arXiv subjects

Noah P. Hammond

Publications and source records attributed to Noah P. Hammond.

5 recordsLinked to original sources

arXiv2018

Compaction and Melt Transport in Ammonia-Rich Ice Shells: Implications for the Evolution of Triton

Ammonia, if present in the ice shells of icy satellites, could lower the temperature for the onset of melting to 176 K and create a large temperature range where partial melt is thermally stable. The evolution of regions of ammonia-rich partial melt could strongly influence the geological and thermal evolution of icy bodies. For melt to be extracted from partially molten regions, the surrounding solid matrix must deform and compact. Whether ammonia-rich melts sink to the subsurface ocean or become frozen into the ice shell depends on the compaction rate and thermal evolution. Here we construct a model for the compaction and thermal evolution of a partially molten, ammonia-rich ice shell in a one-dimensional geometry. We model the thickening of an initially thin ice shell above an ocean with $10\%$ ammonia. We find that ammonia-rich melts can freeze into the upper $5$ to $10$ kilometers of the ice shell, when ice shell thickening is rapid compared to the compaction rate. The trapping of near-surface volatiles suggests that, upon reheating of the ice shell, eutectic melting events are possible. However, as the ice shell thickening rate decreases, ammonia-rich melt is efficiently excluded from the ice shell and the bulk of the ice shell is pure water ice. We apply our results to the thermal evolution of Neptune's moon Triton. As Triton's ice shell thickens, the gradual increase of ammonia concentration in Triton's subsurface ocean helps to prevent freezing and increases the predicted final ocean thickness by up to $50$ km.

astro-ph.EP↗

arXiv2018

Experimental Constraints On The Fatigue of Icy Satellite Lithospheres by Tidal Forces

Fatigue can cause materials that undergo cyclic loading to experience brittle failure at much lower stresses than under monotonic loading. We propose that the lithospheres of icy satellites could become fatigued and thus weakened by cyclical tidal stresses. To test this hypothesis, we performed a series of laboratory experiments to measure the fatigue of water ice at temperatures of $198$ K and $233$ K and at a loading frequency of $1$ Hz. We find that ice is \textit{not} susceptible to fatigue at our experimental conditions and that the brittle failure stress does not decrease with increasing number of loading cycles. Even though fatigue was not observed at our experimental conditions, colder temperatures, lower loading frequencies, and impurities in the ice shells of icy satellites may increase the likelihood of fatigue crack growth. We also explore other mechanisms that may explain the weak behavior of the lithospheres of some icy satellites.

astro-ph.EP↗

arXiv2016

Recent Tectonic Activity on Pluto Driven by Phase Changes in the Ice Shell

The New Horizons spacecraft has found evidence for geologic activity on the surface of Pluto, including extensional tectonic deformation of its water ice bedrock (see Moore et al., 2016). One mechanism that could drive extensional tectonic activity is global surface expansion due to the partial freezing of an ocean. We use updated physical properties for Pluto and simulate its thermal evolution to understand the survival of a possible subsurface ocean. For thermal conductivities of rock less than 3 W m$^{-1}$ K$^{-1}$, an ocean forms and at least partially freezes, leading to recent extensional stresses in the ice shell. In scenarios where the ocean freezes and the ice shell is thicker than $260$ km, ice II forms and causes global volume contraction. Since there is no evidence for recent compressional tectonic features, we argue that ice II has not formed and that Pluto's ocean has likely survived to present day.

astro-ph.EP↗

arXiv2015

A Common Origin for Ridge-and-Trough Terrain on Icy Satellites by Sluggish Lid Convection

Ridge-and-trough terrain is a common landform on outer Solar System icy satellites. Examples include Ganymede's grooved terrain, Europa's gray bands, Miranda's coronae, and several terrains on Enceladus. The conditions associated with the formation of each of these terrains are similar: heat flows of order tens to a hundred milliwatts per meter squared, and deformation rates of order $10^{-16}$ to $10^{-12}$ s$^{-1}$. Our prior work shows that the conditions associated with the formation of these terrains on Ganymede and the south pole of Enceladus are consistent with vigorous solid-state ice convection in a shell with a weak surface. We show that sluggish lid convection, an intermediate regime between the isoviscous and stagnant lid regimes, can create the heat flow and deformation rates appropriate for ridge and trough formation on a number of satellites, regardless of the ice shell thickness. For convection to deform their surfaces, the ice shells must have yield stresses similar in magnitude to the daily tidal stresses. Tidal and convective stresses deform the surface, and the spatial pattern of tidal cracking controls the locations of ridge-and-trough terrain.

astro-ph.EP↗

arXiv2014

Topographic Constraints on the Origin of the Equatorial Ridge on Iapetus

Saturn's moon Iapetus has an equatorial ridge system, which may be as high as 20 km, that may have formed by endogenic forces, such as tectonic and convective forces, or exogenic processes such as debris infall. We use high-resolution topographic data to conduct a topographic analysis of the ridge, which suggests a predominantly triangular morphology, with some ridge face slopes reaching 40 degrees, allowing for an exogenic formation mechanism.

astro-ph.EP↗