A major goal of the Atacama Large Millimeter/submillimeter Array (ALMA) is to make accurate images with resolutions of tens of milliarcseconds, which at submillimeter (submm) wavelengths requires baselines up to ∼15 km. To develop and test this capability, a Long Baseline Campaign (LBC) was carried out from 2014 September to late November, culminating in end-to-end observations, calibrations, and imaging of selected Science Verification (SV) targets. This paper presents an overview of the campaign and its main results, including an investigation of the short-term coherence properties and systematic phase errors over the long baselines at the ALMA site, a summary of the SV targets and observations, and recommendations for science observing strategies at long baselines. Deep ALMA images of the quasar 3C 138 at 97 and 241 GHz are also compared to VLA 43 GHz results, demonstrating an agreement at a level of a few percent. As a result of the extensive program of LBC testing, the highly successful SV imaging at long baselines achieved angular resolutions as fine as 19 mas at ∼350 GHz. Observing with ALMA on baselines of up to 15 km is now possible, and opens up new parameter space for submm astronomy.

Observations of the Pluto-Charon system, acquired with the ALMA interferometer on June 12-13, 2015, have led to the detection of the CO(3-2) and HCN(4-3) rotational transitions from Pluto (including the hyperfine structure of HCN), providing a strong confirmation of the presence of CO, and the first observation of HCN in Pluto's atmosphere. The CO and HCN lines probe Pluto's atmosphere up to ∼450 km and ∼900 km altitude, respectively, with a large contribution due to limb emission. The CO detection yields (i) a much improved determination of the CO mole fraction, as 515±40 ppm for a 12 µbar surface pressure (ii) strong constraints on Pluto's mean atmospheric dayside temperature profile over ∼50-400 km, with clear evidence for a well-marked temperature decrease (i.e., mesosphere) above the 30-50 km stratopause and a best-determined temperature of 70±2 K at 300 km, somewhat lower than previously estimated from stellar occultations (81±6 K), and in agreement with recent inferences from New Horizons / Alice solar occultation data. The HCN line shape implies a high abundance of this species in the upper atmosphere, with a mole fraction > 1.5×10 −5 above 450 km and a value of 4×10 −5 near 800 km. Assuming HCN at saturation, this would require a warm (>92 K) upper atmosphere layer; while this is not ruled out by the CO emission, it is inconsistent with the Alicemeasured CH 4 and N 2 line-of-sight column densities. Taken together, the large HCN abundance and the cold upper atmosphere imply supersaturation of HCN to a degree (7-8 orders of magnitude) hitherto unseen in planetary atmospheres, probably due to a lack of condensation nuclei above the haze region and the slow kinetics of condensation at the low pressure and temperature conditions of Pluto's upper atmosphere. HCN is also present in the bottom ∼100 km of the atmosphere, with a 10 −8 -10 −7 mole fraction; this implies either HCN saturation or undersaturation there, depending on the precise stratopause temperature. The HCN column is (1.6±0.4)×10 14 cm −2 , suggesting a surface-referred vertically-integrated net production rate of ∼2×10 7 cm −2 s −1 . Although HCN rotational line cooling affects Pluto's atmosphere heat budget, the amounts determined in this study are insufficient to explain the well-marked mesosphere and upper atmosphere's ∼70 K temperature, which if controlled by HCN cooling would require HCN mole fractions of (3-7)×10 −4 over 400-800 km. We finally report an upper limit on the HC 3 N column density (< 2×10 13 cm −2 ) and on the HC 15 N / HC 14 N ratio (< 1/125).

Extrasolar satellites are generally too small to be detected by nominal searches. By analogy to the most active body in the Solar System, Io, we describe how sodium (Na I) and potassium (K I) gas could be a signature of the geological activity venting from an otherwise hidden exo-Io. Analyzing ∼ a dozen close-in gas giants hosting robust alkaline detections, we show that an Io-sized satellite can be stable against orbital decay below a planetary tidal Q p 10 11 . This tidal energy is focused into the satellite driving a ∼ 10 5±2 higher mass loss rate than Io's supply to Jupiter's Na exosphere, based on simple atmospheric loss estimates. The remarkable consequence is that several exo-Io column densities are on average more than sufficient to provide the ∼ 10 10±1 Na cm −2 required by the equivalent width of exoplanet transmission spectra. Furthermore, the benchmark observations of both Jupiter's extended (∼ 1000 R J ) Na exosphere and Jupiter's atmosphere in transmission spectroscopy yield similar Na column densities that are purely exogenic in nature. As a proof of concept, we fit the "high-altitude" Na at WASP 49-b with an ionization-limited cloud similar to the observed Na profile about Io. Moving forward, we strongly encourage time-dependent ingress and egress monitoring along with spectroscopic searches for other volcanic volatiles.

Winds in Titan's lower and middle atmosphere have been determined by a variety of techniques, including direct measurements from the Huygens Probe 1 over 0-150 km, Doppler shifts of molecular spectral lines in the optical, thermal infrared and mm ranges 2-4 , probing altogether the ~100-450 km altitude range, and inferences from thermal field over 10 mbar -10 -3 mbar (i.e. ~100-500 km) 5-6 and from central flashes in stellar occultation curves 7-9 . These measurements predominantly indicated strong prograde winds, reaching maximum speeds of ~150-200 m/s in the upper stratosphere, with important latitudinal and seasonal variations. However, these observations provided incomplete atmospheric sounding; in particular, the wind regime in Titan's upper mesosphere and thermosphere (500-1200 km) has remained unconstrained so far. Here we report direct wind measurements based on Doppler shifts of six molecular species observed with ALMA. We show that unlike expectations, strong prograde winds extend up to the thermosphere, with the circulation progressively turning into an equatorial jet regime as altitude increases, reaching ~340 m/s at 1000 km. We suggest that these winds may represent the dynamical response of forcing by waves launched at upper stratospheric/mesospheric levels and/or magnetospheric-ionospheric interaction. We also demonstrate that the HNC distribution is restricted to Titan's thermosphere above ~870 km altitude.

Aims. In spite of considerable progress in the last two decades, Io's atmosphere remains poorly understood. The goal of this work is to improve our understanding of its spatial distribution, temperature and dynamics. Methods. We present millimeter observations of Io's surface and SO 2 atmosphere at 1.4 mm obtained with the IRAM Plateau de Bure Interferometer in January−February 2005. With a synthesized beam of 0.5 × 1.5 , these observations resolve Io's ∼1.0 disk in the longitudinal / local time direction, and sample the leading and trailing hemispheres of Io. Results. The measured continuum total flux and visibilities show that continuum radiation originates from a depth of at least 1 cm in Io's subsurface. On both the leading and trailing sides, emission in the SO 2 216.643 GHz line appears spatially narrower than the continuum, and suggests that the atmosphere covers ∼80% of the surface on the leading side and ∼60% on the trailing. On the leading side, disk-resolved spectra yield Doppler shift measurements, indicating a beam-integrated limb-to-limb velocity difference of 330 ± 100 m/s in the prograde direction. Such a flow allows an improved fit of disk-averaged SO 2 spectra, but its origin remains to be understood. Mean gas temperatures are in the range 130−180 K, in agreement with estimates from IR measurements, and with a tendency for higher trailing vs leading side gas temperatures. On the basis of realistic plume models, we find that the contribution of isolated volcanic plumes to the SO 2 emission is small.