Why is Mars so small?

Ever wonder why Mars is ten times smaller than the Earth?

As you can see, Mars is noticeably smaller than both Earth and Venus

There are many theories, and one theory says that Mars never really "grew up". After initially evolving from dust particles to planetesimals to an early form of the planet, Mars stopped growing while its solar system siblings kept on developing. This is stage is called coined the term planetary embryo, and so while Earth kept on developing to reach "adulthood" 50 million years later, Mars stopped developing after 2-4 millions years after the solar system was formed. 

Representation of the solar system's nebula from which planets were created

Researchers hypothesized this idea when they obtained data in another interesting way. Nicolas Dauphas from the University of Chicago and Ali Pourmand from the University of Miami looked at isotope ratios in meteorites with similar compositions as the Martian mantle. This was done by inductively coupled plasma  mass spectrometry, or ICPMS, which is a power technique which allows the multielemental ultratrace analysis of a variety of samples. The researchers measured decay of hafnium-182 and tungsten-182. When planetismals collide while forming a planet, the metals in the mass of the two colliding objects tend to move towards the inside when they combine together. Therefore, the presence of the tungsten-182 in the Martian mantle millions of years later shows that the planet did not develop much after that. Also through the amount of decay of the isotopes the age of the samples could be calculated. The researchers are calling the state that Mars is in and "embryo" phase, since the planet never fully developed. We can only wonder if, had Mars gotten past the embryo stage, the humans on Earth would be able to have planetary neighbors that existed in all Scifi novels.