Gan De: Science Objectives and Mission Scenarios For China’s Mission to the Jupiter System

<p>To answer key scientific questions about Planetary Systems, it is particularly fruitful to study the Jupiter System, the most complex “secondary” planetary system in the solar system, using the power of in situ exploration. Two key questions should be addressed by future missions:</p><p>A-How did the Jupiter System form? Answers can be found in the most primitive objects of the system: Callisto seems to have been only partly differentiated; its bulk composition, interior and surface terrains keep records of its early eons; the 77 or so irregular satellites, wandering far out beyond the region occupied by the Galilean satellites, are unique and precious remnants of the populations of planetesimals which orbited the outer Solar System at the time of Jupiter’s formation.</p><p>B-How does it work? One can address this question by studying and understanding the chain of energy transfer operating today in the Jupiter System: how is gravitational energy from Jupiter transferred to Io’s interior via tidal heat dissipation to power its volcanic activity? How does this activity in turn store energy into the Io plasma torus to drive the whole magnetosphere into motion? How does the interplay between the Io torus and the solar wind dump energy into heating of Jupiter’s upper atmosphere, or release it into the tail and interplanetary space?</p><p>Starting from the measurement requirements derived from these two objectives, we propose two ambitious mission scenarios, named JCO and JSO, to meet these requirements. Both use the combination of a main spacecraft and one or several specialized small platforms.</p><p>JCO, the Jupiter Callisto Orbiter, first flies by and characterizes several irregular satellites during its Jovian orbital tour. It is then injected into Callisto orbit to characterize its surface and interior, investigate its degree of differentiation and search for the possible existence of an internal ocean.  As an option, JCO could release a lander to Callisto’s surface to perform key measurements of chemical composition, clues to understanding the formation scenario of the Galilean moons.</p><p>JSO, the Jupiter System Observer, performs several fly-bys of Io and visits several irregular satellites during its Jovian orbital tour. As an option, JSO could release one or several small satellites to perform multi-point studies of the dynamics of the Jovian magnetosphere. At the end of its tour it could be injected into a halo orbit around the L1 Lagrangian point of the Sun-Jupiter system to monitor the solar wind upstream of the Jovian magnetosphere, measure Jovian seismic oscillations, and perform a comprehensive survey of the irregular satellites.</p><p>Led by China under the name of GAN De, the first astronomer to have claimed an observation of a moon of Jupiter four centuries BC, and broadly open to international collaboration, a mission flying to Jupiter in the 2030’s according to either one of these scenarios will be able to capitalize on the legacy of previous missions to Jupiter (Juno, JUICE, Europa Clipper) and to trigger a very exciting international collaboration to unravel the mysteries of the origins and workings of the Jupiter system.</p>

Direct imaging of irregular satellite discs in scattered light

ABSTRACT Direct imaging surveys have found that long-period super-Jupiters are rare. By contrast, recent modelling of the widespread gaps in protoplanetary discs revealed by Atacama Large Millimetre Array suggests an abundant population of smaller Neptune to Jupiter-mass planets at large separations. The thermal emission from such lower-mass planets is negligible at optical and near-infrared wavelengths, leaving only their weak signals in reflected light. Planets do not scatter enough light at these large orbital distances, but there is a natural way to enhance their reflecting area. Each of the four giant planets in our Solar system hosts swarms of dozens of irregular satellites, gravitationally captured planetesimals that fill their host planets’ spheres of gravitational influence. What we see of them today are the leftovers of an intense collisional evolution. At early times, they would have generated bright circumplanetary debris discs. We investigate the properties and detectability of such irregular satellite discs (ISDs) following models for their collisional evolution from Kennedy & Wyatt (2011). We find that the scattered light signals from such ISDs would peak in the 10–100 au semimajor axis range implied by ALMA, and can render planets detectable over a wide range of parameters with upcoming high-contrast instrumentation. We argue that future instruments with wide fields of view could simultaneously characterize the atmospheres of known close-in planets, and reveal the population of long-period Neptune–Jupiter mass exoplanets inaccessible to other detection methods. This provides a complementary and compelling science case that would elucidate the early lives of planetary systems.

Accretion Processes

In planetary science, accretion is the process in which solids agglomerate to form larger and larger objects, and eventually planets are produced. The initial conditions are a disc of gas and microscopic solid particles, with a total mass of about 1% of the gas mass. These discs are routinely detected around young stars and are now imaged with the new generation of instruments. Accretion has to be effective and fast. Effective, because the original total mass in solids in the solar protoplanetary disk was probably of the order of ~300 Earth masses, and the mass incorporated into the planets is ~100 Earth masses. Fast, because the cores of the giant planets had to grow to tens of Earth masses to capture massive doses of hydrogen and helium from the disc before the dispersal of the latter, in a few millions of years. The surveys for extrasolar planets have shown that most stars have planets around them. Accretion is therefore not an oddity of the solar system. However, the final planetary systems are very different from each other, and typically very different from the solar system. Observations have shown that more than 50% of the stars have planets that don’t have analogues in the solar system. Therefore the solar system is not the typical specimen. Models of planet accretion have to explain not only how planets form, but also why the outcomes of the accretion history can be so diverse. There is probably not one accretion process but several, depending on the scale at which accretion operates. A first process is the sticking of microscopic dust into larger grains and pebbles. A second process is the formation of an intermediate class of objects called planetesimals. There are still planetesimals left in the solar system. They are the asteroids orbiting between the orbits of Mars and Jupiter, the trans-Neptunian objects in the distant system, and other objects trapped along the orbits of the planets (Trojans) or around the giant planets themselves (irregular satellites). The Oort cloud, source of the long period comets, is also made of planetesimals ejected from the region of formation of the giant planets. A third accretion process has to lead from planetesimals to planets. Actually, several processes can be involved in this step, from collisional coagulation among planetesimals to the accretion of small particles under the effect of gas drag, to giant impacts between protoplanets. Adopting a historical perspective of all these processes provides details of the classic processes investigated in the past decades to those unveiled in the last years. The quest for planet formation is ongoing. Open issues remain, and exciting future developments are expected.