinary mode cutoff frequency, we estimate the characteristic plasma densities (5.1 to 250 cm-3) and lengths (0.6 km to 1.3 × 105 km) of plasma irregularities along the line of propagation from lightning to Juno. These irregularities show a direct link to low plasma density holes with ≤250 cm-3 in the nightside ionosphere.

The search for life in the solar system.

In this presentation I give an overview of the long struggle to answer the age old question, does life exist anywhere else? The focus will be specifically on the search for life in the solar system, since this is the only region currently accessible to direct investigation. A hundred years ago many people believed that life, possibly even intelligent life, existed at the nearby planets Venus and Mars, and possibly elsewhere. The space age exploration of the planets has radically altered that view. We now know that Venus is a very hostile place, with no possibility for life, and that Mars is almost completely barren and very cold, with little prospect for life. The only remaining possibility appears to be in the interior of some of the moons of the outer planets where, due to an unlikely combination of factors, the conditions may be suitable for life.

Subsurface radar sounding of the south polar layered deposits of Mars.

The ice-rich south polar layered deposits of Mars were probed with the Mars Advanced Radar for Subsurface and Ionospheric Sounding on the Mars Express orbiter. The radar signals penetrate deep into the deposits (more than 3.7 kilometers). For most of the area, a reflection is detected at a time delay that is consistent with an interface between the deposits and the substrate. The reflected power from this interface indicates minimal attenuation of the signal, suggesting a composition of nearly pure water ice. Maps were generated of the topography of the basal interface and the thickness of the layered deposits. A set of buried depressions is seen within 300 kilometers of the pole. The thickness map shows an asymmetric distribution of the deposits and regions of anomalous thickness. The total volume is estimated to be 1.6 x 10(6) cubic kilometers, which is equivalent to a global water layer approximately 11 meters thick.

Radar soundings of the subsurface of Mars.

The martian subsurface has been probed to kilometer depths by the Mars Advanced Radar for Subsurface and Ionospheric Sounding instrument aboard the Mars Express orbiter. Signals penetrate the polar layered deposits, probably imaging the base of the deposits. Data from the northern lowlands of Chryse Planitia have revealed a shallowly buried quasi-circular structure about 250 kilometers in diameter that is interpreted to be an impact basin. In addition, a planar reflector associated with the basin structure may indicate the presence of a low-loss deposit that is more than 1 kilometer thick.