Geodynamics of Mars-sized Terrestrial Planets

The early geodynamic evolution of Mars-type planets

Authors:

Zhang et al

Abstract:

It is not clear whether Mars once possessed active tectonics, yet the question is critical for understanding the thermal evolution of Mars, and the origin and longevity of its early dynamo. To address these issues, we have coupled mantle flow simulations, together with parameterized core evolution models, to simulate the early evolution of Mars-like planets, and constrain the influence of early mobile-lid tectonics on core evolution. We have explored a wide parameter suite, encapsulating a range of uncertainties in initial conditions, rheological parameters, and surface strength. We present successful models that experience early mobile-lid behaviour, with a later transition into a stagnant-lid mode, which reproduce core dynamo histories similar to the magnetic history of early Mars.

SETI Institute Google Plus Hangout Discussing Kepler-138b, c & d.

Kepler-138 System: a Mars-sized Terrestrial World (b), a Earth-sized Terrestrial World (c) and a Earth-sized Mini Neptune (d)

The mass of the Mars-sized exoplanet Kepler-138 b from transit timing

Authors:

Jontof-Hutter et al

Abstract:

Extrasolar planets that pass in front of their host star (transit) cause a temporary decrease in the apparent brightness of the star, providing a direct measure of the planet’s size and orbital period. In some systems with multiple transiting planets, the times of the transits are measurably affected by the gravitational interactions between neighbouring planets1, 2. In favourable cases, the departures from Keplerian orbits (that is, unaffected by gravitational effects) implied by the observed transit times permit the planetary masses to be measured, which is key to determining their bulk densities3. Characterizing rocky planets is particularly difficult, because they are generally smaller and less massive than gaseous planets. Therefore, few exoplanets near the size of Earth have had their masses measured. Here we report the sizes and masses of three planets orbiting Kepler-138, a star much fainter and cooler than the Sun. We determine that the mass of the Mars-sized inner planet, Kepler-138 b, is .066 Earth masses. Its density is grams per cubic centimetre. The middle and outer planets are both slightly larger than Earth. The middle planet’s density ( grams per cubic centimetre) is similar to that of Earth, and the outer planet is less than half as dense at grams per cubic centimetre, implying that it contains a greater portion of low-density components such as water and hydrogen.