PARIS: Scientists on Wednesday said they believed they could explain why Mars is so much smaller than Earth, its companion in the Solar System’s innermost “rocky” planets.

The Red Planet put on a growth spurt, developing in as little as two to four million years after the birth of the Solar System, but then suddenly stopped, they reported.

By comparison, Earth grew to its full size over 50 to 100 million years, probably through agglomeration as it collided with other young space rocks.

“Earth was made of embryos like Mars, but Mars is a stranded planetary embryo that never collided with other small space bodies in the Solar System,” said Nicolas Dauphas of the University of Chicago.

The timescale, published in the British journal Nature, is based on calculations of the time it takes for the radioactive isotope hafnium 182 to decay into tungsten 182.

The source material for this is meteorites that have fallen to Earth and whose chemical signature points to infant Mars as the origin.

Previous estimates of the formation of Mars ranged as high as 15 million years.

Mars is little over half of Earth’s size. Its diameter at the equator is 6,792 kilometers (4,245 miles) compared with Earth’s 12,756 kms (7,972 miles) and Venus’ 12,104 kms (7,565 miles), according to NASA’s website.