Do these 3 things before closing this tab:
1Clear out junk files and repair common Windows errors2Scan for outdated or missing drivers - takes under a minute3Repair Windows errors before they cause bigger problemsMars is about half Earth’s radius but only 10.7% of its mass. The leading explanation is that Mars grew from a much smaller supply of solid material: the early Solar System’s distribution of planet-building matter left the region that produced Mars relatively depleted. Giant-planet migration, including the proposed Grand Tack scenario, could have helped create that pattern, but Mars’s exact formation history remains unsettled.
How much smaller is Mars than Earth?
“Smaller” can refer to radius, volume, or mass, and those comparisons give different answers. NASA Goddard Space Flight Center’s National Space Science Data Center lists Mars’s volumetric mean radius as 3,389.5 km, compared with Earth’s 6,371.0 km. Mars is therefore about 53.2% of Earth’s radius. Its mass, however, is 0.107 Earth masses—about one-tenth, not one-half.
| Measurement | Mars | Earth | Mars compared with Earth |
|---|---|---|---|
| Volumetric mean radius | 3,389.5 km | 6,371.0 km | 53.2% |
| Mass | 0.64169 × 1024 kg | 5.9722 × 1024 kg | 0.107 Earth masses |
These are values in NASA NSSDC’s Mars Fact Sheet, accessed in 2026. The radius comparison is a useful visual shorthand; it does not mean Mars contains half as much material as Earth.
How do planets grow, and why does the material supply matter?
Planets form through accretion: gravity gathers dust and other solid material into progressively larger bodies. NASA’s Mars overview describes Mars as forming about 4.5 billion years ago as gravity pulled together swirling gas and dust. That explains the broad process, but not why the material assembled into a relatively low-mass Mars rather than a planet closer in mass to Earth.
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The key question is how much solid matter was available across the early Solar System, and how that matter was distributed. If the zone that supplied Mars contained much less planet-building material than the zone where Earth grew, Mars could end up far less massive even though their radii are not as dramatically different.
Could Jupiter’s migration have left Mars with less material?
The Grand Tack is one proposed way to rearrange the early Solar System’s solid material. In the scenario summarized by NASA Astrobiology, Jupiter migrated inward and then outward. Its movement could have shifted a concentration of material sunward, contributing to the material from which Earth and Venus grew, while reducing the supply near Mars.
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This model does not require every Mars-like planet to have formed at its present distance from the Sun. In a small percentage of the simulations discussed in the NASA account, Mars formed farther out and was later moved inward. Most of those simulations instead suggested a Mars that formed near Earth, so a more distant birthplace is a possibility in the model, not an established history.
What do the asteroid belt and Mars’s composition tell us?
Matching Mars and the asteroid belt is difficult
A 2015 NASA Technical Reports Server study by Andre Izidoro, Sean N. Raymond, Alessandro Morbidelli, and Othon C. Winter found that reproducing the Earth-to-Mars mass ratio in simulations required strong depletion of solid material between 1 and 3 astronomical units (AU). But reproducing Mars’s low mass alone was not enough: the same models also had to account for the asteroid belt’s dynamical structure.
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In the study, shallower disk-density gradients could reproduce the asteroid belt’s dynamical excitation, but they produced Mars analogues that were too massive. A very steep gradient could produce the Earth–Mars mass ratio, but left the simulated asteroid belt too dynamically cold. The authors concluded that none of the disk profiles they tested explained both the terrestrial planets and the asteroid belt, and pointed to an additional process, such as Grand Tack-style giant-planet migration.
Composition adds a separate constraint
Mars and Earth have different elemental and isotopic signatures. NASA Astrobiology’s account explains that this challenges the simple assumption that the two planets formed from identical nearby material. A model must therefore do more than produce the right planetary masses: it must also be consistent with the compositional evidence. The differences constrain possible histories, but do not by themselves identify one definitive birthplace for Mars.
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Meteorites preserve clues to early growth
Shergottite meteorites preserve evidence that researchers use to study Mars’s early differentiation and accretion. A 2018 NASA Astrobiology summary of one study said the meteorite evidence indicated Mars may have accreted before Earth. That is a qualified inference from a particular study, not a settled chronology for the full formation of either planet.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Is Mars’s small size explained by a settled theory?
No single formation pathway is established by the evidence described here. The models show how a depleted supply of solids—and potentially giant-planet migration—could produce a low-mass Mars, while the asteroid belt and compositional signatures impose additional tests. The difficulty of satisfying those constraints together is why Mars’s precise birthplace and migration history remain open questions.
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The explanation is about how much material Mars could gather while forming, not a claim that it formed as large as Earth and later lost most of its mass. As Stephen Mojzsis, a University of Colorado geological sciences professor and co-author of the study, put it in NASA Astrobiology’s account: “Low probability means one of two things: that we don’t have a better physical mechanism to explain Mars’ formation or in the enormous panoply of possibilities we ended up with one that is relatively rare.”
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