The asteroid Vesta, the second largest in our solar system, orbits the Sun between the planets of Mars and Jupiter and is one of the best known. For a long time, asteroids have played an important role in building the popular fascination with space: the first story published by Isaac Asimov, in fact, had Vesta as the protagonist. “When we think of asteroid belts, perhaps we envision Han Solo maneuvering the millennium falcon through a dense set of irregularly shaped gray rocks in space,” reflects Christian Klimczak, a geologist at the University of Georgia, United States. ” While most of the rocks are irregularly shaped and gray, the truth is that they are far apart and NASA’s Dawn spacecraft did not have to maneuver around any other asteroids to reach and explore Vesta .” The aforementioned Dawn mission was a space probe launched by NASA in September 2007 with the mission of studying two of the three known protoplanets of the asteroid belt, Vesta and Ceres.
Vesta was to be a planet
Like Earth, this asteroid contains an iron core and a mantle and crust composed of rock. Due to this composition and its large size, Vesta is considered a planetesimal (the building block from which planets are formed) . The Earth, in fact, is formed by the union of several planetesimals. “Vesta was also on its way to becoming an Earth-like planet, but planet formation stopped at a very early age in the solar system,” Klimczak explains. “So studying Vesta helps us understand the early days of our ‘neighborhood’ and how our own planet was formed.”
The new work, published in the journal Icarus , examines the large-scale impact basins and depressions in Vesta. Vesta is known to have been hit by two other large asteroids that left large impact craters so large that they cover most of its southern hemisphere. Some of these impacts would expel rocky material into space that has reached Earth in the form of meteorites and has allowed us to study its geochemistry.
“The properties of rocks are influenced by the environmental conditions in which they formed, such as the surrounding stresses and the presence of water,” says Jupiter Cheng, co-author of the study. ” Since Vesta is much smaller than Earth, or even the Moon, it has weaker gravity and the rock would warp differently near the surface than we see on Earth .” One of the big questions is the origin of the large channels found on the surface of Vesta, which are concentric around the two basins of mass impact, Rheasilvia and Veneneia, respectively, and are considered to have been formed simultaneously by the events of impact, although this alleged age relationship had never been proven.
“Our work used crater counting methods to explore the relative age of basins and depressions,” explains Cheng. Crater counting is a common method of estimating the age of a planet’s surface, a method based on the assumption that when a part of the planetary surface is new it does not have impact craters; these are subsequently accumulated at a rate that is supposed to be known.
“Consequently, counting the number of craters of various sizes in a given area allows us to determine how long they have accumulated and, consequently, how long ago the surface was formed ,” he said. “Our result shows that the depressions and basins have a similar number of craters of various sizes, indicating that they share a similar age. However, the uncertainties associated with the crater count also leave open the possibility that the depressions have formed long after the impacts.
The origin of depressions
The origin of depressions has long been a point of conjecture within the scientific community. Klimczak hopes that their new geological evidence can promote a more durable theory about the valleys in Vesta and that they hope to publish in a future article.
“The main hypothesis suggests that these depressions are fault-bounded valleys with a distinctive scarp on each side that together mark the fall (slide) of a block of rock. However, rock can also crack and form depressions, an origin that had not been considered before, ”explains Cheng. “Our calculations also show that Vesta’s gravity is not sufficient to induce favorable surrounding stresses for sliding to occur at shallow depth, instead, physics shows that rocks are favored to break,” he says. “Therefore, the formation of these depressions must imply the opening of cracks, which is inconsistent with the main hypothesis of the scientific community. Taken together, the overall project provides alternatives to the previously proposed origin and geological history of Vesta, results that are also important for understanding similar landforms on other small planetary bodies in other parts of the solar system . “
The new theory was presented at the General Assembly of the European Geosciences Union in 2021.
Text: University of Georgia
