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Linda Spilker

Publications and source records attributed to Linda Spilker.

8 recordsLinked to original sources
arXiv2022

Planetary Exploration Horizon 2061 Report, Chapter 4: From planetary exploration goals to technology requirements

This chapter reviews for each province and destination of the Solar System the representative space missions that will have to be designed and implemented by 2061 to address the six key science questions about the diversity, origins, workings and habitability of planetary systems (described in chapter 1) and to perform the critical observations that have been described in chapters 3 and partly 2. It derives from this set of future representative missions, some of which will have to be flown during the 2041-2061 period, the critical technologies and supporting infrastructures that will be needed to fly these challenging missions, thus laying the foundation for the description of technologies and infrastructures for the future of planetary exploration that is given in chapters 5 and 6, respectively.

astro-ph.IM↗
arXiv2020

Captured Small Solar System Bodies in the Ice Giant Region

This white paper advocates for the inclusion of small, captured Outer Solar system objects, found in the Ice Giant region in the next Decadal Survey. These objects include the Trojans and Irregular satellite populations of Uranus and Neptune. The captured small bodies provide vital clues as to the formation of our Solar system. They have unique dynamical situations, which any model of Solar system formation needs to explain. The major issue is that so few of these objects have been discovered, with very little information known about them. The purpose of this document is to prioritize further discovery and characterization of these objects. This will require the use of NASA and NSF facilities over the 2023 2032 decade, including additional support for analysis. This is in preparation for potential future insitu missions in the following decades.

astro-ph.IM↗
arXiv2020

Ice Giant Atmospheric Science

This white paper, written in support of NASA's 2023-2032 Planetary Decadal Survey, outlines 10 major questions that focus on the origin, evolution, and current processes that shape the atmospheres of Uranus and Neptune. Prioritizing these questions over the next decade will greatly improve our understanding of this unique class of planets, which have remained largely unexplored since the Voyager flybys. Studying the atmospheres of the Ice Giants will greatly inform our understanding of the origin and evolution of the solar system as a whole, in addition to the growing number of exoplanetary systems that contain Neptune-mass planets.

astro-ph.IM↗
arXiv2020

Exploration Strategy for the Outer Planets 2023-2032: Goals and Priorities

The outer solar system has a diverse range of objects, holding important clues about the formation and evolution of our solar system, the emergence and current distribution of life, and the physical processes controlling both our own and exoplanetary systems. This White Paper summarizes the Outer Planets Analysis Group's (OPAG's) priorities in the Decadal Survey. Taking into account the science to be achieved, the timing of solar system events, technological readiness, and programmatic factors, our mission recommendations are as follows. OPAG strongly endorses the completion and launch of the Europa Clipper mission, maintaining the science capabilities identified upon its selection, and a Juno extended mission at Jupiter. For the decade 2023-2032, OPAG endorses a new start for two directed missions: first, a mission to Neptune or Uranus with atmospheric probe(s), and second, a life detection Ocean World mission, along with additional technological development for life detection. A Neptune mission is preferred because, while the Neptune and Uranus systems provide equally compelling opportunities, Triton is a higher priority ocean world target than the Uranian satellites. The mission to Neptune or Uranus should fly first because a delay threatens key science objectives, and additional technological development is required for a directed life detection mission. Along with missions, we emphasize the necessity of maintaining a healthy Research and Analysis (R&A) program as well as a robust Earth-based observing program. OPAG's top two technology priorities are rapid development of a next-generation radioisotope power source for a mission to Neptune or Uranus, and development of key life detection technologies in support of an Ocean World mission. Finally, fostering an interdisciplinary, diverse, equitable, inclusive, and accessible community is of top importance to the OPAG community.

astro-ph.EP↗
arXiv2015

Incomplete cooling down of Saturn's A ring at solar equinox: Implication for seasonal thermal inertia and internal structure of ring particles

At the solar equinox in August 2009, the Composite Infrared Spectrometer (CIRS) onboard Cassini showed the lowest Saturn's ring temperatures ever observed. Detailed radiative transfer models show that the observed equinox temperatures of Saturn's A ring are much higher than model predictions as long as only the flux from Saturn is taken into account. This indicates that the A ring was not completely cooled down at the equinox. We develop a simple seasonal model for ring temperatures and first assume that the internal density and the thermal inertia of a ring particle are uniform with depth. The particle size is estimated to be 1-2 m. The seasonal thermal inertia is found to be 30-50 Jm$^{-2}$K$^{-1}$s$^{-1/2}$ in the middle A ring whereas it is $\sim$ 10 Jm$^{-2}$K$^{-1}$s$^{-1/2}$ or as low as the diurnal thermal inertia in the inner and outermost regions of the A ring. An additional internal structure model, in which a particle has a high density core surrounded by a fluffy regolith mantle, shows that the core radius relative to the particle radius is about 0.9 for the middle A ring and is much less for the inner and outer regions of the A ring. This means that the radial variation of the internal density of ring particles exists across the A ring. Some mechanisms may be confining dense particles in the middle A ring against viscous diffusion. Alternatively, the (middle) A ring might have recently formed ($<$ 10$^{8}$ yr) by destruction of an icy satellite, so that dense particles have not yet diffused over the A ring and regolith mantles of particles have not grown thick. Our model results also indicate that the composition of the core is predominantly water ice, not rock.

astro-ph.EP↗
arXiv2012

A multilayer model for thermal infrared emission of Saturn's rings. III: Thermal inertia inferred from Cassini CIRS

The thermal inertia values of Saturn's main rings (the A, B, and C rings and the Cassini division) are derived by applying our thermal model to azimuthally scanned spectra taken by the Cassini Composite Infrared Spectrometer (CIRS). Model fits show the thermal inertia of ring particles to be 16, 13, 20, and 11 Jm$^{-2}$K$^{-1}$s$^{-1/2}$ for the A, B, and C rings, and the Cassini division, respectively. However, there are systematic deviations between modeled and observed temperatures in Saturn's shadow depending on solar phase angle, and these deviations indicate that the apparent thermal inertia increases with solar phase angle. This dependence is likely to be explained if large slowly spinning particles have lower thermal inertia values than those for small fast spinning particles because the thermal emission of slow rotators is relatively stronger than that of fast rotators at low phase and vise versa. Additional parameter fits, which assume that slow and fast rotators have different thermal inertia values, show the derived thermal inertia values of slow (fast) rotators to be 8 (77), 8 (27), 9 (34), 5 (55) Jm$^{-2}$K$^{-1}$s$^{-1/2}$ for the A, B, and C rings, and the Cassini division, respectively. The values for fast rotators are still much smaller than those for solid ice with no porosity. Thus, fast rotators are likely to have surface regolith layers, but these may not be as fluffy as those for slow rotators, probably because the capability of holding regolith particles is limited for fast rotators due to the strong centrifugal force on surfaces of fast rotators. Other additional parameter fits, in which radii of fast rotators are varied, indicate that particles less than $\sim$ 1 cm should not occupy more than a half of the cross section for the A, B, and C rings.

astro-ph.EP↗
arXiv2012

Regolith grain sizes of Saturn's rings inferred from Cassini-CIRS far-infrared spectra

We analyze far-infrared (10-650 cm$^{-1}$) emissivity spectra of Saturn's main rings obtained by the Cassini Composite Infrared Spectrometer (CIRS). In modeling of the spectra, the single scattering albedos of regolith grains are calculated using the Mie theory, diffraction is removed with the delta-Eddington approximation, and the hemispherical emissivities of macroscopic free-floating ring particles are calculated using the Hapke's isotropic scattering model. Only pure crystalline water ice is considered and the size distribution of regolith grains is estimated. We find that good fits are obtained if the size distribution is broad ranging from 1 $μ$m to 1-10 cm with a power law index of $ \sim 3$. This means that the largest regolith grains are comparable to the smallest free-floating particles in size and that the power law indices for both free-floating particles and regolith grains are similar to each other. The apparent relative abundance of small grains increases with decreasing solar phase angle (or increasing mean temperature). This trend is particularly strong for the C ring and is probably caused by eclipse cooling in Saturn's shadow, which relatively suppresses warming up of grains larger than the thermal skin depth ($\sim$ 1 mm) under subsequent solar illumination.

astro-ph.EP↗