DAWN’S SUN­SET

Sky at Night Magazine - - CONTENTS -

NASA’s mis­sion to the two largest ob­jects in the as­ter­oid belt is draw­ing to a close, so what rev­e­la­tions has it un­cov­ered?

As NASA's Dawn space­craft burns up its last fuel re­serves, Jas­min Fox-Skelly looks back at what this 11-year mis­sion has taught us about our So­lar Sys­tem

One of NASA’s most au­da­cious space mis­sions is draw­ing to a close this month. The Dawn space­craft has spent the last 11 years trav­el­ling to and or­bit­ing around not one, but two of the old­est and most mas­sive res­i­dents of the as­ter­oid belt be­tween Mars and Jupiter – the as­ter­oid Vesta and the dwarf planet Ceres.

“Dawn is the first mis­sion to or­bit a body in the main as­ter­oid belt, the first to visit a dwarf planet and the first to or­bit two ex­trater­res­trial bod­ies in a sin­gle mis­sion,” says Carol Ray­mond, prin­ci­pal in­ves­ti­ga­tor of the Dawn mis­sion at NASA’s Jet Propul­sion Lab­o­ra­tory (JPL) in Pasadena, Cal­i­for­nia.

The mis­sion ex­am­ined these two fos­sils – formed 4.6 bil­lion years ago at the be­gin­ning of the So­lar Sys­tem – to shed light on the pro­cesses and en­vi­ron­ments that cre­ated the plan­e­tary sys­tem we know so well. With Dawn com­ing to the end of its fuel re­serves, and the end of its mis­sion, we look back at what the space­craft has taught us about the for­ma­tion of our So­lar Sys­tem.

Dawn launched on 27 Septem­ber 2007, ar­riv­ing at its first tar­get, Vesta, on 16 July 2011. Sci­en­tists

al­ready knew quite a lot about the gi­ant as­ter­oid be­fore Dawn ar­rived, hav­ing pre­vi­ously recorded Vesta’s spec­tra – the pat­terns of light given off by an ob­ject, which re­veal its com­po­si­tion. The re­sults in­di­cated that Vesta was com­po­si­tion­ally very sim­i­lar to hun­dreds of me­te­orites found on Earth, sug­gest­ing they may have orig­i­nated on the as­ter­oid. Im­ages from the Hub­ble Space Tele­scope show­ing a gi­gan­tic crater at Vesta’s south­ern pole fur­ther backed up this idea; a huge im­pact could have blasted pieces of Vesta all over the So­lar Sys­tem, with some of those pieces end­ing up on Earth as me­te­orites.

Dawn al­lowed astronomer­s to zoom into the as­ter­oid and ob­serve its fea­tures in more de­tail. Not only did Dawn’s in­stru­ments con­firm that the sup­posed Vesta me­te­orites had in­deed orig­i­nated on the as­ter­oid, but anal­y­sis of the as­ter­oid’s com­po­si­tion showed that it has tec­tonic fea­tures, in­clud­ing a crust and a sil­i­cate man­tle. Grav­ity data, which can map the as­ter­oid’s in­ter­nal struc­ture, also in­di­cated that Vesta has a dense and pos­si­bly metal­lic core. This means that Vesta was once vol­cani­cally ac­tive, mak­ing it more like a planet than the in­ert piece of rock many other as­ter­oids are thought to be.

Water puz­zle

On top of this find­ing, sci­en­tists were sur­prised to see a lot of water-rich ma­te­ri­als on Vesta’s sur­face, par­tic­u­larly in older ter­rains. That’s odd as it was pre­vi­ously thought that with­out the pro­tec­tion of an at­mos­phere, Vesta was too warm to hold on to any water. Me­te­orites from Vesta are known to be richer in car­bon than one would ex­pect. One ex­pla­na­tion is that the car­bon and water-rich ma­te­rial could have come from out­side Vesta, pos­si­bly when it was struck by two mas­sive chunks of de­bris from much fur­ther out in the So­lar Sys­tem.

With its mis­sion com­plete, Dawn left Vesta on 4 Septem­ber 2012 and headed for Ceres. When it ap­proached the dwarf planet in 2015 the space­craft spot­ted hun­dreds of cu­ri­ous bright spots glint­ing on its sur­face. The spots were mostly con­cen­trated in and around im­pact craters, par­tic­u­larly the large Oc­ca­tor crater. Anal­y­sis showed that the bright spots are made from sodium car­bon­ate, which astronomer­s be­lieve is evidence of an an­cient ocean be­neath the crust in the process of freez­ing.

“Ceres once hosted a global ocean un­til it froze,” ex­plains Ray­mond, “leav­ing car­bon­ates in the shal­low sub­sur­face that form bright spots when ex­posed by im­pacts. The ma­jor­ity of the bright spots are due to these crustal salts, which have been ex­posed by land­slides and small im­pacts.

“How­ever, those spots found in­side the Oc­ca­tor crater have a dif­fer­ent ori­gin. They are pro­duced

by the ex­tru­sion of briny [salty] liq­uid from Ceres’s in­te­rior. When the water evap­o­rates it leaves be­hind a salty de­posit.”

Sci­en­tists think this liq­uid is be­ing forced up from a magma cham­ber be­low, mak­ing Ceres vol­cani­cally ac­tive, though not in the clas­si­cal sense. In­stead of rock, these ‘cry­o­vol­canos’ are made from salt and mud, which erupts as briny water. Only one, Ahuna Mons, is cur­rently ac­tive. There could have been sev­eral other cry­o­vol­canos in Ceres’s past, but ge­ol­o­gists are still in­ter­pret­ing the data.

An­other re­mark­able dis­cov­ery re­lates to the ori­gin of Ceres. Its chem­istry sug­gests that it didn’t form in the as­ter­oid belt, but fur­ther out in the So­lar Sys­tem, be­fore mi­grat­ing in­wards.

“Its am­mo­nia-rich clays in­di­cate that the dwarf planet ac­tu­ally formed in a colder en­vi­ron­ment than where it cur­rently re­sides,” says Ray­mond. “Ceres is sim­i­lar in bulk com­po­si­tion to many moons of the gi­ant plan­ets, like Europa and Ence­ladus. In fact, sim­i­lar salts have been found on Ceres and in the plumes of Saturn’s moon Ence­ladus. This means it may be re­lated to those icy moons.”

What’s lurk­ing in the deeps?

So study­ing Ceres may also help us learn more about these moons, whose struc­tures and in­ter­nal com­po­si­tions are hid­den by ice. Sci­en­tists are par­tic­u­larly in­ter­ested in learn­ing about the en­vi­ron­men­tal conditions in their deep oceans – and now in Ceres’s too – as they con­tain two of the main in­gre­di­ents thought nec­es­sary for life: water and or­ganic car­bon com­pounds. How­ever, ac­cord­ing to Ray­mond the chance of life form­ing

on Ceres is quite un­likely. “Small dwarf plan­ets like Ceres are not ex­pected to have been warm long enough for com­plex or­ganic mol­e­cules to de­velop,” she says.

But that does not to­tally dis­count the pos­si­bil­ity of life grow­ing on, or in, the dwarf planet. “The Dawn data sug­gest Ceres’s early ocean was hab­it­able. That is, it could sus­tain conditions suit­able for life if life were in­tro­duced in its in­te­rior, for ex­am­ple via ma­te­rial ejected from Earth by large im­pacts.”

Af­ter a decade of dis­cov­ery, Dawn is fast ap­proach­ing the end of its mis­sion. Soon the space­craft will breathe its last breath and run out of the hy­drazine fuel that pow­ers the thrusters. But in June the team still had enough left to al­ter Dawn’s or­bit so that it skimmed 34km above the sur­face of Ceres – 10 times closer than the space­craft had ever been be­fore. Here the cam­eras and in­stru­ments could send back the most de­tailed pho­tos of the world yet. This in­cludes record­ing gamma rays and neu­tron spec­tra, which will tell sci­en­tists more about the chem­i­cal makeup of Ceres’s up­per­most layer, in­clud­ing how much water it con­tains.

But now the mis­sion is draw­ing to a close. With­out thrusters to con­trol its move­ments and ori­en­ta­tion, Dawn won’t be able to point its sci­en­tific in­stru­ments at Ceres, or di­rect its an­tenna to­ward Earth to communicat­e. Sci­en­tists don’t know ex­actly when the fuel will run out, but their best guess is Au­gust or Septem­ber. How­ever, the probe will or­bit around Ceres for at least 50 years. Per­haps a fu­ture mis­sion will re­turn to Ceres and be able to watch Dawn ris­ing up over the hori­zon.

Around 500 mil­lion km from Earth, Dawn will fi­nally run out of fuel or­bit­ing Ceres

NASA in­for­mally chris­tened the three craters at the top left of Vesta the Snow­man. The bulge at the bot­tom is the cen­tral peak of the Rheasil­via crater which, at 22km tall, just pips Olym­pus Mons as the tallest moun­tain in the So­lar Sys­tem

Above top: A view of Vesta’s south pole show­ing the gi­ant crater, Rheasil­via, which is about 90 per cent the size of Vesta’s di­am­e­ter

Above bot­tom: One of the me­te­orites that has reached Earth which is be­lieved to have orig­i­nated on Vesta

Vesta im­aged by Dawn on 24 July 2011 from a dis­tance of 5,200km. The as­ter­oid is the sec­ond­largest ob­ject in the as­ter­oid belt, beaten only by Ceres

The bright­est spot on Ceres is Ce­re­alia Fac­ula in the cen­tre of Oc­ca­tor crater. Just to the left is a less bright clus­ter called Vi­na­lia Fac­u­lae

Above left: A close-up of Ce­re­alia Fac­ula. Ceres’s bright spots are cre­ated by de­posits of sodium car­bon­ate

Above right: Ceres’s vol­canic moun­tain Ahuna Mons. On its steep­est side, it is about 3km high, with a di­am­e­ter of about 120km

Dawn re­vealed many land­slides on Ceres, which re­searchers be­lieve have been shaped by a sig­nif­i­cant amount of water ice. The one above has been des­ig­nated a ‘Type 1’ flow fea­ture, which are mostly found at high al­ti­tudes

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