The Hubble telescope has provided new insight on 2 Pallas, one of the largest asteroids² in the Solar System.

哈勃望远镜提供了对2智神星——太阳系中最大的小行星之一——的最新观察资料。

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

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（辨别，看清楚） surface features, including what appears to be a big impact crater³（撞击坑）.

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（居留，属于） some 400 million km from the Sun, in between the orbits of Mars and Jupiter.

It was the second object discovered in the main asteroid¹ belt (hence the name, 2 Pallas), in 1802.

Only Ceres is wider (950km diameter). Another asteroid, Vesta, has a narrower girth（周围长度） but is more massive than Pallas.

All three can be considered "protoplanets（原行星）", an assessment⁴（估价，评估） reinforced（加强） in the case of Pallas by the latest Hubble observations.

Theory holds that planets grow from aggregations⁵ of the dust and rock found circling new-born stars.

Collisions between clumps⁶ of material produce progressively bigger objects.

Eventually, a few will become large enough, and hot enough, to start to undergo differentiation⁷ - a process of layering in which the densest⁸（密集的，浓厚的） materials move to the centre of the object to form a core.

In the case of Pallas, this process appears to have initiated⁹, 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 perfectly¹⁰ 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."

Wet history

Dark features in the Hubble pictures indicate the presence of hydrated（含水的） minerals on the surface of the asteroid（小行星）.

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 silicate¹¹ rock（硅酸盐岩石） 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 relatively¹² low density¹³ (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 intriguing¹⁴（吸引人的，有趣的） depression in the southern hemisphere which Schmidt's team interprets as（把……理解为） a possible impact crater. It is large - about 240km across. It is also near dark terrain¹⁵（地带，地域） which could be material ejected or altered by a collision.

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 entirely¹⁶ 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（旅程，游记）.

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（动态的，有力的）, 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 Pluto¹⁷ and what makes - or doesn't make - a planet. The public are so interested in that."