Question

### The Formation of the Asteroid Belt

The asteroid belt, located between Mars and Jupiter, contains millions of rocky and metallic bodies. However, its total mass is only about $0.05\%$ of Earth's mass. Astronomers have proposed competing models to explain the origin and low mass of the asteroid belt.

#### Model 1 (The Grand Tack Model)
Model 1 proposes that the asteroid belt region initially contained a high density of planetesimals, totaling several Earth masses. Shortly after its formation, Jupiter migrated inward from its birth orbit toward the Sun, reaching a distance of $1.5\text{ AU}$ (near Mars's current orbit). As Jupiter migrated, its strong gravitational influence scattered approximately $99.9\%$ of the planetesimals out of the region. Jupiter then migrated back outward to its current orbit. The remaining $0.1\%$ of the original planetesimals, along with some outer solar system material swept up during Jupiter's return, formed the current asteroid belt.

#### Model 2 (The Low-Mass Disk Model)
Model 2 proposes that the region of the asteroid belt was initially almost empty, containing very little mass due to a physical gap in the early protoplanetary dust disk. As the terrestrial planets (such as Earth and Mars) and the giant planets (such as Jupiter and Saturn) grew, their gravitational interactions scattered nearby leftover planetesimals. Some planetesimals from the inner solar system (silicate-rich) and others from the outer solar system (carbon-rich) were thrown into the gap. These scattered bodies became gravitationally trapped in stable orbits, creating the modern asteroid belt. In Model 2, the giant planets did not undergo large-scale orbital migrations through the inner solar system.

According to the models, which of the following statements describes an assumption about the initial mass of the asteroid belt region that is made by Model 1 but NOT by Model 2?