Mars might be so weird and mysterious because a dwarf planet crashed into it more than 4.4 billion years ago.

The Martian system has long been a head-scratcher for astronomers. One reason is that its northern hemisphere is full of smooth and flat land while its southern hemisphere is pocked with craters and has a ton of high-elevation land and volcanoes — a contrast that is commonly called the Martian dichotomy. That planet also has two moons that look more like asteroids spinning around it. But a new study in the journal Geophysical Research Letters might help solve these mysteries, and it involves a big collision.

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“Once the terrestrial planets had mostly completed their assembly, bombardment continued by planetesimals left over from accretion,” the study says. It’s referring to masses of material floating around and crashing into the bodies in the inner solar system: Mercury, Venus, Earth and Mars. Accretion is the process by which the planets formed and grew, as material came together and settled into a spherical shape with the help of gravity. During the time of late accretion, when much of that work to form the inner planets was finished, objects were still colliding with them — and Mars gained some weight in metals.

According to the researchers, this metallic acquisition could have come from an enormous impact. They say a mass as huge as almost 750 miles in diameter could have hit Mars 4.43 billion years ago, as its crust was forming.

To put that into perspective, 750 miles is longer than the distance between New York City and Chicago; or between Paris and Rome; or between Athens and Cairo. It’s roughly the mileage across Texas at its longest width. It’s longer than the stretch of water between the southern tip of South America and the northernmost icy tentacle of Antarctica.

The study adds that such a collision, in addition to explaining Mars’ composition that includes the rare metals platinum, osmium and iridium in its mantle, would also explain two other Martian mysteries: the fact that its northern and southern hemispheres have such differing geography and the origin of its two oddly shaped moons, Phobos and Deimos.

mars-sphere The origins of Mars has been a mystery for astronomers because the geology of its northern and southern hemispheres are so different, but astronomers are suggesting that a collision with an enormous asteroid explains everything, including the two oddly shaped Martian moons. Photo: NASA/JPL/USGS

“We showed in this paper — that from dynamics and from geochemistry — that we could explain these three unique features of Mars,” researcher Stephen Mojzsis said in a statement from the University of Colorado at Boulder. “This solution is elegant, in the sense that it solves three interesting and outstanding problems about how Mars came to be.”

The study refers to the collision of the 750-mile-diameter object as an asteroid impact, but compares the asteroid culprit as being comparable to the size of Ceres, which is just shy of 600 miles across and is so large that it has been classified as a dwarf planet.

Ceres lives in the asteroid belt between Mars and Jupiter that marks the boundary between the inner and outer solar system. It is the largest object there and the only dwarf planet in the inner solar system, with a dwarf planet being an object massive enough and with enough gravity to be round but not large enough to clear its path around the sun of asteroids and other objects.

Pluto is officially classified as a dwarf planet, although it is much larger than Ceres, measuring close to 1,500 miles in diameter.

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A Ceres-sized asteroid that may have crashed into Mars as it was forming could have “ripped off a chunk of the northern hemisphere and left behind a legacy of metallic elements in the planet’s interior,” the university explained. “The crash also created a ring of rocky debris around Mars that may have later clumped together to form its moons.”

The scientists calculated the size of the impactor using simulations of collisions, to determine how big it would have to be to give Mars that boost of metals that have been found in its mantle.

“The surprising part is how well it fit into our understanding of the dynamics of planet formation,” Mojzsis said. “Such a large impact event elegantly fits into what we understand from that formative time.”