Mars has only 10.7% of Earth’s mass, even though its radius is 53.2% as large. The leading explanation is that Mars formed from a much smaller supply of solid material; models suggest the distribution of that material in the early Solar System was shaped by processes such as giant-planet migration. The exact history is still unsettled.
How much smaller is Mars than Earth?
“Half the size” is a reasonable visual shorthand for Mars, but it describes its radius—not its mass. NASA’s National Space Science Data Center lists the planets’ volumetric mean radii and masses as follows:
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| Measure | Mars | Earth | Mars as a share of Earth |
|---|---|---|---|
| Volumetric mean radius | 3,389.5 km | 6,371.0 km | 53.2% |
| Mass | 0.64169 × 1024 kg | 5.9722 × 1024 kg | 10.7% |
The figures are from NASA Goddard Space Flight Center’s Mars Fact Sheet, accessed in 2026. Radius measures distance from a planet’s center to its surface; mass measures how much matter it contains. The striking formation question is why Mars ended up with so little mass compared with Earth.
How did the planets grow?
Planets formed by accretion: gravity drew together dust and gas in the young Solar System, building larger bodies over time. NASA describes Mars as forming about 4.5 billion years ago through gravity pulling together swirling gas and dust. That explains the broad process, but not why Mars grew so much less than Earth.
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A planet’s final mass depends in part on how much solid material was available in its region and how that material moved and collided while the planets were assembling. If Mars’s growth zone contained or retained far less building material than Earth’s, Mars could stop growing at a much lower mass. NASA’s account of Mars’s formation is at NASA Science: Mars Facts.
Why might Mars’s growth zone have been depleted?
Giant-planet migration: the Grand Tack hypothesis
The Grand Tack is one proposed way to reshape the early Solar System’s supply of planet-building material. In the scenario described by NASA Astrobiology, Jupiter migrated sunward and then changed course; this movement could have shifted material inward toward the regions where Earth and Venus formed while reducing the supply near Mars.
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In a small percentage of the simulations discussed in NASA’s account, Mars formed farther from the Sun and was later moved inward to its present orbit. That is a possible model outcome, not evidence that Mars definitely formed elsewhere. The account also notes that simulations more commonly placed Mars near Earth, while compositional differences between the planets complicate that picture. See NASA Astrobiology: “Mars and Earth May Not Have Been Early Neighbors”.
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Another approach is to propose that the early disk contained progressively less solid material farther from the Sun. A 2015 study recorded by NASA’s Technical Reports Server found that reproducing the large Earth-to-Mars mass ratio in simulations required a strong depletion of solids between 1 and 3 astronomical units (AU), where 1 AU is Earth’s average distance from the Sun.
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But a disk profile that produced a small Mars did not automatically reproduce the asteroid belt. In the study, shallow density gradients could match the belt’s dynamical excitation but produced Mars-like planets that were too massive. A very steep gradient produced the Earth/Mars mass ratio, but left the simulated asteroid belt too dynamically cold. The authors concluded that no disk profile they tested explained both the terrestrial planets and the asteroid belt, suggesting that another process—such as giant-planet migration—might also be needed. Read the study record at NASA Technical Reports Server.
What evidence can distinguish the explanations?
The asteroid belt tests more than Mars’s mass
A model should explain not only why Mars stayed small but also what happened to material elsewhere in the inner Solar System. The asteroid belt’s structure puts a separate constraint on simulations. The 2015 study’s tension between matching the belt and matching the Earth/Mars mass ratio shows why a successful explanation may need more than a simple gradient in the disk’s material.
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Composition and isotopes constrain Mars’s birthplace
Mars and Earth differ in elemental and isotopic signatures. Those differences challenge the simple assumption that both planets formed from identical nearby material. At the same time, the simulations summarized by NASA Astrobiology most commonly placed Mars near Earth. A model therefore has to account for several kinds of evidence together; a proposed birthplace that helps explain mass must also fit what the planets are made of.
Meteorites offer clues to early growth
Martian shergottite meteorites preserve evidence used to study Mars’s early differentiation and accretion. A 2018 NASA Astrobiology summary reported that one study’s evidence indicated Mars may have accreted before Earth. That is a qualified inference from a particular study, not a settled chronology for the planet as a whole. See NASA Astrobiology’s account of the shergottite findings.
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Did Mars form where it is now?
The available accounts do not establish a single answer. Mars could have assembled near its present orbit, or it could have formed farther out and moved inward; the latter appears in only a small subset of the Grand Tack simulations described by NASA Astrobiology. The relative likelihood of these outcomes depends on the model, and evidence from orbital dynamics, the asteroid belt, and planetary composition needs to be considered together.
As Stephen Mojzsis, a University of Colorado geological sciences professor and co-author of the study, put it: “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.”
What planet formation can—and cannot—explain
Formation models offer a plausible explanation for Mars’s low mass: the planet grew from a region with a comparatively limited supply of solids, potentially because migration and other early Solar System dynamics redistributed material. They do not establish that Jupiter’s migration alone caused Mars’s small size, nor do the cited sources settle Mars’s precise birthplace. The supported conclusion is that the amount and distribution of material available during formation likely mattered; a single, proven reconstruction of how that happened remains unavailable.
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