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<jats:title>Volume and shape combine to find a level</jats:title> <jats:p>How can we understand the geological record of sea level change? Sea level varies on time scales from decades to millions of years. These changes have local, regional, and global components and are caused by a wide variety of earth processes. Cloetingh and Haq review how the views of stratigraphers (who interpret the record of marine sediments) and geodynamicists (who consider changes in the shape of Earth caused by lithospheric and mantle processes) have begun to complement each other and are moving toward a more coherent interpretation of the history of sea level. They focus on cyclic sea-level changes 0.5 to 3.0 million years in duration that occurred in the Cretaceous period, approximately 145 to 65 million years ago.</jats:p> <jats:p> <jats:italic>Science</jats:italic> , this issue <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" related-article-type="in-this-issue" xlink:href="10.1126/science.1258375">10.1126/science.1258375</jats:related-article> </jats:p>

<jats:p> Heat transport and ice sublimation in comets are interrelated processes reflecting properties acquired at the time of formation and during subsequent evolution. The Microwave Instrument on the Rosetta Orbiter (MIRO) acquired maps of the subsurface temperature of comet 67P/Churyumov-Gerasimenko, at 1.6 mm and 0.5 mm wavelengths, and spectra of water vapor. The total H <jats:sub>2</jats:sub> O production rate varied from 0.3 kg s <jats:sup>–1</jats:sup> in early June 2014 to 1.2 kg s <jats:sup>–1</jats:sup> in late August and showed periodic variations related to nucleus rotation and shape. Water outgassing was localized to the “neck” region of the comet. Subsurface temperatures showed seasonal and diurnal variations, which indicated that the submillimeter radiation originated at depths comparable to the diurnal thermal skin depth. A low thermal inertia (~10 to 50 J K <jats:sup>–1</jats:sup> m <jats:sup>–2</jats:sup> s <jats:sup>–0.5</jats:sup> ), consistent with a thermally insulating powdered surface, is inferred. </jats:p>

<jats:p> The provenance of water and organic compounds on Earth and other terrestrial planets has been discussed for a long time without reaching a consensus. One of the best means to distinguish between different scenarios is by determining the deuterium-to-hydrogen (D/H) ratios in the reservoirs for comets and Earth’s oceans. Here, we report the direct in situ measurement of the D/H ratio in the Jupiter family comet 67P/Churyumov-Gerasimenko by the ROSINA mass spectrometer aboard the European Space Agency’s Rosetta spacecraft, which is found to be (5.3 ± 0.7) × 10 <jats:sup>−4</jats:sup> —that is, approximately three times the terrestrial value. Previous cometary measurements and our new finding suggest a wide range of D/H ratios in the water within Jupiter family objects and preclude the idea that this reservoir is solely composed of Earth ocean–like water. </jats:p>

<jats:p> Critical measurements for understanding accretion and the dust/gas ratio in the solar nebula, where planets were forming 4.5 billion years ago, are being obtained by the GIADA (Grain Impact Analyser and Dust Accumulator) experiment on the European Space Agency’s Rosetta spacecraft orbiting comet 67P/Churyumov-Gerasimenko. Between 3.6 and 3.4 astronomical units inbound, GIADA and OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System) detected 35 outflowing grains of mass 10 <jats:sup>−10</jats:sup> to 10 <jats:sup>−7</jats:sup> kilograms, and 48 grains of mass 10 <jats:sup>−5</jats:sup> to 10 <jats:sup>−2</jats:sup> kilograms, respectively. Combined with gas data from the MIRO (Microwave Instrument for the Rosetta Orbiter) and ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) instruments, we find a dust/gas mass ratio of 4 ± 2 averaged over the sunlit nucleus surface. A cloud of larger grains also encircles the nucleus in bound orbits from the previous perihelion. The largest orbiting clumps are meter-sized, confirming the dust/gas ratio of 3 inferred at perihelion from models of dust comae and trails. </jats:p>

<jats:p>Images from the OSIRIS scientific imaging system onboard Rosetta show that the nucleus of 67P/Churyumov-Gerasimenko consists of two lobes connected by a short neck. The nucleus has a bulk density less than half that of water. Activity at a distance from the Sun of &gt;3 astronomical units is predominantly from the neck, where jets have been seen consistently. The nucleus rotates about the principal axis of momentum. The surface morphology suggests that the removal of larger volumes of material, possibly via explosive release of subsurface pressure or via creation of overhangs by sublimation, may be a major mass loss process. The shape raises the question of whether the two lobes represent a contact binary formed 4.5 billion years ago, or a single body where a gap has evolved via mass loss.</jats:p>

<jats:p>The Rosetta mission shall accompany comet 67P/Churyumov-Gerasimenko from a heliocentric distance of &gt;3.6 astronomical units through perihelion passage at 1.25 astronomical units, spanning low and maximum activity levels. Initially, the solar wind permeates the thin comet atmosphere formed from sublimation, until the size and plasma pressure of the ionized atmosphere define its boundaries: A magnetosphere is born. Using the Rosetta Plasma Consortium ion composition analyzer, we trace the evolution from the first detection of water ions to when the atmosphere begins repelling the solar wind (~3.3 astronomical units), and we report the spatial structure of this early interaction. The near-comet water population comprises accelerated ions (&lt;800 electron volts), produced upstream of Rosetta, and lower energy locally produced ions; we estimate the fluxes of both ion species and energetic neutral atoms.</jats:p>