Determinación de la fuerza critica

V19 and V31 contain a total o f 16 and 21 craters, w ith 5 and 6 su rfic ia l ‘s p lo tc h ’ im p acts re s p e c tiv e ly . T he c ra te r a v erag e d e n s ity fo r th e c o m b in e d a re a o f th e tw o q u a d ra n g le s ( 1 4 .6 8 3 x 1 0 ^ km ^) is 1 c ra te r p e r 3.97x10^ km^. In c lu d in g splotches, the average density equals 1 im p act related featu re per 3.12x10^ km2.

S urficial splotch im pacts typically have a w eak in terio r rad ar b a c k s c a tte r su rro u n d e d by stro n g e r re tu rn in g m a te ria ls . T he d ep o sits have diffuse b oundaries and are ap p ro x im ately circu lar. Im p a c t c ra te rs w ith in th e stu d y a re a ra n g e in siz e fro m ap p ro x im ately 5 km to 100 km in d iam eter (m easured from the crater centre to the distal edge of the mean ejecta apron).

F o r c ra te rs > 3 0 km in d ia m e te r, th re e u n its m ay be d istin g u ish ed : cen tral p eak m aterials (c p ), c ra ter flo o r m aterials

( c f ) and eje cta m aterials ( c e ). C raters sm aller th an 35 km are m ark ed ce only. Both ce and cp have strong rad ar returns w ith a m o ttled hum m ocky appearance. C rater ejecta m aterials m ay have d eposits w ith a flow -like m orphology extending bey o n d the m ain ejecta m argin (e.g. Cunitz, V31, 14.4® N, 350.8® E, Figure 3.40). The flow m aterials m ay be topographically co n tro lled (e.g. E nid, V 31, 16.40® N; 352.20® E). C rater floor m aterials typically have a w eak ra d a r b ack sc atter. The c rater flo o r m ateria ls o f C u n itz show a p e rv a siv e fa b ric o f rid g es w ith a p p ro x im a te ly 2 km sp acin g (Figure 3.40). Surrounding m any im pact craters are rad ar dark or bright haloes some of w hich are parabolic in shape. In som e cases th e d is trib u tio n o f the d ark h a lo e s is c o n tro lle d by su rface fe a tu re s such as w rin k le rid g e s. E x c e p t w h e re b o u n d e d by topographic obstacles, these haloes have indistinct boundaries.

O f the 37 craters w ithin the m apping area, one is tectonically m odified by fractures (R hoda, V31, 11.50® N, 347.70® E), and one em b ay ed by flow m ateria ls (H eidi, V 31, 20.3 6 N, 3 5 0 .1 0 E) interpreted to originate from S if M ons or a nearby volcanic centre. W h ere a re la tio n can be e stab lish ed , c ra te r m a te ria ls ty p ic a lly postdate wrinkle ridge formation. No correlation between unit and

17.25 N

12.00 N 3 48.70 E

100 km 3 54.00 E

Figure 3.40 Cunitz and Enid craters. Cunitz is surrounded by a radar-bright surHcial deposit, ce - crater ejecta materials (which include flow-like materials to the west of the impact), cm - crater margin materials, cp - central peak materials and cf - crater floor materials. Enid is surrounded by a local radar-dark halo and has materials which are topographically controlled by the local regional plains fabric (south of the crater) (CI- MIDR.15N352;1 browse).

crater density is observed.

Interpretation

A part from H eidi and R hoda, the craters w ithin the m apping q u a d ra n g le s a re in te rp re te d to be lo c a lly y o u n g f e a tu re s . H o w ev er, as w ith tesse ra , an age re la tio n b e tw ee n in d iv id u a l c ra te rs c an n o t be e sta b lish e d . T he a v erag e d e n sity o f im p a c t related features w ithin the m apping quadrangles is sim ilar to th at fo r the global surface area of Venus, calculated as 1 crater related fe a tu re p e r 3.38x10^ km^ (calcu lated u sin g 1333 im p act rela te d featu res id en tified by Strom et al. (1994) from 98% o f the surface o f Venus).

S p lo tc h im p a c ts are in te rp re te d to be s u rfa c e m a te ria ls d istu rb ed by the incom ing shockw ave o f a b o lid e w hich eith er fo rm ed a crater too sm all to be d etected by the M ag ellan SA R ra d a r, or w h ich fa ile d to c re ate a c ra te r o w in g to b o lid e d isin te g ra tio n an d /o r v a p o risatio n b efo re im p a ct {S c h u ltz, 1992). R ad ar b rig h t and dark p arab o lic haloes are co n sid ered to have fo rm ed from crater fallo u t m aterials { S c h u ltz , 1992; C am pbell et a l., 1992). Flow m aterials associated w ith ejecta and the in terio r floors o f the larger craters (e.g. C unitz, B enton and H eilm an) are in te rp re te d as m elt m ateria ls fo rm ed d u rin g and sh o rtly a fte r im p a ct { S c h u lt z , 1992). The interior fabric seen w ithin C unitz is c o n s id e re d to h av e fo rm ed by th e c o o lin g an d su b s e q u e n t contraction of floor melt materials.

3.5 A quantitative M easure of the Amount of M aterials