X1 X1 was a Force - sensitive clone trooper who served the Galactic Republic and later the Galactic Empire . Along with his brother , X2 , he was created by the Kaminoans from the DNA of a Jedi Knight , Falon Grey , and grew up in secret on the planet Kamino . Following the outbreak of the Clone Wars in 22 BBY , X1 and his brother joined the Grand Army of the Republic and fought in many battles of the war , helping to train young clone troopers under the eye of Jedi Master Ferroda . During a battle on the planet Cato Neimoidia , however , Order 66 was enacted , and X1 and his brother were forced to kill Ferroda . As the new Galactic Empire rose in power , X1 and X2 served in its military ranks as stormtroopers . Following a mission to Dantooine to kill a fugitive Jedi , X1 and his brother took separate paths . Not willing to kill innocent people who were harboring their target , X2 defected from the Empire , siding with the Jedi . Following his brother ' s defection , X1 personally led an attack on Dantooine . He killed the Jedi on the planet — none other than his genetic " father , " Falon Grey — and wounded X2 . Although X1 left X2 to die , X2 survived . X1 continued to serve the Imperial Military for many years , playing a minor role in the construction of the first Death Star battlestation and also gaining some mastery of the dark side of the Force . At the Battle of Hoth in 3 ABY , X1 commanded the " Imperial " - class Star Destroyer " Avarice " . Inside the ship , he met his brother — who had joined the Rebel Alliance — dueled him , and fled . After the Battle of Endor and the death of Emperor Palpatine in 4 ABY , X1 became one of the many Imperial warlords fighting for control of the shattered Empire . He also proclaimed himself a Sith Lord and operated from Bast Castle on Vjun , the fortress of the late Darth Vader . During this time , he also gathered around him a sizable military force , including an " Executor " - class Star Dreadnought . Corrupted by power , X1 began to have dreams of galactic domination , hoping to achieve this by creating a personal clone army . He began performing cloning experiments on the planet Dathomir and on the fiery world of Mustafar , to where X2 , now a Jedi Knight in Jedi Master Luke Skywalker ' s New Jedi Order , tracked him . Skywalker went ahead to stop him , but X1 captured the Jedi , intending to use his genes for his clone army of Dark Jedi . Hoping to rescue Skywalker and defeat X1 , X2 and the New Republic — the Rebel Alliance ' s successor state — attacked the planet in a climactic battle . X1 dueled his brother multiple times , first aboard one of his Star Destroyers and then on the surface of the planet . Finally , X1 confronted his brother one last time on a mining platform , and X2 defeated him but refused to strike a final blow . X1 then tried to kill his brother by manipulating his lightsaber with the Force , but X2 evaded the attack , and X1 was stabbed by his own weapon and fell into the planet ' s boiling lava to his death . Biography . Creation . X1 , along with his " brother , " X2 , was a Force - sensitive clone created by the Kaminoans from the DNA of the Jedi Knight Falon Grey . Prior to the outbreak of the Clone Wars , Grey — at that point a Padawan to Jedi Master Rahm Kota — was badly injured during his travels with his Master . Kota was forced to make an emergency landing on the planet Kamino , where the native Kaminoans healed Grey , but in exchange took a sample of Grey ' s genetics without his consent . From the sample , they produced two clones , X1 and X2 . X1 was the older of two " brothers , " having been born an hour earlier . The two were raised in secret by the Kaminoans alongside bounty hunter Jango Fett ' s clones , produced on Jedi Master Sifo - Dyas ' s request . Unaware of their true origins , X1 and X2 trained their clone brothers on Kamino . Clone Wars . In 22 BBY , the Clone Wars began with the Battle of Geonosis . X1 and X2 became clone troopers in the Grand Army of the Republic and fought together many times during the conflict . Due to their exceptional abilities , they were assigned to train young clone troopers in the art of warfare . In 19 BBY , during the latest stages of the war , X1 and X2 went to the planet Tatooine in order to oversee the training of new clone troopers in a training outpost along with Jedi Master Ferroda . However , the first training session was abruptly stopped after the discovery of a Confederate recon droid inside the outpost . X1 ordered his brother to destroy the droid , and X2 did so . The droid turned out to be a part of a larger droid reconnaissance patrol , which X2 dispatched as well . On X1 ' s orders , X2 took a V - wing starfighter into space and destroyed the transport ship from which the droids had been deployed . X1 realized that an invasion of Tatooine was imminent , and he and X2 began preparing for the ensuing battle . As the Confederacy attacked in full force , X1 and X2 gave support to clone pilots who had been trapped inside a Republic hangar . The Confederacy continued its attack , sending spider walkers onto the battlefield . Lacking heavy weapons necessary to destroy the spider droids , X1 and his brother went through a nearby cantina and destroyed B1 battle droids inside . On the other side of the cantina , X2 was provided with a rocket launcher by one of the clones , with which he managed to take down the walkers . In the meantime , X1 sent in reinforcements , allowing the Republic to gain the upper hand . Following X1 ' s commands , X2 used an ion cannon to take down the shields of a Confederate " Providence " - class carrier / destroyer flying above Tatooine . After clearing the Republic hangar , X1 and his brother took V - wings into space and fought Vulture droid starfighters covering the unshielded ship . X1 then sent X2 into the ship ' s interior in order to sabotage its reactor core and therefore destroy the vessel , which X2 succeeded in doing . In the end , the Republic won the battle of Tatooine . Order 66 . Following their exploits on Tatooine , Supreme Chancellor Palpatine invited X1 and X2 to the galactic capital of Coruscant for a special award ceremony . Palpatine gave them medals for their bravery , though he initially confused X1 with X2 . The ceremony was abruptly stopped after the Confederacy attacked the planet . After ordering a group of clone troopers to get the Chancellor to safety , X1 and X2 joined the battle , taking separate routes through Coruscant ' s streets . X2 managed to track down the Separatist General Grievous and even injured him in combat , though he was unable to stop Grievous from kidnapping the Chancellor and transferring him to his flagship , the " Invisible Hand " . X1 rejoined his brother and they took off into space , taking nearby ARC - 170 starfighters and battling Separatist Vulture droids . Soon , they were joined by Jedi Knights Anakin Skywalker and Obi - Wan Kenobi , who had arrived to rescue the Chancellor , and , with X2 ' s help , they managed to do so . The Clone Wars neared its conclusion , but the Republic was still fighting on many important worlds . X1 and X2 were sent to battle alongside Ferroda on the planet Cato Neimoidia at the end of the war . X1 led clone troopers on the ground in order to capture Trade Federation Viceroy Nute Gunray ' s palace . X2 , having fought in space over the planet , soon joined him in the ground battle , providing support for the attacking Republic clone troopers . As the troops advanced on the citadel , X1 sent his brother to deactivate the battle droids from inside the palace . X2 completed the task , and the droids were shut down . However , a message from Coruscant was then transmitted to the clones . Palpatine told the clone troopers that Ferroda , and all other Jedi , had betrayed the Republic , and the Chancellor ordered the clones to execute Order 66 and kill the Jedi Master . Though X2 was reluctant , X1 forced his brother to comply , and the two of them , along with the other clone troopers , attacked the Jedi Master . Ferroda , though initially caught off - guard by the attack , put up a fierce resistance . He wounded X1 and killed most of the clones in the platoon , leaving only X2 to battle him . Angered at what the Jedi had done to his brother , X2 killed Ferroda . Service to the Empire . Taking separate paths . Following the issuing of Order 66 and the extermination of most of the Jedi , Palpatine reformed the Republic into the Galactic Empire with himself as Emperor . X1 and X2 became stormtroopers in the new Imperial Military and served the Empire , gaining a reputation of two of the Empire ' s most trusted agents . During this time , X1 slew a number of Jedi , but he also noticed signs that X2 had been concerned and insecure ever since Order 66 . At some point between 17 BBY and 15 BBY , Dark Lord of the Sith Darth Vader sent the two brothers to find and kill a fugitive Jedi who was hiding among villagers on the planet Dantooine . X2 was reluctant to embark on the mission , but X1 ordered him to do so . The two brothers , along with other troopers , took ARC - 170 starfighters and headed for Dantooine ' s surface . X1 ordered the troopers under his command to enter the planetary atmosphere and destroy the village where the Jedi was hiding . X2 refused , saying that the villagers were innocent , but X1 strongly believed that they deserved to die for harboring a Jedi . X2 continued arguing , and X1 ordered Zeta Squadron to escort his brother back to their carrier ship . X1 ' s commands angered X2 , however , and he defected from the Empire . X1 , enraged by his brother ' s treachery , ordered the stormtroopers in the squadron to kill X2 , but the latter dispatched them . X2 reached the planet surface and met with the brothers ' genetic father , Falon Grey , who was Vader ' s target . X2 assisted Grey in fighting off initial Imperial attacks . The two of them helped Dantooine villagers to reach a secret hangar with a LAAT / i gunship in an effort to escape . X1 arrived with a group of stormtroopers aboard several gunships , however , killing all the villagers and wounding X2 . Grey repulsed the stormtroopers under X1 ' s command , and X1 , who was armed with a force pike , engaged in a duel with his father . X1 was soon at the mercy of the victorious Grey , but the Jedi spared his life and even returned his force pike to him , believing that X1 could still be redeemed from the dark path he had taken . X1 , however , seized the opportunity to strike back and took Grey ' s lightsaber , cutting X2 with it and then shooting Grey in the back with his blaster . Leaving Grey and X2 to die , X1 and his stormtroopers departed the planet . Unbeknownst to X1 , X2 survived , having been healed by Grey before the Jedi ' s own death . The early Galactic Civil War . With his brother out of the way , X1 rose in the ranks of the Imperial Military , serving the Empire during the Galactic Civil War against the Alliance to Restore the Republic . Between 2 BBY and 0 BBY , X1 was assigned to oversee the operation of Desolation Station , an asteroid - based facility in the Atrivis sector that utilized slave labor to produce superlaser components for the new Imperial superweapon , the Death Star . The last of those components , the tributary beam , was eventually completed and was to be delivered to the Death Star , which was undergoing construction over the prison planet of Despayre . Yet , the transport carrying the tributary beam was hijacked by a Rebel saboteur , who destroyed Desolation Station with its help . The station ' s commander managed to survive the facility ' s destruction and was brought before X1 with a report . Not giving the man a chance to explain himself , X1 executed the commander for his failure . Little did X1 know that the Rebel saboteur who had destroyed Desolation Station was in fact X2 , who had joined the Rebel Alliance . When a replacement tributary beam was commissioned , X1 reported the fact in a holorecording to the under - construction Death Star . This record was witnessed by X2 , who was aboard the Death Star on a reconnaissance mission on behalf of the Rebels , and who therefore learned that X1 was still alive . The Death Star was later destroyed by the Rebellion during the Battle of Yavin , but X1 continued to serve the Empire and started to develop his Force powers inherited from Grey ' s DNA , becoming a Dark Side Adept . He also kept Grey ' s lightsaber as a trophy . Although he did not know how to use it at first , X1 eventually learned to handle the weapon . In 3 ABY , X1 was involved in the Battle of Hoth , a major Imperial victory , where he commanded the " Imperial " - class Star Destroyer " Avarice " in orbit around the ice planet . X2 , having helped the Rebels escape their base on Hoth , boarded the " Avarice " along with his military unit , Grey Squadron , in an attempt to destroy the ship . X1 sensed his brother ' s presence aboard the starship and was surprised at the feeling , as he believed that he had killed X2 almost two decades earlier on Dantooine . Ambushing his brother , X1 used telekinesis and Force lightning on X2 and brandished Grey ' s lightsaber . The two brothers dueled , and X1 offered X2 a chance to " redeem " himself and return to the Empire , but X2 declined and soon defeated X1 . When Grey Squadron used Class - A thermal detonators to deal major damage to the " Avarice " engines , a series of explosions rocketed the ship . X1 Force - pushed his brother away and escaped from the crumbling starship , promising to kill X2 if they met again . X2 was also able to escape , just before the " Avarice " was destroyed . Only a year later , the Empire was dealt a major defeat at the Battle of Endor , and Emperor Palpatine and Darth Vader were both killed . In the wake of their deaths , X1 began to dream about creating his own Empire , with himself as the new Emperor . Sith Lord . In order to realize his dream , X1 assembled a fleet consisting of " Imperial " - class Star Destroyers and his own " Executor " - class Star Dreadnought . Under X1 ' s command were also former CIS battle droids , numerous stormtroopers , Imperial officers , dark trooper s , and members of the Emperor ' s Royal Guard . X1 was in possession of TX - 130T fighter tank s , AT - STs , and AATs . X1 ' s starfighter corps included TIE Fighters , TIE Bombers , Vulture droids , and even costly TIE Defenders . X1 ' s own starfighter of choice was a TIE Advanced x1 similar to Darth Vader ' s own . X1 also established a base on the planet Dathomir , where he conducted cloning experiments . He eventually abandoned the facility , however , leaving a large contingent of troops there . X1 ' s base was then overrun by the planet ' s native Nightsisters . In the midst of the fighting between X1 ' s troops and the Nightsisters , Grey Squadron , led by X2 , arrived at the planet , hoping to apprehend X1 . X2 managed to defeat the Nightsisters , though his unit suffered heavy casualties inflicted by one of X1 ' s men . In the meantime , X1 proclaimed himself a Sith Lord and sought to learn the secrets of the dark side of the Force . To this end , he traveled to the planet Vjun and occupied Bast Castle , which had once belonged to the now - deceased Darth Vader . Inside Bast Castle , X1 stored Grey ' s lightsaber , having switched to a red - bladed weapon . X1 also created a holocron , wherein he kept personal information . Preparing to set his plans for galactic domination in motion , X1 took his fleet and journeyed to the fiery world of Mustafar , where he created another cloning laboratory , which he hoped to use to create his own clone army . In almost no time , he produced a force of fierce cloned Wookiee warriors . Soon enough , however , X2 raided X1 ' s base on Vjun , retrieving Grey ' s lightsaber and the holocron and discovering X1 ' s current location . At some point between 11 ABY and 19 ABY Jedi Master Luke Skywalker went to Mustafar to confront X1 , but the latter managed to capture him , intent on using Skywalker as a prime clone for an entire army of Dark Jedi . Confrontation on Mustafar . X2 and his troops soon arrived at Mustafar to rescue Skywalker and defeat X1 . A battle began as X1 ' s fleet was attacked by the forces of the New Republic , the galactic government that had succeeded the Rebel Alliance . One of X1 ' s Star Destroyers was boarded by X2 , who destroyed the ship ' s reactor core . As soon as he had done this , an enraged X1 arrived in the reactor control room , along with several dark troopers and Royal Guards , and engaged X2 in a duel . As soon as X2 gained the upper hand , X1 retreated aboard his TIE Advanced . X2 followed him , and the two brothers engaged in a dogfight . After receiving some damage , X1 escaped to the surface and sent a swarm of TIE Defenders to cover his retreat . All the while , X1 continued to goad X2 into giving in to his anger , trying to seduce his brother to join the dark side . X2 defeated the TIE Defenders and joined the ground battle . On the ground , X2 and his soldiers were able to capture an ion cannon control bunker , shifting the tide of the battle to the New Republic ' s advantage and forcing X1 to try to attack his brother from a distance . He sent stormtroopers , as well as dark troopers and hovertanks , which X2 also defeated before proceeding to the cloning laboratory . X1 ambushed X2 and began another brief duel with him . X1 soon dismissed his brother as an unworthy opponent and retreated , but not before releasing his Wookiee clones to combat X2 . X1 waited for his brother in the ion cannon control room , backed by more Emperor ' s Royal Guards . After X2 killed the Wookiees , he reached X1 ' s location and the two engaged in yet another fight . Soon enough , X1 escaped to a mining platform , but X2 followed him , hoping to stop his brother once and for all . Once X2 arrived on the platform , the brothers ' final duel began . The two fought with lightsabers and various Force powers , but , finally X2 managed to overpower X1 . X1 admitted defeat and asked his brother to kill him , but X2 followed the Jedi Code and refused to do so . Then , in a final act , X1 levitated his fallen lightsaber and threw it at X2 , trying to kill him . The latter jumped over the lightsaber , though , causing X1 to be impaled by his own blade . With the lightsaber falling out of his chest , X1 stepped back over the platform ' s edge and fell down into a lava river to his death . His army was defeated , and Skywalker was rescued , earning X2 a promotion to Jedi Master . With X2 and the New Republic victorious , X1 ' s dreams of galactic conquest were over . Personality and traits . From the time of his birth on Kamino , X1 was trained to serve in the military , and he developed strong leadership skills , which were noted by Palpatine during the award ceremony on Coruscant . X1 held great respect for the Supreme Chancellor , and he was certain that he would meet him someday . During the Clone Wars , X1 felt that the Jedi ' s pacifist views slowed down the war effort , and he believed that the Jedi should use their power to its full extent instead of relying on clone troopers to fight for them . It was during the war that he developed thoughts of an army of cloned Jedi , jokingly telling X2 that this was how the galaxy should be run . X1 ' s relationship with X2 affected both brothers ' lives . While the two were at first very similar in personality , X1 in time developed a taste for power , often acting with authority toward his brother . In contrast to X2 , X1 was more inclined to take risks to achieve an objective . Because X1 was born an hour earlier than X2 , he referred to X2 as " little brother " and used this name as late as the time of their confrontation on Mustafar . After the rise of the Empire , X1 became even more violent and power - hungry . He strongly supported the New Order , developed a strong anti - Jedi sentiment , and took pride in the murders he committed on behalf of the Emperor . X1 thought that the best way to make peace with one ' s enemies was to destroy them . During the battle on Dantooine , X1 was ready to kill innocents if it was necessary to enforce the Emperor ' s will , and he murdered Grey after the latter refused to kill him . Later , X1 killed Desolation Station ' s commander in cold blood after he failed to prevent the destruction of the station . X1 was a hypocrite , a fact that he admitted himself . He was also egotistical , keeping a large picture of himself standing over a defeated beast in his office on Dathomir . X1 was a loyal servant of the Empire during its height , though after the Battle of Endor , he went on to create his own empire as a Sith Lord , dreaming of galactic domination . Noticing a large number of stormtroopers guarding X1 ' s base on Dathomir , X2 believed that his brother had become paranoid . Exploring his Force - sensitivity , X1 developed a passion for the dark side , craving to learn its secrets and traveling to Vjun in order to do so . X1 originally had light skin , brown hair and eyes . Proclaiming himself a Sith , X1 shaved and tattooed his head , while his eyes started glowing red as a result of his corruption by the dark side . He fed on anger , which fueled his Force abilities . He was also fascinated with chaos as a Sith Lord and believed that the galaxy should be a large battlefield like the clashing lava on Mustafar . Powers and abilities . Having been raised as an elite clone trooper , X1 , like his brother , developed fighting and leadership skills for use on the battlefield . He and X2 often commanded groups of soldiers in battle , and X1 strategized to bring the Republic to victory . While fighting , X1 was apt in the use of various weapons . He was a good pilot , capable of handling various starfighters . As a Force - sensitive , X1 was skilled in various Force powers . Although he was at first ignorant of his origins , X1 became a Dark Side Adept by the time of the Galactic Civil War . He gained some proficiency in various forms of telekinesis , as well as in Force lightning , which he used in his duel with X2 over Hoth . He was also able to shield himself from attacks for a limited amount of time . X1 had a developed Force sense ability , being able to sense his brother ' s presence on the " Avarice " , and later feeling a tint of anger in X2 during the battle on Mustafar . After becoming a Sith Lord , X1 studied many dark side secrets and created his own holocron . He also learned the process of cloning , intending to use this knowledge to create his own army . X1 showed proficiency with melee weapons , being able to withstand Grey ' s attacks for some time armed only with a force pike . He was a practitioner of Dun Möch , taunting X2 during their duels . X1 was also a quick study . He learned to handle Grey ' s lightsaber despite having no prior knowledge on how to operate such a weapon , and he was able to develop the skill of throwing his lightsaber with the Force . This backfired during his final duel with X2 , however , as , when the latter jumped over X1 ' s blade , the weapon returned and impaled X1 , killing him . Equipment . X1 proved to be apt in the use of various weapons when in combat situations . During the Clone Wars , he used a blaster rifle and a shotgun , as well as grenades . X1 was also proficient with melee weapons and used a force pike to ward off an attack by Falon Grey . Following the events on Dantooine , X1 fought with a lightsaber . His first weapon was the one confiscated from Grey in the attack . He later switched to a red - bladed lightsaber . X1 also piloted various starfighters during his combat experience . During the Clone Wars , he used V - wings and ARC - 170s . He also piloted TIE Fighters and the TIE Advanced x1 craft during the Galactic Civil War . Even aside from combat matériel and spacecraft , X1 possessed a holocron that he made after proclaiming himself a Sith . This hand - crafted item contained information regarding his plans and whereabouts . Behind the scenes . X1 , along with X2 , made his only appearance in the 2009 video game " " , released for the PlayStation Portable and the Nintendo DS and developed by Rebellion Developments and N - space , respectively . During the game ' s campaign , in which the player controls X2 , X1 ' s role changes as the storyline advances . In the Clone Wars missions , X1 acts as a comrade and a mentor to the player , giving instructions on what to do and helping in battle . During the mission to Dantooine , X1 becomes the antagonist in the game . X1 is essentially absent during the early Galactic Civil War missions , but during the Battle of Hoth mission , the player duels him on the " Avarice " . The plotline in the final missions centers around X2 tracking X1 down to Mustafar , ending with the final duel between the two brothers . X1 is a playable character in the PSP version ' s Instant Action mode of the game , which lacks a specific storyline . He is available in three skins , depending on the era and planet chosen by the player . Clone trooper X1 is available on Hoth , Imperial X1 is available on Coruscant and Kashyyyk , and Sith Lord X1 is available on Hoth , Dantooine , as well as on Yavin 4 and Kashyyyk , though only in the Heroes and Villains mode . Free Radical Design , a development company that had been working on the " Star Wars : Battlefront III " video game before it was cancelled , has leaked unfinished material indicating that X1 and X2 were originally set to be featured in that game . The storyline , as it appears in the revealed material for " Battlefront III " bears a lot of similarity to the final version featured in " Elite Squadron " . Differences between versions of " Elite Squadron " . Because the game was released on two handheld platforms , some missions are present in one version but absent in the other . In addition , there are numerous differences between the missions as they are presented in both versions . This article incorporates elements from both versions unless they blatantly contradict each other . In the latter cases , as there has not been any official word on which version is more canonical , the article approaches each inconsistency on a case - by - case basis to create a coherent storyline , choosing the version that focuses more heavily on X1 and better fits story - wise between the events immediately prior to and following the mission in question . The alternative portrayals of the contradictory events are noted below and may actually be the canonical versions . The Battle of Tatooine , for example , is described in this article according to the PSP mission . While X1 and X2 train troopers in the PSP mission , they supervise the construction of an outpost in the Nintendo DS version . The two are busy working at the outpost when the Separatist attack begins , and the brothers are initially caught off - guard . They receive a distress call from Ferroda , make their way to an invaded Republic garrison , and clear it from droids . Taking BARC speeders to another base across the desert , they rendezvous with Ferroda . Alongside the Jedi , X1 and X2 make their way to the Republic hangar and take ARC - 170s into space . There , X2 infiltrates a " Lucrehulk " - class battleship instead of a " Providence " - class destroyer . Additionally , the PSP version claims that Tatooine was an " early " victory for the Republic , while the Nintendo DS version explicitly provides the 19 BBY date . The Battle of Coruscant is described here as it is portrayed in the Nintendo DS version , as Palpatine ' s award ceremony is absent in the PSP version . In the latter edition , X1 has a very minor role and only participates in the space battle , flying an ARC - 170 starfighter . This article uses the PSP version ' s storyline to describe the Battle of Cato Neimoidia . The Nintendo DS version states that months have elapsed between the Battle of Coruscant and the brothers ' assignment to Cato Neimoidia , while the canonical elapsed time is much shorter . Cato Neimoidia is also protected by an energy shield , which X1 and X2 take down by destroying several orbital satellites before proceeding with the ground assault . During the battle on Dantooine in the Nintendo DS version , after Grey defeats him , X1 pretends to admit that he was wrong and regrets his past actions . When Grey turns his back on him , X1 stabs him in the back with the force pike , instead of killing him with a blaster shot as depicted in the PSP version . The Battle of Hoth follows the Nintendo DS portrayal in this article . In the PSP version , X1 uses a blaster rifle instead of Grey ' s lightsaber . Additionally , the " Avarice " is not clearly shown being destroyed . The battle on Mustafar is presented here as it is in the PSP version . In the DS version , all references to the clone army are cut and X1 ' s reason to capture Skywalker is unspecified . Instead , X1 uses reactivated Confederate battle droids left on the planet after the Clone Wars . Another discrepancy is that Mustafar is protected by an energy shield , forcing X2 to crash X1 ' s " Executor " - class Star Dreadnought into the shield to pierce through it and reach the planet ' s surface .