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NASA’s Astronomy Picture of the Day on Nov. 2, 2026, features a 2018 image of the Arsia Mons Elongated Cloud, a seasonal water-ice cloud on Mars. It forms near the volcano during spring and summer mornings, then can stretch more than 1,500 kilometers downwind; how it forms remains uncertain.
NASA’s Astronomy Picture of the Day for Nov. 2, 2026, features a 2018 image of the Arsia Mons Elongated Cloud, a water-ice cloud that forms near a large Martian volcano and can trail more than 1,500 kilometers downwind. The image brings attention to a recurring atmospheric feature whose formation process is still not fully understood.
The NASA Science explanation identifies the feature as the Arsia Mons Elongated Cloud, or AMEC. It says the cloud forms only during Martian spring and summer, and only in the morning hours. After developing near Arsia Mons, wind carries it downstream, producing the long tail that gives the cloud its distinctive appearance.
The cloud is made of water ice, according to the APOD entry. The explanation says the familiar process in which ice crystals form around dust particles would not work under the conditions high in Mars’ atmosphere: the air is thin and dust is scarce there. One leading speculation is that dust is not involved, but the source does not present that explanation as settled fact.
The image featured by APOD dates to 2018 and is credited to the European Space Agency, the German Aerospace Center and Freie Universität Berlin. NASA’s entry says the cloud was first recorded by NASA’s Viking Orbiter 2 in 1977 and has since been imaged in greater detail by the European Space Agency’s Mars Express spacecraft.
A Mars Cloud With an Open Mechanism
AMEC offers researchers a visible example of how clouds can form and change in a planetary atmosphere with conditions unlike Earth’s. Its regular seasonal and morning appearance, combined with its great length, makes it a recurring feature rather than a one-time atmospheric event. Yet the NASA explanation says the usual dust-based account of ice-crystal formation does not fit the thin, relatively dust-poor air where the cloud develops.
Understanding the process could add to scientific knowledge of cloud formation on Mars and, as the APOD account says, may support a more complete understanding of cloud formation on Earth. That is a potential scientific benefit, not a claim that studying this Martian cloud will directly change forecasts or explain all terrestrial clouds. The immediate value is a chance to investigate how water ice behaves in a different atmospheric setting.
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From Viking Orbiter to Mars Express
The cloud’s history spans several generations of Mars observation. NASA’s Viking Orbiter 2 recorded it in 1977, establishing that the elongated feature was not simply a recent discovery. Later observations by ESA’s Mars Express provided more detailed images, according to NASA’s account. The image selected for this APOD entry was taken in 2018; it is not described as a new observation made for the Nov. 2, 2026, publication.
Arsia Mons is the volcano near which the cloud forms. The source describes a sequence in which the cloud develops near the volcano and then is extended downwind by wind. The cloud’s seasonal and morning timing is also part of the reported pattern. These details help distinguish AMEC from a stationary plume: the long visible tail forms as air carries the cloud away from its origin.
NASA’s Astronomy Picture of the Day pairs an image with an explanation written by a professional astronomer. For this entry, the listed image credits are ESA, DLR and FU Berlin, with processing and license credited to J. Cowart. The APOD authors and editors are Robert Nemiroff, Jerry Bonnell, Cecilia Chirenti and Keighley Rockcliffe.
How the Ice Cloud Forms
The physical process that initiates AMEC remains unclear in the supplied NASA explanation. It says that the familiar dust-particle route to forming cloud crystals would fail in the thin, dust-poor conditions high in Mars’ atmosphere, and offers the absence of dust as a leading speculation. It does not identify a confirmed alternative process or describe evidence that settles the question.
The APOD entry also does not provide measurements or details about the 2018 image’s observation conditions, the cloud’s exact dimensions on that occasion, or the methods used to determine its water-ice composition. Its statement that the tail can extend over 1,500 kilometers describes the cloud’s potential extent, not a specific measurement supplied for the featured image. Further observations and research are needed to clarify the cloud’s development and the conditions behind its seasonal pattern.
Further Observations of AMEC
NASA’s APOD explanation says efforts to understand the Arsia Mons Elongated Cloud will continue, but it gives no dates for a future observation, mission or research publication. ESA’s Mars Express has already imaged the feature in greater detail than the early Viking record, according to the source; what new observations will be made and when is not specified.
The next scientific steps will need to clarify how the ice cloud forms in the high Martian atmosphere and how its appearance relates to season, morning conditions and winds near Arsia Mons. Until more findings are reported, the dust-free explanation remains speculation and the cloud’s precise formation mechanism remains open.
Key Questions
What is the long cloud on Mars called?
It is the Arsia Mons Elongated Cloud, abbreviated AMEC, named for the volcano near which it forms.
How long can the cloud become?
NASA’s APOD explanation says its downwind tail can extend over 1,500 kilometers. The entry does not give a separate measurement for the featured 2018 image.
When does the cloud appear?
The source says it forms during spring and summer on Mars, and only during morning hours.
Do scientists know how it forms?
Not yet, according to the APOD explanation. It identifies the cloud as water ice and says a familiar dust-based formation process would not fit the thin, dust-poor conditions described. The idea that dust is not involved is a leading speculation, not a confirmed mechanism.
Is the APOD image a new Mars observation?
No. NASA identifies the featured image as dating from 2018. The APOD item itself was published on Nov. 2, 2026.
Source: science.nasa.gov
