4 Vesta - Geology

Geology

There is a large collection of potential samples from Vesta accessible to scientists, in the form of over 200 HED meteorites, giving insight into Vesta's geologic history and structure. NASA Infrared Telescope Facility (NASA IRTF) studies of asteroid (237442) 1999 TA₁₀ suggest that it originated from the interior of Vesta.

Vesta is thought to consist of a metallic iron–nickel core 214–226 km in diameter, an overlying rocky olivine mantle, with a surface crust. From the first appearance of Ca-Al-rich inclusions (the first solid matter in the Solar System, forming about 4,567 million years ago), a likely time line is as follows:

Timeline of the evolution of Vesta

2–3 million years	Accretion completed
4–5 million years	Complete or almost complete melting due to radioactive decay of 26Al, leading to separation of the metal core
6–7 million years	Progressive crystallization of a convecting molten mantle. Convection stopped when about 80% of the material had crystallized
Extrusion of the remaining molten material to form the crust, either as basaltic lavas in progressive eruptions, or possibly forming a short-lived magma ocean.
The deeper layers of the crust crystallize to form plutonic rocks, while older basalts are metamorphosed due to the pressure of newer surface layers.
Slow cooling of the interior

Vesta is the only known intact asteroid that has been resurfaced in this manner. Because of this, some scientists refer to Vesta as a protoplanet, rather than an asteroid. However, the presence of iron meteorites and achondritic meteorite classes without identified parent bodies indicates that there once were other differentiated planetesimals with igneous histories, which have since been shattered by impacts.

Composition of the Vestan crust (by depth)

A lithified regolith, the source of howardites and brecciated eucrites.

Basaltic lava flows, a source of non-cumulate eucrites.

Plutonic rocks consisting of pyroxene, pigeonite and plagioclase, the source of cumulate eucrites.

Plutonic rocks rich in orthopyroxene with large grain sizes, the source of diogenites.

On the basis of the sizes of V-type asteroids (thought to be pieces of Vesta's crust ejected during large impacts), and the depth of the south polar crater (see below), the crust is thought to be roughly 10 kilometres (6 mi) thick.