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2. CHARACTERIZING THE FACTORS THAT CONTROL THE HABITABILITY OF PLANETS AND PLANETARY SATELLITES

2.1 Overview

	Figure 2.1. From top to bottom: Mercury, Venus, Earth (and its Moon), Mars, Jupiter, Saturn, Uranus and Neptune. Image courtesy of NASA/JPL/Caltech.

Here we describe research plans in those areas that pertain to the habitability of planets, including Earth. The five areas of research concerning habitability at UCLA include assessments of the habitability of icy bodies within our Solar System, the potential links between planet-scale dynamics and long-term habitability, the influences of orbital and rotational dynamics on habitability in extrasolar planetary systems, the role of impacts on survival and evolution of life, and the volatile history of Mars.

The most direct means for investigating the habitability of planets and their satellites is through the study of bodies in the Solar System. Lead-team members Schubert, Moore, Veradi, and Nimmo will Examine the potential for sustaining life within the Galilean moons of Jupiter. These bodies may owe their potential for habitability to the immense tidal forces they experience by virtue of their gravitaitional interactions with Jupiter and one another. These forces produce heat that may be capable of sustaining liquid water beneath their surfaces. The mere prospect of the existence of these subsurface oceans demonstrates that an Earth-like planet in circumstellar orbit is but one of a large number of possible hosts for life in the Galaxy and underscores the need to study non-Earth-like systems for gauging their habitability. Schubert, Nimmo, and Moore will study the implications of tidal interactions for oceans within the icy satellites Callisto, Europa, and Gaynmede. They will also refine estimates of the thicknesses of the icy crusts of these bodies.

Lead team members Paige, Newman, Lyons, Young and Varadi will study the volatile inventory of Mars and the Martian climate. Emphasis will be placed on linking Martian climate to rotational and orbital dynamics of the planet, interpreting the stable isotope data for Mars, and on shaping future missions to Mars.

The issue of long-term habitability of planets is being addressed through numerical simulations of planetary-scale dynamics by lead team members Tackley and Aurnou. Earth has been habitable for billions of years while Mars may have been so for a much shorter interval. Based on comparisons between Earth, Mars, and Venus, it seems likely that the long-term habitability in general may depend on the dynamics of the host planet. Plate tectonics may be necessary to sustain an appropriate level of carbon in the atmosphere, for example, which in turn influences surface temperatures. The long-term stability of an atmosphere suitable for life may depend on the presence of a magnetic field of sufficient intensity. Through a program of numerical simulations, Tackley and Aurnou envision establishing criteria that will afford predictions about the likelihood for plate tectonics and magnetic fields based on size and perhaps ages of extrasolar planets. The result of this research has been likened to a rocky planet analogue to the Hertzsprung-Russell diagram (HR diagram) that summarizes the evolution of stars. The HR diagram is used to summarize what is known about stellar evolution. The rocky planet version of the HR diagram would serve an analogous role and would be a useful tool for gauging the potential habitability of extrasolar rocky planets.

The impact of asteroids, comets, and other objects must play a fundamental role in the origin, evolution, and extinction of life. Impacts are a primary mechanism of planetary accretion and are responsible for the delivery of water and organic matter to young planets. Large-body impacts may inhibit the formation of life in the early history of planetary formation. Once life has taken hold, impacts can play an important role in the path followed by evolution, such as the mass extinctions that are now known to be coincident with the Chicxulub impact event at the Cretaceous-Tertiary (K-T) boundary. This is not just a terrestrial problem. If life exists on Mars, Europa, or other planets outside our solar system, impacts must have played a fundamental role there as well. Impacts may even play a role in transporting organisms between planetary objects.

Orbital and rotational dynamics is a central theme in several on-going studies by UCLA lead-team members. The factors that cause changes in the frequency of asteroid impacts in the inner Solar System, coupling between Mars’ orbital and rotational dynamics and climate, and the influences of giant planet orbital eccentricities on prospects for life in extrasolar planet systems are all being investigated by lead team member Varadi and colleagues using computer codes developed at UCLA. These specialized codes accurately reconstructs the orbital and rotational history of planets and asteroids for up to 100 million years. The physical model is successively refined to take into account small corrections in the equations of motion due to General Relativity, the finite size of the lunar orbit, and so forth. Varadi and Runnegar are using this code to investigate the possibility that impacts that caused mass extinctions might be the result of inner Solar System orbital chaos resulting in asteroid impacts. Albeit controversial, it might be said that because the dynamics of the inner Solar System depend on subtle interactions involving relativistic effects, the extent to which the geological record of extinctions on Earth can be explained by changes in planet orbits serves as a test of General Relativity.

The details of these habitability-related research plans are described below.