Pallas is not quite a sphere, and has a huge crater

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This was quite interesting.

Pallas is 'Peter Pan' space rock

By Jonathan Amos
Science reporter, BBC News

2 Pallas is not quite a sphere

The Hubble telescope has provided new insight on 2 Pallas, one of the largest asteroids in the Solar System.
The nearly 600km-wide rock is an example of an object that started out on the process of becoming a planet but never grew up into the real thing.

Researchers have published a 3D model of the grapefruit-shaped mini-world in Science magazine.
Hubble's data makes it possible to discern surface features, including what appears to be a big impact crater.
The new information is expected to help scientists better understand how planets evolve in their earliest phases.
"Pallas is a unique piece of the puzzle of how our Solar System formed," said Britney Schmidt, of the University of California, Los Angeles, who led the observations.

Layering process

Pallas resides some 400 million km from the Sun, in between the orbits of Mars and Jupiter.
It was the second object discovered in the main asteroid belt (hence the name, 2 Pallas), in 1802.
Only Ceres is wider (950km diameter). Another asteroid, Vesta, has a narrower girth but is more massive than Pallas.


Found by Heinrich Olbers in 1802
Initially thought to be a true planet
Now classed as a B-type asteroid
May have a major impact crater

All three can be considered "protoplanets", an assessment reinforced in the case of Pallas by the latest Hubble observations.

Theory holds that planets grow from aggregations of the dust and rock found circling new-born stars.
Collisions between clumps of material produce progressively bigger objects.
Eventually, a few will become large enough, and hot enough, to start to undergo differentiation - a process of layering in which the densest materials move to the centre of the object to form a core.
In the case of Pallas, this process appears to have initiated, but its slightly irregular shape suggests it never quite moved to completion.

"Whether you could form a core is something we can't really determine with these observations, but Pallas is big enough and round enough that it's very possible that its interior started to separate out," Ms Schmidt told BBC News.
"Ceres is perfectly round and so there's a really good chance that that happened. For Pallas, it may be just that this process got started but never finished."

Dark features in the Hubble pictures indicate the presence of hydrated minerals on the surface of the asteroid.
If Pallas formed early in the asteroid belt, it would probably have incorporated large quantities of water-ice.
As the rock heated up, this water would have melted. Not only would this have aided differentiation but it also would have altered the silicate rock to produce the type of mineralogical signal obvious today.

It was highly unlikely, though, that Pallas got hot enough to melt silicate rock, said the California researcher.
The presence even now of a lot of water-ice in the asteroid might help to explain its relatively low density (2,400-2,800kg per cubic metre), she added.

The Science paper describing the Hubble observations includes a 3D representation of Pallas. This was built up from a series of snapshots of the asteroid's outline as it rotated in the view of the telescope.

It reveals an intriguing depression in the southern hemisphere which Schmidt's team interprets as a possible impact crater. It is large - about 240km across. It is also near dark terrain which could be material ejected or altered by a collision.

Schmidt and colleagues suspect the presence of an impact crater (ringed)

Pallas is known to share orbital characteristics with a group of rocks that could have been blown off the asteroid. The largest of this "family" is called Ioffe and has a diameter of 22km. It is entirely possible Ioffe originated in the assumed crater.

Nasa has sent a spacecraft to the asteroid belt to visit Ceres and Vesta. The Dawn probe will arrive first at the smaller of the two objects in 2011.

Pallas, unfortunately, is not on the itinerary.
But Britney Schmidt believes interest in the asteroids and what they represent can only grow and is hopeful that her protoplanet could one day become a target for a space mission.
"We are really changing our perspective on these objects. When you say asteroid people don't tend to think of big, dynamic, evolved bodies; but that's probably what we have in the case of [Ceres, Vesta and Pallas].
"I'm trying to evolve people's thinking, taking them from 'big rocks to little planets'.
"It's also timely, with public discussion about Pluto and what makes - or doesn't make - a planet. The public are so interested in that."

Ceres and Vesta will be visited by the Dawn mission in the next few years


Recent HST Observations of 2 Pallas

The Dawn team was granted time near Pallas' opposition in September 2007 at a distance of ~ 2.25 AU. I'm sure they would also been given time during the much closer opposition in Dec 2008 (~ 1.56 AU), but Hubbles brain failure ruined that opportunity, so these observations remain the best view of the 2nd largest and 3rd most massive asteroid in the main belt. The observations were made through 5 filters.

Ceres is ~ 950 km, Vesta ~ 530 km in average diameter.

It led to a refined size for Pallas of diameter of 582 x 556 x 500 (all +/- 18) km (550 average), comparing favorably with the best occultation measurements of 574 x 526 x 500 (+/- 20) km made in in 1990.
The refined density is 2400 +/- 250 kg/m^3 midway between Ceres' 2077 and Vesta's 3480.

A probable crater 250 +/- 25 km crater was detected, much darker in UV than the remainder of the surface. In addition, using the new size and density measurements, hydrostatic equilibrium would result in a body of ~ 586 x 503 km at it's current rotation speed of 7.1 hours. It therefore appears that Pallas is differentiated, and an evolved body, like Ceres and Vesta, with planet-like properties. The Dawn mission will of course orbit Vesta and Ceres for extended periods...perhaps afterward a pass might be made near Pallas, but due to it's large inclination (34 degrees) and eccentricity, orbiting it would be impossible.

Most of this info is from an article in the Oct 9, 2009 Science by Schmidt, et al.

Edit, missed that silylene had already started a thread, so I merged my post in.
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