Mars' Oceans Disappeared 3.2 Billion Years Ago! — Localized Lake Environments Persisted Even After 2.1 Billion Years Ago —
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Kochi University
Okayama University
Nagasaki University
Faculty of Science and TechnologyA research group led by Professor Seishun Hasegawa has successfully recreated the delta landforms*1 found on Mars through tank experiments and elucidated the conditions under which they formed. Furthermore, by examining the distribution and formation ages of these Martian delta landforms, the team determined that a vast ocean existed in the northern hemisphere of Mars from 3.8 billion to 3.2 billion years ago, and that this ocean had disappeared approximately 3.2 billion years ago.Furthermore, it was revealed that small-scale lake environments also emerged after approximately 2.1 billion years ago. This indicates that habitable environments for life—which may have existed on ancient Mars—persisted even after the ocean disappeared approximately 3.2 billion years ago.
Key Points of the Research
- Using an experimental tank at Nagasaki University, the researchers determined the conditions under which the delta-like landforms found on the surface of Mars were formed.
- An investigation into the distribution and age of deltaic landforms formed during periods of sea regression revealed that a vast ocean existed from 3.8 billion to 3.2 billion years ago.
- The findings suggest that small lakes continued to exist sporadically even after approximately 2.1 billion years ago, indicating that habitable environments on ancient Mars may have persisted even after the oceans disappeared 3.2 billion years ago.
Figure 1: Delta landforms formed by marine regression (lob-extending deltas) and their Martian analogues.
Mars today is an extremely cold and dry environment, and there is no liquid water on its surface.However, NASA’s satellite observations and other research have revealed numerous features on the Martian surface—such as deltas and rivers—indicating that liquid water existed on Mars approximately 3.8 billion years ago, and that a vast ocean once covered the northern hemisphere. Nevertheless, debate continues regarding when the ocean disappeared and whether it ever actually existed, and no consensus has yet been reached on the history of Mars’s ancient water environment.
Consequently, a research group led by Associate Professor Hasegawa focused on the “deltas” (landforms formed where rivers flow into the sea) observed on the surface of Mars. Through a series of experiments using the experimental water tank at Nagasaki University—developed by Professor Emeritus Muto—the group sought evidence of the existence of oceans on Mars and attempted to determine when these oceans existed.As a result, by investigating the formation conditions of the delta landforms observed on the Martian surface, they succeeded in identifying delta landforms formed by large-scale sea regression*2 as well as those formed in small lakes during periods of rising water levels. Furthermore, using images from NASA’s Mars satellite surveys ( CTX*3, MOLA*4 ), the research group conducted a survey of the distribution of these delta landforms and performed dating based on crater analysis*5.
As a result, it has been proven that a vast ocean did indeed exist in the northern hemisphere of Mars from 3.8 billion years ago to 3.2 billion years ago. Mars’s ocean existed for a longer period than previously thought. Furthermore, it has become clear that small-scale lake environments frequently appeared even after approximately 2.1 billion years ago. This indicates that habitable environments on ancient Mars persisted even after the ocean disappeared approximately 3.2 billion years ago.
These findings were published in the scientific journal *npj Space Exploration*, published by Nature Publishing Group, on October 1, 2026.
<用語＞
注１）三角州（デルタ） ： 海や湖の河口付近で川から運ばれた物質が堆積することでできる地形。
注２）海退 ： 海面が低下し、海が陸側から沖側へ後退する現象。
注３）CTX ： NASAの火星探査機Mars Reconnaissance Orbiter (MRO)に搭載されたカメラ。Context Cameraの略。火星表面を約6 mの解像度で広範囲に撮影している。
注４）MOLA ： NASAの火星探査機Mars Global Surveyorに搭載されたレーザー高度計。Mars Orbiter Laser Altimeterの略。１ピクセル約200m。
注５）クレーター解析 ： 対象領域内にあるクレーターの密度分布から年代を推定する方法。月のクレーターの年代分布に基づき、大きなクレーターが多く、クレーター密度が高い場所ほど古いという考えを基盤にしている。
Paper Information
Title: Delta morphology reveals timings of early ocean regression and hydrologic shift on ancient Mars
Journal: npj Space Exploration
Authors: Ren Kito, Hitoshi Hasegawa*, Trishit Ruj *, Tetsuji Muto, Gian Gabriele Ori, Goro Komatsu *Corresponding author
DOI: https://doi.org/10.1038/s44453-026-00056-7
Research Inquiries
Kochi University Faculty of Science and Technology Associate Professor Sei Hasegawa
Tel: 088-844-8321
E-mail: hito_hase★kochi-u.ac.jp Please replace ★ with @ when sending an email.