J ournalojClac iologl l, Fol. +4, No . 148, 1998

Hinge-line Illigration of Peterll1ann G letscher, north , 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 (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 comb