11 May 2012

The center of attention in the latest issue of Science journal were the six articles devoted to Dawn, the NASA mission that is studying the asteroid Vesta. The Italian instrument VIR-MS has contributed decisively in collecting detailed data on the surface and structure of the asteroid that has enabled the scientific team involved in the mission to draw some initial conclusions. The image reproduced on the left shows the south pole of Vesta, dominated by the two impact craters Veneneia and Rheasilvia.

The data collected in about 10 months of observations provide clear indications on the protoplanetary nature of Vesta, demonstrating that it is actually a celestial body that has undergone a differentiation process similar to the rocky planets, with an iron core formed since the first days of the solar system and has survived the strong and frequent impacts within the asteroid belt. In short, viewed more closely, Vesta looks more like a small planet or our moon than an asteroid.

The probe is completing the final phase of operations around Vesta before heading to another body in the asteroid belt of Ceres. "The NASA team responsible for the Dawn mission, in accordance with the directions of the scientific team, has prolonged the operational period for the observation of Vesta", says Raffaele Mugnuolo, ASI chief for the involvement in the NASA mission.

"It is remarkable - continues Mugnuolo – to emphasize this particular aspect of Dawn: the ion propulsion that equips the probe, offers high flexibility that allows to reformulate the mission underway, which has never been seen in other missions".

In light of the observations made and the scientific results obtained, the team can decide to change the schedule of the mission in order to study certain aspects more in depth. In this case, a short extension in orbit of Vesta is allowing us to "await" the more advantageous positioning of the Sun and thereby to observe the area around the North Pole. Again, this confirms the integrity of this propulsion system, which offers a new way of conceiving future scientific missions.

The image on the right illustrates the mosaic that shows the coverage of data collected by VIR-MS during the observations called HAMO.

An article published in the American journal seems to confirm the thesis for which a particular class of meteorite has originated from asteroids of the main belt and in particular from Vesta. These are HED class meteorites (Howardite–Eucrite–Diogenite), "space rocks" that represent 6 percent of all those that fall to Earth, and thus it seems to confirm that Vesta is a major source of meteorites.

The traces of pyroxene, an iron ore rich in magnesium, contained in these meteorites are in fact the same traces that VIR finds in the rocks on the surface of Vesta.

But this is not all. Scientists today know that Vesta has a very varied and steep topography, not flat as previously thought by Dawn. Some of the craters on Vesta are formed on very steep slopes and have virtually vertical sides, with landslides occurring more frequently than expected.

This very diverse morphology is due to numerous collisions that have affected the surface of the asteroid during the course of its life. For example, Dawn scientists are now able to date the two intense impacts that have occurred in the southern hemisphere of Vesta: the Veneneia basin was probably created at least 2 billion years ago and the Rheasilvia basin was formed about 1 billion years ago.

The geological evolution of Vesta tells us that we are looking more at a planetary behavior, characterized by an evolutionary history also due to endogenous factors, rather than that of an asteroid: for this reason Vesta may be in the range of proto-planets.

"These results are of great interest to add fundamental elements to the overall picture of the history of the solar system’s formation", points out Enrico Flamini, ASI's chief scientist. "In particular – he continues - the period in which the nuclei of materials were aggregating, which would have firstly determined the formation of planetesimals and then, continuing the construction process, the planets as we know them today.

Dawn and our instrument VIR-MS, a visible and infrared spectrometer that testifies to the excellence of the Italian industry in this field, are contributing brilliantly to clarify the role of protoplanets. "I remember - continues Flamini – that the IAU conference that downgraded Pluto, instead upgraded Ceres: now they are both 'dwarf planets'. Vesta was halfway there, but with these observations it could also be upgraded. We'll see what surprises Ceres has in store in a few years".

We therefore have a missing piece in the learning path of the asteroid that will reveal the true identity of Vesta. Dawn and the VIR instrument are assigned this task.