Decoding the past

Decoding the past

Decoding the past

The gap between the orbits of Mars and Jupiter in the Copernican model of the solar system was large enough to get the attention of many great people like Johannes Kepler, Isaac Newton and Immanuel Kant. The Italian astronomer, Giuseppe Piazzi working in Palermo, Sicily discovered a small planet in that gap in 1801. This coincidentally was in the exact position predicted by astronomers Johann Daniel Titius and Johann Elert Bode. Giuseppe named the object ‘Ceres’ after the patron goddess of Sicily.

Later, many objects were discovered moving in that orbit, the most important of them being Vesta (smaller than Ceres). It turns out that a planet, which existed in the beginning of the solar system, broke up completely due to the gravitational tug from the nearby giant planet of Jupiter and the broken objects were called asteroids. Basically, asteroids are the leftovers of the solar system formation. There are roughly 1.5 million of these objects with diameter larger than 1 km and only 5% of them have names.

Flying through space
Since asteroids fly through space uninhibited, they sometimes crash into surrounding planets. Their high-velocity impact creates craters in the planets. Many meteors from Vesta have been found on Earth, which is supposed to be due to the  huge impacts on Vesta itself. Ceres and Vesta have surface areas sightly less than that of India and Pakistan, respectively. Ceres has about 7% of the mass of Pluto and Vesta’s mass is about one-third of Ceres. While Ceres cannot be seen through naked eyes, the very reflective surface of Vesta makes it visible without a telescope.

Ceres and Vesta are at twice and thrice the distance of Mars from Earth respectively. Ceres is the only asteroid that has a shape rounded by the force of its own gravity. Therefore, Ceres was also included as dwarf planet along with Pluto and Eris in 2006.

A study of these asteroids will tell us how our Earth was in the dawn of the solar system. The planets of the solar system took long time to get to their present shape. For instance, the planets Earth and Mars went on accruing mass for 50 and 30 million years respectively.

However, asteroids acquired their present shape quite soon: Ceres and Vesta in 10 and 7 million years respectively. They could not acquire more mass because of the overwhelming presence of Jupiter. With this aim, there have been many specific observations of asteroids in the last few decades. As for Ceres, there were observations by the Hubble telescope in 2004 and by the Herschel IR telescope in 2014.

An aptly-termed space vehicle called Dawn departed Earth in 2007 for the exploration of Ceres and Vesta. This was the first ever vehicle to orbit 2 celestial bodies beyond Earth. It was also the first to orbit an object in the main asteroid belt of the solar system. With a speed of 40,000 km per hour, it passed Mars in 2009 and reached Vesta in July 2011. It surveyed the asteroid from about 200 km from the surface for several months. It left Vesta in September 2012 and took about 3 years to reach Ceres.

After travelling nearly 5 billion km, it started orbiting Ceres from March 2015. In December 2015, it started surveying the planet from a height of 385 km and has provided magnificent views of the celestial body. While the mission could end in 2016, Dawn will remain in orbit forever around Ceres though it won’t be sending any signals. In its lowest-altitude mapping orbit, at a distance of 385 km from Ceres, Dawn is nearer to the planet surface than International Space Station is to Earth!

Sighting water
An important feature of Ceres from the earlier days has been the presence of large bright spots suggesting watery ice on the surface. Dawn has been able to take a very close look at these bright spots from a close distance. The two big ones are the  the Occator and the Haulani craters with breadths of 100 km and 34 km. One has a  dome-like structure in the pit and the other has a central ridge. The intricate geometry of the craters and the colour of the ejected material suggest geological activities and impacts in the recent past.

The spots could be due to briny water erupting from the interior and later sublimating, leaving behind the salt deposits. There is also the possibility that the interiors of the asteroid is supplying fresh salts (containing Magnesium, Ammonia etc) to the region. Even the polygonal shape of the craters can shed light on many things as most of the craters elsewhere are circular in nature.  

There are also further evidences to believe that Ceres has large amount of ice and may even have subsurface liquid water. A few years ago, Herschel IR telescope  showed water vapour coming from 2 dark patches of ground with the estimate that the loss was about 6 kg of water per second. This proves that Ceres has a icy surface and also an atmosphere. Dawn has shown in the past few months that the 10-km-wide Oxo crater , the second-brightest feature on Ceres, harbours water either in the form of ice or hydrated minerals.

It is possible that Ceres has more water than all the fresh water on Earth. However, Ceres’s water, unlike Earth’s, would be in the form of water ice and located deep in the mantle. While the presence of water has been noted in several comets, finding  of water on Ceres shows that impact of asteroids also must have brought water to Earth. For this reason, the idea of colonising Ceres someday has some appeal, compared to distant objects like Europa and Titan.

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