Engineering topics

Tonks, W. B.

Publications and source records attributed to Tonks, W. B..

5 records

NASA NTRS1993

Magma ocean formation due to giant impacts

The thermal effects of giant impacts are studied by estimating the melt volume generated by the initial shock wave and corresponding magma ocean depths. Additionally, the effects of the planet's initial temperature on the generated melt volume are examined. The shock pressure required to completely melt the material is determined using the Hugoniot curve plotted in pressure-entropy space. Once the melting pressure is known, an impact melting model is used to estimate the radial distance melting occurred from the impact site. The melt region's geometry then determines the associated melt volume. The model is also used to estimate the partial melt volume. Magma ocean depths resulting from both excavated and retained melt are calculated, and the melt fraction not excavated during the formation of the crater is estimated. The fraction of a planet melted by the initial shock wave is also estimated using the model.

Tonks, W. B.↗

NASA NTRS1993

Core formation by giant impacts: Conditions for intact melt region formation

Among the many effects of high-speed, giant impacts is widescale melting that can potentially trigger catastrophic core formation. If the projectile is sufficiently large, the melt pools to form an intact melt region. The dense phase then segregates from the melt, forming a density anomoly at the melt region's base. If the anomoly produces a differential stress larger than a certain minimum, it overcomes the mantle's long-term elastic strength and rapidly forms a core. It was previously shown that giant impacts effectively trigger core formation in silicate bodies by the time they grow to the mass of Mercury and in icy bodies by the time they grow larger than Triton. In order for this process to be viable, an intact melt region must be formed. Conditions under which this occurs is examined in more detail than previously published.

Tonks, W. B.↗

NASA NTRS1992

Core formation by giant impacts

The present model for the timing and mechanisms of planetary core formation argues that once a planet reaches a certain minimum mass, the large impacts that are typical of late accretion can trigger core formation. This model circumvents the difficulties posed by the large-scale segregation of molten iron into diapirs, and the displacement of the cold, elastic interior of the planet by the iron. The analytical melting model used is based on the Hugoniot equations, the empirical relationship for the decline of particle velocity with distance, and the linear shock-particle velocity relationship.

Tonks, W. B.↗

NASA NTRS1992

Magma ocean formation due to giant impacts

The effect of giant impacts on the initial chemical and thermal states of the terrestrial planets is just now being explored. A large high speed impact creates an approximately hemispherical melt region with a radius that depends on the projectile's radius and impact speed. It is shown that giant impacts on large planets can create large, intact melt regions containing melt volumes up to a few times the volume of the projectile. These large melt regions are not created on asteroid sized bodies. If extruded to the surface, these regions contain enough melt to create a magma ocean of considerable depth, depending on the impact speed, projectile radius, and gravity of the target planet.

Tonks, W. B.↗

NASA NTRS1991

Core formation by giant impacts

Ideas about the accretion and early evolution of the Earth and the other terrestrial planets have recently undergone a number of revolutionary changes. It has become clear that giant impacts were far from rare events. In the later stages of accretion any given planetary embryo is liable to be struck several times by other bodies of up to half its own diameter. Such an impact may have the ability to trigger core formation. Traditional accretion models have had great difficulty explaining the formation of the core. If one admits the importance of infrequent large events that may melt an entire hemisphere, the core formation difficulty vanishes. Millimeter-size iron blebs in the melted region will rain out due to their density difference with the silicate melt. Core formation may not require the melting of the entire hemisphere of the planet. The conditions are explored under which impact induced core formation may occur.

Tonks, W. B.↗