![]() There is strong evidence that they are associated with the formation of the largest crater on Phobos. Remarkable linear grooves, typically 100 metres (330 feet) wide and 20 metres (65 feet) deep, cover much of the surface. This fact and the satellite’s low mean density (1.9 grams per cubic cm) are consistent with the composition of carbonaceous chondrite meteorites, suggesting that Phobos may be a captured asteroid-like object. The heavily cratered surface of Phobos is covered with a very dark gray regolith (unconsolidated rocky debris) that reflects only about 6 percent of the light falling on it-about one-half that of the Moon’s surface. Learn about Phobos and Deimos, Mars's two tiny moons See all videos for this article SpaceNext50 Britannica presents SpaceNext50, From the race to the Moon to space stewardship, we explore a wide range of subjects that feed our curiosity about space!.Learn about the major environmental problems facing our planet and what can be done about them! Saving Earth Britannica Presents Earth’s To-Do List for the 21st Century.Britannica Beyond We’ve created a new place where questions are at the center of learning.100 Women Britannica celebrates the centennial of the Nineteenth Amendment, highlighting suffragists and history-making politicians.COVID-19 Portal While this global health crisis continues to evolve, it can be useful to look to past pandemics to better understand how to respond today.Student Portal Britannica is the ultimate student resource for key school subjects like history, government, literature, and more.This Time in History In these videos, find out what happened this month (or any month!) in history.#WTFact Videos In #WTFact Britannica shares some of the most bizarre facts we can find.Demystified Videos In Demystified, Britannica has all the answers to your burning questions.Britannica Explains In these videos, Britannica explains a variety of topics and answers frequently asked questions.Britannica Classics Check out these retro videos from Encyclopedia Britannica’s archives.Other chances to achieve a phase angle of under one will not occur until April and September 2020 (in the latter case, Mars Express may achieve a phase angle of precisely zero).Īs such, Mars Express takes every opportunity to view this small and intriguing moon from this angle, to shed light on its properties, behaviour, possible origin, orbital characteristics and location in space – and to probe its potential as a mission destination. This arrangement – of the Sun, Mars Express and Phobos where the latter is observed at a phase angle of near zero – is very rare, and happens only three times a year at most. The lowest phase angle in this animation is not precisely zero, but 0.92 degrees. In this alignment, all of the light illuminating Phobos hits the surface vertically and is thus largely reflected back into space, causing the target object to brighten up noticeably, as seen in the animation, and shadows to disappear. ![]() ![]() Different shadows are cast as the Sun’s position changes relative to the target object: this illuminates and highlights the surface features and enables calculations of feature height, depth and relief, and reveals much about the roughness, porosity and reflectivity of the surface material itself.Ī phase angle of zero degrees occurs when the Sun is directly behind the observer. Images acquired across a range of phase angles, as shown here, are incredibly useful for scientists. The slight up-and-down motion of the moon is caused by the slight oscillation of Mars Express, as the spacecraft has rotated from its normal position (pointing towards Mars) to this new one (pointing towards Phobos) in order to acquire these images. The image sequence shows Phobos at a number of angles – the moon can be seen rotating, and slowly lightens up before it begins to darken again. The origin of these features is uncertain: they may have been carved out by debris rolling across the moon’s surface, or created as the moon was pulled in different directions by tidal forces driven by its parent planet. The largest of these is Stickney crater, which can be seen towards the centre of the frame and measures 10 km across.Īlso visible are a number of linear marks and furrows that resemble long, deep grooves or scratches. A number of impact craters can be seen, created as the 26 km-long Phobos was hit by small bodies and rocky debris during its travels through space. This opportunity allowed the spacecraft to view myriad features across the moon’s surface.
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