Unknown class of water-rich asteroid identified

New astronomical measurements in the infrared range have led to the identification of a hitherto unknown class of asteroids. An international research team including geoscientists from Heidelberg University has succeeded in characterizing these small planets using infrared spectroscopy.

They are located in the asteroid belt between Mars and Jupiter and – like the dwarf planet Ceres – are rich in water. According to computer models, complex dynamical processes moved these asteroids from the outer regions of our solar system into today’s asteroid belt shortly after they were created.

With an equatorial diameter of approximately 900 kilometers, the dwarf planet Ceres is the largest object in the asteroid belt between Mars and Jupiter. Many other small planets also orbit this region.

“These are the remains of the building materials from which the planets in our solar system were created four and a half billion years ago. In these small bodies and their fragments, the meteorites, we find many relics that point directly to the process of the planet’s formation,” explains Prof. Dr. Mario Trieloff from the Institute of Earth Sciences at Heidelberg University. The current study shows that the small astronomical bodies originate from all regions of the early solar system.

With the help of small bodies from the outer solar system, water could have reached the still-growing Earth in the form of asteroids, because the building blocks of the planets in the inner solar system tended to be dry, according to Prof. Trieloff, who leads the research group Geo- and Cosmochemistry.

The new infrared spectra were measured by Dr. Driss Takir at the NASA Infrared Telescope Facility at the Mauna Kea Observatory in Hawaii (USA). “The astronomical measurements allow the identification of Ceres-like asteroids with a diameter as small as 100 kilometers, which are currently located in a restricted region between Mars and Jupiter near the orbit of Ceres,” explains Dr. Takir, astrophysicist at NASA Johnson Space Center and lead author of the study.

At the same time, the infrared spectra support conclusions about the body’s chemical and mineralogical composition. Just like Ceres, there are minerals on the surface of the discovered asteroids that originate from an interaction with liquid water.

The small astronomical bodies are quite porous. High porosity is another characteristic shared with the dwarf planet Ceres and an indication that the rock material is still quite original.

“Shortly after the formation of the asteroids, the temperatures were not high enough to transform them into a compact rocky structure; they retained the porous and primitive character typical of the outer ice planets located far from the Sun,” explains Dr. Wladimir Neumann, a member of Prof. Trieloff’s team. He was responsible for the computer modeling of the thermal evolution of the small bodies.

The properties of these Ceres-like objects and their presence in a relatively narrow zone in the outer asteroid belt suggest that these bodies first formed in a cold region at the edge of our solar system. Gravitational disturbances in the orbits of large planets such as Jupiter and Saturn – or “giant planet instability” – changed the orbits of these asteroids so that the objects were “implanted” in today’s asteroid belt. This was demonstrated through numerical calculations carried out by the researchers on the orbital evolution in the early solar system.

The results were published in Natural astronomy.