The im­pact of im­pacts on small moons

The fall­out from large me­te­ors could make moons ap­pear much older than their years

Sky at Night Magazine - - BULLETIN -

When a me­teor im­pacts a moon, it kicks up a lot of ma­te­rial, or ejecta, which then ei­ther rains back down on the sur­face or es­capes into space. Some­times, how­ever, the con­di­tions are just right and the ejecta en­ters or­bit around the moon’s par­ent planet be­fore re-im­pact­ing the moon, cre­at­ing what are known as sesquinary craters.

As well as be­ing in­her­ently pleas­ing to say, sesquinary craters are also an im­por­tant fac­tor shap­ing the sur­faces of moons in the So­lar Sys­tem. Sesquinary im­pacts of­ten form dis­tinc­tive chains of craters, known as cate­nae, and can con­sti­tute a sig­nif­i­cant frac­tion of the to­tal im­pactors on a moon’s sur­face. Pre­vi­ous stud­ies have looked at the process of sesquinary cra­ter­ing on our Moon, as well as Ganymede, Io and Europa or­bit­ing Jupiter, Tethys around Saturn, and Mars’s moons Deimos and Pho­bos. In his pa­per, Michael Nayak, at the Air Force Re­search Lab­o­ra­tory, on Maui, Hawaii, fo­cuses on Pho­bos.

Nayak used a com­puter model of pri­mary im­pacts slam­ming into Pho­bos and tracked how the ejecta fragments or­bited Mars be­fore hit­ting the moon again. He fo­cused on pri­mary craters 1km across, which throw off ejecta streams trav­el­ling at up to 100m/s (360km/h). What he found from his cal­cu­la­tions is that 85 per cent of the de­bris blasted off at es­cape ve­loc­ity (around 40km/h on Pho­bos) even­tu­ally reac­cretes back onto the moon’s sur­face. Nayak says that this high per­cent­age of re­turn­ing ma­te­rial is be­cause Pho­bos or­bits so deeply within the grav­i­ta­tional field of Mars. Pho­bos or­bits nearer to its planet than any other moon, so close, in fact, that it or­bits faster than Mars spins. If you were to stand on the sur­face of Mars, you’d see Pho­bos rise in the west twice ev­ery Mar­tian day.

Pho­bos is grad­u­ally spi­ralling to­wards Mars, but Nayak cal­cu­lates that the im­por­tance of sesquinary im­pacts on the moon has re­mained true for the past 25 mil­lion years. This means that the top 30cm of Pho­bos’s re­golith is mostly made up of ejecta that has sub­se­quently reac­creted as sesquinary im­pactors. In essence, Pho­bos is mix­ing up its top sur­face by cycling it through or­bit around Mars.

But even more in­ter­est­ing than that is the cas­cade ef­fect trig­gered by this process. Some of the ma­te­rial

“85 per cent of the de­bris blasted off at es­cape ve­loc­ity even­tu­ally reac­cretes back onto Pho­bos’s sur­face”

ejected by a sesquinary im­pact will be large enough that when it falls back down onto Pho­bos it trig­gers sesquinary im­pacts of its own. So a pri­mary im­pact cre­ates sesquinary craters that sub­se­quently cause fur­ther sesquinary craters. The com­bi­na­tion of Pho­bos’s low grav­ity and tight or­bit around Mars means that the moon is ex­cep­tion­ally good at stir­ring up its sur­face over time.

The fact that the sur­face of Pho­bos has be­come so thor­oughly re­dis­tributed by this sesquinary im­pact process is cru­cial to ap­pre­ci­ate when con­sid­er­ing the ge­ol­ogy and land­scape of the moon, ar­gues Nayak, es­pe­cially for land­ing probes on Pho­bos in the fu­ture. It’s also im­por­tant to take the ef­fects of sesquinary im­pacts into ac­count when count­ing craters to es­ti­mate the age of re­gions of the moon’s sur­face – these ad­di­tional craters formed by the same pri­mary im­pact would cause the sur­face to ap­pear older than it re­ally is.

LEWIS DARTNELL was read­ing… Sesquinary reim­pacts dom­i­nate sur­face char­ac­ter­is­tics on Pho­bos by Michael Nayak Read it on­line at http://dx.doi.org/10.1016/j.icarus.2017.08.039

Pho­bos’s prox­im­ity to Mars makes the moon par­tic­u­larly prone to craters be­ing cre­ated by sesquinary im­pacts

(www.the-knowl­edge.org)

LEWIS DARTNELL is an astro­bi­ol­ogy re­searcher at the Univer­sity of West­min­ster and the au­thor of The Knowl­edge: How to Re­build our World from Scratch

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