J ournalojClac iologl l, Fol. +4, No . 148, 1998 Hinge-line Illigration of Peterll1ann G letscher, north Greenland, detected using satellite-radar interferoll1etry ERIC RIGNOT J et Pro/J1I lsion Labomtol),. Califo rnia illstilule ojTed7l7 olog1'. Pasaden a, California 9lJU9, C S A. ABSTRACT. The synthetic-aperture rada r-inte rferometry tech nique is used to d etect the migratio n of the limit oCtida l fl exing, or hinge line, of the Il oating ice tong ue o fPetcr­ m ann Gle tsch er, a major o utlet g lacier of north Gree nl and. The hinge line is d e tected a utomMica ll y from differential inter[erogra m s using a model-lilling technique based on a n elastic-beam theory. The statistical noise o f the model fit is less tha n 3 mm, a nd the hinge line is m a pped with a precision of 30 m. ro ll owing a uto m a tic registrati on o f multi­ date image d a ta to a precisio n of 5 m , hinge-line mig rati on is subseque ntly detected with a precision o f 4 0 m in the ho rizonta l pl ane across the glacier width. The res ults show that the hinge line ofPeterma nn Gletscher mig ra tes back a nd fo rth w ith tide by ± 70 m , in excell cnt agreement II'ith the mig ration calcula ted frolll ocean tidcs prcdictcd by a tidal model combincd II'ith the g lacier surface a nd b asal slope measured by a n ice-sounding rad a r. Supc rimposed on the sho rt-term hinge-line migrati o n due to tide, I'IT d e tect a hinge-line retreat of 270 m in 3.87 years which ""tries across the g lacie r width by ± 120 m. Thc rctreat s uggests glacier thinning at a ra te oC 78±35 cm ice a 1. Coincid enta ll y, an analvsis of ice-volume flu xes indicates that the hi nge-line ice flux oCP etermann Gle tscher e xce~ d s its bala nce flu x by 0.88 ± I km:l ice a I, which in turn implies g lacier thinning at 83 ± 95 cm ice a 1 in the g lacie r lower reach es. Both Ill etho d s the refore suggest th at Petermann Gletsc her is curre ntly losing m ass to the ocean. INTRODUCTION p e riod. Thc ra d a r d a ta lI'ere coll ected by the European Space Agency (ESA)'s E a rth Re m ote Sensing Sa tellites The transitio n region between the grounded p a rt of a n ice (ERS-I a nd ERS-2) instruments. The m apping of the hinge sheet a nd its fl oating part, ofte n rcferredto as the g ro unding line is repeated a t different epochs a nd at diflc re nt tida l line, is fund a m enta l to the stability or a n ice sheet (Weert­ phases to sepa ra te the elTeCl of sho rt-te rm I'a ri ati on s in sea m an, 197+; H ug hes, 1977; Thomas a nd Bentl ey, 1978). Because level (associated with ocean tidc) from that or longer-te rm the glacier slo p es a rc typica ll y sm a ll at the g ro unding line, ch a nges in glacie r thickness which wc seek to es tima te. \Ve the hori zonta l p osition of the gr o unding line is sen sitive to a na lyze the errors associated with the m apping of the hi nge changes in ice thickness, sea level or elevatio n of the sea line a nd the detectio n of hinge-line mig rati on. The inCe rred bed (Hughes, 19 77; Thom as, 19 79). ch a nge in glacie r thickness is subsequently compa red to a n Locating a g lacier g ro unding line in the field, or using a na lys is of ice-\'o lume flu xes to p rO\'ide compleme nta ry remote-se nsing techniques, rem a ins a cha lleng ing exercise info rmation on the c urrent sta te oC the mass ba la nce of (e.g. Va ug ha n, 1994). As a result, the grounding lines of Petermann Gle tsche r. Anta rcti c a nd Greenland glacie rs a rc kn own o nl y with con­ siderabl e u ncerta i nt y. Syntheti c-a pe rture rada r illle rferometry is a new techni­ STUDY AREA que 10 1' measuring glacier displacements II'om sp ace at the millimete r lew'l over a period of just a few d ays (G oldstein Pe te rma nn Gle tsch er, located a t 8 1 ~ a nd 60 \ V o n the a nd others, 1993). O\'C r flo a ting tong ues and ice sh elves, the no rthwestern flank oCthe Gn:enl a nd ice shcl'l, is na med afte r measured g lacier displacem ent is a combinatio n oC creep the German geogra pher Dr A. Pete rma nn (Koch, 1928), Pr- ­ fl oll' a ndtida llllo ti on which can be se pa rated using differen­ termann Fj ord w as discol'C red o n 27 August 1871 during ti al (multiple ) interferogra m s (H a rtl and others, 199+). \\'ith H a ll 's U. S. Steam er Polaris ex peditio n (Bessels, 1876) but it difTc rentia l inte rferometry, the limit of tida l fl exing of the was not untilJune l876 that Coppinger a nd Fullo rd realized glacier, or hinge line, may be m easured at a n unprecedented tha t the fj ord was fill ed II'ith a glacie r. In 1892. Peary disco\,­ IeI TI of spa ti a l detail a nd accuracy, simulta neously across e redthatthe g lacier reached fa r into the "Inlandsis" (K och, the entire g lacie r width, with a uniform sampling scheme, 1940). over large a reas (Ri gnot, 199 6, 1998; Rig no t a nd others, Petermann Gletscher is one o f the longest glacie rs in the 1997). N o rthern Hemisphe re. Tt delTlops a n extensive fl oating to n­ Satell i te-ra dar interferom e try is used here to re fi ne our g ue (70 km long ), with a terminus o nl y a few mete rs a bove earli er m a pping of the hinge line of Peterma nn Gletsc her sea Ic\ -c l. In 1917, K och (1928) noted that the outermost pa rt (Rignot, 1996), a m aj or o utlet g lacier of north Greenland, o f the glacier beyond a line drawn between Cap e A g n es a nd deteCl its hinge-line mig ration o\'er a 4 year time (w here Porsild Gle tscher meets with Peterma nn Gle tschcr) Downloaded from https://www.cambridge.org/core. 01 Oct 2021 at 04:51:51, subject to the Cambridge Core terms of use. 469 JOlllllalofGlaciologv a nd Cape Coppinger was afl oat, with a smooth surface free Rignot (1996). In brief, we combinc two passages of the from cre,'asses (Fig. 1). The glacier grounding line is m ore or ERS satellite cohcrently to form a radar interferogram , less in the same position today (Rignot, 1996). Histori cal which is then corrcctcd for surface topography using a photographs a lso suggest th at little change in the glacier prior-determined precision digital el evati on model (DEM) ice-fro nt position occurred in the pas t 50 years (Higgins, of no rth Greenl and assembled by Ekholm (1996). DEM con­ 1991). trol points are uscd to refin e th e ini tial estimates of the orbit Although the histori cal record is suggestive of glacier se pa ration between thc successive passages of the satellite stability, Petermann Gl etse her is far from being a sluggish (the so-called interferometric base lines )obtained from the glacier (Weidick, 1995). It fl ows a t more than I km a I into ERS prccise orbit da ta distributed by the German Archi ve the Arcti c O cean (Higgins, 1991), fas ter than a ny other a nd Processing Facility (DPAF). glacier in north Greenl and, a nd is the largest dischargcr of Two to pography-corrected in te rferograms spanning the ice in north Greenla nd (Ri gnot a nd others, 1997). Like mos t same time interval arc then differenced to yield a difference other northern outlet glaciers, Pctermann Gletscher loses intcrferogram which measures onl y the tidal displ acement mass to the occan mostl y throug h basal melting of its fl oat­ of the glacier. We refer to this difference interferogram as a ing tongue (Rignot, 1996). Yet, nowhere in the north and "tide interferogram" in the res t of the paper. The success of northeast sectors of Greenl and a rc the inferred basal-melt diITcrencing relies o n the assumption that the glacier creep rates hi gher than on Petermann G letscher (Rignot and now rcmain s steady a nd continuous during the period of others, 1997). How the glacier can m aintain its m ass ba lance obser vation so that the displacement signal associated with while at the samc time fl owing rapidly to the ocean a nd sus­ creep remains the sam e in both topography-corrected inLer­ taining mass iyc rcmoval of ice from basal melting r emains ferograms and thereby cancels out when computing their unclear.