By Lexa W. Lee Updated Aug 30, 2022
Picture: Pitris/iStock/GettyImages
Both Earth and Mars formed roughly 4.6 billion years ago from a massive cloud of hot gases orbiting the Sun, according to the European Space Agency (ESA). Since that epoch, each planet has gradually cooled and settled into the near‑spherical shapes we observe today. They share dense iron‑nickel cores, rigid silicate crusts, and a history in which water played a pivotal role.
Mars is slightly over half the diameter of Earth—6,786 km (4,217 mi) compared to Earth's 12,756 km (7,926 mi)—yet its mass is only about 10 % of Earth's. The planet’s smaller size and lower gravity caused it to cool more rapidly after formation, which, combined with the loss of volatile gases, left it with a thin, tenuous atmosphere.
Mars’ atmosphere is dominated by carbon dioxide, with an average surface pressure of just 7 millibars versus Earth's 1,013 millibars. The thin envelope offers little insulation, so temperatures are harsh: mid‑latitude averages hover around –50 °C (–58 °F), with nighttime lows dipping to –60 °C (–76 °F) and daytime highs reaching 0 °C (32 °F). Water, once abundant on the Martian surface, has long since vanished—likely due to the planet’s weak gravity and thin atmosphere. In contrast, Earth's oceans cover roughly two‑thirds of its surface.
Earth’s lithosphere is split into tectonic plates that continually shift, reshaping the surface and recycling materials. Mars, however, has a geologically stable crust, though recent data suggest subsurface magma currents. This stability preserves ancient surface features that can date back to around 4 billion years, offering a window into the early solar system.
